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The characteristic of delayed airfoil stalls caused by the bio-inspired Wavy Leading-Edges (WLEs) has attracted extensive attention. This paper investigated the effect of WLEs on the aerodynamic performance and flow topologies of the airfoil through wind tunnel experiments, while also discussing the flow control mechanism of WLEs. The result shows that, at small Angle of Attack (AOA), the flow through the WLEs exhibits periodic and symmetrical characteristics, where flow vortices upwash at the trough and downwash at the crest, resulting in flow from the crest to the trough. Upwash leads to the formation of a localized three-dimensional laminar separation bubble (LSB) structure at the leading edge of the trough section. At large AOA after baseline airfoil stall, the flow on the airfoil surface of WLEs presents a two-period pattern along the spanwise direction, and the separation zone and the attachment zone appear alternately, indicating that the control effect of delayed stall is accomplished by reducing the separation zone on the airfoil surface. The alternating occurrence of the separation and attachment zones is the result of intricate interactions among flows passing through multiple WLEs. This interaction causes the convergence of high-momentum attached airflows on both sides, thereby constraining the spread of the separation from the leading edge and enabling the re-attachment of separated air. The research results of this paper provide a reference for researchers to reveal the flow control mechanism of WLEs more comprehensively. [ABSTRACT FROM AUTHOR]

One of the essential issues for spacecraft is maintaining the combustion chamber wall from exceeding acceptable temperatures. The cooling channels' thermal resistance and flow topology can be augmented by adding ribs to the heated surface. The present work proposed a Novel X-shape rib to enhance the thermal efficiency of hydrogen regenerative-cooling channels. The numerical investigation was carried out for the cooling system with rectangular and trapezoidal X-shape ribs at various attack angles (30 ° , 45 ° , and 60 °). The investigation findings demonstrated that both X-shape rib constructions optimized the cooling system's thermal efficiency. Since the trapezoidal X-shape ribs' chamfering angle reduces the resistance to coolant flow, the pressure loss of these ribs is less than rectangular ribs. Additionally, the heat transfer rate is maximum for 30 ° X-shape ribs, decreasing gradually with the increase of the X-shape rib's attack angle. Furthermore, compared to a smooth channel, the rectangular 30 ° X-shape ribs enhance the thermal efficiency by 44 % while reducing the wall temperature by 24 %. • The thermal efficiency of the channel with X-shape rib was numerically calculated. • Six X-shape ribs' performance was evaluated and compared to the smooth channel. • Heat transfer rate is maximum for 30 ° X ribs, inversely proportional to attack angle. • The rectangular 30 ° X ribs enhance thermal efficiency by 44 % relative to the flat channel. [ABSTRACT FROM AUTHOR]

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Moderate or Intense Low-oxygen Dilution (MILD) combustion has potential to achieve both high energy efficiency and ultra-low emissions. This analysis adopts the critical point theory to characterise the Flame-Self Interaction (FSI) events and flow topologies in turbulent, homogeneous mixture, n-Heptane MILD combustion using Direct Numerical Simulations (DNS) with reduced chemical mechanism. The local flame geometry has also been categorised using the mean and Gauss curvatures. It was found that the Tunnel Formation (TF) and Tunnel Closure (TC) topologies are the most probable FSI events at all values of the reaction progress variable c, while the Unburned Pocket (UP) and Burned Pocket (BP) topologies were mostly present towards the unburned and burned mixtures of the flame, respectively. Moreover, increasing the turbulence intensity did not result in any significant changes in the distribution of FSI events. Investigation of the flow topology distribution showed that the features associated with non-zero dilatation rate did not exist in the MILD cases considered. This is a consequence of the negligible thermal expansion effect due to the small temperature rise in MILD combustion cases. Increasing the dilution factor caused a reduction in the frequency of FSI events for all c levels. The distributions of flame self-interaction events in homogeneous mixture MILD combustion have been found to be significantly different from previously reported distributions for conventional turbulent premixed combustion. [ABSTRACT FROM AUTHOR]

In recent years there has been vigorous debate whether asymptotic transverse macrodispersion exists in steady three‐dimensional (3D) groundwater flows in the purely advective limit. This question is tied to the topology of 3D flow paths (termed the Lagrangian kinematics), specifically whether streamlines can undergo braiding motions or can wander freely in the transverse direction. In this study we determine which Darcy flows do admit asymptotic transverse macrodispersion for purely advective transport on the basis of the conductivity structure. We prove that porous media with smooth, locally isotropic hydraulic conductivity exhibit zero transverse macrodispersion under pure advection due to constraints on the Lagrangian kinematics of these flows, whereas either non‐smooth or locally anisotropic conductivity fields can generate transverse macrodispersion. This has implications for upscaling locally isotropic porous media to the block scale as this can result in a locally anisotropic conductivity, leading to non‐zero macrodispersion at the block scale that is spurious in that it does not arise for the fully resolved Darcy scale flow. We also show that conventional numerical methods for computation of particle trajectories do not explicitly preserve the kinematic constraints associated with locally isotropic Darcy flow, and propose a novel psuedo‐symplectic method that preserves these constraints. These results provide insights into the mechanisms that govern transverse macrodispersion in groundwater flow, and unify seemingly contradictory results in the literature in a consistent framework. These insights call into question the ability of smooth, locally isotropic conductivity fields to represent flow and transport in real heterogeneous porous media. Plain Language Summary: The spreading of solutes, colloids and pollutants in groundwater flow is key to both risk assessment and a fundamental understanding of chemical, biological and geophysical transport in the subsurface. In many scenarios the spreading of solutes transverse to the mean flow direction is dominated by hydrodynamic transport from diverging streamlines and the contribution from local dispersion is relatively small. Although it is well understood that such hydrodynamic transport (termed transverse macrodispersion) is zero in steady two‐dimensional groundwater flow for purely advective transport, there has been much debate in recent years as to whether transverse macrodispersion can occur in steady three‐dimensional groundwater flows. In this study we address this question by considering the kinematics associated with Darcy flow of heterogeneous porous with different conductivity structures and identify the structures that generate non‐zero transverse macrodispersion in the absence of local dispersion. We show that numerical predictions of transverse macrodispersion can be highly sensitive to the details of the numerical solvers used and propose a novel numerical method to mitigate against spurious predictions of transverse macrodispersion. These findings are used to reconcile seemingly contradictory results in the literature and provide insights into the suitability of commonly used models of groundwater flow and transport. Key Points: We prove that the transverse macrodispersion does not exist when the hydraulic conductivity field is smooth and locally isotropicWe show that conventional numerical methods do not preserve this constraint, and propose novel numerical methods that doThese insights reconcile previous contradictory results and question the ability of some conductivity models to represent real porous media [ABSTRACT FROM AUTHOR]

The characteristic of delayed airfoil stalls caused by the bio-inspired Wavy Leading-Edges (WLEs) has attracted extensive attention. This paper investigated the effect of WLEs on the aerodynamic performance and flow topologies of the airfoil through wind tunnel experiments, while also discussing the flow control mechanism of WLEs. The result shows that, at small Angle of Attack (AOA), the flow through the WLEs exhibits periodic and symmetrical characteristics, where flow vortices upwash at the trough and downwash at the crest, resulting in flow from the crest to the trough. Upwash leads to the formation of a localized three-dimensional laminar separation bubble (LSB) structure at the leading edge of the trough section. At large AOA after baseline airfoil stall, the flow on the airfoil surface of WLEs presents a two-period pattern along the spanwise direction, and the separation zone and the attachment zone appear alternately, indicating that the control effect of delayed stall is accomplished by reducing the separation zone on the airfoil surface. The alternating occurrence of the separation and attachment zones is the result of intricate interactions among flows passing through multiple WLEs. This interaction causes the convergence of high-momentum attached airflows on both sides, thereby constraining the spread of the separation from the leading edge and enabling the re-attachment of separated air. The research results of this paper provide a reference for researchers to reveal the flow control mechanism of WLEs more comprehensively.

