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

Mesh is hexahedral and consists of 6.8 million elements.

Step2. Models

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.