[Batou]

Hello Guys.

Since I will be examined about CFD in a few days I have some questions I hope you can answer for me.

1. When do I need additional numerical dissipation? It is clear that i need numerical dissipation for supersonic inviscid flows with pressure waves and central difference schemes for the convective term, but do i also need it when solving, i.E. incompressible flows or compressible flows with small Mach-number? are there also some discontinuities that need to be dampend ?
And is numerical dissipation also needed when i use the NS-Equations (instead of Euler) which already have a physical diffusion term?

2. (Perhaps a rather silly question)

In external flows, it is told that you commonly use velocity conditions at the inlet and static pressure conditions at the outlet (with boundaries far away enough from i.E. an airfoil).
In cascade flows you can use total pressure and total temperature conditions at the inlet and static pressure conditions at the outlet and the mesh does not have to be as big around the turbine blades as around the airfoil above.
Does it make sense to switch the boundary conditions for the two cases? Why is it unphysical?I mean with the latter BC you can save a lot of time because u need less cells.

Thank you very much in advance, this forum is a goldmine for me