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The heating of oil and oil products is widely used to reduce energy losses during transportation. An approach is developed to determine the effective length of the heat exchanger and the temperature of the cold coolant (oil) at its outlet in the case of a strong dependence of oil viscosity on temperature. Oil from the Uzen field (Kazakhstan) is considered as a heated coolant, and water is considered as a heating component. The method of the log–mean temperature difference, modified for the case of variable viscosity, and the methods of computational fluid dynamics (CFD) are used for calculations. The results of the numerical calculations are compared with the data obtained on the basis of a theoretical approach at a constant viscosity. When using a theoretical approach with a constant or variable viscosity, the heat transfer coefficients to cold and hot coolants are found using criterion dependencies. The Reynolds-averaged Navier–Stokes (RANS) and a turbulence model that takes into account the laminar–turbulent transition are applied. In the case of variable oil viscosity, a transition from the laminar flow regime to the turbulent one is manifested, which has a significant effect on the effective length of the heat exchanger. The obtained results of the CFD calculations are of interest for the design of heat exchangers of a new type, for example, helicoid ones.

Direct numerical simulations were carried out to investigate three-dimensional, unsteady magnetohydrodynamic (MHD) convection in a spherical electromagnetically levitated molten Cu-Co droplet with a diameter of 5 mm under different strengths of the static magnetic field. The strength of the static magnetic field ranged from 0 to 3 T. MHD convection under lower static magnetic fields presented turbulent flow. The velocities in the levitated droplet decreased with increasing strength of the static magnetic field. At more than 1.5 T, the convective flow pattern became axisymmetric and the turbulent energy reached almost zero, i.e., MHD convection presented laminar flow. Comparisons of the present numerical results with past experimental results demonstrated that the marked change in phase separation structures in undercooled Cu-Co alloys at 1.0 to 1.5 T was closely related to the laminar-turbulent transition of MHD convection in the levitated droplet.

The influence of thermal nonequilibrium on the laminar–turbulent transition is studied with the use of the $$\mathrm{e}^N$$ -method for two widespread flow regimes in a supersonic boundary layer at the Mach number $$\textrm{M}=4.5$$ . The set of the actual frequencies of spatial disturbances is determined on the basis of the neutral curves for temporal disturbances. Families of the curves of $$N$$ -factors are calculated for selected frequencies. Then, based on the envelopes of these curves, the transition Reynolds number $$\mbox{Re}_{\delta T}$$ for a given transition factor $$N_T$$ is determined. The calculations show that the transition region in the case with $$N_T=8$$ and vibrational excitation level below the dissociation limit is located 12–14% downstream as compared to the transition region in a perfect gas.

The interaction of a laminar boundary layer with a shock wave at a Mach number M = 1.43 is studied by numerical simulation. The results obtained by direct numerical simulation are compared with the results of calculations using the Reynolds-averaged Navier–Stokes (RANS) equations supplemented with different turbulence models describing laminar–turbulent transition. The possibility of determining the position of the flow turbulence zone based on linear stability theory and the $$\mathrm{e}^{N}$$ -method is estimated. Comparison of the numerical simulation with experimental data shows that engineering RANS methods can be used to study supersonic flows in which transition to turbulence occurs in regions of shock wave–boundary layer interaction.

Flight experiments aimed at studying the transition from the laminar to turbulent flow in the boundary layer on the glider wing at different levels of atmospheric turbulence performed in the 1980s are reviewed. The results are obtained by means of hot-wire measurements and visualization of the flow on the wing surface by the method of sublimating coatings. The transition is found to proceed in several stages: emergence, evolution, and failure of a discrete packet of instability waves in the region of the adverse pressure gradient. The data obtained in the study are compared with the results of similar investigations carried out in a large wind tunnel on the same real glider wing at realistic Reynolds numbers.

A new experimental method for panoramic contactless determination of the laminar-turbulent transition position in a three-dimensional boundary layer is described. It is demonstrated that this method allows accurate determination of the transition region boundaries at both low and enhanced levels of free-stream turbulence.

In this paper, known probabilistic methods for estimating the thickness of the boundary layer of a two-dimensional laminar flow of viscous incompressible fluid are extended to three-dimensional laminar flows of a viscous compressible medium. Their applicability to the problems of boundary-layer stability is studied with the LOTRAN3 software package, which allows us to compute the position of laminar-turbulent transition in three-dimensional aerodynamic configurations.

Results of direct numerical simulations of the roughness-induced development of instability and transition to turbulence in a supersonic boundary layer on a blunted cone for the free-stream Mach number M∞ = 5.95 are presented. The flow parameters and model geometry are consistent with the conditions of the experiments performed in the study. The following roughness types are considered: random distributed roughness, isolated roughness elements of different shapes, and a group of regularly arranged roughness elements. The processes of the instability development and transition for different roughness types are compared, and possible mechanisms of the roughness influence on the stability of boundary layers on blunt bodies are discussed.

