Currently, the Named Expression cannot be set as a parameter. However, the required functionality can be achieved with the help of a dummy parameter as follows:
1. Open the geometry and go to groups panel. Right click and select "Create Script Parameter". You can see a parameter "DS_Re" created in below image using this option.
2. Close the geometry session and update the Geometry task. This will import the newly created parameter.
3. Click the parameter button for this imported parameter.
4. Next, create a Calculated value for this imported parameter as shown below.
5. Now edit the expression of "v" and replace "Re" with the newly created calculated value (DS_Re). See below image.
This will set the Reynolds Number as an input parameter for your study. See below image.
Let me know if you have any questions.