A numerical investigation of flow topology in open-type cavities with length-to-depth ratio L/h1 = 4 was performed in the Reynolds number range of 10–1000. Cavities with differently rounded corners were chosen for simulation. Three-dimensional numerical simulations were performed to analyse flow topology in different planes. A series of experiments was performed to ensure the validity of numerical simulations. Both numerical simulations and physical experiments were conducted with water as the working fluid. Since the results agreed acceptably, further simulations were performed. The main goal of this study was to investigate and highlight the influence of rounded cavity corners on the topology and stability of flow. Analysis revealed that fully rounded upper cavity corners decrease pressure loss compared to other investigated cases; therefore, the velocity of the main flow is increased. Additionally, fully rounded upper corners form a notably smaller recirculation zone compared to other investigated cases. Flow stability analysis showed that fully rounded cavity bottom corners negatively impact flow stability by increasing the intensity of turbulence. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a potent ozone‐depleting greenhouse gas produced by incomplete denitrification. Recent works on riverine N2O emissions focus mainly on contributions from in‐channel, benthic, and fluvial hyporheic environments under assumptions of steady‐state conditions and homogeneous sediment hydraulic conductivity (K). However, riparian floodplains are also a potentially important N2O source characterized by complex sediment heterogeneity and dynamic surface and groundwater interactions. We use numerical flow and reactive transport models to investigate the influence of complex sedimentary architecture and high‐flow events (e.g., storms) on N2O production. We interpret the correlation between flow and solute fields with the flow topological Okubo‐Weiss metric (OW) and the scalar dissipation rate weighted by soil organic matter (OM) fraction and soil saturation. We model a heterogeneous riparian floodplain based on field observations from the Theis Environmental Monitoring and Modeling Site, Ohio, USA. N2O production is greatest within intermediate‐K sediments (e.g., sands) where denitrification rates are highest, and emissions increase by more than an order of magnitude during storms. Sensitivity analysis reveals that the denitrification rate is most influential for N2O flux, accounting for nearly 46% of the variance in production rates. Denitrification rates adapt to spatial changes in the flow topology (measured by OW) related to sediment heterogeneity and are strongly influenced by subsurface mixing dynamics. Mixing is greatest in shear strain‐dominated regions, while vorticity promotes OM dissolution and prolongs residence times. Accurate lithologic representation is imperative for characterizing subsurface N2O production dynamics, especially given growing concern regarding climate change driven hydrologic changes within watersheds worldwide. Key Points: Subsurface N2O production increases in response to storms, but sediment heterogeneity controls the location and magnitude of emissionsAccurate representation of heterogeneity is important for characterizing subsurface nitrogen transformations and N2O production dynamicsHeterogeneity introduces shear into the groundwater flow topology, enhancing mixing and increasing the spatial extent of denitrification [ABSTRACT FROM AUTHOR]

This article presents a preliminary approximation of the effect in blood flow produced by tortuosity in the blood vessels. This roughness is related to certain diseases (such as tumours) and the modelling of the flow properties can help to characterize the existence of such diseases. Malignancy provokes local changes to vessel shape and characteristic vessel tortuosity appears early during tumour development. Numerical simulations have been carried out, where the elastic behaviour modelling the surface of blood vessels has been represented using several geometries with different wavy shapes. The complexity of the topology patterns, which could be related with the different particle paths found in different geometries, encourages to continue this research using novel machine learning tools to predict and connect the main flow instabilities with the tumour evolution.

The flow topology and thermohydraulic performance of a novel designed punched delta-winglet (P-DW) placed on the absorber of a solar air heater duct are numerically explored. The effects of geometrical parameters, namely, the relative winglet pitch, PR = 1–2 and the relative punched hole size, dR = 0–0.583 at a single value of blockage ratio, BR = 0.48 and attack angle, α = 30° on thermal characteristics are proposed for Reynolds number from 4000 to 24,000. Among several turbulence models, the simulation has shown that the realizable k–ε turbulence model is favorable with respect to measurements. For flow patterns, the P-DW produces several counter-spinning vortices helping induce the impinging jets onto the absorber surface whilst for thermal behaviors, the decline of PR and dR leads to the rise in the friction factor (f) and Nusselt number (Nu). The P-DW provides greater Nu and f than the plain flat plate by 17.1–78.21 and 3.92–5.9 times, respectively and gives the highest performance around 2.1. Further, the P-DW is modified by covering the punched hole partially with a circular flap, called the flapped delta-winglet (F-DW) and this F-DW yields the greatest performance around 2.16 higher than the P-DW about 2.9%.

• LBM simulations were carried out to explore pore-scale dispersion and mixing in a two-phase co-flow system in porous media. • The spatial-temporal evolution of tracer fronts is resolved over a range of saturation values and Péclet numbers. • At low S w , ramified dispersive finger structures were observed, regulating solute transport into oriented channels while weakening its transport into diffusive barriers of disconnected/isolated clusters. • At high S w , much uniform invading fronts with finer fingers were discovered, strengthening the dispersion process through the merging of fingertips and transverse mixing. Phase distributions and hydrodynamic flow field at the pore scale can greatly affect the tracer mass transfer process in a two-phase co-flow system, yet the underlining mechanism is not well understood. In this work, dispersion of non-reactive tracers in the aqueous-phase flow field in porous structures (reconstructed from high-resolution Micro-CT images) is numerically investigated under an immiscible two-phase co-flow condition. The spatial-temporal evolution of tracer fronts is observed over a range of S w from 0.45 to 0.95, and Péclet numbers from 130 to 650. The topology of the two-phase flow field is evaluated in terms of the fluid-phase morphology, rescaled Eulerian velocity distributions, and the WP fractional flow rate (f w)-saturation (S w) curve, etc. Meanwhile, solute dispersion and mixing are comprehensively studied through tracer breakthrough curves, residence time distribution, dispersion coefficient, mean concentration gradient, and dilution index. A well-defined Fickian transport regime was achieved for S w ≥ 0.75, where uniform invading fronts with finer fingers were discovered, strengthening the dispersion process through merging of fingertips and transverse mixing. In contrast, multipeak and strong tails were discovered for lower saturations of S w = 0.45 and S w = 0.55, where ramified dispersive finger structures were observed, regulating solute transport into oriented channels while weakening its transport into diffusive barriers of disconnected/isolated clusters. In addition, transverse mixing contributes to 25 %-50 % of the overall mixing, which increases with S w due to finger merging. The majority of mixing takes place in the well-connected branches, while ganglia and singlet may act as sources and sinks that weakens tracer transport especially for low S w. The dilution index is 2 or 3 orders larger in the branches than that in the isolated clusters of ganglia or singlet. This research provides insight into the relationship between solute transport properties and two-phase hydrodynamics at the pore scale. [ABSTRACT FROM AUTHOR]

A numerical investigation of flow topology in open-type cavities with length-to-depth ratio L/h1 = 4 was performed in the Reynolds number range of 10–1000. Cavities with differently rounded corners were chosen for simulation. Three-dimensional numerical simulations were performed to analyse flow topology in different planes. A series of experiments was performed to ensure the validity of numerical simulations. Both numerical simulations and physical experiments were conducted with water as the working fluid. Since the results agreed acceptably, further simulations were performed. The main goal of this study was to investigate and highlight the influence of rounded cavity corners on the topology and stability of flow. Analysis revealed that fully rounded upper cavity corners decrease pressure loss compared to other investigated cases; therefore, the velocity of the main flow is increased. Additionally, fully rounded upper corners form a notably smaller recirculation zone compared to other investigated cases. Flow stability analysis showed that fully rounded cavity bottom corners negatively impact flow stability by increasing the intensity of turbulence.

