External bluff-body flow-cfd simulation using ANSYS Fluent External flow over a bluff body is complex, three-dimensional, and vortical. It is massively separated and it exhibits vortex shedding. Thus, appropriate numerical simulation is needed. Steady RANS simulation is applicable only on statistically steady flow. Flow in which coherent vortex shedding occurs is statistically unsteady, therefore unsteady RANS (URANS) must be used. Bluff-body flow represents this kind of flow. This video tutorial demonstrates application of unsteady RANS simulation on 3D geometry. Barrier consisting of horizontal bars is the bluff body. This tutorial includes: - Mesh import and scaling - Turbulence model selection - Boundary condition set-up - Solver set-up for transient simulation - Monitor set-up - Monitor and residual set-up - Time-step selection - Monitoring convergence of solution during calculation - Post-processing of solution
Step1: Grid 1. Read the mesh file (*.msh) File-Read-mesh 2. Check the mesh Make sure the minimum volume is a positive number 3. Check the scale of the mesh Mesh-Scale Check the domain size so that it corresponds to actual dimensions 4. Mesh display Display-Mesh Mesh size check Mesh display
Barrier mesh Step2. Models Mesh is hexahedral and consists of 6.8 million elements. Enable SST k-omega turbulence model Define-Models-Viscous-SST k-omega
Step3: Boundary Conditions 1. Set boundary conditions for inlet and outlet Define-Boundary Conditions a) Set boundary conditions for inlet In ICEM-CFD meshing software upstream face was already set to velocity-inlet. Specify value for x-velocity 20 (m/s) Select intensity and length scale from the drop-down list. Specify value for turbulent intensity 0.1 %. Specify value for length scale 0.145 m. b) Set boundary condition for outlet In ICEM-CFD meshing software downstream face was already set to pressure-outlet. Select intensity and length scale from the drop-down list Specify value for turbulent intensity 0.1 %
Specify value for length scale 0.145 m Backflow parameters are in case of reverse flow. Step4: Solution At first, simulation is performed with steady solver. First 300 iterations are with first order accurate methods and then switch to second order for another 250 iterations. This is done in order to avoid instabilities in calculations or divergence. After the steady solver, it is switched to transient solver and continues to calculate. To set-up steady state solver: General-Solver-Time-Steady Set-up steady solver with first order accurate methods: Pressure-Velocity coupling: SIMPLE Spatial discretization: Gradient-Least Squares Cell Based Pressure-Standard Momentum-First order Upwind Turbulence Kinetic energy- First order Upwind Specific dissipation rate- First order Upwind
Set-up steady solver with first order accurate methods: Pressure-Velocity coupling: SIMPLE Spatial discretization: Gradient-Least Squares Cell Based Pressure-Second order Momentum- Second order Upwind Turbulence Kinetic energy- Second order Upwind Specific dissipation rate- Second order Upwind Enable high order term relaxation-to help with calculation stability
Set-up transient solver: General-Solver-Time-Transient Pressure-Velocity coupling: PISO
Specify value for pressure under-relaxation factor to 1 in order to speed-up calculation. Step5: Set-up monitor Alongside residuals, monitoring variable or integral variable is helpful in determining if solution converged. We will set-up monitor for drag on the barrier. Monitors-Create-Drag
Select print to console, plot and write with name cd-barrier. Select barrier in wall zones. Force vector is: 1 0 0. Step6: Solution initialization Select hybrid initialization. Initialize. Step7: Run calculation For steady state, only number of iterations are specified.
For transient simulation, one needs to specify time step size, number of time steps, maximum iterations per time step. Specify 0.00003 for time step size. Number of tiem step depends on converegeny of the solution. Simulation should be run until reaches statistically staionary state. After 250 iterations with transient solver, scaled residual graph should look similar to:
Scaled Residuals after 550 iterations with steady solver and 250 iteration with transient solver As one can see, first 300 iterations are steady state solver with first order methods. Next, 250 iterations are with steady state solver with second order methods. After 550 iterations with steady solver, transient solver is applied. Step8: Post-processing There are many options of data visualization in Ansys Fluent. There are contours, vectors, pathlines, and animation. For example, one can visualize contour of x-component of velocity field on a plane in the middle of the domain. Create plane: Surface-Plane
Select point and normal. Specify 0 for x0, y0,z0. Specify 0 for ix, iy, and 1 for iz. Select Create. Results-Graphics and Animation-contours-set-up Select plane-14. Select Display.
Contours of x-velocity on middle plane Step9: Summary In this tutorial Ansys Fluent was used to calculate transient simulation of a flow through bluff body. It was shown mesh import and modification, set-up solver, turbulence models, and monitors. Post-process was used to analyze data.