13.8. Case Comparison

The Compare Cases command enables you to compare results from two distinct cases, or between two steps of a single case. The Compare Cases command is available in the Tools menu in any of the following situations:

  • You have loaded two or more cases using the Load Results File dialog box option Keep current cases loaded

  • You have loaded a single transient case (with results available for at least two time steps)

  • You have loaded a multi-configuration case, or a case with run history, using the Load Results File dialog box option Load complete history as (either as a single case or as separate cases), so that results for two or more steps are available through the timestep selector.

Selecting Compare Cases displays the Case Comparison details view.

The following options are available:

Case Comparison Active

Enables the Case Comparison function; the comparison occurs when you click Apply.

In Case Comparison mode:

  • Difference variables are computed as the variable values from Case 1 minus the variable values from Case 2. The latter are interpolated onto the mesh from Case 1 before the subtraction. As a result, the difference variables are located on the mesh from Case 1.

    To reverse the order of subtraction, swap the specifications for Case 1 and Case 2 in the Case Comparison details view.

    Note:

    • CFD-Post does not support comparison of the following variables:

      • Connectivity Number

      • Edge Length Ratio

      • Element Volume Ratio

      • Force

      • Length

      • Mass Flow

      • Maximum Face Angle

      • Minimum Face Angle

    • When comparing variables on interior walls in cases where meshes are not identical, you may see unexpected differences in difference plots. This can happen because during mesh interpolation, variable values may get picked up from one or the other side of the interior boundary. If the two sides do not have the same values, the interpolated values could randomly oscillate between values of the two sides, producing additional difference in the plot.

    • When comparing cases with a large number of mesh nodes, CFD-Post may take a long time to produce the difference variable and as a result may appear to be unresponsive.

  • A Difference view is shown in a new view (in addition to the Case 1 (<case_name>) view and the Case 2 (<case_name>) view). In that view, differences are shown on the mesh from Case 1.

  • Each difference variable is named by appending ".Difference" to the end of the variable name from which it was derived. For example, the difference variable for the variable Pressure is Pressure.Difference.

  • The difference variables can be used anywhere that variables can normally be used. The Function Calculator and Table Viewer have special support for the difference variables, enabling you to easily see functions and tables (respectively) of difference values. In addition, a chart that is based on locators that exist in both Case 1 and Case 2 will have a "Difference" chart line. See Example: Comparing Differences Between Two Files.

  • CFD-Post refers to the cases as "Case 1" and "Case 2" rather than as the original case names (which are usually based on the results filename).

Case 1 and Case 2

Enables you to select the cases to be compared. If you want to compare two steps from within the same case (that is, two time steps from a transient case) then you should select the same case for both Case 1 and Case 2. The timestep selector that is embedded into the Case Comparison details view then enables you to select which steps you want to compare. In this circumstance, CFD-Post needs to load the results from the selected case a second time, so you will see a second case appearing in the tree view. After the comparison has been initialized, the steps used for the comparison can be changed either by using the embedded timestep selector on the Case Comparison details view, or by using the usual timestep selector (which now has separate entries for each of the two copies of the case being compared).

Tip:  When comparing two 2D cases, set the case that is extruded less as Case 1. This enables CFD-Post to match nodes between the two cases for one of the symmetry boundaries and to define difference plots.

Options: Synchronize camera in displayed views

Causes changes in orientation of one view to be duplicated in the other. If the views are initially in different orientations, the first movement of any view will align all views to the same orientation.

Options: Use absolute difference for scalar variables

Causes all scalar variable differences to be reported as positive numbers.

Mesh Detection

Enables you to control whether or not CFD-Post needs to determine whether the meshes in the two cases are identical. If you know beforehand that the meshes are the same or different, you can save processing time by enabling the appropriate mesh detection setting. Your options are:

  • Auto-detect same mesh causes CFD-Post to analyze the two meshes to determine whether they are the same or different before performing any interpolation.

  • Meshes are identical and Meshes are different enable CFD-Post to perform interpolation immediately, which saves processing time when cases are large.

    Note:  When you know meshes to be topologically identical but the node numbering may be different, use Meshes are identical. This setting causes CFD-Post to ignore node numbering and just use the topology of the mesh. In such cases do not use the Auto-detect same mesh setting because this fails when node numbering is not the same.

    For example, when comparing a case from Ansys CFX with a case from Fluent, the node numbering may differ even between apparently identical meshes, so the Meshes are identical setting is required.

Note:

  • If you run a case comparison on a file that contains solver-generated difference variables (such as Volume Porosity.Difference), these variables will become unviewable when you enter case comparison mode. However, the variables will be viewable again if you reload the results file.

  • Global ranges of difference variables are updated as domains are used. For example, if a multi-domain case is loaded and a difference variable colors a locator that is in a single domain, the range displayed will reflect the range of the difference variable only in that domain. If the locator is moved to another domain (or a new locator colored by the same variable is added), the global range for that difference variable is updated to reflect both domains.

  • When using expressions in case-comparison situations, the expression syntax is:

    <function>()@CASE:[1|2].<location>

    For example, area()@CASE:2.myplane

  • Case comparison is supported only for General mode. As a result, case comparison initiated from the Turbo tab will revert to General mode.

  • When using Variable Minimum or Variable Maximum option on a point in multi-file or comparison mode, the point is placed at the location of the overall minimum/maximum. If you want to place the point at the minimum/maximum value for the individual cases, select the appropriate case in the point's Domain List selector.