Learning Forum

[rfblumen]

Modeling shock waves in an axial turbine involves prescribing the correct boundary conditions.  It's also helpful in most cases to start from a subsonic solution and gradually modify the boundary conditions to the prescribed inlet/outlet conditions that will achieve supersonic flow in the turbine.  A typical case is a supersonic turbine stage where the flow enters the stator as subsonic flow, is accelerated to surpersonic flow at the stator outlet and enters the rotor.  If the rotor is designed well, there should be no shocks within the rotor passage except perhaps near the trailing edge. Running this type of case off-design might trigger shocks in the rotor passage due to leading edge incidence effects.

The approach to this case where the flow is entering the model as subsonic flow and exiting as supersonic flow is the following:

-Start with the prescribed inlet total pressure, temperature using  a subsonic inlet condition

-Set the outlet pressure to a value that would achieve subsonic conditions in the turbine (e.g. use a value >0.5*Inlet total pressure)

-Gradually decrease the outlet pressure in stages until the outlet flow reaches Ma>=1 over the entire outlet.  Switch from a subsonic outlet pressure boundary condition to a supersonic (i.e. extrapolation) boundary condition.

In some cases, using a Local Time Scale Factor may help with convergence if both subsonic and supersonic flow are present in the model.  Use a value between 3-10.