a problem about thermo-acoustic instabilities
 

hi,I face a problem about thermo-acoustic instabilities in my numerical simulation. Because my computational domain is not as long as the real combustion tube, I need to define the boundry condifition for my computational domain.But i don't konw how to do this because the bc at the computational domain edge is not the bc value in the environment and maybe these values are related to the combustion domain because this is an unsteady computation.Thanks a lot!!!