Made available in DSpace on 2019-10-04T12:38:21Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-05-01 Several flows of practical interest are of viscoelastic fluids and it is desirable to know if these flows are in a laminar or turbulent state. In this paper, the laminar-turbulent transition is studied in which the convection of Tollmien-Schlichting waves is investigated for the incompressible two-dimensional flow between two parallel plates. The viscoelastic fluid adopted is modeled by the Oldroyd-B constitutive equation. Direct Numerical Simulation (DNS) and Linear Stability Theory (LST) were used to verify the stability of the viscoelastic fluid flow to unsteady disturbances. In the DNS formulation, the governing equations are discretized by high-order compact finite difference schemes for the spatial derivatives, while time integrations are carried out by a fourth order Runge Kutta method. For the LST analysis, the Orr-Sommerfeld equation is modified to include viscoelastic effects and solved by a shooting method. In order to evaluate the flow stability when submitted to unsteady disturbances and to find the neutral stability curves, a range of numerical simulations was performed with different dimensionless parameters for the viscoelastic fluid flow and the results were compared with the Newtonian fluid flow. The influence of the elastic forces and the amount of the polymer concentration in the fluid were investigated. A wide range of polymer concentration was studied. The results show that the variation of the critical Reynolds number may be monotonic for certain conditions. The DNS simulations were carried out, and the amplification rates obtained were compared with the LST results, with a very good agreement. The DNS results allow also the visualization of the disturbances variation in streamwise and wall-normal directions, showing the behavior of the velocity components, vorticity and the non-Newtonian extra-stress tensor components. The DNS results show a significant change in flow structure for certain combinations of the polymer concentration and elastic forces parameters. Univ Estadual Paulista, Dept Matemat & Comp, Rua Roberto Simonsen 305, BR-19060900 Presidente Prudente, Brazil Inst Aeronaut & Espaco, Praca Marechal Eduardo Gomes 50, BR-12228904 Sao Jose Dos Campos, Brazil Univ Sao Paulo, Dept Matemat Aplicada & Estat, Ave Trabalhador Sao Carlense 400, BR-13566590 Sao Carlos, SP, Brazil Univ Estadual Paulista, Dept Matemat & Comp, Rua Roberto Simonsen 305, BR-19060900 Presidente Prudente, Brazil

Hypersonic Inflatable Aerodynamic Decelerator (HIAD) has shown its great potential for future planetary explorations. However, the HIAD surface deflections could both promote boundary-layer transition early and augment heating levels sharply, which poses challenges for survivability of a Thermal Protection System (TPS). The goal of this work is to assess different transition models for the prediction of hypersonic transitional flows over scalloping deformed HIAD surface and to seek the critical factor for their capabilities to provide a reference for the application and advancement. Three representative transition models are considered: γ-Reθ, k-ω-γ and kT-kL-ω. The results show that the undulating surface causes flow separations and reattachments in valleys and strong crossflow along the leeward. k-ω-γ model can correctly predict both the shapes and locations of the transition onsets. The transition zone predicted by γ-Reθ model is much small, while kT-kL-ω is incapable of transition prediction for such a complex and irregular configuration. Moreover, this study reveals that the crossflow instability plays a dominant role in the transition. The crossflow Reynolds number Recf, whose distributions are approximately consistent with the transition zone, could be a feasible crossflow strength indicator for the transition onsets on the undulating surface. Once k-ω-γ model excludes effects of crossflow instability, it predicts incorrect transition results for the deformed surface of HIAD. Besides, a detailed analysis shows that both the crossflow instability mode and the first disturbance mode in valleys are the major contributors to the transition on the leeward surface. Near the leeward ray, the transition is mainly determined by the first disturbance mode.

This paper presents the phenomenon of laminar turbulent transition simulated by modified algebraic intermittency model based on well known k-ω turbulence model by Wilcox for flows in parallel-plate channels. Results of simulations using described model were obtained for low Reynolds numbers 1000 ≤ Re ≤ 75000 with different turbulence intensity in inlet cross-section of the parallel-plate channel. Results were compared to and validated with existing data available in literature both experimental and theoretical. Implementation of described model with pre-processing and post-processing was made using open source software OpenFOAM.

The role of free-stream turbulence (FST) in the hydrodynamic instability mechanisms and transition to turbulence in laminar separation bubbles (LSBs) was investigated using direct numerical simulations (DNS). Towards this end, a set of highly resolved DNS have been carried out, where isotropic FST fluctuations with intensities from 0.1 % to 3 % are introduced to investigate the relevant physical mechanisms governing the interaction of separation and transition in LSBs. For disturbance-free simulations, i.e. without FST, laminar–turbulent transition involves a Kelvin–Helmholtz (KH) instability of the separated shear layer. For LSBs subjected to FST, vortical FST fluctuations penetrate the approaching attached laminar boundary layer upstream of the separation location and induce slowly growing low-frequency disturbances, so-called Klebanoff (K) modes, which cause a spanwise modulation with a distinct spanwise wavelength. Simultaneously, the FST enhances the initial levels of instability waves with frequencies in the frequency range of the KH instability, but at much smaller amplitude levels compared to the K-modes. Results from the calculations based on the linearized Navier–Stokes equations and comparison with DNS results reveal that the K-mode exhibits exponential growth in the separated shear layer until it reaches a peak amplitude. At the same time, two-dimensional (2D) disturbance waves are also exponentially amplified, in fact at larger growth rate compared to the K-mode, due to the primary (convective) shear-layer instability mechanism until they saturate downstream of the peak amplitude associated with the K-mode. Therefore, based on detailed spectral analysis and modal decompositions for the separation bubbles investigated, the transition process is the result of two different mechanisms: (i) strong amplification of high-frequency (order of the shedding frequency), essentially 2D or weakly oblique fluctuating disturbances and (ii) low-frequency, three-dimensional K-modes caused by FST. Depending on the intensity of the FST, one of these mechanisms would dominate the transition process, or both mechanisms act together and contribute simultaneously. The net effect of these two events is an acceleration of transition for an increased level of FST intensity, which in turn leads to a reduction of the extent of the separation bubble in streamwise and wall-normal directions. The ‘roll-up’ into spanwise large-scale vortical structures resulting from the shear-layer instability, and the eventual breakdown of these structures, strongly contribute to the reattachment process. The spanwise coherence of these ‘rollers’ deteriorates due to the presence of large-amplitude K-modes, thus effectively weakening their strength for high levels of FST intensities ($Tu>1\,\%$).

