Step 8: Solution

  In the steps that follow, you will set up and run the calculation using the task pages listed under the Solution heading in the navigation pane.

1.   Enable the plotting of residuals during the calculation.

Monitors Residuals Edit...

(a)   Make sure that Plot is enabled in the Options group box.

(b)   Enter 1e-05 for the Absolute Criteria of continuity, as shown in the Residual Monitor dialog box.

(c)   Click OK to close the Residual Monitors dialog box.

Note:   By default, all variables will be monitored and checked by ANSYS FLUENT as a means to determine the convergence of the solution. It is a good practice to also define a surface monitor that can help evaluate whether the solution is truly converged. You will do this in the next step.

2.   Define a surface monitor at the outlet ( pressure-outlet-7).

Monitors Create... (Surface Monitors)

(a)   Retain the default entry of surf-mon-1 for the Name of the surface monitor.

(b)   Enable the Plot and Write options for surf-mon-1.

(c)   Retain the default entry of surf-mon-1.out for File Name.

(d)   Set Get Data Every to 3 by clicking the up-arrow button.

  This setting instructs ANSYS FLUENT to update the plot of the surface monitor and write data to a file after every 3 iterations during the solution.

(e)   Select Mass-Weighted Average from the Report Type drop-down list.

(f)   Select Temperature... and Static Temperature from the Field Variable drop-down lists.

(g)   Select pressure-outlet-7 from the Surfaces selection list.

(h)   Click OK to save the surface monitor settings and close the Surface Monitor dialog box.

  The name and report type of the surface monitor you created will be displayed in the Surface Monitors selection list in the Monitors task page.

3.   Initialize the flow field, using the boundary conditions settings at the cold inlet ( velocity-inlet-5) as a starting point.

Solution Initialization

(a)   Select velocity-inlet-5 from the Compute from drop-down list.

(b)   Enter 1.2  ${\rm m/s}$ for Y Velocity in the Initial Values group box.

Note:   While an initial X Velocity is an appropriate guess for the horizontal section, the addition of a Y Velocity component will give rise to a better initial guess throughout the entire elbow.

(c)   Click Initialize.

4.   Check to see if the case conforms to best practices.

Run Calculation Check Case

(a)   Click the Solver tab and examine the Recommendation in the Manual Implementation group box.

  The only recommendation for this case file is to use discretization of a higher order. This recommendation can be ignored for the time being, as it will be performed in a later step.

(b)   Close the Case Check dialog box.

5.   Save the case file ( elbow1.cas.gz).

File $\rightarrow$ Write $\rightarrow$ Case...

(a)   (optional) Indicate the folder in which you would like the file to be saved.

  By default, the file will be saved in the folder from which you read in elbow.msh (i.e., the introduction folder). You can indicate a different folder by browsing to it or by creating a new folder.

(b)   Enter elbow1.cas.gz for Case File.

  Adding the extension .gz to the end of the file name extension instructs ANSYS FLUENT to save the file in a compressed format. You do not have to include .cas in the extension (e.g., if you enter elbow1.gz, ANSYS FLUENT will automatically save the file as elbow1.cas.gz). The .gz extension can also be used to save data files in a compressed format.

(c)   Make sure that the default Write Binary Files option is enabled, so that a binary file will be written.

(d)   Click OK to save the case file and close the Select File dialog box.

6.   Start the calculation by requesting 150 iterations.

Run Calculation

(a)   Enter 150 for Number of Iterations.

(b)   Click Calculate.

Note:   By starting the calculation, you are also starting to save the surface monitor data at the rate specified in the Surface monitors dialog box. If a file already exists in your working folder with the name you specified in the Define Surface Monitor dialog box, then a Question dialog box will open, asking if you would like to append the new data to the existing file. Click No in the Question dialog box, and then click OK in the Warning dialog box that follows to overwrite the existing file.

Figure 1.3: Convergence History of the Mass-Weighted Average Temperature

Note:   The solution will be stopped by ANSYS FLUENT after approximately 140 iterations, when the residuals reach their specified values. The exact number of iterations will vary, depending on the platform being used. An Information dialog box will open to alert you that the calculation is complete. Click OK in the Information dialog box to proceed.

Since the residual values vary slightly by platform, the plot that appears on your screen may not be exactly the same as the one shown here.

  As the calculation progresses, the residuals will be plotted in the graphics window (Figure  1.4).

You can display the residuals history (Figure  1.4), by selecting it from the graphics window drop-down list.

Figure 1.4: Residuals for the First 140 Iterations

7.   Examine the plots for convergence (Figures  1.4 and 1.3).

Note:   There are no universal metrics for judging convergence. Residual definitions that are useful for one class of problem are sometimes misleading for other classes of problems. Therefore it is a good idea to judge convergence not only by examining residual levels, but also by monitoring relevant integrated quantities and checking for mass and energy balances.

There are three indicators that convergence has been reached:

• The residuals have decreased to a sufficient degree.

  The solution has converged when the Convergence Criterion for each variable has been reached. The default criterion is that each residual will be reduced to a value of less than $10^{-3}$, except the energy residual, for which the default criterion is $10^{-6}$.

• The solution no longer changes with more iterations.

  Sometimes the residuals may not fall below the convergence criterion set in the case setup. However, monitoring the representative flow variables through iterations may show that the residuals have stagnated and do not change with further iterations. This could also be considered as convergence.

• The overall mass, momentum, energy, and scalar balances are obtained.

  You can examine the overall mass, momentum, energy and scalar balances in the Flux Reports dialog box. The net imbalance should be less than 0.2 ${\rm\%}$ of the net flux through the domain when the solution has converged. In the next step you will check to see if the mass balance indicates convergence.

8.   Examine the mass flux report for convergence.

Reports Fluxes Set Up...

(a)   Make sure that Mass Flow Rate is selected from the Options list.

(b)   Select pressure-outlet-7, velocity-inlet-5, and velocity-inlet-6 from the Boundaries selection list.

(c)   Click Compute.

  The individual and net results of the computation will be displayed in the Results and Net Results boxes, respectively, in the Flux Reports dialog box, as well as in the console.

The sum of the flux for the inlets should be very close to the sum of the flux for the outlets. The net results show that the imbalance in this case is well below the 0.2 ${\rm\%}$ criteria suggested previously.

(d)   Close the Flux Reports dialog box.

9.   Save the data file ( elbow1.dat.gz).

File $\rightarrow$ Write $\rightarrow$ Data...

  In later steps of this tutorial you will save additional case and data files with different prefixes.