The purpose of this paper is to demonstrate the effects of thermal expansion, as a result of heat release arising from exothermic chemical reactions, on the underlying turbulent fluid dynamics and its modelling in the case of turbulent premixed combustion. The thermal expansion due to heat release gives rise to predominantly positive values of dilatation rate within turbulent premixed flames, which has been shown to have significant implications on the flow topology distributions, and turbulent kinetic energy and enstrophy evolutions. It has been demonstrated that the magnitude of predominantly positive dilatation rate provides the measure of the strength of thermal expansion. The influence of thermal expansion on fluid turbulence has been shown to strengthen with decreasing values of Karlovitz number and characteristic Lewis number, and with increasing density ratio between unburned and burned gases. This is reflected in the weakening of the contributions of flow topologies, which are obtained only for positive values of dilatation rate, with increasing Karlovitz number. The thermal expansion within premixed turbulent flames not only induces mostly positive dilatation rate but also induces a flame-induced pressure gradient due to flame normal acceleration. The correlation between the pressure and dilatation fluctuations, and the vector product between density and pressure gradients significantly affect the evolutions of turbulent kinetic energy and enstrophy within turbulent premixed flames through pressure-dilatation and baroclinic torque terms, respectively. The relative contributions of pressure-dilatation and baroclinic torque in comparison to the magnitudes of the other terms in the turbulent kinetic energy and enstrophy transport equations, respectively strengthen with decreasing values of Karlovitz and characteristic Lewis numbers. This leads to significant augmentations of turbulent kinetic energy and enstrophy within the flame brush for small values of Karlovitz and characteristic Lewis numbers, but both turbulent kinetic energy and enstrophy decay from the unburned to the burned gas side of the flame brush for large values of Karlovitz and characteristic Lewis numbers. The heat release within premixed flames also induces significant anisotropy of sub-grid stresses and affects their alignments with resolved strain rates. This anisotropy plays a key role in the modelling of sub-grid stresses and the explicit closure of the isotropic part of the sub-grid stress has been demonstrated to improve the performance of sub-grid stress and turbulent kinetic energy closures. Moreover, the usual dynamic modelling techniques, which are used for non-reacting turbulent flows, have been shown to not be suitable for turbulent premixed flames. Furthermore, the velocity increase across the flame due to flame normal acceleration may induce counter-gradient transport for turbulent kinetic energy, reactive scalars, scalar gradients and scalar variances in premixed turbulent flames under some conditions. The propensity of counter-gradient transport increases with decreasing values of root-mean-square turbulent velocity and characteristic Lewis number. It has been found that vorticity aligns predominantly with the intermediate principal strain rate eigendirection but the relative extents of alignment of vorticity with the most extensive and the most compressive principal strain rate eigendirections change in response to the strength of thermal expansion. It has been found that dilatation rate almost equates to the most extensive strain rate for small sub-unity Lewis numbers and for the combination of large Damköhler and small Karlovitz numbers, and under these conditions vorticity shows no alignment with the most extensive principal strain rate eigendirection but an increased collinear alignment with the most compressive principal strain rate eigendirection is obtained. By contrast, for the combination of high Karlovitz number and low Damköhler number in the flames with Lewis number close to unity, vorticity shows an increased collinear alignment with the most extensive principal direction in the reaction zone where the effects of heat release are strong. The strengthening of flame normal acceleration in comparison to turbulent straining with increasing values of density ratio, Damköhler number and decreasing Lewis number makes the reactive scalar gradient align preferentially with the most extensive principal strain rate eigendirection, which is in contrast to preferential collinear alignment of the passive scalar gradient with the most compressive principal strain rate eigendirection. For high Karlovitz number, the reactive scalar gradient alignment starts to resemble the behaviour observed in the case of passive scalar mixing. The influence of thermal expansion on the alignment characteristics of vorticity and reactive scalar gradient with local principal strain rate eigendirections dictates the statistics of vortex-stretching term in the enstrophy transport equation and normal strain rate contributions in the scalar dissipation rate and flame surface density transport equations, respectively. Based on the aforementioned fundamental physical information regarding the thermal expansion effects on fluid turbulence in premixed combustion, it has been argued that turbulence and combustion modelling are closely interlinked in turbulent premixed combustion. Therefore, it might be necessary to alter and adapt both turbulence and combustion modelling strategies while moving from one combustion regime to the other. [ABSTRACT FROM AUTHOR]

In traditional reaction flasks, the reaction rate of macromolecular compounds is dictated by the concentrations and distribution of reactants along with their intrinsic reaction kinetics. Controlling reaction kinetics in microfluidic droplets has been proposed through the regulation of flow dynamics, but is yet to be demonstrated experimentally. Here, this hypothesis is verified by accelerating or suppressing macromolecular reactant mixing in microfluidic droplets. The control of reaction kinetics through the modulation of flow topology in microdroplets may enable further developments in modeling macromolecular systems. [ABSTRACT FROM AUTHOR]

Tortuosity in coronary artery has been found to be greatly related to the potential sites of stenosis in these last years. Many investigations have been carried out based on the tool of Computational Fluid Dynamics (CFD) mainly focusing on the influences of curved artery in blood flow. Within the limited investigations of coupling between stenosis and tortuosity, the stenosis has been considered to be located at the tortuous segment. However, with recent clinical studies, the case of stenosis occurred at non-tortuous segment before tortuosities has been confirmed which has not been paid enough attention yet. Therefore, the present study aims to investigate the disturbed streamlines and hemodynamics in curved and spiral artery considering symmetrical and asymmetrical stenosis upstream these tortuosities. Different stenosis severities, pulse rates and distances between stenosis and tortuosity as controlling parameters have been studied. The distribution of time averaged wall shear stress (TAWSS) and streamlines through tortuous segment have been displayed in order to determine the potential disease sites. Artery surface of TAWSS below critical value has been quantified as well to evaluate the risks of atherosclerosis. The results reveal that larger artery surface of TAWSS below critical value generally goes with smaller pulse rate, larger stenosis severity and distance between stenosis and tortuosity both for curved and spiral artery. However, exceptions were found in the cases of distance of 6 mm in curved artery with symmetrical stenosis and stenosis severity of 50% in spiral artery. Moreover, the spiral tortuosity tends to suppress the potential risks of atherosclerosis compared to curved tortuosity. [ABSTRACT FROM AUTHOR]

In the present work, three-dimensional turbulent non-premixed oblique slot-jet flames impinging at a wall were investigated using direct numerical simulation (DNS). Two cases are considered with the Damköhler number (Da) of case A being twice that of case B. A 17 species and 73-step mechanism for methane combustion was employed in the simulations. It was found that flame extinction in case B is more prominent compared to case A. Reignition in the lower branch of combustion for case A occurs when the scalar dissipation rate relaxes, while no reignition occurs in the lower branch for case B due to excessive scalar dissipation rate. A method was proposed to identify the flame quenching edges of turbulent non-premixed flames in wall-bounded flows based on the intersections of mixture fraction and OH mass fraction iso-surfaces. The flame/wall interactions were examined in terms of the quenching distance and the wall heat flux along the quenching edges. There is essentially no flame/wall interaction in case B due to the extinction caused by excessive turbulent mixing. In contrast, significant interactions between flames and the wall are observed in case A. The quenching distance is found to be negatively correlated with wall heat flux as previously reported in turbulent premixed flames. The influence of chemical reactions and wall on flow topologies was identified. The FS/U and FC/U topologies are found near flame edges, and the NNN/U topology appears when reignition occurs. The vortex-dominant topologies, FC/U and FS/S, play an increasingly important role as the jet turbulence develops. [ABSTRACT FROM AUTHOR]

This numerical study reports the aerodynamic properties of a hybrid airship. The hybrid airships were designed by combining two semi-ellipsoids with a semi-discoid as the base model. From the base model, three different geometrics were identified to study their aerodynamic characteristics. A circular slot was provided between the pressure side and the suction side of the airship. The objective of this study was to realize the flow behavior, aerodynamic characteristics, and stability properties of such slotted hybrid flying vehicles. Interestingly, the results imply that the lift coefficient increases with an increase in the angle of attack for the slotted configurations; this is because the flow separation is delayed due to the slot opening, which in turn is due to the flow of energies from the high-pressure region to the bottom through the slots. The delayed stall angle was 50 degrees, which was 10% more than that of the base model. Aerodynamic characteristics are discussed based on surface pressure, coefficient of lift, and coefficient of drag for various slotted hybrid airships.

