Search

This paper introduces a novel numerical approach for tackling the nonlinear fractional Phi-four equation by employing the Homotopy perturbation method (HPM) and the Adomian decomposition method (ADM), augmented by the Shehu transform. These established techniques are adept at addressing nonlinear difierential equations. The equation's complexity is reduced by applying the Shehu Transform, rendering it amenable to solutions via HPM and ADM. The eficacy of this approach is underscored by conclusive results, attesting to its proficiency in solving the equation. With extensive ramifications spanning physics and engineering domains like fluid dynamics, heat transfer, and mechanics, the proposed method emerges as a precise and eficient tool for resolving nonlinear fractional difierential equations pervasive in scientific and engineering contexts. Its potential extends to analogous equations, warranting further investigation to unravel its complete capabilities. [ABSTRACT FROM AUTHOR]

Analysis of the slipstream development around the high-speed trains in tunnels would provide references for assessing the transient gust loads on trackside workers and trackside furniture in tunnels. This paper focuses on the computational analysis of the slipstream caused by high-speed trains passing through double-track tunnels with a cross-sectional area of 100 m2. Three-dimensional unsteady compressible Reynolds-averaged Navier-Stokes equations and a realizable k-ε turbulence model were used to describe the airflow characteristics around a high-speed train in the tunnel. The moving boundary problem was treated using the sliding mesh technology. Three cases were simulated in this paper, including two tunnel lengths and two different configurations of the train. The train speed in these three cases was 250 km/h. The accuracy of the numerical method was validated by the experimental data from full-scale tests, and reasonable consistency was obtained. The results show that the flow field around the high-speed trains can be divided into three distinct regions: the region in front of the train nose, the annular region and the wake region. The slipstream development along the two sides of train is not in balance and offsets to the narrow side in the double-track tunnels. Due to the piston effect, the slipstream has a larger peak value in the tunnel than in open air. The tunnel length, train length and length ratio affect the slipstream velocities; in particular, the velocities increase with longer trains. Moreover, the propagation of pressure waves also induces the slipstream fluctuations: substantial velocity fluctuations mainly occur in front of the train, and weaken with the decrease in amplitude of the pressure wave. [ABSTRACT FROM AUTHOR]

The aim of this paper is to present a developed detailed model for the simulation of automotive fin-and-tube heat exchangers (Compact Heat Exchanger Simulation Software, CHESS). The simulation strategy and the mathematical methodology are described in detail. The model is based on a 3D discretization around the tubes as small heat exchangers, where the appropriate governing equations (mass, momentum and energy) are applied for each control volume on the air-side, the solid elements, and the coolant side. Some verification and illustrative results are also provided to show the features of the model. A comparison between numerical simulation results and experimental data is presented in a companion paper (Part 2). [ABSTRACT FROM AUTHOR]

This paper presents a set of numerical analyses carried out on a model combustor equipped with a prefilming airblast injection system using a multi-coupled approach that includes the solution of the liquid film over the prefilming surface. The main objective of this study is to perform a systematic investigation of all the relevant aspects involved in the liquid fuel preparation of airblast atomizers, ranging from the interaction between the gas phase and the liquid film to the effect of velocity fluctuations on the dispersion of droplets downstream of the injector exit. Measurements at high pressure and reacting conditions are available for the case considered here, therefore, allowing to perform such investigation at engine-relevant conditions. The solution of the liquid film evolution over the prefilming surface suggests that the interaction between the gas phase and the liquid film is an important aspect to be considered for a reliable simulation of prefilming airblast systems since it has a strong impact on both velocity and fuel temperature at the atomizing edge. The role of primary breakup has been investigated by performing a sensitivity analysis to different theoretical and correlation-based models. Results obtained from this analysis, performed using Reynolds averaged Navier-Stokes simulations, show that the various formulations predict a quite different diameter, affecting the mixing field in the downstream region and therefore pointing out the necessity of more advanced and robust formulations. A comparison between experimental measurements and a scale-adaptive simulation of the combustor, performed using the spray setup determined in the sensitivity analysis, demonstrates the necessity of including in the simulation time-resolved velocity fluctuations to improve the prediction of the dispersion of droplets and therefore give a reliable prediction of fuel location and mixing. [ABSTRACT FROM AUTHOR]

In this paper, an improved delayed detached eddy simulation method combined with shear-stress transport (SST) model was used to study the three-dimensional turbulent characteristics in a small rotary engine with a peripheral port. The turbulent characteristics including instantaneous velocity, turbulent fluctuation, coherent structure and velocity circulation were analysed based on a dynamic model of the small rotary engine. Three sets of conclusions on the basis of computational results were obtained. First, it was found that large-scale vortex structures with high intensity were distributed in the center of the chamber in the intake process and broke into lots of small vortex structures in the compression process. Second, flow stability in the X direction decreased from the leading to the trailing in the small rotary engine. The fluctuation velocity of the Y direction showed the paraboloid feature and its peak position moved from the mid-back to the middle of the chamber during the operation process. Third, during the intake process, two vortices occurred in the cross section parallel to the covers and were located at the leading and trailing of the cross section, respectively. Compared to the intake process, more vortices occur at cross sections which were far away from the central section during the compression process. [ABSTRACT FROM AUTHOR]

