I am simulating a sonic underexpaned jet. The flow expands up to M = 2.8 and then there is shock. Well my question is about grid generation, in the formula for yplus=yu*/nu, (nu= viscosity), What should the u* value be based on M = 1 or M =2.8? Also is assuming u*=U/30 a good choic for u*? where U speed of sound or based on M = 2.8.

Thank you very much.

friction velocity (utau/u*) values are usually calculated from some initial runs and by computing the wall shear stress. Try to run your code with different levels of grid refinement for around one period of the relevant time scale and computed yplus.