Simulation Of Unsteady Viscous Flow Research Articles

A GPU-based 2D viscous flow model with variable density and heat exchange

Numerical simulation of unsteady viscous flow over variable topography under the influence of temperature changes is a challenge. In order to apply the model to large domains with complex topography, it becomes mandatory to reduce the model complexity from 3D to 2D by depth averaging the equations and to apply massive parallelization techniques for an efficient simulation. The depth averaged mass continuity, momentum and internal energy equations, combined with suitable friction laws, can be used for this type of flows. Variations in temperature can be accounted for from internal energy changes with density changing accordingly. The resulting system is solved using a finite volume technique on unstructured triangular grids well suited for problems over variable topography. A generic model applicable to a wide range of viscous fluids and validated with synthetic cases is presented to evaluate the performance of the numerical solution in presence of both external thermal forcing functions and discontinuous initial conditions. Finally, the model is applied to a realistic application and calibration to a particular case of lava flow taking into consideration variable density, viscosity and yield stress with temperature . A heat transfer with the air is included to consider the lava cooling. The numerical results of the lava front advance are compared to the Copernicus satellite observations at different dates. The efficiency of the GPU implementation allows to simulate a 11 day event in less than 1.7 h of simulation.

Switching mechanism investigation for the supersonic jet element: Deflection, attachment and adjustment stages

Based on the unsteady viscous flow simulation, the flow characteristics inside the supersonic jet Element have been investigated numerically. The results have revealed that once the specific structures of the supersonic jet element are finalized, even if the boundary conditions remain unchanged, the corresponding internal flow will also shows strong unsteady in a certain range of primary gas source pressure. The instability of the main vortex center is a main reason to make the output thrust of the supersonic jet components fluctuate all the time in the attached wall condition. At the deflection of the jet stage, once the static pressure for the right side of the wedge exceeds the left side, the transverse expansion for the stripping zone near the right output channel entrance can play a significant role in making the primary jet deflect successfully. When the jet starts to attach the wall layer, due to the “Coanda” effect, the jet also can attach the layer successfully even though the corresponding control flow is totally closed. When the jet enters the adjustment stage, the thrust changes process for the left and right outputs will experience two typical stages: adjusting and adjusted stages. The corresponding vortex structures at different switching time together with the force variations etc. inside the jet Element have been obtained computationally and analyzed in details.

Modeling of inertial-admixture accumulation zones in vortex ring-like flows by fully Lagrangian method

A new combined fully Lagrangian approach to numerical simulation of axially symmetric vortex ring-like flows (in the absence of swirl) is proposed. The method is applicable to simulation of unsteady viscous flows with a dilute admixture of non-colliding particles which do not affect the carrier phase. The novel approach is based on a modification and combination of two Lagrangian methods: the carrier phase parameters are calculated using the vortex method based on diffusion velocity and the dispersed phase parameters (including the concentration) are calculated using the full Lagrangian approach. The application of the method is illustrated by two examples of numerical calculations of transient two-phase vortex ring-like flows.

Numerical investigation of switching mechanism for the supersonic jet element

Based on the unsteady viscous flow simulation, the flow characteristics inside the supersonic jet element have been investigated numerically. The corresponding initial switching process has been overall divided into two major stages. The results have shown that the switching process for the supersonic jet flow is an extremely complex process, which can include the complex shock system evolution, the free shear layers together with the boundary layers evolution and multiple vortex region unsteady evolution etc. The presence of the switching oblique shock near the control port is not the necessary condition to make the jet deflect, but its formation is good for the early transverse extension of the stripping vortex zone. A new concept named minimum control mass flow rate has also been proposed and emphasized. The Viscosity and adverse pressure gradient have been found to be the key factors for the occurrence of flow separation to shorten the switching time. The vortex structures at different switching time together with the variations (thrust, static pressure as well as oblique shock waves etc.) inside the supersonic jet element have been obtained computationally and analyzed in details.

Meshfree simulation of flow around airfoil using different turbulent models

The moving least square meshfree method is used to solve the Reynolds-averaged Navier-Stokes equations with two different turbulence models: the renormalisation group (RNG) k-ε model and the Wilcox k-ω model. The moving least square approach with clouds-of-points reconstruction, which can provide results from highly anisotropic clouds of points, is adopted to approximate the spatial derivatives. Both the subsonic and transonic viscous flows around RAE2822 airfoil are simulated. The pressure coefficients of the airfoil surface are calculated, the Mach number contours of the flow fields are presented, and the mean velocity profiles of the boundary layer at different locations are investigated. Some unsteady viscous flow simulation results are also presented. The simulation results show a good agreement with the experiment results and other numerical simulation results. Overall, the simulation results demonstrate that the proposed meshfree numerical algorithm is accurate and capable of modelling turbulence flows around airfoils.