The laminar-turbulent transition of flows of Newtonian (viscous) and non-Newtonian liquids in pipes was investigated on the basis of the combined analysis of the generalized equation of the first and second laws of thermodynamics for a simple system and the Darcy–Weisbach equation. It is shown that a reason for the crises of the laminar flows of such liquids in pipes is the disturbance of the balance between the positive entropy production and the negative entropy flow in them. An analysis of calculation and experimental data on the disturbance of the laminar flow of a non-Newtonian liquid in a pipe and the transformation of this flow into a turbulent flow has shown that the crises of a laminar flow of an elastoviscoplastic liquid in a pipe happens at a larger critical Reynolds number compared to that of a laminar flow of a viscoplastic liquid at one and the same values of the Hedstrom parameter.

Nanofluids present enhanced thermal conductivity compared to their base fluid and then have become good candidates for heat transfer fluids. In this study, the convective heat transfer coefficient and the pressure drop are measured in a circular tube of inner diameter 4.4 mm and outer diameter 5 mm. The experiments were conducted from Reynolds number R e D of 500 up to 4500. After making sure that the results for water were in good agreement with the literature, five concentrations of a γ-Al2O3-water nanofluid were tested. The results show that the concentration has no impact on the critical Reynolds number corresponding to the laminar-turbulent transition for this nanofluid up to 5w%. Moreover, a slight enhancement of the convective heat transfer coefficient is noted in laminar flow with increasing mass concentration while no significant amelioration is observed in turbulent regime compared to water. On the other hand, the pressure drop strongly increases with concentration whatever the flow regime. As a result, the thermal performance (Convective heat transfer coefficient at constant pumping power) of this nanofluid is lower than pure water.

The presence of large surface irregularities such as humps, where the height is similar to the local boundary-layer (BL) displacement thickness, introduces regions of localized strong streamwise gradients in the base flow quantities. These large gradients can significantly modify the spatial development of incoming disturbances that lead to laminar–turbulent transition in wall-bounded flows [e.g., Tollmien–Schlichting (TS) waves]. Techniques such as Parabolized Stability Equations (PSE) are not suited for BL instability analysis in such regions: their formulation assumes that streamwise variations of base flow and disturbance quantities are small, allowing a marching procedure for their resolution. On the other hand, the Adaptive Harmonic Linearized Navier–Stokes (AHLNS) equations can handle these large streamwise gradients by using a fully elliptic system of equations, similar to Linearized Navier–Stokes (LNS), Harmonic LNS (HLNS), or Direct Numerical Simulation (DNS). Moreover, in AHLNS (as in PSE), a wave-like character of the instabilities is assumed, leading to a significant reduction in the number of streamwise grid points required compared with LNS, HLNS, or DNS computations. In the present study, an efficient combination of PSE and AHLNS is used to investigate the effect of height, length, and shape of a single hump placed on a flat plate in a two-dimensional flow field at Ma=0.5 without pressure gradient. The effect of this hump on the spatial evolution of TS waves, in terms of N-factors, is presented. An expected laminar–turbulent transition onset, via the eN methodology, is also described. It is shown that the shape of the surface irregularity, together with the height and length, plays an important role for the location of laminar–turbulent transition onset in convectively unstable flows.

Superhydrophobic surfaces dramatically reduce the skin friction of overlying liquid flows, providing a lubricating layer of gas bubbles trapped within their surface nano-sculptures. Under wetting-stable conditions, different models can be used to numerically simulate their effect on the overlying flow, ranging from spatially homogeneous slip conditions at the wall, to spatially heterogeneous slip–no-slip conditions taking into account or not the displacement of the gas–water interfaces. These models provide similar results in both laminar and turbulent regimes, but their effect on transitional flows has not been investigated yet. In this work we study, by means of numerical simulations and global stability analyses, the influence of the modelling of superhydrophobic surfaces on laminar–turbulent transition in a channel flow. For the K-type scenario, a strong transition delay is found using spatially homogeneous or heterogeneous slippery boundaries with flat, rigid liquid–gas interfaces. Whereas, when the interface dynamics is taken into account, the time to transition is reduced, approaching that of a no-slip channel flow. It is found that the interface deformation promotes ejection events creating hairpin heads that are prone to breakdown, reducing the transition delay effect with respect to flat slippery surfaces. Thus, in the case of modal transition, the interface dynamics must be taken into account for accurately estimating transition delay. Contrariwise, non-modal transition triggered by a broadband forcing is unaffected by the presence of these surfaces, no matter the surface modelling. Thus, superhydrophobic surfaces may or not influence transition to turbulence depending on the interface dynamics and on the considered transition process.