Flamelet-based combustion models have been extensively used in the modeling of turbulent non-premixed combustion due to their notable advantages in reducing computational cost. The classical one-dimensional flamelet equations are derived based on an assumption: flame-normal transport of thermochemical state variables in mixture fraction gradient direction dominates over flame-tangential transport along iso-surfaces of mixture fraction. The motivation of this work is to quantify the contribution of tangential diffusion (TD) and seek the source of TD, since several recent studies have shown that TD effects may play an important role in the regions with large curvature and finite-rate chemistry. In this work, the most probable cause and effect chain that elucidates the source of TD in a turbulent non-premixed jet flame is proposed for the first time from a flow topology perspective. To this end, local flow topology and its effects on scalar structure are first investigated. It is found that the local flow structures with unstable focus compressing (UFC) topology have the highest probability of large compressive strain and that the negative correlation between curvature and strain strongly depends on local flow topology, scalar structure and reaction. Then, the influence of flow topology and scalar structure on the relevance of TD is assessed with the angle between reactive scalars and mixture fraction. The results show that TD effects mainly exist in the regions with high curvatures and low strain rates or scalar dissipation rates. These findings suggest that the local flow structures with low probability of UFC topology tend to cause significant flame-tangential diffusion in the turbulent non-premixed flame. [ABSTRACT FROM AUTHOR]

Three different computational fluid dynamics (CFD) methods are assessed for their ability to predict topological flow features in idealized street canyons with uneven building heights. Mean velocity-fields from step-up (i.e., a high-rise building downwind of a low-rise building) and step-down (i.e., a low-rise building downwind of a high-rise building) street canyons are evaluated against high-spatial-resolution wind-tunnel data. Each method represents a different level of flow physics using: a mass-conserving model entitled Quick Urban & Industrial Complex wind model (QUIC-URB), a Reynolds-averaged Navier-Stokes (RANS) model, and a large-eddy simulation (LES) model. A new metric that represents the equally weighted trade-off between accuracy and efficiency is introduced to evaluate the CFD methods' capabilities to capture major-flow topological features in uneven building height street canyons. For step-up street canyons, all three methods qualitatively predict primary topological features, however, none simultaneously capture all secondary features. For step-up street canyons and step-down street canyons with narrow-streets, QUIC-URB captures most of the primary flow topological features including stagnation and saddle points and rooftop recirculation zones. RANS captures primary vortices for step-up street canyons and step-down street canyons with wide-streets. LES is computationally costly but it is the only method that successfully predicts secondary flow topological characteristics for step-down street canyons with wide-streets. When examining our trade-off metric, QUIC-URB has the highest score for step-up street canyons, while QUIC-URB and RANS have equally high trade-off scores for step-down street canyons with narrow-streets, and QUIC-URB and LES have nearly equal trade-off scores for step-down street canyons with wide-streets. [ABSTRACT FROM AUTHOR]

Abstract The heat transfer rate, flow topology, friction loss and thermal performance factor (TEF) of newly design wavy-triangular ribs mounted on an absorber plate in a solar air heater channel were numerically investigated. The effects of the blockage ratio, B R = 0.02–0.1 and the pitch ratio, P R = 0.4–1.5 were studied in the turbulent flow region, Re = 3000–20,000. The numerical results showed that the RNG k-ε turbulent model is recommended for the present case since the model gives the most accurate predictions compared to other turbulent models. The wavy-triangular ribs induced vortex flows that effectively disrupted the thermal boundary layers leading to enhanced heat transfer rate from the absorber plate to the air. Increasing the Reynolds number (Re) and blockage ratio (B R), while decreasing the pitch ratio (P R) led to increased vortex strength that helps in reducing the thermal boundary layer thickness, thus promoting heat transfer. Over the range investigated, the channel with wavy-triangular ribs provided higher friction factors (f) and Nusselt numbers (Nu) than a smooth channel by 5.8–304% and 13.8–171%, respectively. In addition, the ribs with a blockage ratio of B R = 0.07 and a pitch ratio of P R = 0.5 gave the highest TEF of 2.62 at Re = 3000. Graphical abstract Image 1 Highlights • Solar air heater channel with wavy-triangular ribs mounted on an absorber plate is proposed. • Influence of blockage ratio (B R) and the pitch ratio (P R) is analyzed. • Wavy-triangular ribs induced vortex flows that effectively disrupted the thermal boundary layers leading to enhanced heat transfer rate. • The ribs with B R = 0.07 and P R = 0.5 gave the highest TEF of 2.62. [ABSTRACT FROM AUTHOR]

Characterization of flow topology is essential to understand the effects of the heterogeneity and dimensionality of geological formations on the mixing of inert solute clouds in these same geological formations. In this work, we numerically study two indicators of flow topology, the averaged vorticity magnitude | ω | av and the averaged positive second invariant Q av of the deformation tensor ∇ u , in steady Darcy flows through exponentially correlated lognormal hydraulic conductivity fields K. Our numerical results allow us to establish the relationships between the two indicators considered here and the hydraulic conductivity variance σ 2 in 2D and 3D, highlighting the role played by the spatial structure of these porous media on flow topology, and indirectly on mixing. This work leads us to assess the maximum dilution index E max , indicator of mixing, theoretically known to increase monotonically in steady Darcy flows through isotropic heterogeneous porous media. Our numerical results allow us to test this hypothesis by establishing the relationship between the slope a of maximum dilution index E max and the averaged positive second invariant Q av of deformation tensor ∇ u. The parameters of this relationship depend on molecular diffusion and dimensionality of problem considered. [ABSTRACT FROM AUTHOR]

Abstract The Favre-averaged scalar dissipation rate transport conditional on local flow topologies in premixed turbulent flames has been analysed based on a detailed chemistry Direct Numerical Simulation database of statistically planar turbulent hydrogen-air premixed flames with an equivalence ratio of 0.7 representing the corrugated flamelets, thin reaction zones and broken reaction zones regimes of premixed turbulent combustion. The local flow topologies have been categorised by the values of the three invariants of the velocity gradient tensor and the statistical behaviour of the Favre-averaged scalar dissipation rate conditional on these flow topologies has been analysed in detail for different choices of reaction progress variable. The qualitative behaviour of the scalar-turbulence interaction term in the Favre-averaged scalar dissipation rate transport equation has been found to be affected by the regime of combustion, whereas the chemical reaction rate gradient contribution to the scalar dissipation rate transport has been found to be affected by the choice of the reaction progress variable. The topologies, which exist for all values of dilatation rate, contribute significantly to the Favre-averaged scalar dissipation rate transport in premixed turbulent flames for all regimes of combustion. In addition, the flow topologies, which are obtained only for positive values of dilatation rate, contribute significantly to the Favre-averaged scalar dissipation rate transport in the case representing the corrugated flamelets regime combustion. An unstable nodal flow topology, which is representative of a counter-flow configuration, has been found to be a dominant contributor to the Favre-averaged scalar dissipation rate transport for all regimes of combustion irrespective of the choice of reaction progress variable. [ABSTRACT FROM AUTHOR]

The importance of stagnation points for characterizing groundwater flow patterns has long been recognized. However, the possible streamline configurations near stagnation points under the constraints of Darcy's law have not been thoroughly investigated. We fill this gap by conducting a systematic analysis of groundwater flow patterns near stagnation points in two and three dimensions. The approach borrows ideas from dynamical systems theory, as often done in fluid mechanics. The most general form of Darcy's law, which applies to variable‐density, compressible fluids flowing through heterogeneous, anisotropic, compressible porous media, is first considered. Under these conditions, there are no major restrictions on the possible flow patterns near stagnation points. The common types of stagnation points are thus minimums (spiral or nonspiral), maximums (spiral or nonspiral), and saddles (in three dimensions, these can be converging or diverging and spiral or nonspiral). The implications of dealing with more restrictive fluid and porous medium properties are then systematically investigated. In particular, important restrictions on the possible types of stagnation points exist when the flow is constant density or divergence free. The theoretical analysis is complemented by a series of examples of groundwater flow fields in which different types of stagnation point arise. The findings highlight key differences between two‐dimensional and three‐dimensional flows and provide new insights on the patterns of variable‐density groundwater flow. The fundamental knowledge on groundwater flow patterns gained from this study will benefit both theoretical and applied studies of groundwater flow and solute transport. Plain Language Summary: Groundwater flow patterns reveal how water flows and solutes are transported in the subsurface. Therefore, characterizing them allows for understanding the origin and fate of water and solutes in the subsurface. Such information is absolutely essential for developing appropriate water resources management strategies. This paper investigates stagnation points in groundwater flow fields, where the flux is zero. Although the flux at these points is zero, these points are extremely important to identify and analyze in order to characterize the flow patterns. The reason is that at stagnation points, groundwater flow can converge, diverge, or abruptly change its course. This is in contrast with other (nonzero flux) points, at which groundwater locally flows uniformly. This article provides for the first time a thorough investigation of the possible flow patterns near stagnation points in porous media in two and three dimensions. It thus advances fundamental knowledge on groundwater flow patterns that will benefit both theoretical and applied studies of groundwater flow and solute transport. Key Points: Flow patterns near stagnation points are systematically studied and classified in general Darcy flow in two and three dimensionsRestrictions on the possible flow patterns near stagnation points are identified depending on fluid and porous medium propertiesDifferent types of stagnation points are illustrated in practical examples of groundwater flow [ABSTRACT FROM AUTHOR]