Meshfree Simulation of Flow around Airfoil using Different Turbulent Models

The moving least square meshfree method is used to solve the Reynolds-averaged Navier-Stokes equations with two different turbulence models: the renormalisation group (RNG) k-e model and the Wilcox k-ω model. The moving least square approach with clouds-of-points reconstruction, which can provide results from highly anisotropic clouds of points, is adopted to approximate the spatial derivatives. Both the subsonic and transonic viscous flows around RAE2822 airfoil are simulated. The pressure coefficients of the airfoil surface are calculated, the Mach number contours of the flow fields are presented, and the mean velocity profiles of the boundary layer at different locations are investigated. Some unsteady viscous flow simulation results are also presented. The simulation results show a good agreement with the experiment results and other numerical simulation results. Overall, the simulation results demonstrate that the proposed meshfree numerical algorithm is accurate and capable of modelling turbulence flows around airfoils.

Viscous-Inviscid Coupling Methods for Advanced Marine Propeller Applications

The paper reports the development of coupling strategies between an inviscid direct panel method and a viscous RANS method and their application to complex propeller ows. The work is motivated by the prohibitive computational cost associated to unsteady viscous flow simulations using geometrically resolved propellers to analyse the dynamics of ships in seaways. The present effort aims to combine the advantages of the two baseline methods in order to reduce the numerical effort without compromising the predictive accuracy. Accordingly, the viscous method is used to calculate the global flow field, while the inviscid method predicts the forces acting on the propeller. The corresponding reaction forces are employed as body forces to mimic the propeller influence on the viscous flow field. Examples included refer to simple verification cases for an isolated propeller blade, open-water validation simulations for a complete propeller, and more challenging investigations of a manoeuvring vessel in seaways. Reported results reveal a fair predictive agreement between the coupled approach and fully viscous simulations and display the efficiency of the coupled approach.

Standard Unstructured Grid Solutions for Cranked Arrow Wing Aerodynamics Project International F-16XL

Steady and unsteady viscous flow simulations of a full-scale, semispan, and full-span model of the F-16XL-1 aircraft are performed with three different computational fluid dynamics codes using a common unstructured grid. Six different flight conditions are considered. They represent Reynolds and Mach number combinations at subsonic speeds, with and without sideslip. The steady computations of the flow at these flight conditions are made with several Reynolds-averaged Navier-Stokes turbulence models of different complexity. Detached-eddy simulation, delayed detached-eddy simulation, and an algebraic hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation model are used to quantify unsteady effects at the same flight conditions. The computed results are compared with flight-test data in the form of surface pressures, skin friction, and boundary-layer velocity profiles. The focus of the comparison is on turbulence modeling effects and effects of unsteadiness. The overall agreement with flight data is good, with no clear trend as to which physical modeling approach is superior for this class of flow. The Reynolds-averaged Navier-Stokes turbulence models perform well in predicting the flow in an average sense. However, some of the flow conditions involve locally unsteady flow over the aircraft, which are held responsible for the scatter between the different turbulence modeling approaches. The detached-eddy simulations are able to quantify the unsteady effects, although they are not consistently better than the Reynolds-averaged Navier-Stokes turbulence models in predicting the flow in an average sense in these flow regions. Detached-eddy simulation fails to predict boundary-layer profiles consistently over a range of flow regimes, with delayed detached-eddy simulation and hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation models offering a remedy to recover some of the predictive capabilities of the underlying Reynolds-averaged Navier-Stokes turbulence model. Nonetheless, the confidence in the predictive capabilities of the computational fluid dynamics codes with regard to complex vortical flowfields around high-performance aircraft of this planform increased significantly during this study.

Deforming Grid Technique Applied to Unsteady Viscous Flow Simulation by a Fully Implicit Solver

Introduction T HE unsteady flows associated with moving boundaries have long been of interest to aerodynamic research. For most unsteady problems, the computational grid has to conform to the instantaneous shape of a moving body. Thereby, grid regeneration, or movement techniques, becomes an important issue. Rotating the grid system rigidly can easily treat rigid-body motions, but this approach is inapplicable if the body deforms, if the block boundaries have to be fixed for a multiblock grid system, or if the relative motion of a multicomponent configuration is to be considered. A simple way is to regenerate a grid at each physical time step. Obviously, this process will be very time consuming. It is a much more relatively difficult task when a three-dimensional problem must to be taken into account. An efficient grid-movement technique seems to be crucial for such problems, by which the grid system will be only deformed once per time step rather than regenerated completely in unsteady flow simulations.1 The present work attempts to develop an unsteady viscous flow simulating method for moving boundary problems with deforming grids in a multiblock grid system. The grid movement is mainly based on a transfinite interpolation (TFI) algorithm. The TFI tech-