In this paper, a new type of finned plate heat exchanger (FPHE) is presented to recover the waste heat from exhaust flue gases. A finned plate configuration causes low pressure drop and it is especially appropriate for heat transfer at the flue gas side. Meanwhile, this paper presents a detailed experimental and numerical study of convection heat transfer and pressure drop of the new structure. Three-dimensional numerical simulation results using the CFD code FLUENT6.3 were compared with experimental data to select the best model. The heat transfer and pressure drop with different geometry pattern was then studied numerically using the selected model. And the velocity field and temperature distribution of air flow in the finned plate channel are presented with different geometry patterns. These results provide insight into improved designs of FPHEs. [ABSTRACT FROM AUTHOR]

Double-skin façade (DSF) is an environmental friendly architectural feature. However, fire hazard is a concern. A scenario of having a flashover room fire adjacent to the façade was identified. Heat and mass would be trapped in the façade cavity. This paper examines air flow driven out of a flashover room fire to the cavity of a DSF by Computational Fluid Dynamics. The software Fire Dynamics Simulator developed at the Building and Fire Research laboratory, National Institute of Standards and Technology, USA was selected as the simulation tool. Three DSF features labeled as DSF1, DSF2 and DSF3 were considered. Detailed simulations were carried out to understand the fire-induced aerodynamics in a 5-level model DSF1 with a fire room at the third level. Hot gas spreading out to the façade cavity was simulated under two heat release rates of 1 MW and 5 MW. Air cavity depths of 0.5 m, 1 m, 1.5 m and 2 m were considered. Three stages of flame spreading out to a DSF with a wide air cavity depth were identified. Results suggested that wider air cavity depths would be more dangerous, with higher risk of the upper interior glass pane's breaking. To study spreading of heat and mass up the façade cavity as vertical channel flow, two taller DSF façade features DSF2 and DSF3 with differing air cavity depths were simulated. Both features were of height 24 m but of differing fire room height. Vertical temperature profiles with and without the DSF feature were compared. [ABSTRACT FROM PUBLISHER]

Identifying vortices is the key to understanding the turbulence in plasma shear layers. This paper aims to provide general guidelines for identifying 3‐D vortex structures. Currently, no single precise definition of a vortex is universally accepted, despite the significance of vortices in fluid and plasma dynamics. Recently, various vortex identification methods using Galilean invariance have been proposed by numerous researchers. These methods are general for different fluid and plasma visualization applications. In the present paper, we describe how we have identified 105 vortex structures by applying these methods to Cluster data near the duskside of the magnetopause. Four sets of Cluster satellite magnetic field data are used to linearly approximate the magnetic field. We identify the 3‐D magnetic vortex structures by using various vortex identification criteria as follows: (i) the first criterion is Q‐criterion that defines vortices as regions in which the vorticity energy prevails over other energies; (ii) the second criterion is the λ2‐criterion that is related to the minus eigenvalue of the Hessian matrix of the pressure terms; and (iii) the third criterion called the geometrical line‐type method requires the existence of Galilean‐invariant vortex core inside the four Cluster tetrahedral regions. In reality, both Q‐ and λ2‐criteria are also related to Galilean invariance. The present analysis evidences that the geometrical line‐type method is more precise than the other two using Cluster satellite magnetic field data. Key Points: 3‐D vortex detection technique is developed and applied to tetrahedral satellite configurationFirst time in the world, 3‐D vortex structures are detected using Cluster satellite in space at/around magnetopauseWe found almost the same number of clockwise and counterclockwise rotation vortices that suggests possible existence of double vortex rows [ABSTRACT FROM AUTHOR]

In previous work, a time-stepped numerical algorithm to compute particle trajectories consisting of lunar soil blown by the engine exhaust of a lunar lander was developed. The time-stepped integration method relies on the gas density, velocity, and temperature fields, calculated by computational fluid dynamics simulations, to compute the forces and accelerations acting on single noninteracting particles. In this paper, a computationally efficient particle ballistics model is presented where the trajectory is estimated by computing the vertical position (axial coordinate x) as a function of the horizontal position (radial coordinate y) using a constant horizontal velocity and a vertical acceleration approximated as a power-law. The unknown parameters of the model are determined by fitting the ballistics trajectory path to a matrix of trajectories generated by the time-step integration method using the rocket exhaust gas properties predicted by computational fluid dynamics software. Also in previous work, a strictly empirical trajectory model was developed to satisfy the need for a computationally efficient method of computing particle trajectories. This new model (like the previous model) is expressed as a time-independent trajectory path function. However, the method of this current work is based on physical laws of motion, unlike the previous empirical model. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical simulations are carried out to examine the problem of transient electroconvection stability of dielectric liquids subjected to unipolar injection. The entire set of electrohydrodynamics equations associated with the electroconvective phenomena that occur in a layer of a dielectric liquid between two parallel electrodes subjected to a potential difference are solved numerically. We first validate the numerical simulation by comparing our linear stability electroconvection criteria with those obtained by other authors with a stability approach. In this paper, we restrict the study to the strong injection case, which corresponds to values of the non-dimensional injection parameter C greater than or equal to 10. The numerical solution of the electroconvective problem is then presented for rigid lateral boundary conditions. A detailed analysis of the scenario that occurs for different characteristic values of the stability parameter T is provided. The flow structure and its behaviour highlight the existence of different regimes, from laminar to chaotic. The development of charged plumes has been observed in particular. We compute the electrical Nusselt number for different values of the stability parameter and ion mobility. The electrical Nusselt number saturates with increasing T, a fact that it is in agreement with available experimental data. Finally, a spectral analysis is conducted for different aspect ratios of the computational domain. The spectral analysis gives an insight into the physical origin of the velocity and current oscillations. [ABSTRACT FROM AUTHOR]