Direct numerical simulations (DNS) of oblique wall quenching of a turbulent V-flame and head-on quenching (HOQ) of a statistically planar flame by isothermal inert walls have been utilized to analyze the statistics of wall heat flux, flame quenching distance in terms of the distributions of flow topologies and their contributions to the wall heat flux. The flow topologies have been categorized into eight generic flow configurations (i.e., S1-S8) in terms of three invariants of the velocity gradient tensor (i.e., first, second and third P, Q, and R, respectively). It has been found that nodal (i.e., strain rate dominated) flow topologies are major contributors to the wall heat flux when it attains large magnitude in the HOQ configuration, whereas focal (i.e., vorticity-dominated) topologies contribute significantly to the wall heat flux in the case of oblique flame quenching. These differences in the heat transfer mechanisms contribute to the differences in wall heat flux and flame quenching distance between HOQ and oblique quenching configurations. The maximum wall heat flux magnitude in the case of oblique flame quenching has been found to be greater than that in the corresponding turbulent HOQ case. By contrast, the minimum wall Peclet number, which quantifies the flame quenching distance, in the case of oblique quenching has been found to be smaller than that in the case of HOQ. [ABSTRACT FROM AUTHOR]

Highlights • Endwall heat transfer and flow topologies around a short pin-fin subjected to an impingement jet were investigated. • Different endwall flow exist depending on jet-pin-fin spacing and pin-fin relative size. • Short spacing or large diameters cause jets on the endwall radiating from cylinder surface. • Flow field and heat transfer distribution is strictly different to that of horseshoe vortex system. • Longer spacing or smaller pin-fins cause flow and heat transfer distribution to resemble that of horseshoe vortex system. Abstract A short pin-fin array has been used to improve the internal cooling characteristics at the trailing edge of some gas turbine blade designs. In such a cooling scheme, the pin-fin array that is sandwiched by the turbine blade's inner surfaces, experiences a uniform-like coolant stream. The local elevation of internal heat transfer especially on the end-walls (i.e., inner blade surfaces) at the trailing edge is achieved predominantly by horseshoe vortex-type secondary flows whose fluidic behavior has been well established. A modification to this cooling scheme has been made by introducing a blockage upstream, causing multiple jets to impinge on the pin-fins – the blockage jets. Previous studies on the internal cooling scheme employing the blockage jets have assumed that the end-wall flow topology is similar to that formed by the horseshoe vortex-type secondary flows due to similar local heat transfer distributions. However, there is no detailed and sufficient acknowledgement made of the lack of an approaching boundary layer. Therefore, the present study experimentally investigates local heat transfer around a single short pin-fin subjected to a fully turbulent jet impingement simulating the blockage jet impingement and demonstrates that the end-wall flow topology loosely resembles that formed by a horseshoe vortex system and is strictly different, depending on the distance between the jet exit and the pin-fin, relative to the length of the jet's potential core. [ABSTRACT FROM AUTHOR]

Abstract The flow around a Ground Transport System (GTS), is numerically investigated using steady RANS model; k − ω SST, at a Reynolds number of ∼ 2 × 10 6 . This paper focuses on the effect of crosswinds on the near-wake structure of the GTS with and without boat-tails. Upon the emanation of crosswinds, a quadratic increase in the drag coefficient ( C D ) was observed, as a function of the yaw angle. Such an increase is attributed to the break in the symmetry of near-wake structure and three, streamwise vortices emanating from the leading edges of the GTS. Boat-tail with a slant angle of 15 ° , at zero yaw, has resulted in a ( C D ) reduction of up to ∼ 50 % , relative to the baseline GTS. The reduction is consistent with previous studies on various simplified geometries. Such reduction is a resultant of a smaller wake length, coupled with an overall increase in the pressure in the wake, consequently increasing the mean base pressure coefficient. Higher boat-tail angles have resulted in an increase in C D , whereas, under crosswind, ∼ 40 % reduction in ( C D ) is observed. Boat-tails have additionally resulted in a quasi-symmetric near-wake structure, under crosswinds, acting as a blockage and preventing the interaction between the three streamwise vortices and the near-wake. Highlights • Effect of crosswinds on the near-wake structure of the GTS is investigated. • Three streamwise vortices result in a change in the near-wake structure. • The effect of a boat-tail's length and slant angle are additionally investigated. • Under no crosswinds, the slant angle with highest drag reduction was identified. • Under crosswinds, boat-tails result in restoring the symmetry in the near-wake. [ABSTRACT FROM AUTHOR]

Highlights • Flow topology was built on the boattail surfaces. • Three distinctive flow regimes were observed. • The separation bubble develops in size with growth of the angles. • The length of separation bubble increases linearly with the angles. Abstract The surface flow pattern over a conical boattail on an axisymmetric body was investigated experimentally under low-speed and turbulent-boundary-layer conditions. Seven conical boattails with the same length but different angles from 10° to 22° were tested at a Reynolds number around 4.3 × 104, based on the model diameter. The study used the global luminescent oil-film (GLOF) skin friction measurement technique. The skin friction fields were measured and the corresponding flow topologies were extracted from the GLOF measurements. The effect of oil-film thickness on the separation position was also evaluated. Experimental results showed three different flow types on the boattail surface: (1) flow without separation, (2) flow with a separation bubble, and (3) fully separated flow. The critical angles for the transitions are discussed and compared with classic results for similar boattail models. The separation bubble generated at moderate boattail angles was observed for what we believe to be the first time under low-speed conditions, and the flow topology was clearly shown by the GLOF results. The azimuthally-averaged skin friction projected on the centerline showed different trends inside and behind the reattachment position when the boattail angle increased. [ABSTRACT FROM AUTHOR]

Balancing accuracy and computational cost is a challenge in the modelling of turbulent flows. A widely used method for turbulence modelling is large–eddy simulation (LES). LES allows one to describe large scale flow features at a reasonable computational cost compared to the more accurate direct numerical simulation (DNS), making it a popular choice for engineering applications. One strategy to balance accuracy and cost with LES is through the use of mesh adaptivity, which allows the degrees of freedom in a problem to be reduced by changing spatial discretisation. However, mesh adaptivity can affect accuracy when using the standard Smagorinsky LES model with an implicit filter, considering that the parameter C s is highly dependent on the filter width, which depends on mesh resolution. This work is aimed to develop an LES model that does not require any user–defined parameters and is suitable for mesh adaptivity with implicit filter. In this study we introduce a parameter–free LES model incorporating an anisotropic eddy–viscosity formulation combined with anisotropic mesh adaptivity. In our model, the parameter C s in the eddy–viscosity formulation of the Smagorinsky model, is replaced by a function that evaluates the relative location of turbulence fluctuations in each element with respect to the turbulence spectrum inertial range. The anisotropic formulation of the eddy–viscosity allows for the application of an appropriate filter width in different directions, improving accuracy. Additionally, the mesh adaptivity algorithm assesses the local turbulence fluctuations via local Reynolds number and vortex identification criteria. This assessment leads to the refinement of regions with higher turbulence fluctuations down to the smallest scale limit in the inertial range in the corresponding direction, and also leads to the coarsening of regions without turbulence fluctuation up to largest scale limit in the inertial range. This method is tested using a flow past a sphere test case. The results are compared both qualitatively and quantitatively to results obtained with the standard Smagorinsky model, and demonstrate the better performance of our method with lower computational cost. This allows us to simulate large Reynolds number cases and compare our quantitative results to experimental results found in the literature, emphasising that our method produces good results at reasonable computational cost. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to develop design guidelines for flood control channel height in the vicinity of the confluence of a submerged drainage pipe and a flood control channel. The water exchange in the confluence of an open channel with a lateral drainage pipe produces unique hydraulic characteristics, ultimately affecting the water surface elevation in the channel. An accurate prediction of the water surface elevation is essential in the successful design of a high-velocity channel. By performing several experiments, and utilizing a numerical model (FLOW-3D), this study investigated the impact of submerged lateral drainage pipe discharges into rectangular open channels on flow topology in the confluence hydrodynamics zone (CHZ). The experiments were conducted in different flume and junction configurations and flow conditions. Moreover, the simulations were performed on actual size channels with different channel, pipe, and junction configurations and flow conditions. The flow topology in the CHZ was found to be highly influenced by the junction angle, as well as the momentum ratios of the channel flow and the pipe flow. The findings of this study were used to develop conservative design curves for channel confluences with lateral drainage pipe inlets. The curves can be used to estimate water surface elevation rise in different channel and pipe configurations with different flow conditions to determine the channel wall heights required to contain flows in the vicinity of laterals.

