Thanks. I now understand this a little better and able to do some computations. Are there any characteristic-based boundary conditions available in cgcns ? What type of BC would I use for compressible flow for a curved outer (farfield) boundary. I may not know the direction of the velocity there since the boundary is curved. Thanks praveen praveen On Wed, Jul 15, 2009 at 9:05 AM, Bill Henshaw <henshaw@xxxxxxxx> wrote: > Dear Praveen, > Sorry, that error message is not very good. Try using > > backGround(1,0)=subSonicOutflow mixedDerivative(1.*t+1.*t.n=300.) > > as in cg/cns/cmd/cicSubSonic.cmd > > Regards, > Bill > > >