A fully implicit method for steady and unsteady viscous flow simulations

AbstractIn this paper a time‐accurate, fully implicit method has been applied to solve a variety of steady and unsteady viscous flow problems. It uses a finite volume cell‐centred formulation on structured grids and employs central space discretization with artificial dissipation for the residual computation. In order to obtain a second‐order time‐accurate implicit scheme, a Newton‐like subiteration is performed in the original LU‐SGS method to converge the calculations at each physical time step by means of a dual‐time approach proposed by Jameson. The numerical experiments show that the present method is very efficient, reliable, and robust for steady and unsteady viscous flow simulations, especially for some low speed flow problems. Copyright © 2003 John Wiley & Sons, Ltd.

Experiment-based numerical simulation of unsteady viscous flow in stenotic collapsible tubes

A numerical method using generalized finite differences is introduced to solve a nonlinear axisymmetric model with a free moving boundary to study unsteady viscous flow in collapsible stenotic tubes simulating blood flow in stenotic carotid arteries. The Navier–Stokes equations are used as the governing equations for the fluid. The tube wall is treated as a free moving boundary whose elastic properties (the tube law) are determined experimentally using a Polyvinyl Alcohol (PVA) hydrogel artery stenosis model. Longitudinal tension is included in the tube law. Physiologically-relevant pressure conditions and parameters are used in the simulation. Our results indicate that severe stenoses cause cyclic pressure changes between positive and negative values at the throat of the stenosis, cyclic tube compression and expansions, and shear stress changing directions in the region just distal to the stenosis under unsteady conditions. These critical mechanical conditions may be related to excessive artery fatigue and possible plaque cap rupture. Computational and experimental results are compared and reasonable agreement is found.

Unsteady Viscous Flow Simulation around the Blade of Rowing Boat

Some aspects of unsteady viscous flow field around an oar blade of a rowing boat are studied numerically by means of laminar, incompressible unsteady full 2D Navier-Stokes simulation. Main objectives are to reveal the flow phenomena in order to understand the mechanism of generation of the propulsive force and to study the influence of different parameters on the flow pattern and efficiency of rowing. The main parameters under consideration are : 1) angular velocity of the blade (speed of rowing), 2) blade geometry and 3) outboard length of the oar. Special attention is payed to the unsteady effects taking place during the blade stroke. An analysis of the accuracy and reliability of the results with respect to the limited Reynolds number and 2D simulation is provided.Analysis of the results for the flow simulation and hydrodynamical forces acting on the blade during its stroke shows, that the flow is very sensitive even to the small changes of main motion or geometry parameters, although, a general flow pattern is valid for all of the studied cases.Performed flow simulation shows that for all studied cases, strong and stable vortical structures, formed near the catch position, attend the blade motion and have strong influence on the further flow evolution. This fact determines the strong unsteady effects on the flow pattern and the exerted hydrodynamical forces.

Simulation of unsteady viscous flow around a longitudinally oscillating circular cylinder in a uniform flow

A finite difference simulation method for the time-dependent viscous incompressible flow around a longitudinally oscillating circular cylinder at Reynolds numbers of Re = 1000 and 4000 is presented. The Navier-Stokes equations in finite difference form are solved on a moving grid system, based on a time-dependent coordinate transformation. Solution of the vortex street development behind the cylinder is obtained when the cylinder remains stationary and also when oscillating. Evolution with time of the flow field, by means of streamlines, pressure contours, and vorticity contours, is studied. The computer results predict the lock-in phenomenon, which occurs when the oscillation frequency is around double the natural vortex-shedding frequency. The time-dependent lift and drag coefficients are obtained by the integration of the pressure and shear forces around the body. The drag, lift, and the displacement relations are also discussed.

Numerical simulation of unsteady viscous flow around a circular cylinder

A finite difference solution is obtained for the time-dependent viscous incompressible 2-dimensional flow past a circular cylinder by direct integration of the Navier-Stokes equations expressed in a general curvilinear coordinate system. The solution describes the development of the vortex street developed behind the cylinder. Evolution of flow configuration is studied by means of streamlines, pressure contours, and vorticity contours for different Reynolds numbers. The time-dependent lift and drag coefficients are also obtained.