« Back to Video Tutorials

Calculate Q-Criterion with Tecplot 360


Q-Criterion is an important calculation used to identify vortices. In this video we’ll show you how to calculate Q-Criterion, plot the results, and compare the performance of Plot3D and SZL file formats for this work.

In this example, we are using the final time-step of a transient simulation of a wind turbine. An overset mesh was used and is composed of 5863 zones with a total of 263 million elements. The output is in Plot3D file format and the grid and solution files combine for a total of 21 GB.

To calculate Q-Criterion go to Analyze > Calculate Variables and select Q Criterion from the list. The Calculate Variables dialog has a unique feature called Calculate-on-Demand – if you check this toggle, the formula will simply be registered and the calculation will not occur until it is required. This can save a significant amount of time particularly for a data set such as this which has many zones, because the calculation will only be performed on the zones required for the desired plot.

Use an iso-surface, at a positive value near zero, to view the resulting Q-Criterion calculation. Some adjustment of the iso-surface value will be required to see the vortices. Increasing Q reduces the complexity of the iso-surface, but too high of a value  makes an iso-surface that is too sparse. It is important to find a value that results in an iso-surface that is neither too dense nor too sparse.

When run in batch mode, the total time to load the data, calculate Q and generate an image with an iso-surface at Q=0.01, without Calculate-on-Demand, was 517 seconds for the Plot3D file and 409 seconds for the SZL file. Using Calculate-on-Demand was 27% faster, taking 372 seconds for the Plot3D file and 299 seconds for the SZL file. Additionally the SZL file is 35% smaller at only 13GB.

This concludes the tutorial on Q-Criterion.

Thanks for watching!