The influence of the molecular structure of gas flows on the characteristics of turbulent flows and the influence of the properties of molecules on turbulent processes have been studied. A review of the results of studies of turbulent processes is presented. Data on flows at the boundary of a supersonic jet and in a pipe with a divergent inlet section, and Hagen–Poiseuille flow are given. Experimental study of Hagen–Poiseuille flow has shown that the molecular properties of the medium have an effect on the critical Reynolds number. It is shown that in comparing the critical Reynolds numbers for flows of different gases at different pressures, the common determining parameter is the second virial coefficient.

International audience; Boundary layer transition over an isolated surface roughness element is investigated by means of numerical simulation. Large Eddy Simulation (LES) flow-modeling approach is employed to study flow characteristics and transition phenomenon past a roughness element immersed within an incoming developing boundary layer, at a height-based Reynolds number of 1170. LES numerical results are compared to experimental data from literature showing the time-averaged velocity distribution, the velocity fluctuation statistics and the instantaneous flow topology.Despite slight difference in the intensity of streamwise velocity fluctuations, the present LES results and experimental data show very good agreement. The mean flow visualization shows streamwise counter-rotating vortices pairs formation downstream of the obstacle. The primary pair induces an upwash motion and a momentum deficit that creates a Kelvin-Helmholtz type flow instability. The instantaneous flow topology revealsthe formation of coherent K-H vortices downstream that produce turbulent fluctuations in the wake of the roughness element. These vortices are streched and lifted up when moving downstream. The velocity fluctuations results show that the onset of the turbulence is dominated by the energy transfer of large-scale vortices.

The effects of isolated, cylindrical roughness elements on laminar–turbulent transition in a flat-plate boundary layer are investigated in a laminar water channel. Our experiments aim at providing a comparison to global linear stability theory (LST) by means of hot-film anemometry and particle image velocimetry. Although the critical Reynolds number from theory does not match the transition Reynolds number observed in experiments, there are distinct experimental observations indicating a changeover from purely convective to absolute/global instability very close to the critical Reynolds number predicted by theory. Forcing with a vibrating wire reveals the evolution of the system dynamics from an amplifier to a wavemaker when the critical Reynolds number is exceeded. The mode symmetry is varicose for thick roughness elements and a changeover from varicose to sinuous modes is observed at the critical Reynolds number for thin roughness elements. Therefore, most predictions by global LST can be confirmed, but additional observations in the physical flow demonstrate that not all features can be captured adequately by global LST.

The shock wave boundary layer interaction on the suction side of transonic compressor blade is one of the main objectives of TFAST project (Transition Location Effect on Shock Wave Boundary Layer Interaction). The experimental and numerical results for the flow structure investigations are shown for the flow conditions as the existing ones on the suction side of the compressor profile. The two cases are investigated: without and with boundary layer tripping device. In the first case, boundary layer is laminar up to the shock wave, while in the second case the boundary layer is tripped by the step. Numerical results carried out by means of Fine/Turbo Numeca with Explicit Algebraic Reynolds Stress Model including transition modeling are compared with schlieren, Temperature Sensitive Paint and wake measurements. Boundary layer transition location is detected by Temperature Sensitive Paint.

We analyze modern methods for calculating heat and hydrodynamic flow parameters in a boundary layer during the laminar–turbulent transition. The main approaches for describing the phenomenon of laminar–turbulent transition are examined. Each approach is analyzed. The manner in which different factors influence the laminar–turbulent transition is studied. An engineering model of the laminar–turbulent transition in a high-velocity flow is presented.

A sample will undergo T1 relaxation at continuously changing applied magnetic field strengths when flowing from the Earth magnetic field outside an NMR instrument to the much stronger applied magnetic field inside an NMR instrument. The interaction between fluid flow, T1 relaxation and magnetic field distributions in the case of negligible T1 dispersion can be described by considering two length scales involved: the characteristic polarization length vT1, and the effective polarization magnet length Lm⁎. By comparing water flow experiments on a 0.9 m long Halbach magnet array with simulations using the Bloch-Torrey equation, we determined that Lm⁎ is given by the integral along flow direction x of the normalized axial polarization field distribution, p(x) = B0(x)/B0(xROI), in which xROI is the center of the polarization magnet. The fluid magnetization level Mz in a region-of-interest located at xROI is a function of the ratio s = vT1/Lm⁎ and can be generally expressed as Mz,ROI = M0⁎(s)[1–exp(-1/2 s)]. The effective equilibrium magnetization function M0⁎(s) was found to have both a sample independent contribution from the axial polarization field p(x) and a sample dependent contribution from the flow velocity profile at a given flow rate. In our experiments, the axial Halbach field was found to contribute with a weight of approximately 1/3 to M0⁎(s) and the remaining 2/3 wt is contributed by the flow velocity distribution. Based on this analysis, a set of general principles for magnetization build up modeling and online NMR instrument design has been derived.

Stability of plane and cylindrical Poiseuille flows of nanofluids to comparatively small perturbations is studied. Ethylene glycol-based nanofluids with silicon dioxide particles are considered. The volume fraction of nanoparticles is varied from 0 to 10%, and the particle size is varied from 10 to 210 nm. Neutral stability curves are constructed, and the most unstable modes of disturbances are found. It is demonstrated that nanofluids are less stable than base fluids; the presence of particles leads to additional destabilization of the flow. The greater the volume fraction of nanoparticles and the smaller the particle size, the greater the degree of this additional destabilization. In this case, the critical Reynolds number significantly decreases, and the spectrum of unstable disturbances becomes different; in particular, even for the volume fraction of particles equal to 5%, the wave length of the most unstable disturbances of the nanofluid with particles approximately 20 nm in size decreases almost by a factor of 4.