Abstract In this paper, Energy absorption characteristics of tapered multi-cell tubes improved by foams are studied. For the energy absorption process, a theoretical formulation is developed to represent the mean crush load of the foam-filled tubes considering a combination of mean crush loads due to non-filled tube, the foam-filler, and a frictional resistance between them (interaction effects). The formulation is based on the assumptions including equivalent segmented non-tapered single-cell tubes as well as a parallel resisting force of the contributors (non-filled tubes, foam-filler, and interaction effects). Moreover, an extensive numerical analysis on the crushing of the foam-filled tapered multi-cell tubes, with a taper angle ranging from 0 to 7 degrees, a wall thickness ranging from 0.25 to 3 mm, and a cell number ranging from 11 to 10 × 10, is conducted to evaluate the effects of side wall tapering, cross section's division into multiple cells and wall thickness enlargement on the crashworthiness characteristics. Analyses indicate that there are significant enhancements in the energy absorption behavior of the foam-filled tubes with respect to the non-filled ones. As a special case, the force-displacement results obtained in the numerical simulation are verified against those experimentally observed and reported in the literature for a foam-filled non-tapered single-cell tube. [ABSTRACT FROM AUTHOR]

Abstract: This paper identifies cooling-induced phenomena that can follow pipeline shut-in. Resultant shrinkage flow can affect the viscosity of gelled semi-solids that develop in pipelines containing waxy fluids. Pressure gradients develop in accordance with flow rate and direction. These same processes may also induce gel fracture and fragmentation. The relationship between these thermally driven events and easily measured parameters is used in numerical calculation for predicting gel viscosity and the distribution of void space and intact gel. The resultant map of the gel state within the line thus delineates the conditions prior to restart. Simulation allows parametric testing to provide a powerful design tool. Laboratory tests are described that support such predictions. The insights provided by this novel approach can indicate cost effective solutions to pipeline design and operation, and possible sources of laboratory testing inaccuracy. The findings reported in this paper represent part of an ongoing investigation into aspects of waxy crude restart that may lead to predictive errors. [Copyright &y& Elsevier]

Rigid-lid approximation is usually used to replace the free surface in scour simulation. The influence of the rigid lid assumption on the prediction precision of scour hole in steady flow is studied in this paper. Firstly, a local scour model was constructed based on the open sources Computational Fluid Dynamics (CFD) model OpenFOAM, where both the bed load and suspended load were considered. In the present model, the bed shear stress was calculated by the Newton shear stress formula, instead of the traditional calculation method with the assumption that the flow velocity in vertical direction complies with a logarithmic distribution. The Volume of Fluid (VOF) method was used to capture the free surface and a moving-mesh method was used to track the change of bed surface. Then, several experiments were chosen to validate the model, and the modeling results fitted well with the measured data. Lastly, the effect of the rigid lid assumption on surface elevation, bed shear stress and the profile of the scour hole in steady flow are studied. The result shows that the surface elevation suffers a drop above the pipeline, and the difference of surface elevation between the upstream and downstream increases with decreasing dimensionless depth. Compared with the free surface condition, the bed shear stress and scour hole depth computed with the rigid lid approximation were underestimated. [ABSTRACT FROM AUTHOR]

Abstract: Magnetohydrodynamics (MHD) laminar flows through circular pipes are studied in this paper by numerical simulation under the conditions of Hartmann numbers from 18 to 10000. The code is developed based on a fully developed modeling and validated by Samad's analytical solution and Chang's asymptotic results. After the code validation, numerical simulation is extended to high Hartmann number for MHD circular pipe flows with conducting walls, and numerical results such as velocity distribution and MHD pressure gradient are obtained. Typical M-type velocity is observed but there is not such a big velocity jet as that of MHD rectangular duct flows even under the conditions of high Hartmann numbers and big wall conductance ratio. The over speed region in Robert layers becomes smaller when Hartmann numbers increase. When Hartmann number is fixed and wall conductance ratios change, the dimensionless velocity is through one point which is in agreement with Samad's results, the locus of maximum value of velocity jet is same and effects of wall conductance ratio only on the maximum value of velocity jet. In case of Robert walls are treated as insulating and Hartmann walls as conducting for circular pipe MHD flows, there is big velocity jet like as MHD rectangular duct flows of Hunt's case 2. [Copyright &y& Elsevier]

