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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 magnetic fluid seal is one of the best known and widely used technical applications of magnetic fluid. However, the boundary and the flow regime with the rotation shaft cannot be observed by the experiment method. This paper presents a novel numerical simulation method to obtain the boundary and the velocity of the fluid in seals. The magnetic field of the seal is solved by the finite-element method, and the Navier–Stokes equations of the magnetic fluid based on the influence of magnetic force are solved by the finite-volume method. The two-phase flow is solved by the volume of fluid method. The seal capacities of the rectangle tooth and the trapezoid tooth with different rotation velocities are obtained. The result shows that the seal capacity decreases with the increase in the rotation velocity. [ABSTRACT FROM AUTHOR]

Purpose - The purpose of this paper is to investigate, experimentally and numerically, the pressure pulse characteristics and unsteady flow behavior in a Francis turbine runner for moderate flow heads. The pressure pulses in the runner blade passage were predicted numerically for both moderate and high heads. The calculations were used to partition the turbine operating regions and to clarify the various for the unsteady flow behavior, especially the blade channel vortex in the runner. Design/methodology/approach - Experimental and numerical analyses of pressure pulse characteristics at moderate flow heads in a Francis turbine runner were then extended to high heads through numerical modeling with 3D unsteady numerical simulations performed for a number of operating conditions. The unsteady Reynolds-averaged Navier-Stokes equations with the k-?-based shear stress transport turbulence model were used to model the unsteady flow within the entire flow passage of a Francis turbine. Findings - The dominate frequency of the predicted pressure pulses at runner inlet agree with the experimental results in the head cover at moderate flow heads. The influence of the blade passing frequency causes the simulated peak-to-peak amplitudes in the runner inlet to be larger than in the head cover. The measured and predicted pressure pulses at different positions along the runner are comparable. At the most unstable operating condition of 0.5a0 guide vane opening, the pressure pulses in the runner blade passage are due to the blade channel vortex and the rotor-stator interference. The predictions show that the frequency of the blade channel vortex is relatively low and it changes with the operating conditions. Originality/value - The paper describes a study which experimentally and numerically investigated the pressure pulses characteristics in a Francis turbine runner at moderate flow heads. The pulse characteristics and unsteady flow behavior due to the blade channel vortex in the runner at high heads were investigated numerically, with the turbine operating regions then partitioned to identify safe operating regions. [ABSTRACT FROM AUTHOR]

The flows around a typical open cavity with a store are numerically simulated in this paper. A fully implicit unfactored method is employed in the solver of RANS equations, where the S-A turbulent model is implemented to calculate the turbulent eddy viscosity. The flow-induced oscillation of the free shear layer around the cavity lip is captured and analyzed. Comparisons for the different cases of the cavity flow are made. The calculated results show that the pressure fluctuation is pertinent to the different positions of the store inside the cavity. The power spectral density (PSD) of the pressure by fast Fourier transformations (FFT) is also displayed in the paper. For the Rossiter's oscillatory mode, the numerical results compare well with the experimental data. [ABSTRACT FROM AUTHOR]

We consider methods for the numerical simulations of variable density incompressible fluids, modelled by the Navier–Stokes equations. Variable density problems arise, for instance, in interfaces between fluids of different densities in multiphase flows such as appearing in porous media problems. We show that by solving the Navier–Stokes equation for the momentum variable instead of the velocity the corresponding saddle point problem, arising at each time step, no special treatment of the pressure variable is required and leads to an efficient preconditioning of the arising block matrix. This study consists of two parts, of which this paper constitutes Part I. Here we present the algorithm, compare it with a broadly used projectiontype method and illustrate some advantages and disadvantages of both techniques via analysis and numerical experiments. In addition we also include test results for a method, based on coupling of the Navier–Stokes equations with a phase-field model, where the variable density function is handled in a different way. The theory including stability bounds and a second order splitting method is dealt with in Part II of the study. [ABSTRACT FROM AUTHOR]