Remarkable feature of the phase diagram of sulfur hydrides, the record-high T c superconductors, is a sharp increase near P ≈ 150 GPa from T c ≈ 120 to ≈ 200 K. This increase is a signature of the structural transition. The present study is concerned with the nature of this phase transition. One can demonstrate that the symmetry analysis along with an analysis of the impact of lattice deformations lead to the conclusion that we are dealing with the first-order transition. Such a transition is manifested in an abrupt appearance of small pockets on the Fermi surface and, correspondingly, the two-gap energy spectrum.

The algorithm of stage-by-stage qualitative modeling of the mechanism of a semiconductor–metal phase transition in vanadium dioxide has been proposed. The basis for the model is a statement that the transition is complex in character and consists of the anhysteretic, purely electronic Моtt transition occurring over a wide temperature range, and the temperature-abrupt structural Peierls transition having a thermal hysteresis. The initial stage of the model is based on the solution of a quantum-mechanical problem of an electronic spectrum of a linear vanadium-ion's chain. The model is completed by consideration of correlation effects and a martensitic character of the structural transition through taking consecutively account of results obtained by X-ray, spectroscopic, impedansmetric and magnetic resonance methods.

Direct numerical simulations of instability development and transition to turbulence in a supersonic boundary layer on a flat plate are performed. The computations are carried out for moderate supersonic (free-stream Mach number M = 2) and hypersonic (M = 6) velocities. The boundary layer development is simulated, which includes the stages of linear growth of disturbances, their nonlinear interaction, stochastization, and turbulent flow formation. A laminar–turbulent transition initiated by distributed roughness of the plate surface at the Mach number M = 2 is also considered.

The influence of a cylinder-shaped single roughness element on the laminar–turbulent transition in the presence of an entropy layer is experimentally studied. The experiments are performed on a blunted cone model at the Mach number M = 5. The roughness element is located on the blunted tip of the model. Information about the mean and fluctuating parameters of the boundary layer in the wake behind the roughness element is obtained by using hot-wire anemometry. It is shown that flow turbulization behind the roughness elements occurs at the local Reynolds number calculated on the basis of the roughness element height and equal to 400–500. It is found that the presence of the roughness element exerts a significant effect on the unsteady characteristics of the boundary layer if the roughness element height is smaller than the effective value.

Some types of mixed subgrid-scale (SGS) models combining an isotropic eddy-viscosity model and a scale-similarity model can be used to effectively improve the accuracy of large eddy simulation (LES) in predicting wall turbulence. Abe (2013) has recently proposed a stabilized mixed model that maintains its computational stability through a unique procedure that prevents the energy transfer between the grid-scale (GS) and SGS components induced by the scale-similarity term. At the same time, since this model can successfully predict the anisotropy of the SGS stress, the predictive performance, particularly at coarse grid resolutions, is remarkably improved in comparison with other mixed models. However, since the stabilized anisotropy-resolving SGS model includes a transport equation of the SGS turbulence energy, k SGS , containing a production term proportional to the square root of k SGS , its applicability to flows with both laminar and turbulent regions is not so high. This is because such a production term causes k SGS to self-reproduce. Consequently, the laminar–turbulent transition region predicted by this model depends on the inflow or initial condition of k SGS . To resolve these issues, in the present study, the mixed-timescale (MTS) SGS model proposed by Inagaki et al. (2005) is introduced into the stabilized mixed model as the isotropic eddy-viscosity part and the production term in the k SGS transport equation. In the MTS model, the SGS turbulence energy, k es , estimated by filtering the instantaneous flow field is used. Since the k es approaches zero by itself in the laminar flow region, the self-reproduction property brought about by using the conventional k SGS transport equation model is eliminated in this modified model. Therefore, this modification is expected to enhance the applicability of the model to flows with both laminar and turbulent regions. The model performance is tested in plane channel flows with different Reynolds numbers and in a backward-facing step flow. The results demonstrate that the proposed model successfully predicts a parabolic velocity profile under laminar flow conditions and reduces the dependence on the grid resolution to the same degree as the unmodified model by Abe (2013) for turbulent flow conditions. Moreover, it is shown that the present model is effective at transitional Reynolds numbers. Furthermore, the present model successfully provides accurate results for the backward-facing step flow with various grid resolutions. Thus, the proposed model is considered to be a refined anisotropy-resolving SGS model applicable to laminar, transitional, and turbulent flows.

