1. Locate the Warnings & Errors button above the runtime plots. It means warnings are present.
2. After the meshblock mismatch errors discussed in There is one warning, which is displayed below.
Solver Messages:
convective flux exceeded stability limit
at t= 3.0009E-02 cycle= 1 iter= 51 delt= 3.0009E-02 mesh block 4 restarting cycle with smaller time step
maximum failure ratio = 2.26944E+00 is in x-coordinate direction at cell ( 82, 34, 17) mesh block 4
3. The meaning is as follows:
a. At t = 0.03 seconds, a packet of fluid tried to move more than one cell at a time. The packet probably accelerated unexpectedly, which is why the solver didn’t predict the velocity of the packet. The stability limit is the requirement that fluid packets only move one cell per time step. The solver takes care of the problem by restarting the time step with a smaller dt. b. The error occurred in Mesh Block 4, the conforming block.
c. The offending cell number is i, j, k = 82, 34, 17. Remember that i,j,k values are block-specific, while x,y,z values are global.
d. The problematic fluid packet was moving in the x-direction. In the next step you will find the problem cell and identify why the problem occured.
4. Use the small X in the upper right corner of the dialog to close the Warnings & Errors.
5. Scroll up through the text output displayed below the plots and find the same warning. Notice that the cell location and mesh block are not given in this summary.
6. Select Diagnostics > Solver Errors from the menu at the top of the screen: the same information is displayed. Some errors report additional information to the Solver Errors file, so it should always be checked.
The only run-time error reported in this simulation is a single convective flux error which does not affect solver accuracy. Except for the purposes of this exercise, you can safely ignore isolated errors. If
convective flux errors recur frequently, they will at best slow down the simulation, and at worst cause it to terminate before completion. Errors should be addressed when they are repetitive.
Use 2-D Plots to Diagnose Errors
1. Go to the Analyze > 2-D tab.
2. Click the Mesh Block button at the lower right and select Mesh Block 4 only. When you click OK, the Limits sliders will reset to show the I J K cell number in the selected block at their sides.
3. Set plotting extents for Block 4:
a. Set Contour Variable = X-Velocity and Vector Type = Plain Velocity Vectors. b. Select Plane = XZ.
c. Select the Mesh checkbox to plot the grid lines. d. Select Data Source = Selected.
e. Slide the Max Time Frame slider (or use the associated arrow) so that t = 0 to 0.7 seconds. f. Slide the X Limits sliders so that I = 78 to 86. The problem cell (I = 61) will be in the center. g. Move the Y Limits sliders so that both Min and Max J = 34. This will display one planar slice
along the cell centers of Block 4. The slice will contain the problem cell. h. Slide the Z Limits sliders so that K = 2 to 32.
4. Select contour plotting options:
a. Click the Contour Options button in the bottom central portion of the tab.
b. Activate the No Contour Smoothing checkbox to turn off the aesthetic blurring of cell- centered values.
c. Select Blanking Variable = None. This clears the Blanking Variable Value entry. All cell values will be displayed, whether they contain fluid or not.
d. Select OK to close the dialog.
5. Check that you’ve made the selections above (marked below for reference) and click Render. The two plots closest in time to the error should be similar to the ones on the next page.
6. Locate the cell at I = 82, J = 34, K = 17:
a. Select multi as the plot type and then select the time frames for t = 0.35 and t = 0.73 seconds from the list at the right. The problem occurred before these plots, but they still show why.
b. Check that the text below the plot shows the range ix = 78 to 86 and kz = 2 to 32.
c. Start with the bottom-right cell at K = 2 and count up to 17. From the left, start at I = 78 and count up to 82. This is the cell specified by the warning message.
The problem cell is just inside the top of the pipe. Notice the recirculation zone to the right above the pipe discharge jet. Convective flux errors are associated with sudden accelerations. The flow is making a corner here, so it can be induced that the discharge, forming in quiet water, interacted with the pool and experienced an unexpected acceleration at t = 0.03 seconds. Even at t = 0.35 and 0.7 seconds you can still see where the recirculating pool flow accelerates from the downward direction to the horizontal direction as it’s caught by the pipe discharge. By the visible time steps, however, the flow is predictable enough that the dynamically chosen time step accounts for the change in direction.
Identifying the problem cell is useful when there are many errors in a row. In this case the failure is of a type that can usually be ignored, especially since it occurs only once. Common causes of serious (that is, numerous) errors include:
bad geometry files: fix with pyAdmesh and netfabb or re-export from CAD,
unresolved or poorly resolved gaps between geometry: use finer cells or different geometry, incorrect boundaries or sudden changes in boundary conditions or mass/momentum sources, incorrect initial conditions, and/or
flying droplets/splashing of fluid.