A surrogate modeling strategy, using effective interpolation and sampling methods, facilitates a reduction in the number of computational fluid dynamics simulations required to construct an aerodynamic model to a specified accuracy. In this paper, two adaptive sampling strategies are compared for generating surrogate models, based on Kriging and radial basis function interpolation, respectively. The relationships between the two model formulations are discussed, and three test cases are considered, including analytic functions and recovery of aerodynamic coefficients for two example applications: longitudinal flight mechanics analysis for the DLR-F12 aircraft and structural loads analysis of an RAE2822 airfoil. For the airfoil example, models of CL, CD, and CM were constructed with the two sampling strategies using Euler/boundary-layer-coupled computational fluid dynamics and a three-dimensional flight envelope of incidence, Mach, and Reynolds number. The two sampling approaches direct some samples toward exploration of the domain by minimizing model uncertainty and some toward refinement of local nonlinearities, by adapting to model curvature or extrema. The results provide some evidence that, for certain functions, in certain scenarios, each update scheme could be useful. Both methods were at least better than traditional space-filling sampling for all the test cases presented. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional (3D) numerical model is established to simulate the flow field around fishing plane nets in a current. The realizable k–ε turbulence model was chosen to describe the flow, and the governing equations are solved by using the finite volume method. In this model, the net is modeled using the porous media model. The unknown porous coefficients are determined from the hydrodynamic forces on the net and from the flow velocities and the attack angles using the least squares method. In order to validate the numerical model, the numerical results were compared with the data obtained from two physical model tests. The comparisons show that the numerical results are in good agreement with the experimental data. This numerical model is applicable for the simulation of the flow field around a fishing net. Using the proposed method, this paper presents the flow field around a plane net with different plane net inclination angles, plane net heights, spacing distances between two plane nets and plane net numbers. This study provides information about the flow field around the fishing net cage. [Copyright &y& Elsevier]

Abstract: In this paper, the problem of natural convection in a circular eccentric annulus filled with micropolar fluid has been numerically investigated using Fourier Spectral Method. The annulus inner wall is heated and maintained at constant temperature while the outer wall is cooled and kept at constant temperature. The full governing equations of momentum, angular momentum and energy have been solved to give the details of flow and thermal fields. The heat convection process in the annulus is mainly controlled by Rayleigh number Ra, Prandtl number Pr, radius ratio Rr, eccentricity, e and material parameters of micropolar fluid. The material parameters are dimensionless spin gradient viscosity λ, dimensionless micro-inertia density B and dimensionless vortex viscosity D. This study considers Ra up to 5×104 and the eccentricity is varied between −0.6 and +0.6 while dimensionless material parameters D of micropolar fluid is considered between 0 and 10. Both Pr, and Rr are fixed at 0.7, 2.6, respectively while B and λ are assigned the value of 1. The study considers the effect of controlling parameters on flow and thermal fields with emphasis on the effect of these parameters on mean Nusselt number. The study has shown that for certain controlling parameters the heat transfer in the annulus is minimum at a certain eccentricity. The study also has shown that as parameter D increases the rate of heat transfer through the annulus decreases. [Copyright &y& Elsevier]

Abstract: Various pseudo functions or general functions were introduced into multidimensional flow in turbo machinery aerodynamics to simplify governing equations, but in fact little progress has been made in the use of pseudo functions for numerical simulation. The difficulty of using pseudo functions for numerical calculation is discussed in the paper. [Copyright &y& Elsevier]

Abstract: In this paper, the variation of the fiber orientation distribution along the flow of fresh UHPCC was studied. In order to describe the rotational motion of a single fiber, Jeffery’s equation was adopted, in which the interaction among fibers is neglected. Two cases of flow patterns were considered: shear flow and radial flow. Starting with a three-dimensional random distribution of fibers, the fiber orientation distribution along the flow distance was simulated. These results reveal that fibers gradually become more parallel (in the case of shear flow) and perpendicular (in the case of radial flow) to the flow direction as the flow distance increases. This approach will be useful to predict flow-dependent tensile behavior considering the change of fiber orientation distribution. [Copyright &y& Elsevier]