The transition from laminar to turbulent flows has challenged the scientific community since the seminal work of Reynolds (Phil. Trans. R. Soc. Lond. A, vol. 174, 1883, pp. 935–982). Recently, experimental and numerical investigations on this matter have demonstrated that the spatio-temporal dynamics that are associated with transitional flows belong to the directed percolation class. In the present work, we explore the analysis of laminar–turbulent transition from the perspective of the recent theoretical development that concerns viscoelastic turbulence, i.e. the drag-reducing turbulent flow obtained from adding polymers to a Newtonian fluid. We found remarkable fingerprints of the variety of states that are present in both types of flows, as captured by a series of features that are known to be present in drag-reducing viscoelastic turbulence. In particular, when compared to a Newtonian fully turbulent flow, the universal nature of these flows includes: (i) the statistical dynamics of the alternation between active and hibernating turbulence; (ii) the weakening of elliptical and hyperbolic structures; (iii) the existence of high and low drag reduction regimes with the same boundary; (iv) the relative enhancement of the streamwise-normal stress; and (v) the slope of the energy spectrum decay with respect to the wavenumber. The maximum drag reduction profile was attained in a Newtonian flow with a Reynolds number near the boundary of the laminar regime and in a hibernating state. It is generally conjectured that, as the Reynolds number increases, the dynamics of the intermittency that characterises transitional flows migrate from a situation where heteroclinic connections between the upper and the lower branches of solutions are more frequent to another where homoclinic orbits around the upper solution become the general rule.

Extensive combined experimental and theoretical investigations of the linear evolution of unsteady (in general) Cross-Flow (CF) and three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. CF-instability characteristics are investigated in detail at an angle of attack of −5° when this kind of instability dominates in the laminar-turbulent transition process, while the 3D TS-instability characteristics are studied at an angle of attack of +1.5° when this kind of instability is predominant in the transition process. All experimental results are deeply processed and compared with results of calculations based on several theoretical approaches. For the first time, very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved both for unsteady CF- and 3D TS-instability modes for the case of a boundary layer developing on a real swept airfoil. The first part of the present study (this paper) is devoted to the description of the case of CF-dominated transition, while the TS-dominated case will be described in detail in a subsequent second part of this investigation.

The transient process of turbulence development and vortex breakdown from a marginal flow state dominated by streaky velocity patterns is not only essential for understanding the bypass transition into turbulence, but – in the context of viscoelastic fluids – also offers unique insight into the dynamics at high-extent and maximum drag reduction (HDR and MDR). Shooting trajectories connecting the edge state and following its unstable manifold to the turbulent basin are generated. In Newtonian flow, the growth of turbulence starts with the intensification of velocity streaks and a sharp rise in the Reynolds shear stress. It is followed by a quick breakdown into high-intensity small-scale fluctuations before entering the basin of statistical turbulence. Adding drag-reducing polymers does not affect the initial growth of turbulence but stabilizes the primary streak-vortex structure. As a result, the vortex breakdown stage is circumvented. Polymer deformation is insignificant until the vortex breakdown, after which polymer stress rapidly shoots up. At high Weissenberg number Wi, loss of turbulent kinetic energy through polymer elastic conversion is comparable to viscous dissipation. Beyond bypass transition, the transient process studied here closely resembles the bursting phase of the self-sustaining cycle of turbulence. Our results indicate that at high Wi (i.e., HDR) polymer effects can significantly reduce bursting by rerouting the trajectory of turbulent dynamics. (NSERC) Natural Sciences and Engineering Research Council of Canada: No. RGPIN-2014-04903; (NSF) National Science Foundation: No. NSF PHY11-25915.

International audience; Superhydrophobic surfaces are capable of trapping gas pockets within the micro-roughnesses on their surfaces when submerged in a liquid, with the overall effect of lubricating the flow on top of them. These bio-inspired surfaces have proven to be capable of dramatically reducing skin friction of the overlying flow in both laminar and turbulent regimes. However, their effect in transitional conditions, in which the flow evolution strongly depends on the initial conditions, has still not been deeply investigated. In this work the influence of superhydrophobic surfaces on several scenarios of laminar–turbulent transition in channel flow is studied by means of direct numerical simulations. A single phase incompressible flow has been considered and the effect of the micro-structured superhydrophobic surfaces has been modelled imposing a slip condition with given slip length at both walls. The evolution from laminar, to transitional, to fully developed turbulent flow has been followed starting from several different initial conditions. When modal disturbances issued from linear stability analyses are used for perturbing the laminar flow, as in supercritical conditions or in the classical K-type transition scenario, superhydrophobic surfaces are able to delay or even avoid the onset of turbulence, leading to a considerable drag reduction. Whereas, when transition is triggered by non-modal mechanisms, as in the optimal or uncontrolled transition scenarios, which are currently observed in noisy environments, these surfaces are totally ineffective for controlling transition. Superhydrophobic surfaces can thus be considered effective for delaying transition only in low-noise environments, where transition is triggered mostly by modal mechanisms.

We report a theoretical study of the properties of a supersonic boundary layer and its linear stability under conditions of surface material sublimation. Calculations were performed for an adiabatic boundary layer for a flat plate with a naphthalene coating at a free-stream Mach number of M = 3 (for the first instability mode disturbances). In the boundary layer, surface sublimation generates a binary mixture flow (air and foreign vapors). This flow is studied using local self-similar boundary layer equations, and it is shown that the rise in the flow stagnation temperature and the corresponding evaporation of the wall material cause significant wall cooling and an increase in the near-wall density of the binary mixture. This modification of the boundary layer profiles leads to a decrease in the disturbance amplification rates. This is confirmed by calculations based on linear stability theory (LST). Boundary-layer stabilization occurs with an increase in stagnation temperature. The influence of surface sublimation on the position of the laminar–turbulent transition was estimated by means of the LST-based e N method. The possibility of increasing the transition Reynolds number by application of the sublimation coating is demonstrated. The results of pilot boundary layer transition experiments performed in a hot-shot wind tunnel are reported. For the first time, a delay in the transition due to the application of a naphthalene coating was experimentally demonstrated. It is also shown that surface sublimation leads to an increase in the growth rates of the second and third instability modes for a Mach 8 boundary layer.