Abstract: The overall stability of tidal stream generators can only be assured if the maximum loads expected to arise during the lifetime of the structures are accounted for. These loads typically consist of a predicted maximum wave scenario and a maximum current scenario, which could potentially occur simultaneously. In this paper, combined wave–current offshore conditions are analyzed using a numerical procedure that is based on the unsteady Reynolds-averaged Navier–Stokes equations and incorporates the Volume of Fluid method and Fenton's wave theory. A procedure is introduced that allows for the simulation of non-linear waves in combination with a depth varying current under full consideration of wave–current interaction. The methodology is utilized to investigate the change in the overall loading on a structure subjected to combined wave–current conditions in comparison to a linear superposition of pure wave and pure current loads. It is shown that the peak loads on the structure are significantly higher for the combined load case than those computed for the superimposed individual load scenarios. Finally, the numerical wave channel is applied as part of the design process of a tidal stream generator prototype recently constructed near Jindo Island in South Korea. [Copyright &y& Elsevier]

Abstract: The lattice Boltzmann method (LBM) for fluid flow has already proven itself a viable alternative to classical CFD (computational fluid dynamics) methods based on discrete scheme application to Navier Stokes equations governing the fluid flow. In this paper, we aim to verify the applicability of the LBM in a special case of a Couette–Taylor photobioreactor—a device comprised of two coaxial cylinders with a rotating inner wall. An appropriate numerical approach to the curved moving wall boundary condition is presented and verified by velocity flow field comparison with analytical solution. The parallelism potential of the LBM is exploited on the parallel platform of CUDA (Compute Unified Device Architecture). Microalgae growth in the flow field simulated by the LBM is based on the photosynthetic factory (PSF) model treated in a parallel stochastic manner and implemented also on the parallel platform of CUDA. The parallel stochastic PSF solver has been validated by comparison with an analytical solution of the PSF model at constant irradiance. [Copyright &y& Elsevier]

Abstract: In the last decade much research works has been carried out in order to improve the operation and speed of the underwater vehicles. In this study important supercavitation features including formation, evolution and loss of supercavitation and partial cavitation for an underwater moving body are both experimentally and numerically investigated. To achieve this goal the natural supercavitation experiment was conducted through a moving body and the trajectory of the body and the shape of supercavity profile were recorded by a high speed camera. For the numerical simulation of the underwater moving body the unsteady Reynolds averaged Navier–Stokes equations are coupled to a six-degree-of-freedom (6DOF) rigid body motion model. The numerical results are validated by comparing the predicted trajectory and velocity of the body with presented experimental data. A good agreement between the predicted and experimental values ensures the accuracy of the prepared numerical algorithm that used in the present study. The results reported in this paper illustrate that the above experimental method and numerical algorithm can be one of the most powerful and beneficial tools for the supercavitating underwater vehicles optimization and performance analysis. [Copyright &y& Elsevier]

In this paper, we determined a numerical solution of the incompressible Navier-stokes equations for the flow inside six contraction geometry on the V-shape with different upper and lower sharpe edges, we illustrate the solutions on the singular points on the V-shape wich have upper and lower kinks through the stream and vorticity functions. The governing equations are written in the vorticity-stream function formulations. We study the numerical solution is based on a technique of automatic numerical generation of a curvilinear coordinate system by transforming the governing equation into computational plane. The transformed equations are approximated using central differences and solved simultaneously by the Alternating Direction Implicit method(ADI)and successive-over relaxation iteration method(SOR). [ABSTRACT FROM AUTHOR]

Abstract: A numerical and theoretical study of sound generation by planar, axisymmetric and spherically symmetric premixed laminar flame annihilation is presented in this paper. The compressible Navier–Stokes, energy, and progress variable equations are solved using Direct Numerical Simulation with one-step chemistry. A theory is developed to relate the pressure amplitude to the flame’s propagation velocity and consumption speed, which were identified as key parameters in the generation of sound in our previous study (Talei et al., 2011[38]). The results show that by obtaining the propagation velocity of the flame from the simulations, the sound generation can be predicted accurately for unity Lewis number. Use of Markstein’s linear theory relating flame speed and curvature is also investigated as a more practical way of applying this theory. It is shown that this leads to under-prediction of the radiated sound in the spherical and axisymmetric configurations, but it is able to capture the qualitative trends. The effects of laminar flame speed, temperature ratio and Zel’dovich number are then investigated. These are interpreted in terms of a scaling argument put forward in our previous study, and also in terms of the flame’s propagation velocity and consumption speed. Finally, cases involving non-unity Lewis number are investigated. As the flames approach annihilation, either flame acceleration or deceleration is observed depending on the Lewis number, and this is shown to affect the radiated sound significantly. For Lewis numbers greater than unity, the annihilation of the flame results in a local increase in consumption speed at annihilation, leading to more sound production compared with that of unity Lewis number. Lewis numbers less than unity exhibit the opposite behaviour. [Copyright &y& Elsevier]

