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4. Guide to a Successful Simulation Using FLUENT

The following guidelines can help you make sure your CFD simulation is a success. Before contacting your technical support engineer, make sure you do the following:

1.   Examine the quality of the mesh.

There are two basic things that you should do before you start a simulation:

  • Perform a grid check to avoid problems due to incorrect mesh connectivity, etc.

  • Look at maximum cell skewness (e.g., using the Compute button in the Contours panel). As a rule of thumb, the skewness should be below 0.98.

If there are mesh problems, you may have to re-mesh the problem.

2.   Scale the grid and check length units.

In FLUENT, all physical dimensions are initially assumed to be in meters. You should scale the grid accordingly. Other quantities can also be scaled independent of other units used. FLUENT defaults to SI units.

3.   Employ the appropriate physical models.

4.   Set the energy under-relaxation factor between 0.95 and 1.

For problems with conjugate heat transfer, when the conductivity ratio is very high, smaller values of the energy under-relaxation factor practically stall the convergence rate.

5.   Use node-based gradients with unstructured tetrahedral meshes.

The node-based averaging scheme is known to be more accurate than the default cell-based scheme for unstructured meshes, most notably for triangular and tetrahedral meshes.

6.   Monitor convergence with residuals history.

Residual plots can show when the residual values have reached the specified tolerance. After the simulation, note if your residuals have decreased by at least 3 orders of magnitude to at least 10 $^{-3}$. For the pressure-based solver, the scaled energy residual must decrease to 10 $^{-6}$. Also, the scaled species residual may need to decrease to 10 $^{-5}$ to achieve species balance.

You can also monitor lift, drag, or moment forces as well as pertinent variables or functions (e.g., surface integrals) at a boundary or any defined surface.

7.   Run the CFD simulation using second order discretization for better accuracy rather than a faster solution.

A converged solution is not necessarily a correct one. You should use the second-order upwind discretization scheme for final results.

8.   Monitor values of solution variables to make sure that any changes in the solution variables from one iteration to the next are negligible.

9.   Verify that property conservation is satisfied.

After the simulation, note if overall property conservation has been achieved. In addition to monitoring residual and variable histories, you should also check for overall heat and mass balances. At a minimum, the net imbalance should be less than 1% of smallest flux through domain boundary.

10.   Check for grid dependence.

You should ensure that the solution is grid-independent and use grid adaption to modify the grid or create additional meshes for the grid-independence study.

11.   Check to see that the solution makes sense based on engineering judgment.

If flow features do not seem reasonable, you should reconsider your physical models and boundary conditions. Reconsider the choice of the boundaries location (or the domain). An inadequate choice of domain (especially the outlet boundary) can significantly impact solution accuracy.

You are encouraged to collaborate with your technical support engineer in order to develop a solution process that ensures good results for your specific application. This type of collaboration is a good investment of time for both yourself and the FLUENT support engineer.

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