external bluff-body flow-cfd simulation using ansys fluent
TRANSCRIPT
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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
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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
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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
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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 %
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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
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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
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Set-up transient solver:
General-Solver-Time-Transient
Pressure-Velocity coupling: PISO
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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
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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.
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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:
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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
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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.
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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.