The effect of gas non-ideality on the laminar-turbulent transition is studied experimentally as the flow in a long circular tube at room temperature. The gases SF6 and Ar, differing significantly in the value of the second virial coefficient, were chosen for this study. Experiments were carried out by varying the pressure at the tube inlet (the maximum pressure of 105 Pa) and at the tube outlet up to the chock flow (formation of a supersonic flow at the outlet). The difference between the critical Reynolds numbers in the flow of SF6 and Ar was found. The largest difference was observed for the maximum pressures; with a decrease in pressure, the critical Reynolds numbers become closer. The conclusion is an effect of the non-ideal character of gas exists on the laminar-turbulent transition in Hagen–Poiseuille flow. Some experiments were suggested to confirm this conclusion.

An experimental study of the laminar-turbulent transition between two coaxial rotating cylinders with the inner cylinder rotates and outer one stationary is presented in this paper. Special attention is given to the onset of various flow modes in tilted and partially filled system. The effect of the inclination angle was investigated on the different flow regimes occurring at fully and/ or partially filled space between two rotating cylinders. The flow behavior, the transitional phenomena and the features of various flow modes are discussed for different inclination angles, filling ratio and Taylor numbers. It is established that the different filling ratio and inclination angles of the Taylor-Couette system deeply affect the flow patterns. Furthermore, the most significant result concerns the relaminarization of the flow when the aspect ratio is decreased and inclination angle is increased for a given value of Taylor number.

This manuscript presents a theoretical study on the transition from laminar to turbulent natural convection flow of gas ( Pr ≈ 1 ) along a vertical isothermal plate. The analysis of the transition to unstable flows is based on an approximate solution, deduced from the integral formulation of the boundary layer equations for Pr ≈ 1 (i.e. thickness of the momentum boundary layer is near to that of the thermal boundary layer), and on the Rayleigh equation. An approximate disturbance profile is then calculated. It allows the determination of the parameter kδ (product of the wave number by the boundary layer thickness) corresponding to retrograde, neutral and progressive wave modes. The Figures 7 to 13 and the Annexes A 1 to A 5 of the ref. [4], edited in 1998 by Elsevier Science Ltd, give to the reader all the necessary information.

The shock wave boundary layer interaction on the suction side of transonic compressor blade is one of the main objectives of TFAST project (Transition Location Effect on Shock Wave Boundary Layer Interaction). In order to investigate the flow structure on the suction side of a profile, a design of a generic test section in linear transonic wind tunnel was proposed. The experimental and numerical results for the flow structure investigations are shown for the flow conditions as the existing ones on the suction side of the compressor profile. Near the sidewalls the suction slots are applied for the corner flow structure control. It allows to control the Axial Velocity Density Ratio (AVDR), important parameter for compressor cascade investigations. Numerical results for Explicit Algebraic Reynolds Stress Model with transition modeling are compared with oil flow visualization, schlieren and Pressure Sensitive Paint. Boundary layer transition location is detected by Temperature Sensitive Paint.

The transition induced by the natural convection with high temperature differences can be easily observed in many engineering applications. The compressible solver has to be taken into consideration because the usual way of using the incompressible solver with Boussinesq approximation is only available within the temperature difference of 30 K. However, speeds in natural convection are always several orders of magnitude lower than the speed in compressible regions. A modified compressible solver which can deal with the buoyancy-induced turbulence is proposed here. In this solver, an all speed preconditioned Roe (APRoe) is adopted to appropriately treat flows at extremely low speed regions. And then, a reconstruction method for the MUSCL scheme at low Mach numbers (M5LM) is applied to attenuate the dissipation. The numerical dissipation of the compressible solver is utilized as a subgrid scale (SGS) model for the implicit large eddy simulation (LES). Based on qualitative agreement with direct numerical simulation and considering the capability of numerically predicting a -3 decay law for the energy transfer induced by the natural convection in the temporal power spectrum of the temperature fluctuation, this solver is potentially a good candidate for laminar–turbulent transition in natural convection.

Cardiovascular diseases are the leading cause of death globally and there is an unmet need for effective, safer blood-contacting devices, including valves, stents and artificial hearts. In these, recirculation regions promote thrombosis, triggering mechanical failure, neurological dysfunction and infarctions. Transitional flow over a backward facing step is an idealised model of these flow conditions, the aim was to understand the impact of non-Newtonian blood rheology on modelling this flow. Flow simulations of shear-thinning and Newtonian fluids were compared for Reynolds numbers ( R e ) covering the comprehensive range of laminar, transitional and turbulent flow for the first time. Both unsteady Reynolds Averaged Navier&ndash, Stokes ( k &minus, &omega, SST) and Smagorinsky Large Eddy Simulations (LES) were assessed, only LES correctly predicted trends in the recirculation zone length for all R e . Turbulent-transition was assessed by several criteria, revealing a complex picture. Instantaneous turbulent parameters, such as velocity, indicated delayed transition: R e = 1600 versus R e = 2000, for Newtonian and shear-thinning transitions, respectively. Conversely, when using a Re defined on spatially averaged viscosity, the shear-thinning model transitioned below the Newtonian. However, recirculation zone length, a mean flow parameter, did not indicate any difference in the transitional Re between the two. This work shows a shear-thinning rheology can explain the delayed transition for whole blood seen in published experimental data, but this delay is not the full story. The results show that, to accurately model transitional blood flow, and so enable the design of advanced cardiovascular devices, it is essential to incorporate the shear-thinning rheology, and to explicitly model the turbulent eddies.