Abstract: Direct numerical simulations (DNS) are performed to study the behavior of a swarm of rising air bubbles in water, employing the front tracking method, which allows to handle finite-size bubbles. The swarms consist of monodisperse deformable 4mm bubbles with a gas fraction of 5% and 15%. This paper focuses on the comparison of the liquid energy spectra and bubble velocity probability density functions (PDFs) with experimental data obtained by phase-sensitive constant-temperature anemometry (CTA) and three-dimensional particle tracking velocimetry (PTV), respectively. The numerical simulations confirm that the spectra of the velocity fluctuations driven by the rising bubbles follow a power law with slope close to −3, supporting the idea that the dissipation of the bubble wake is the origin of this spectral scaling, as previously proposed by Lance and Bataille. The computed PDFs of the bubble velocity show non-Gaussian features, as is also observed in the experiments. The agreement with experimental measurements is especially good in the peak region, whereas the tails of the experimental PDFs show more intermittency in comparison to the numerical results. This can be explained by the lack of large-scale flow structures in the simulations, and by the large difference in measurement time. [Copyright &y& Elsevier]

Abstract: Recent progress concerning experimental and numerical methods for practical investigations of vehicle soiling are reviewed in this paper. Wind tunnel tests and available measurement techniques are analyzed and compared with respect to their possible applications and to the results they may deliver. Different computational models for liquid films generated by spray impingement on vehicle surfaces are then considered. Models relying either on a dispersed or on a continuous approach are compared, resulting advantages and shortcomings are discussed. It is finally shown that, in spite of considerable progress during the last years, substantial work remains to be done in order to capture the complexity of real film flow dynamics for an accurate representation of wetting processes, film separation and break-up interacting with turbulent flows. [Copyright &y& Elsevier]

A numerical method that employs a combination of contour advection and pseudo-spectral techniques is used to simulate shear-induced instabilities in an internal solitary wave (ISW). A three-layer configuration for the background stratification, in which a linearly stratified intermediate layer is sandwiched between two homogeneous ones, is considered throughout. The flow is assumed to satisfy the inviscid, incompressible, Oberbeck–Boussinesq equations in two dimensions. Simulations are initialized by fully nonlinear, steady-state, ISWs. The results of the simulations show that the instability takes place in the pycnocline and manifests itself as Kelvin–Helmholtz billows. The billows form near the trough of the wave, subsequently grow and disturb the tail. Both the critical Richardson number (${\mathit{Ri}}_{c} $) and the critical amplitude required for instability are found to be functions of the ratio of the undisturbed layer thicknesses. It is shown, therefore, that the constant, critical bound for instability in ISWs given in Barad & Fringer (J. Fluid Mech., vol. 644, 2010, pp. 61–95), namely ${\mathit{Ri}}_{c} = 0. 1\pm 0. 01$, is not a sufficient condition for instability. It is also shown that the critical value of ${L}_{x} / \lambda $ required for instability, where ${L}_{x} $ is the length of the region in a wave in which $\mathit{Ri}\lt 1/ 4$ and $\lambda $ is the half-width of the wave, is sensitive to the ratio of the layer thicknesses. Similarly, a linear stability analysis reveals that ${\bar {\sigma } }_{i} {T}_{w} $ (where ${\bar {\sigma } }_{i} $ is the growth rate of the instability averaged over ${T}_{w} $, the period in which parcels of fluid are subjected to $\mathit{Ri}\lt 1/ 4$) is very sensitive to the transition between the undisturbed pycnocline and the homogeneous layers, and the amplitude of the wave. Therefore, the alternative tests for instability presented in Fructus et al. (J. Fluid Mech., vol. 620, 2009, pp. 1–29) and Barad & Fringer (J. Fluid Mech., vol. 644, 2010, pp. 61–95), respectively, namely ${L}_{x} / \lambda \geq 0. 86$ and ${\bar {\sigma } }_{i} {T}_{w} \gt 5$, are shown to be valid only for a limited parameter range. [ABSTRACT FROM AUTHOR]

Abstract: Ventilation system with air recirculation is designed to conserve energy, yet at the same time may result in transporting hazardous substance among different rooms in the same building, which is a concern in indoor air quality control. There is a lack of effective methods to predict indoor contaminant distribution primarily because of uncertainty of the contaminant concentration in supply air which in turn due to the mixing ratio of fresh and recirculation air. In this paper, a versatile numerical method to determine the pollutant distribution of ventilation system with recirculation at steady state is proposed based on typical ventilation systems with accessibility of supply air (ASA) and accessibility of contaminant source (ACS). The relationship is established between contaminant concentrations of supply air and return air in a ventilated room or zone. The concentrations of supply air and contaminant distribution in each room can be determined using such parameters as ASA and ACS. The proposed method is validated by both experimental data and numerical simulation result. The computing speed of the proposed method is compared with the iteration method. The comparisons between the proposed method and the lumped parameter model are also conducted. The advantages of the proposed method in terms of accuracy, speed and versatility make it advantageous to be applied in air quality control of complex ventilation systems with recirculation. [Copyright &y& Elsevier]

