Step 10: Enabling Second-Order Discretization

  The elbow solution computed in the first part of this tutorial uses first-order discretization. The resulting solution is very diffusive; mixing is overpredicted, as can be seen in the contour plots of temperature and velocity distribution. You will now change to second-order discretization for all listed equations.

1.   Change the solver settings.

Solution Methods

(a)   Select Second Order from the Pressure drop-down list.

(b)   Select Second Order Upwind from the Momentum, Turbulent Kinetic Energy, Turbulent Dissipation Rate, and Energy drop-down lists.

  You will need to scroll the Spatial Discretization group box down to find Energy.

2.   (optional) Check the case to confirm that there are no recommendations for revisions to the setup.

Run Calculation Check Case

3.   Continue the calculation by requesting 150 more iterations.

Run Calculation

Extra:   To save the convergence history of the surface monitor for this set of iterations as a separate output file, you would need to change the File Name in the Surface Monitor dialog box to surf-mon-2.out prior to running the calculation.

(a)   Make sure that 150 is entered for Number of Iterations.

(b)   Click Calculate.

  The solution will converge in approximately 63 additional iterations (Figure  1.11). The convergence history is shown in Figure  1.12.

Figure 1.11: Residuals for the Second-Order Energy Calculation

Note:   You should expect to see the residuals jump whenever you change the solution control parameters.

Figure 1.12: Convergence History of Mass-Weighted Average Temperature

4.   Save the case and data files for the second-order solution ( elbow2.cas.gz and elbow2.dat.gz).

File $\rightarrow$ Write $\rightarrow$ Case & Data...

(a)   Enter elbow2.gz for Case/Data File.

(b)   Click OK to save the files and close the Select File dialog box.

  The files elbow2.cas.gz and elbow2.dat.gz will be saved in your default folder.

5.   Examine the revised temperature distribution (Figure  1.13).

Graphics and Animations Contours Set Up...

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

(b)   Select Temperature... and Static Temperature from the Contours of drop-down lists.

(c)   Make sure that symmetry is selected from the Surfaces selection list.

(d)   Click Display (Figure  1.13) and close the Contours dialog box.

  Figure  1.13 shows the thermal spreading of the warm fluid layer near the outer wall of the bend. To see the effects of second-order discretization, compare Figure  1.13 with Figure  1.6.

Figure 1.13: Temperature Contours for the Second-Order Solution

6.   Display and save an XY plot of the temperature profile across the centerline of the outlet for the second-order solution (Figure  1.14).

Plots XYPlot Set Up...

(a)   Disable Write to File in the Options group box.

  The button that was labeled Write... will change to Plot.

(b)   Make sure that Temperature... and Static Temperature are selected from the Y Axis Function drop-down lists.

(c)   Make sure that z=0_outlet is selected from the Surfaces selection list.

(d)   Click Plot.

Figure 1.14: Outlet Temperature Profile for the Second-Order Solution

(e)   Enable Write to File in the Options group box.

  The button that was labeled Plot will change to Write....

(f)   Click Write... to open the Select File dialog box.

i.   Enter outlet_temp2.xy for XY File.

ii.   Click OK to save the temperature data.

(g)   Close the Solution XY Plot dialog box.