The distribution of flow topologies within the flame, and their evolution with flame quenching have been analysed using a Direct Numerical Simulation (DNS) database of head-on quenching of statistically planar turbulent premixed flames by isothermal inert walls for different values of turbulence intensity and global Lewis number. It has been found that dilatation rate plays a key role in determining the flow topology distribution within the flame and this dilatation rate field is significantly affected by the flame quenching in the vicinity of the wall. The influence of the wall on the dilatation rate field in turn affects the statistical behaviour of all three invariants of the velocity gradient tensor and the distribution of flow topologies. The effects of heat release and thermal expansion strengthen with decreasing Lewis number which give rise to an increase in the probability of obtaining topologies which are specific to high positive values of dilatation rate. As the magnitude of positive dilatation rate and the likelihood of obtaining it decrease with flame quenching, the probability of finding the topologies, which are obtained only for positive values of dilatation rate, decreases close to the wall. The interrelation between the flow and flame topologies has been analysed in terms of Gaussian flame curvature and mean of principal flame curvatures. The contributions of individual flow topologies on the mean behaviour of wall heat flux magnitude, and the scalar-turbulence interaction and vortex-stretching terms in the scalar dissipation rate and enstrophy transport equations, respectively have been analysed in detail and dominant flow topologies which dictate the mean behaviours of these quantities have been identified. Detailed physical explanations have been provided for the observed flow topology distribution and its contribution to the scalar-turbulence and vortex-stretching terms. The nodal flow topologies have been found to be the significant contributors to the wall heat flux magnitude during head-on quenching of turbulent premixed flames irrespective of the value of global Lewis number. [ABSTRACT FROM AUTHOR]

The behaviours of the three invariants of the velocity gradient tensor and the resultant local flow topologies in turbulent premixed flames have been analysed using three-dimensional direct numerical simulation data for different values of the characteristic Lewis number ranging from 0.34 to 1.2. The results have been analysed to reveal the statistical behaviours of the invariants and the flow topologies conditional upon the reaction progress variable. The behaviours of the invariants have been explained in terms of the relative strengths of the thermal and mass diffusions, embodied by the influence of the Lewis number on turbulent premixed combustion. Similarly, the behaviours of the flow topologies have been explained in terms not only of the Lewis number but also of the likelihood of the occurrence of individual flow topologies in the different flame regions. Furthermore, the sensitivity of the joint probability density function of the second and third invariants and the joint probability density functions of the mean and Gaussian curvatures to the variation in Lewis number have similarly been examined. Finally, the dependences of the scalar-turbulence interaction term on augmented heat release and of the vortex-stretching term on flame-induced turbulence have been explained in terms of the Lewis number, flow topology and reaction progress variable. [ABSTRACT FROM AUTHOR]

A numerical study has been carried out to elucidate the effects of rounding the sharp edges on flow-induced forces on a square cylinder immersed in a laminar cross flow. Rounding reduces both the upstream stagnation pressure and the downstream base pressure. Consequently, competition between these two pressure reductions yields the minimum drag on the cylinder when its edges are partially rounded. It was also found that the leading-edge rounding is mainly responsible for the topological change thus the drag reduction, while the trailing- edge rounding alone just enhances lift fluctuation. However, trailing-edge rounding plays a role of stabilizing the flow when all of the four edges are rounded. [ABSTRACT FROM AUTHOR]

• The flow inside viscoelastic droplets in a rectangular microchannel is studied by means of micro Particle Image Velocimetry. • Well characterised Boger fluids are used to decouple the effects of elasticity and viscosity. • A distinct change in flow topology is observed for the most elastic droplets. • The change is captured well by the criterion of Pakdel - McKinley for pure elastic instabilities. • Spatiotemporal maps reveal velocity fluctuations inside the droplets suggesting the presence of an elastic instability. The role of elasticity on the flow topology inside viscoelastic (Boger) droplets moving in a rectangular microchannel is examined experimentally by means of micro Particle Image Velocimetry (μPIV). Polyacrylamide (PAAM) water -glycerol solutions of different concentrations are employed to vary droplet elasticity. Varying the Wi number alters the flow topology inside the Boger microdroplets, progressively reducing the number of vortical structures observed until their complete disappearance. The flow structure resembles that of Newtonian inelastic droplets for low Wi and elasticities. However, when the Wi and elastic number increase above one-i.e. elastic effects become more important- a new flow structure is observed at the front of the droplets characterized by two recirculating regions either side of the droplet centreline. Spatiotemporal maps show that the flow in this new regime fluctuates periodically around the centreline indicating the onset of an elastic instability. This flow transition is attributed to the well documented coupled effects of polymer stretching and curved streamlines and resultant hoop stresses. The findings highlight the importance elasticity can have on the nature of microdroplet flows of complex fluids and the potential of tuning elasticity to engineer flow structures for given microfluidic applications. [ABSTRACT FROM AUTHOR]

• Flow on the slant surface, force and pressure are analyzed for Ahmed body with deflectors. • At the deflection angle above −5°, total drag, flow fields, and pressure distribution on the slant change suddenly. • Maximum drag reduction obtains at 8%, lift almost cancels out at deflection angles above 0°. • Flow on the slant is fully separated with D-shape attachment line for deflection angles above −5°. • The pressure drag acting on the slant reaches around 40% of the total drag of the model. This study investigated the surface flow and the aerodynamic characteristics of a 7/10 scale Ahmed model equipped with deflectors at several angles. The experiments were conducted at Reynolds number of 2.45 × 105 based on the model height. A global luminescent oil film skin-friction measurement technique was applied for the flow fields on the slant and base surfaces. The aerodynamic forces of the model were measured to understand the effect of the deflection angle on the aerodynamic characteristics. The pressure was measured and pressure drags acting on the slant and base surfaces were also calculated. The results showed that the drag and lift of the model suddenly dropped at the deflection angle above −5°. The pressure distribution on the slant and base surfaces became flat with a maximum drag reduction of 8% for the model with a deflection angle of 0°. The skin-friction measurement indicated that the drag reduction was connected to the breakdown of the separation bubble and longitudinal vortices on the slant. At the fully separated flow state, the skin-friction results illustrated two small vortices and a counter-clockwise vortex on the slant. Additionally, the wake vortices shifted from horizontal to vertical structures. The relation among the flow fields on the slant surface, flow on the symmetric plan, pressure distribution, and drag was illustrated for the model with different deflection angles. [ABSTRACT FROM AUTHOR]

The aerodynamic performances of three-dimensional compressor cascade are mainly influenced by secondary flow effects, such as the cross flow on the side wall and the comer separation at the wall-blade junction. Often, these secondary flows can produce a blockage and losses in the blade passage. A numerical study of active flow control using blowing technique in a linear compressor cascade has been carried out, in order to eliminate the 3D boundary layer stall and enhance the aerodynamic performance. The numerical simulation is performed using steady and incompressible RANS (Reynolds averaged Navier-Stokes) equations with Realizable k-e turbulence model. Good agreement is found between numerical results and experimental data, in particular pressure coefficient (Cp) and total pressure loss coefficient (to). A detailed flow topology analysis is mentioned and gave the flow structure and the separation zone behavior. It was found that the blowing slot should be located upstream the dual nature critical point which represents the origin of separation. Furthermore, three suction slot configurations are used to control the separation, whereas one configuration on the suction side of the blade and two configurations are on the endwall. The first endwall slot is located parallel to the suction side of the blade and the second one is located perpendicular to the axial chord of the blade. Results indicate that the removal of the comer separation was not achieved using the slots on the blade suction side and on the endwall parallel to the suction side. Contrariwise, the perpendicular endwall slot was found to be most effective and the 3D separation is completely disappeared. [ABSTRACT FROM AUTHOR]