Abstract: The paper is focused on the investigation of fluid flow and heat transfer characteristics in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall. In contrast to the new microchannel heat sink, the corresponding conventional rectangular microchannel heat sink is chosen. The computational fluid dynamics is used to simulate the flow and heat transfer in the heat sinks. The steady, laminar flow and heat transfer equations are solved in a finite-volume method. The SIMPLEX method is used for the computations. The effects of flow rate and heat flux on pressure drop and heat transfer are presented. The results indicate that the microchannel heat sink with offset fan-shaped reentrant cavities in sidewall improved heat transfer performance with an acceptable pressure drop. The fluid flow and heat transfer mechanism of the new microchannel heat sink can attribute to the interaction of the increased heat transfer surface area, the redeveloping of the hydraulic and thermal boundary layers, the jet and throttling effects and the slipping over the reentrant cavities. The increased heat transfer surface area and the periodic thermal developing flow are responsible for the significant heat transfer enhancement. The jet and throttling effects enhance heat transfer, simultaneously increasing pressure drop. The slipping over the reentrant cavities reduces pressure drop, but drastically decreases heat transfer. [Copyright &y& Elsevier]

Abstract: Bubbly flows exist extensively in industrial processes, so it is very meaningful to study hydrodynamic characteristics of them to improve efficiency of bubbly flow equipments. This paper introduces a numerical method of the Euler–Lagrange two-way model for the air–water bubbly flows in detail. The flow field is simulated by using direct numerical simulations (DNS) in Euler frame of reference, while the bubble dynamics are fully analyzed by integration of Newtonian equations of motion taking into account interphase interaction forces including drag force, lift force, wall lift force, pressure gradient force, virtual mass force, gravity force, buoyant force, and inertia force in Lagrange frame of reference. The coupling between phases is considered by regarding the interphase interaction forces as a momentum source term of the continuous phase. Bubbles distribution and turbulent statistics of the liquid phase are comprehensively analyzed. The results show that an overwhelming majority of bubbles cluster near the walls, and turbulent structures of the liquid phase are modified to some certain by addition of bubbles, namely, the mean streamwise velocity become increased at the core of the channel, the wall-normal and spanwise turbulent intensities and Reynolds stress are reduced. Redistribution of turbulent energy from the streamwise velocity components to wall-normal and spanwise velocity components is also suppressed due to the addition of bubbles. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a technique for enhancing the mixing of electroosmotic flows in a microchannel by means of aperiodic spatio-temporal variations in the zeta potential. The effects of the magnitude of the time-varying zeta potential, the length of the heterogeneous surface zeta potential patterns, and the aperiodic switching frequency of the zeta potential on the fluid flow characteristics and mixing performance are analyzed by performing a series of numerical simulations in which the aperiodic oscillating source used to modulate the switching frequency of the zeta potential over time is derived using the Sprott system. The results show that the aperiodic spatio-temporal variations in the zeta potential generate irregularly alternating flow recirculations over time, which in turn produce a stirring of the species. In addition, it is shown that an effective enhancement in the mixing performance can be obtained through the application of a suitable switching frequency. Moreover, the mixing performance can be further improved by increasing the strength of the heterogeneous surface zeta potential or the length of the heterogeneous surface zeta potential patterns, respectively. Overall, it is shown that an average mixing efficiency of over 90% can be obtained when the length of the heterogeneous surface zeta potential patterns is equal to twice the channel width, the magnitude of the heterogeneous surface zeta potential is twice that of the homogeneous surface zeta potential, and the value of the oscillating frequency used to modulate the variation of the zeta potential is assigned within an appropriate range. [Copyright &y& Elsevier]

Abstract: This paper describes a numerical procedure for the prediction of the homogenizing performance of high pressure homogenizing valves used in diary plants. The method is based on a strict interaction between a complex CFD code and a simple homogenizing simulation code developed by the authors. This latter implements a mathematical model for the evaluation of droplets break-up, that needs an accurate evaluation of few significant fluid dynamic parameters in the whole fluid dynamic domain inside the valve. Due to the relevant pressure gradients within the flow and to the possibility of cavitation, particular attention has been paid in defining the fluid model, the mesh and the parameters required for CFD simulations. Notwithstanding the quite simple model of the homogenizing process, comparatively with the complexity of phenomena involved, the first results obtained are in general agreement with the experimental data available. These results point out the potential of the procedure proposed as a starting point for further implementation of more complex effects. [Copyright &y& Elsevier]