A helical fin in a circular pipe is a means to generate swirling flow and the swirling motion will in turn modulate the turbulence field. This paper reports on a direct numerical simulation aimed to explore such flow phenomena. The full Navier-Stokes equations expressed in a cylindrical coordinate system are integrated numerically in time and the helical fin is embedded in the structured grid by means of an immersed boundary method. The statistically steady three-dimensional flow field exhibits a helical symmetry with an azimuthal mean velocity which amounts to about 50% of the axial mean velocity component. The variation of the mean flow over the cross-sections can be explained by the variations of the nine non-zero components of the Reynolds stress tensor. Particular attention is paid to the contribution of the Reynolds stresses to the skin-friction coefficient. Appreciable levels of fluctuating helicity are observed in the vicinity of the pipe wall and reflect that the swirling motion breaks the structural symmetry in conventional pipe flow. [ABSTRACT FROM AUTHOR]

Abstract: The simulation of cavitating flows is a challenging problem both in terms of modelling the physics and developing robust numerical methodologies. Such flows are characterized by important variations of the local Mach number, compressibility effects on turbulence and involve thermodynamic phase transition. To simulate these flows by applying homogeneous models and Reynolds averaged codes, the turbulence modelling plays a major role in the capture of unsteady behaviours. This paper presents a one-fluid compressible Reynolds-Averaged Navier–Stokes (RANS) solver with a simple equation of state (EOS) for the mixture. A special focus is devoted to the turbulence model influence. Unsteady numerical results are given for Venturi geometries and comparisons are made with experimental data. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, effectiveness of aerodisk/aerospike assemblies as retractable drag-reduction devices for large-angle blunt cones flying at hypersonic Mach numbers is simulated numerically at various angles of attack. Different lengths of spike have been chosen to investigate the effect of the aerospike on the surrounding flowfield of nose cone. The compressible, 3-D full Navier–Stokes equations are solved with k−ω (SST) turbulence model for free stream Mach number of 5.75 and different angles of attack namely 3, 7, 10, and 12. The visualized shock structure and the computed drag on the blunt cone with aerospike agree well with the experimental result. The effect of reattachment of point on the surface convective heat-load reduction is investigated for different kind of spikes with varying length to diameter ratio. Additional modifications to the tip of the spike are examined in order to obtain different bow shocks, including a cut spike, and a flat and hemispherical aerodisk mounted on the tip of the spike. The numerical results presented the pressure distributions and the surface heat reduction for different type of spike. Some discrepancies existed, especially on the length of the recirculation region. [Copyright &y& Elsevier]

Abstract: This paper deals with the numerical three-dimensional prediction of the induced flow patterns around and inside a building, which is cross-ventilated in a natural way. The air change rate is controlled by two opposite openings on the building envelope, as a function of wind velocity and its incidence angle. The numerical methodology is based on the finite volume numerical solution of the Navier–Stokes equations, using the CFD commercial code FLUENT. The numerical results are compared with available experimental data regarding the refresh rate of the building''s room and the relative velocity profiles at the window openings, indicating a good agreement. Furthermore, a detailed description of the natural ventilation process is provided, whilst additional information regarding the induced velocity and pressure field is presented; information which cannot be easily extrapolated by experimental methodologies. Finally, the impact of the inner topology of the building on the induced flow field is investigated. [Copyright &y& Elsevier]