Numerical simulation and theoretical analysis were performed to investigate the upstream topology of a jet-crossflow interaction. The numerical results were validated with mathematical theory as well as a juncture flow structure. The upstream critical point satisfies the condition of occurrence for a saddle point of attachment in the horseshoe vortex system. In addition to the classical topology led by a saddle point of separation, a new topology led by a saddle point of attachment was found for the first time in a jet-crossflow interaction. The degeneration of the critical point from separation to attachment is determined by the velocity ratio of the jet over the crossflow, and the boundary layer thickness of the flat plate. When the boundary layer thickness at the upstream edge of the jet is close to one diameter of the jet, the flow topology is led by a saddle point of attachment. Variation of the velocity ratio does not change the topology but the location of the saddle point. When the boundary layer thickness is less than 0.255 of the jet flow diameter, large velocity ratio can generate a saddle point of attachment without spiral horseshoe vortex; continuously decreasing the velocity ratio will change the flow topology to saddle point of the separation. The degeneration of the critical point from attachment to separation was observed. [ABSTRACT FROM AUTHOR]

Three computational fluid dynamics (CFD) methods with different levels of flow-physics modelling are comprehensively evaluated against high-spatial-resolution wind-tunnel velocity data from step-down street canyons (i.e., a short building downwind of a tall building). The first method is a semi-empirical fast-response approach using the Quick Urban Industrial Complex (QUIC-URB) model. The second method solves the Reynolds-averaged Navier-Stokes (RANS) equations, and the third one utilizes a fully-coupled fluid-structure interaction large-eddy simulation (LES) model with a grid-turbulence inflow generator. Unlike typical point-by-point evaluation comparisons, here the entire two-dimensional wind-tunnel dataset is used to evaluate the dynamics of dominant flow topological features in the street canyon. Each CFD method is scrutinized for several geometric configurations by varying the downwind-to-upwind building-height ratio ( $$H_\mathrm{d}/H_\mathrm{u}$$ ) and street canyon-width to building-width aspect ratio ( S / W) for inflow winds perpendicular to the upwind building front face. Disparities between the numerical results and experimental data are quantified in terms of their ability to capture flow topological features for different geometric configurations. Overall, all three methods qualitatively predict the primary flow topological features, including a saddle point and a primary vortex. However, the secondary flow topological features, namely an in-canyon separation point and secondary vortices, are only well represented by the LES method despite its failure for taller downwind building cases. Misrepresentation of flow-regime transitions, exaggeration of the coherence of recirculation zones and wake fields, and overestimation of downwards vertical velocity into the canyon are the main defects in QUIC-URB, RANS and LES results, respectively. All three methods underestimate the updrafts and, surprisingly, QUIC-URB outperforms RANS for the streamwise velocity component, while RANS is superior to QUIC-URB for the vertical velocity component in the street canyon. [ABSTRACT FROM AUTHOR]

The spatial variability of hydraulic conductivity is known to have a strong impact on solute spreading and mixing. In most investigations, its local anisotropy has been neglected. Recent studies have shown that spatially varying orientation in sedimentary anisotropy can lead to twisting flow enhancing transverse mixing, but most of these studies used geologically implausible geometries. We use an object-based approach to generate stacked scour-pool structures with either isotropic or anisotropic filling which are typically reported in glacial outwash deposits. We analyze how spatially variable isotropic conductivity and variation of internal anisotropy in these features impacts transverse plume deformation and both longitudinal and transverse spreading and mixing. In five test cases, either the scalar values of conductivity or the spatial orientation of its anisotropy is varied between the scour-pool structures. Based on 100 random configurations, we compare the variability of velocity components, stretching and folding metrics, advective travel-time distributions, one and two-particle statistics in advective-dispersive transport, and the flux-related dilution indices for steady state advective-dispersive transport among the five test cases. Variation in the orientation of internal anisotropy causes strong variability in the lateral velocity components, which leads to deformation in transverse directions and enhances transverse mixing, whereas it hardly affects the variability of the longitudinal velocity component and thus longitudinal spreading and mixing. The latter is controlled by the spatial variability in the scalar values of hydraulic conductivity. Our results demonstrate that sedimentary anisotropy is important for transverse mixing, whereas it may be neglected when considering longitudinal spreading and mixing. [ABSTRACT FROM AUTHOR]

Paper presents a computational study of energy transformation in two geometrical configurations of Kaplan turbine elbow draft tube. Pressure recovery, hydraulic efficiency and loss coefficient are evaluated for a series of flow rates and swirl numbers corresponding to operating regimes of the turbine. These integral characteristics are then correlated with local flow field properties identified by extraction of topological features. Main focus is to find the reasons for hydraulic efficiency drop of the elbow draft tube.

Flame structure, flow topology and the relative alignments of principal strain rates with scalar gradients and the vorticity vector in turbulent spray flames arising from mono-disperse droplets have been analysed using three-dimensional Direct Numerical Simulations (DNS) data. An extensive parametric analysis has been performed for a range of values of droplet equivalence ratio ( ϕ d ) , droplet diameter ( a d ) and turbulence intensity ( u ′ ). The results have been analysed to analyse the statistical behaviours of the alignments of the gradients of mixture fraction and reaction progress variable and of the vorticity vector ( ω → ) with the local principal strain rates. The resulting alignments of reactive scalar gradient and vorticity with local principal strain rates in droplet-laden flames have been compared to those observed for a gaseous stoichiometric premixed flame under the same turbulent flow conditions. The differences in behaviour have been identified in terms of the statistical behaviours of scalar gradient and vorticity alignments with local principal strain rates in terms of the eight local flow topologies, classified as either focal (four topologies) or nodal (four topologies), in four zones across the flame: leading edge, preheat zone, burning reactants and trailing edge. This information has in turn been utilised to offer detailed physical explanations for the observed differences in alignment statistics of reactive scalar gradient and vorticity between turbulent premixed and spray flames. The implications of scalar gradient and vorticity alignment on the statistical behaviours of scalar–turbulence interaction and vortex stretching terms have been discussed in detail along with explanations for their dependences on droplet size, droplet equivalence ratio and turbulence intensity. [ABSTRACT FROM AUTHOR]

This numerical study describes the eddy emergence and transformations in a slow steady axisymmetric air-water flow, driven by a rotating top disk in a vertical conical container. As water height $$H_{\mathrm{w}}$$ and cone half-angle $$\beta $$ vary, numerous flow metamorphoses occur. They are investigated for $$\beta =30^{\circ }, 45^{\circ }$$ , and $$60^{\circ }$$ . For small $$H_{\mathrm{w}}$$ , the air flow is multi-cellular with clockwise meridional circulation near the disk. The air flow becomes one cellular as $$H_{\mathrm{w}}$$ exceeds a threshold depending on $$\beta $$ . For all $$\beta $$ , the water flow has an unbounded number of eddies whose size and strength diminish as the cone apex is approached. As the water level becomes close to the disk, the outmost water eddy with clockwise meridional circulation expands, reaches the interface, and induces a thin layer with anticlockwise circulation in the air. Then this layer expands and occupies the entire air domain. The physical reasons for the flow transformations are provided. The results are of fundamental interest and can be relevant for aerial bioreactors. [ABSTRACT FROM AUTHOR]

The stationary band complex (SBC) is a distinct, quasi-stationary convective asymmetry outside the eyewall of a tropical cyclone (TC) and plays an important role for TC intensity. Seminal work by Willoughby et al. related the SBC to the kinematic boundary between the TC and its environment and proposed a formation mechanism for the SBC. These results have been derived from observational data that are limited in radial and azimuthal coverage. The current study revisits these results using data from an idealized numerical experiment, which are not subject to such limitations. Consistent with Willoughby et al., the SBC in the idealized experiment is located near the boundary between the TC's 'moist envelope' and the lower θe environment. In contrast, however, the SBC is not associated with the kinematic boundary between the TC and the environmental flow. Furthermore, the proposed formation mechanism does not operate in the idealized experiment. I propose an alternative formation mechanism that links the SBC to two fundamental consequences of environmental vertical wind shear: (i) the distortion of the 'moist envelope' and (ii) the tilt of the TC vortex. In the idealized experiment, the respective regions of high- θe air and a positive vorticity anomaly overlap at low levels in the semicircle to the right of the shear vector well outside the eyewall. A simple slab boundary-layer model indicates that frictional convergence associated with the vorticity asymmetry is a viable, dynamical forcing mechanism for deep convection. While ascent out of the boundary layer occurs over a radially broad region, the large radial shear of the TC's swirling winds promotes the organization of the ensuing convection into a band-like pattern. I emphasize that this formation mechanism considers only the meso- β-scale envelope of the SBC. As shown previously by others, convective-scale processes contribute further to the organization of the SBC within this envelope. [ABSTRACT FROM AUTHOR]

