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Step 5: Solution
Let's now investigate how we can achieve a solution in FLUENT. One must keep in mind that the governing equations we are attempting to find an approximate solution to are non-linear. This means that in order for a CFD program, such as FLUENT to solve it, it must go through an iterative process. This process is briefly described in the flow-chart below (click for a clearer image).
figure 5.1 - solution process
Before we can start the solution process, we must provide FLUENT with some initial information. Firstly, we will be describing the solution method we want FLUENT to follow. The options can be found with:
Solution > Solution Methods
In this case, we are using the first order solvers. (If time permits, try using second order solvers and determine what kind of difference it makes to the convergence time and the final solution).
Before we begin, we must also set the convergence limit. The convergence limit tells FLUENT how close the number must be to 0 before we accept the result as a solution to the governing equations. Note there is a trade-off between computational time and the convergence criteria. Also, if the grid size of the mesh is large, having a very small convergence limit would not make any difference to the solution. Most of the inaccuracies would be generated by the large mesh. Keep this in mind when optimizing results by refining the mesh (if the mesh size is reduced, often the convergence limit also needs to be reduced to achieve a closer approximation to the exact solution).
In FLUENT we can adjust the convergence limit by:
Solution > Monitors > Residuals - Print, Plot > Edit...
For this example, we will use the default convergence criteria is of 0.001. Also make sure Plot box is checked. Click OK when the value has been inputted.
Next, we need to set an initial guess at the solution. Note the better or closer the guess to the converged solution, the less time it takes to compute. The initial guess / condition can be entered by:
Solution > Solution Initialization
For this example, we know the conditions at the inlet of the pipe. Therefore, we can set the inlet conditions under Compute from, select Inlet and click Initialize.
This essentially assumes that the initial guess for the entire pipe area to be the inlet conditions. From here we can now use FLUENT to solve the problem.
Please refer back to (figure 5.1 - solution process). FLUENT solves this non-linear problem by taking an initial guess at the solution, solving the equations with the initial guess, comparing the convergence of the solution and if the criteria is not met, it will repeat the process over many iterations until the criteria is met. However, there are cases when the solution does not converge (this is usually the result of a mistake in any of the previous sections). To prevent the computer iterating indefinitely, we need to set an iterations limit.
This can be done by:
Solution > Run Calculation
Enter 500 for Number of Iterations and click Calculate. You will see a window message saying Calculating the solution... Wait for FLUENT to finish the calculation. You should see a residual plot on screen as the computation is being performed. It should look something like this:
Now that the computation is completed, we can go check out the results!