An adaptive numerical method is employed to simulate shear instabilities in open-ocean internal solitary-like gravity waves. The method is second-order accurate, employs block-structured adaptive mesh refinement, solves the incompressible Navier-Stokes equations and allows for the simulation of all of the length scales of interest by dynamically tracking important regions with recursively-nested finer grids. Two-dimensional simulations are performed over a range of parameters, which allows us to assess the conditions under which the shear instabilities in the waves occur, including a method to evaluate the critical Richardson number for instability based on the bulk wave parameters. The results show that although the minimum Richardson number is well below the canonical value of ¼ in all simulations, this value is not a sufficient condition for instability, but instead a much lower Richardson number of 0.1 is required. When the Richardson number falls below 0.1, shear instabilities develop and grow into two-dimensional billows of the Kelvin-Helmholtz type. A linear stability analysis with the Taylor-Goldstein equation indicates that an alternate criterion for instability is given by σ¯iTw >5, where σ¯i is the growth rate of the instability averaged over Tw, the period in which parcels of fluid are subjected to a Richardson number of less than ¼. A third criterion for instability requires that Lw/L>0.86, where Lw is half the length of the region in which the Richardson number falls below ¼ and L is the solitary wave half-width. An eigendecomposition of the rate-of-strain tensor is performed to show that the pycnocline thickness increases within the wave because of pycnocline-normal strain and not because of diffusion, which has important ramifications for stability. A three-dimensional simulation indicates that the primary instability is two-dimensional and that secondary, three-dimensional instabilities occur thereafter and lead to strong dissipation and mixing. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional, non-isothermal, polymer melt flow in sudden expansion is numerically investigated. The main goal of the paper is to study the general features of the flow field. The generalized Newtonian formulation is adopted, and the mathematical model corresponds to the laminar, incompressible Navier–Stokes equations. A second order finite difference scheme is used to discretize the governing equations in a collocated mesh. The non-Newtonian flow behaviour is modelled by the Cross constitutive relation, in which temperature effects are accounted for. The simulations show that, despite the high viscosity exhibited by polymer melts, a complex three-dimensional flow structure is found close to the expansion section, characterized by a spiral motion. The results also indicate that viscous dissipation causes limited effect in temperature rise and viscosity variations. The latter was found mostly affected by the shear rate. [Copyright &y& Elsevier]

Abstract: The flow of non-Newtonian liquid polymers through fibrous reinforcements is a phenomenon which is often encountered during polymer composites manufacturing. In a previous work, we have proposed from a multiscale theoretical approach a method to model this phenomenon when the polymer can be regarded as a generalised Newtonian fluid [Orgéas et al. J. Non-Newtonian Fluid Mech. 2007; 145]. In this paper, the capability of the method is tested with power-law fluids flowing through deformed plain weave fabrics. For that purpose, the flow problem is firstly analysed at the mesoscale from numerical simulations performed on representative elementary volumes of the fabrics. The influences of both the current deformation of the fabrics and the fluid rheology on the macroscopic flow law are emphasised. Secondly, it is shown that the proposed method allows a nice fit of numerical results. [Copyright &y& Elsevier]

According to multi-node model, the dynamics equations of conical parachute system for simulating shape deformation process of the flexible canopy in the opening process were established. With the combination of dynamics equations code and computational fluid dynamics (CFD) software, the fluid-structure interaction investigation of the conical parachute was carried out. Also the change of parachute shape and flow field, inflation time, the rate of descent, the distance of descent, and other relevant data were achieved. This paper has focused on analysing vortex structure of the flow field in the opening process of conical parachute, and laid the foundation for studying mechanics mechanism of flow field variation of conical parachute in future. [ABSTRACT FROM AUTHOR]

Abstract: Unsteady natural convection in a reservoir model subject to constant cooling at the water surface is considered in this paper. In such a case, the unequal heat loss resulting from a varying water depth creates a horizontal temperature gradient which in turn drives a horizontal exchange flow between the coastal region and the main water body of the reservoir. Understanding of the flow mechanisms pertinent to this flow is important for predicting the transport of nutrients and pollutants across the reservoir. The present numerical simulations impose random perturbations at the water surface in order to trigger flow instabilities. First, numerical tests are carried out to investigate the flow response to different perturbation amplitudes. These tests reveal a linear range of the flow response to the perturbations. Subsequently, the transient flow development at different stages is described in details based on numerical data, and the effects of the Grashof number on the unsteady flow are examined. [Copyright &y& Elsevier]

Numerical simulation is used for studying the formation and outflow of a jet of low-temperature plasma generated by water evaporation induced by a high-energy electric pulse discharge in a half-closed cylindrical volume. It is shown that a stable bounded comet-shaped high-pressure zone is formed in the leading part of the jet moving in the vertical direction. [ABSTRACT FROM AUTHOR]

This paper describes the parallel implementation of a numerical model for the simulation of problems from fluid dynamics on distributed memory multiprocessors. The basic procedure is to apply a fully explicit upwind finite difference approximation on a staggered grid. A theoretical time complexity analysis shows that a perfect speedup is achieved asympotically. Experimental results on the Intel Touchstone Delta System confirm the analytical performance model. © 1997 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A mean-line compressor performance calculation method is presented that covers the entire operating range, including the choked region of the map. It can be directly integrated into overall engine performance models, as it is developed in the same simulation environment. The code materializing the model can inherit the same interfaces, fluid models, and solvers, as the engine cycle model, allowing consistent, transparent, and robust simulations. In order to deal with convergence problems when the compressor operates close to or within the choked operation region, an approach to model choking conditions at blade row and overall compressor level is proposed. The choked portion of the compressor characteristics map is thus numerically established, allowing full knowledge and handling of inter-stage flow conditions. Such choking modelling capabilities are illustrated, for the first time in the open literature, for the case of multi-stage compressors. Integration capabilities of the 1D code within an overall engine model are demonstrated through steady state and transient simulations of a contemporary turbofan layout. Advantages offered by this approach are discussed, while comparison of using alternative approaches for representing compressor performance in overall engine models is discussed. [ABSTRACT FROM AUTHOR]