Abstract  This paper deals with the dynamics of the flowfield resulting from the interaction of twin inclined elliptic jets with an oncoming crossflow. It will particularly focus on the impact of the initial streamwise inclination angle of the emitted jets on the different flow dynamic features. This particular side of the question is of an extremely high interest as it allows a better understanding of the mixing of the different interacting flows. A well understanding of this mechanism is likely to enable us to control the jets’ trajectories, expansion, confinement, etc. all these processes are in tight relationship with the atmospheric pollution and the pollutants’ dispersion which are nowadays alarming questions needing urgent constraining measures and viable solutions. The consideration of the question was first carried out experimentally by means of the particle image velocimetry (PIV) technique in order to track the evolution of the jets among the environing flow. The second step of this work consisted in the numerical simulation of the same configuration by solving the Navier-Stokes equations with the finite volume method and the Reynolds stress model (RSM) second-order turbulent model. The use of this particular model is in itself a new contribution in the “jets in crossflow” examination; and its merit comes from its high ability to detect the shear stresses that are precisely determinant in the mixing enhancement. The efficiency of this model was proved in our work by the satisfying matching of the numerical results and the experimental data. Once the validation obtained, we enhanced our model by introducing a temperature gradient between the interacting flows and by injecting a non reactive fume through the jet nozzles. We also varied the initial angle of the emitted jets in order to evaluate the impact of this parameter on some dynamic characterizing features such as the global jets’ plumes, the windward and the leeward jets’ spread, the size, the location and the magnitude of the reverse flow region, the penetration and the deflection of the jets’ trajectories, the mass entrainment of the discharged pollutants from the jets’ nozzles, some of the shear stress components, etc. a thorough description of these parameters is likely to well characterize the exact progression of a given particle contained within the jets (in the case of the polluted jets) which will help us find the adequate way to control it. [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: This paper takes a fresh look at the geometric conservation law (GCL) from the perspective of the finite element method (FEM) for incompressible flows. The GCL arises naturally in the context of Arbitrary Lagrangian Eulerian (ALE) formulations for solving problems on deforming domains. GCL compliance is traditionally interpreted as a consistency criterion for applying an unsteady flow solution algorithm to simulate exactly a uniform flow on a deforming domain. We introduce an additional requirement: the time integrator must maintain its fixed mesh accuracy when applied to deforming meshes. A review of the literature shows that while many authors use an ALE FEM, few of them discuss the GCL issues. We show how a fixed mesh unsteady FEM using high order time integrator (up to fifth order in time) can be transposed to solve problems on deforming meshes and preserve its fixed mesh high order temporal accuracy. An appropriate construction of the divergence of the mesh velocity guarantees GCL compliance while a separate construction of the mesh velocity itself allows the time-integrator to deliver its fixed mesh high order temporal accuracy on deforming domains. Analytical error analysis of problems with closed form solutions provides insight on the behavior of the time integrators. It also explains why high order temporal accuracy is achieved with a conservative formulation of the incompressible Navier–Stokes equations, while only first order time accuracy is observed with the non-conservative formulation and all time-integrators investigated here. We present thorough time-step and grid refinement studies for simple problems with closed form solutions and for a manufactured solution with a non-trivial flow on a deforming mesh. In all cases studied, the proposed reconstructions of the mesh velocity and its divergence for the conservative formulation lead to optimal time accuracy on deforming grids. [Copyright &y& Elsevier]

Abstract: The scale effect of form factor is investigated via a numerical approach in this paper, where the turbulent ship flow is computed by solving the steady and incompressible Reynolds-averaged Navier–Stokes and continuity equations. A wall function approach is employed to bridge the near-wall and outer turbulent flow region. The numerical scheme based on a finite-volume formulation is applied to discretize the coupled governing equation. For the sake of numerical stability, accuracy and economy, an identical grid is employed to compute ship flow at different Reynolds number, where the grid is optimized for the medium Reynolds number of the investigated range. Four surface ships and two sub-bodies with notably different geometrical characteristics are chosen as the investigated cases, where double-model flow without appendages is computed. The calculated total resistance coefficient shows a decreasing tendency against Reynolds number among all studied hulls. Similar to the calculated total resistance coefficient, the calculated friction resistance coefficient decreases with the Reynolds number and varies relatively little for a given Reynolds number among different hulls. The viscous pressure resistance coefficient is less insensitive to the Reynolds number but apparently depends on hull form. Compared with the form factor calculation based on empirical friction lines, the flat-plate friction prediction based on CFD approach clearly gives smaller Re-dependent form factor, which should more realistically reflect the scale effect of form factor. The form factor exhibits a near linear and increasing dependence on Reynolds number. The numerical results show that the dependence of rP on Reynolds number mainly governs the scale effect of form factor. [Copyright &y& Elsevier]