Select the parent part.

Select Create → Vortex cores...

Bring up the Vortex Core Variable Settings dialog by clicking Define vortex Variables....

Define either (Density and Momentum) or Velocity, as well as the Ratio of Specific Heats. The variables can be set by either typing them into the fields, or selecting them from the list above and clicking the Set button.

Click OK to finish the variable setup.

Click Create with selected parts.