In this study, a quadtreeadaptive viscous numerical wave tank in which the free surface is resolved by the volume of fluid method is established to solve Bragg resonance caused by Stokes and cnoidal waves. Numerical wavemakers of third-order cnoidal and fifth-order Stokes waves are implemented. Additionally, a numerical damping zone is implemented such that the numerical grid in that area is automatically coarsened under a prescribed low resolution in which the time-matching is more efficient without an excessively small time-step. Furthermore, the areas with intense vorticity, boundary layers, and free surfaces are automatically refined to prescribed high resolutions. The model is shown as stable even when multiple adaptation () criteria are implemented in the computation. With respect to the simulation of Bragg resonance caused by Stokes waves, the obtained reflection and transmission coefficients, velocity in the boundary layers as well as velocity and vorticity fields are compared with other results obtained by extant studies. The model is then applied for solving Bragg resonance caused by cnoidal waves which show stronger scouring on the lee side of the submerged obstacles. The numerical results exhibit higher resolution for examining the secondary eddies compared with the conventional numerical methods. In addition, the computing time of the model with an adaptive grid is at least six times faster than that with a semi-uniform grid while their results are in a good agreement. [ABSTRACT FROM AUTHOR]

Racetrack flumes are established experimental settings in ecohydraulics and sediment studies. Their experimental results are often coupled with numerical simulations. The two-equation turbulence closures of the Reynolds-Averaged Navier-Stokes equations are applied widely in such engineering applications. They are preferred for their ease of use and low computational costs compared with more sophisticated turbulence models involving large-scale fluvial simulations. Here, three variants of two-equation models, i.e. the standard k-ϵ model and two variants of k-ω models, were applied to tackle mean flow and turbulence in a racetrack flume and a river reach. Regarding model performances, we found an overall reasonable agreement between simulated and measured mean velocity values. Nonetheless, the simulated turbulent kinetic energy exhibited discrepancies to the measured values. The computational costs of all investigated model variants are comparable. Therefore, there is no significant preference among models. The findings confirm the application of these model approaches for mean flow-related investigations (e.g. habitat modeling) but suggest exercising caution for applications sensitive to turbulent processes. [ABSTRACT FROM AUTHOR]

The steady shear rheology of nondilute suspensions of noncolloidal, rigid spheres in highly elastic, constant viscosity fluids is studied via numerical simulation using three-dimensional, finite-volume methods and via experiments using cone-and-plate measurements. In the numerical study, we use an immersed boundary method to simulate an ensemble of particles as a function of time until they achieve steady average bulk properties, and the simulations include fully resolved particle-scale hydrodynamics and fluid stresses. The simulations show that for low volume fraction, nondilute suspensions, the viscosity thickens at high shear and this effect can be fully determined by considering a single particle's interactions with the suspending fluid. In fact, we show that experimentally measured viscosities of suspensions up to ϕ ∼ 0.25 , where ϕ is the volume fraction of particles, can be characterized by a shift factor that determines the zero-shear viscosity and a master curve that describes the viscosity thickening. The master curve is determined by single particle–fluid interactions that depend critically on the extensional properties of the suspending fluid. Thus, the "per particle" thickening contribution to the viscosity, at large values of the Weissenberg number, is not dependent on particle–particle interactions when plotted versus the suspension stress. This "master curve" for the thickening of the shear viscosity is not only demonstrated in the simulations, but also we show that it is consistent with all available experimental data including our own. We show that the first normal stress difference coefficient can be described similarly by a shift factor and a master curve. Our numerical simulations do not quantitatively reproduce the experimental data and in general underpredict the experimentally measured shear-thickening of the rheological properties. We suggest, and provide supporting evidence, that this can be explained by the inability of the constitutive model employed in the simulations to adequately describe the extensional properties of the suspending fluid. [ABSTRACT FROM AUTHOR]

A smoothed particle hydrodynamics (SPH) framework is developed for modelling multiphase interactions at meso-scale, including the liquid-solid interaction induced deformation of the solid phase. With an inter-particle force formulation that mimics the inter-atomic force in molecular dynamics, the proposed framework includes the long-range attractions between particles, and more importantly, the short-range repulsive forces to avoid particle clustering and instability problems. Three-dimensional numerical studies have been conducted to demonstrate the capabilities of the proposed framework to quantitatively replicate the surface tension of water, to model the interactions between immiscible liquids and solid, and more importantly, to simultaneously model the deformation of solid and liquid induced by the multiphase interaction. By varying inter-particle potential magnitude, the proposed SPH framework has successfully simulated various wetting properties ranging from hydrophobic to hydrophilic surfaces. The simulation results demonstrate the potential of the proposed framework to genuinely study complex multiphase interactions in wet granular media. [ABSTRACT FROM AUTHOR]