Critical Values

Critical Values are evaluated on Summary entities (Parts, Planes, and Points) from the Design Study. They are used to compare important results quantities to evaluate the performance of design alternatives. Results are presented in a simple data table and plotted with bar graphs. This allows easy comparison across multiple scenarios against known design constraints or targets.

An example where Critical Values is helpful involves a design study aimed at selecting the optimal heat sink design in a high-power electronics module. The heat sink is placed on a chip that has a 60°C failure temperature. Several design alternatives are analyzed, and the chip is selected as a Summary Part.

The resultant temperature on the chip in each Scenario is tabulated in a Critical Value table, and 60°C is specified as a Reference Value. By examining the tabulated data and the resultant bar graph, it is clear which designs cause the chip temperature to exceed the limit, as well as which one produces the lowest temperature.

Note: Scenarios do not have to be from the same Design to be compared in the Decision Center, but they must exist in the same Design Study.

Step 1: Create Summary Entities

Critical Values are calculated on Summary Entities created in the Results task:

Summary Planes are planar cutting surfaces positioned at critical locations in the model. Data assessed on a summary plane is presented in the Critical Values table. Use these to assess and compare flow rate, pressure drop, and bulk temperatures through specific passages or openings.

Summary Parts are selected parts from the model. A typical use is to compare the temperature on a part to assess if a known limit has been exceeded.

Summary Points are specific locations selected within the model. All of the available scalar quantities are available in the Critical Values table. Use these to assess and compare results values at specific locations in the model.

Step 2: Create Critical Value Table in the Output Bar

Each summary entity is listed in the appropriate branch of the Decision Center (Summary Parts, Summary Planes, or Summary Points, respectively)

Right click on an entity branch, and select Update critical values to propagate the data across all scenarios. This updates all Critical Values.

If this step is not performed, the table will not contain any data values. If the warning symbol is displayed on a branch, the results in the model do not match the results in the table. To resolve this, click Update critical values.

The Critical Values tab of the Output bar lists the results for each summary entity.

Note: By default, results for each summary object are computed for all scenarios. To change the scenarios, right click on a particular object, and pick Select scenarios... Select only the scenarios you wish to compare from the list.

If a summary entity is moved or a new one added, the data in the table does not match the summary entity, and the warning symbol will appear on the branch:

Right click on the entity and select Update critical values.

Step 3: Customize the Output Table

To customize the data presentation in the Critical Values Table:

Add a row below a selected row by clicking the Add row button
Delete a row by clicking the Select bullet, and clicking the Delete row button
Change the result quantity of a row by selecting it from the Result Quantity column
Change the units of a result by selecting an entry from the Units column
Set a critical value (or a value important to your design) in the Reference column. Examples include the maximum allowable temperature or the minimum flow rate. This is very useful when plotting the data to assess which scenarios are within the allowed design constraints.

Note: To select a row, click the Select bullet on the left side of the table.

Step 4: Plot and Compare the Results

Use plots to visually compare results as well as to evaluate performance relative to a design constraint or target Reference value.

Check the Plot box for each desired result quantity.
Click the Plot button to generate the bar graph. Each result quantity is plotted on a separate tab.

To plot all active quantities, click Plot all values. Each quantity is plotted in a separate tab.

When a Reference value is specified, it is drawn as a horizontal line across the plot. For each scenario, the difference between the Critical value and the Reference value is plotted.

On the Plot dialog, to change the range of the Y-axis, click the Scale button, and input new minimum and maximum values.

To save the plot to an image file, click the Save button.

Note: Once a Results Part, Results Plane, or Results Point is designated as a Summary entity, it can only be deleted from the Decision Center on the Design Study Bar. Right click on its branch under the appropriate Summary branch, and click Remove.

For an example of the use of Critical Values