This paper investigates the requirements for turbulence models, in application to heat transfer simulation of turbine part, from a thermal stresses point-of-view. The investigation was based on thermal-fluid-structure interaction (thermal-FSI) analysis of the convectively-cooled turbine vane C3X, for which experimental heat transfer data is available and a laminar-turbulent transition occurs in the boundary layer. First, the production of turbulence energy at the stagnation point was investigated based on multiple turbulence models able to capture the transition phenomena. Next, the influence of turbulence modelling on thermal stresses was addressed. The stresses produced by experimental temperature field were compared with the modelled temperature-induced stresses. The presented results show that even relatively small discrepancies in temperature in an area where gradients of temperature are high can lead to large discrepancies in thermal stresses.

Extensive combined experimental and theoretical investigations of the linear evolution of three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. The flow under consideration is the boundary layer over an airfoil at 35 degrees sweep and an angle of attack of +1.5 degree. At these conditions, TS instability is found to be the predominant one. Perturbations with different frequencies and spanwise wavenumbers are generated in a controlled way using a row of elastic membranes. All experimental results are deeply processed and compared with results of calculations based on theoretical approaches. Very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved.

The dynamics of the phase transition in antiferroelectric ${\mathrm{PbZrO}}_{3}$ which is the subject of a decades-long debate, is examined using first-principles-based simulations. This is achieved through the development of a computational approach that allows calculations of generalized complex susceptibilities at an arbitrary point of the Brillouin zone. Application of this approach to the case of ${\mathrm{PbZrO}}_{3}$ predicts the temperature evolution of many of its lattice modes, some of which remain elusive or even ``invisible'' in experiments. The computational data suggest that two lattice modes are primarily responsible for the antiferroelectric phase transition in this material: the one associated with oxygen octahedra tilts dynamics and the other due to lead ions antipolar vibrations.

We study computationally the effects of dissipations on correlated two-photon transport in waveguide--quantum-electrodynamic systems. We show that in the dissipative regime, the two-photon correlation signatures remain prominent even when the two-photon transport can be described by a single-photon picture. Moreover, by varying the dissipation, the photonic correlation can have an induced transition from bunching to antibunching, and vice versa, depending on the input quantum state.

Stratified cocurrent flow of air and water was studied experimentally in a 5 cm diameter horizontal pipe. The velocity in the liquid phase was measured using planar particle image velocimetry, and the instantaneous interfacial profile was recorded using a separate camera. The resulting velocity fields extended from the pipe wall to the wavy interface. The principal aims of the study were to investigate the laminar–turbulent transition of the liquid phase in stratified gas–liquid flow, and to explore the interaction between the transition process and the interfacial waves. The boundaries of transition were determined in both the smooth and the wavy region. The occurrence of waves had the effect of increasing the Reynolds numbers at the end of transition. On the other hand, the transition to turbulence caused a change from the ‘2D small-amplitude’ to the ‘3D small-amplitude’ wave pattern, which were seen to correspond to the capillary–gravity and gravity–capillary solutions of the dispersion relationship respectively. In light of this, the flowmap of the wavy region was recast into Weber number–Froude number coordinates, which provided a physical interpretation of the interaction between the developing turbulence and the changing wave patterns.

The supersonic core length of microjets and the influence of the laminar–turbulent transition on the core length are considered. Axisymmetric mini- and micronozzles with diameters from 341 to 10.4 μm are used. The microjet is studied with the use of a Pitot microtube, shadow flow visualization and hot-wire anemometry. It is demonstrated that the laminar–turbulent transition in the jet mixing layer exerts a dominating effect on the supersonic core length. The increasing of the supersonic core length is associated with the laminar flow in microjet. Decreasing of the supersonic core length is connected with the laminar–turbulent transition in microjet. Based on experimental results, a chart of microjet regimes is constructed. The influence of the Pitot tube diameter on the accuracy of supersonic core length determining is considered. The effect of the nozzle edge roughness on the supersonic core length is examined.

This paper concerns the study of direct numerical simulation (DNS) data of a wavepacket in laminar turbulent transition in a Blasius boundary layer. The decomposition of this wavepacket into a set of "modes" (a basis that spans an approximate solution space) can be achieved in a wide variety of ways. Two well-known tools are the fast Fourier transform (FFT) and the proper orthogonal decomposition (POD). To synergize the strengths of both methods, a hybrid POD-FFT is pioneered, using the FFT as a tool for interpreting the POD modes. The POD-FFT automatically identifies well-known fundamental, subharmonic and Klebanoff modes in the flow, even though it is blind to the underlying physics. Moreover, the POD-FFT further separates the subharmonic content of the wavepacket into three fairly distinct parts: a positively detuned mode resembling a Lambda-vortex, a Craik-type tuned mode and a Herbert-type positive-negative detuned mode pair, in decreasing order of energy. This distinction is less widely recognized, but it provides a possible explanation for the slightly positively detuned subharmonic mode often observed in previous experiments and simulations., 38 pages, 13 figures