Helical flow can occur in porous media if the hydraulic conductivity tensor is anisotropic. We study the structure of steady-state flow fields in three-dimensional anisotropic porous media formed by two homogeneous layers, one of which is anisotropic. We simulate transient transport of a conservative scalar in such flow fields by a hybrid streamline/smoothed particle hydrodynamics method and analyze dilution. We use stretching and folding metrics to characterize the flow field and the dilution index of a conservative scalar divided by the volume of the domain to quantify plume dilution. Based on the results of detailed numerical simulations, we conclude that nonlinear deformation triggers dilution and that plume dilution is controlled by two parameters: the contrast between the principal directions of the anisotropic layer, and the orientation of the hydraulic conductivity tensor with respect to the main flow direction. Furthermore, we show that in this kind of flow fields transverse dispersion is responsible for an increase in plume dilution, while the effect of longitudinal dispersion is negligible. [ABSTRACT FROM AUTHOR]

Innovations in conventional surface and planar laser scattering visualizations revealed complex structures in the Mach 2.5 flow past a sharp-edged, sub-boundary-layer ramp with swept sides that is one type of micro vortex generator (MVG). The incoming flow separated over the leading edge despite the ramp angle being below the threshold for incipient separation. The separation produced a weak trailing horseshoe vortex system. The flow over the top of the MVG separated off the slant edges to produce a large primary vortex pair. Extra details were revealed at the trailing edge with at least two pairs of singularities. Vortex filaments spring from these singularities. Symmetry breaking from the confluence of the two primary vortices was observed as an unsteady wake to result in a train of possibly ring or hairpin vortices trailing downstream. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The successful design of pillow-plate condensers requires specific knowledge of the falling liquid film characteristics. Due to the typical wavy, “pillow-like” surface of pillow plates, the film flow here differs from the flow over vertical smooth walls. In this work, we visualized the topology and free surface waviness of a falling liquid film over a vertical pillow plate by using a fluorescent dye and a high speed camera. First observations show that the liquid film preferably flows down the rows of vertically aligned welding spots (zone 1). As a result, the wall surface between these rows is covered by a thinner film (zone 2). In order to quantify this “2-zone” topology, we measured the local mass flow rate in zone 1 over the height of the pillow plate. The results confirmed that the liquid film is mainly “drained” in zone 1. The findings of this study will help to develop flow-pattern-oriented modeling methods and to design pillow-plate falling film equipment. [ABSTRACT FROM AUTHOR]

Three-dimensional Direct Numerical Simulations combined with Particle Image Velocimetry experiments have been performed on a hemisphere-cylinder at Reynolds number 1000 and angle of attack 20°. At these flow conditions, a pair of vortices, so-called “horn” vortices, are found to be associated with flow separation. In order to understand the highly complex phenomena associated with this fully three-dimensional massively separated flow, different structural analysis techniques have been employed: Proper Orthogonal and Dynamic Mode Decompositions, POD and DMD, respectively, as well as critical-point theory. A single dominant frequency associated with the von Karman vortex shedding has been identified in both the experimental and the numerical results. POD and DMD modes associated with this frequency were recovered in the analysis. Flow separation was also found to be intrinsically linked to the observed modes. On the other hand, critical-point theory has been applied in order to highlight possible links of the topology patterns over the surface of the body with the computed modes. Critical points and separation lines on the body surface show in detail the presence of different flow patterns in the base flow: a three-dimensional separation bubble and two pairs of unsteady vortices systems, the horn vortices, mentioned before, and the so-called “leeward” vortices. The horn vortices emerge perpendicularly from the body surface at the separation region. On the other hand, the leeward vortices are originated downstream of the separation bubble, as a result of the boundary layer separation. The frequencies associated with these vortical structures have been quantified. [ABSTRACT FROM AUTHOR]

Saddle points (SPs), recirculation centres (RCs), and reattachment points (RPs) are examples of key points of the flow, also called critical or stationary points. Finding the location of these null velocity points is essential to the understanding of a flow pattern and may even spare measurements of the whole flow field. To that purpose, laser based techniques may render inappropriate, especially because of the difficulty in conveying seed particles to the vicinity of those points. The article details an original technique based on the measurement of pairs of profiles with Pitot tubes and finding their intersection. Two flows are taken as case-studies. These consist of jets emanating from the base of a cylindrical chamber: multiple jets arranged in a circular crown and a single central jet. While the location of SP and RP could be done in a single step, due to the symmetry of the flow, pinpointing RCs required an iterative procedure, in which the radial and longitudinal coordinates were determined alternately. The location of the RP was the least accurate. The technique has pedagogical value and will particularly interest young researchers and practicing engineers without access to elaborate and expensive techniques such as particle image velocimetry. [ABSTRACT FROM AUTHOR]

A step-down street canyon is a street canyon in which the upwind building height ( $$H_{u}$$ ) is greater than the downwind building height ( $$H_{d}$$ ) ( $$H_{u}>H_{d}$$ ). Here, the effect of downwind building height and canyon width on the flow structure in isolated step-down canyons is investigated through wind-tunnel measurements. The measurements were acquired along the vertical symmetry plane of the model buildings using two-dimensional particle image velocimetry for normal approach flow. For the present study, $$H_{u}$$ was kept constant at $$120$$ mm, and $$H_{d}$$ was increased in increments of $$\approx $$ 0.08 $$H_{u}$$ , to span the range: $$0.08\le H_{d}/H_{u}\le 1$$ . The configuration $$H_{d}/H_{u} \approx 1$$ corresponds to a deep canyon. The footprints of the buildings were square, with the widths ( $$W$$ ) and lengths ( $$L$$ ) being, $$W (= L) \approx 32$$ mm. Four different street-canyon widths ( $$S$$ ) were considered, with $$S/W \approx 2.5, 2, 1.5, 1$$ . This resulted in a total of 48 test cases, with 12 cases for every street-canyon width. The flow topology in the near-wake of an isolated tall building ( $$H_{d}=0$$ ) is characterised by a bow-shaped structure comprising the vortex core, saddle point, and ground originating shear layer. For $$S/W \approx 2.5, 2$$ , and $$1.5$$ , increasing the downwind building height from $$H_{d}/H_{u} \approx 0.08$$ to $$1$$ resulted in the in-canyon flow structure transitioning from wake dominated to deep canyon wake interference regimes. Similar increase of the downwind building height for $$S/W \approx 1$$ resulted in the flow structure transitioning from wake dominated to deep canyon skimming flow regime. The results indicate that in step-down canyons formed by tall and slender buildings, momentum transport into and out of the canyon around the building sidewalls plays a crucial role in the determining the overall flow patterns in the canyon. [ABSTRACT FROM AUTHOR]

Transverse mixing by local-scale dispersion plays a pivotal role in mixing-controlled reactions of continuously injected compounds and controls the length of steady-state plumes. In two-dimensional heterogeneous media, high-permeability inclusions cause a net enhancement of transverse mixing because the effect of reducing the transverse diffusion lengths in such inclusions prevails over the effect of reduced diffusion time. A simple scaling law for effective transverse mixing in two-dimensional domains could be derived by transferring the steady-state transport equation into streamline-coordinates. The net enhancement factor does not depend on scale. Closed-form expressions for effective mixing and its uncertainty could be derived for uniform-in-the-mean velocity in a multi-Gaussian log-conductivity field with isotropic, exponential covariance function. These expressions could be used to estimate the uncertainty of steady-state concentrations and reactive-plume lengths. The picture is more complex in three-dimensional domains where spatially variable orientation of anisotropy can lead to twisting streamlines. Macroscopically helical flow can lead to an enhancement of transverse mixing by about an order of magnitude more than the flow-focusing effects in two-dimensional flows. [ABSTRACT FROM AUTHOR]