
Estimate the length of the fully extended jet (L). Divide the mesh along the axis of symmetry into parts (subgrids) each length 1.5*L, then join all parts. Inactivate all parts except one in front of the jet, based on the average velocity of the jet, estimate the length of time required for the jet tip to reach about half way into the next part, and run the calculation for that length of time. Activate the next part and if appropriate deactivate the part behind the jet. Restart the problem and run until the jet trip reaches about half way into the next part and repeat the process. While this may be a little more labor intensive, it could speed up the total calculation time required by a factor of 3 to 5.

