In the tutorial, the Mach number of the golf ball is less than 0.3, then how come is it compressible at all?
Hey @2102044
That’s a great question and you’re right that typically, a Mach number below 0.3 indicates that the flow can be assumed to be ‘incompressible’.
However, in reality, all flows are compressible, just negligibly compressible at lower speeds. If you switch to the density solution field you will see slight variation in density around the golf ball as follows:
A slightly higher velocity at inlet would have increased the speed of the flow around the golf ball even more leading to a Mach higher than 0.3 over the golf ball but not necessarily around it, for example near the inlet.
In such cases, since you do not know the exact Mach number around the ball before looking at the first simulation results, it makes sense to use the compressible solver even if you only expect the flow to reach Mach 0.3 at any point in the domain. This also highlights the iterative nature of simulation and CFD, in my opinion.
I hope this helps clarify things a little bit! For more information on the compressible flow solver in SimScale, you can check out the documentation here.
Cheers,
Satvik
Thank you very much for making it that much clear 
.