I have a quick question. If one uses total engery with variable desity option CFX takes it as compressible flow. But in reality the mach number is <0.15. Is the solution acceptable? Will there be numerical problem? Please comment..

Also in the domain if at certain places Ma<0.2 and an other places Ma>0.3, what options to use in the CFX solver and how/why they are correct.

Thanks in advance. Kali

Hi Kali,

That's fine. Basically, if the flow is low speed you can get away with simpler assumptions. In this case solving for Temperature instead of Enthalpy. However, the physics will still be correct. There might be slightly more computational effort, although probably negligable. Since the Total Energy equation involves more terms, there is a risk of it being more unstable, but it is not likely to be a problem.

If you have high speed flow anywhere in the domain, solve for the Total Energy equation. Note that if you are using ideal gas, the fluid is always compressible (i.e. density varies with pressure). Total Energy just accounts for this appropriately in the energy equation.

Regards, Robin

Thanks Robin for your lucid explanation. Kali