A while ago, I posted a NRBC thread and got useful feedback for the characteric boundary conditions from papers by K.W.Thompson etc. (i.e. dU/dt at boundaries)

Q = Q + dU/dt

dU/dt = -1/delta_x*delta_X(F, 2) - 1/delta_y*delta_Y(G, 2);

Is there another way of imposing the nonreflecting boundary condition at the end of each step on Q? (like a Neumann bounday condition)

Thanks Shuo

Yes there is a way of implementing it.

" an absorbing buffer zone technique for acoustic wave propagation " -AIAA-1995 Ta'asan and Nark.

THere are many different ways of implementing non reflecting BCs. See for example the review paper:

Givoli, D., 1991, Journal oc Computational Physics, vol. 94, page 1.

The best way to do it is to impose the boundary conditions on the charactersitics of the flow (equivalent to the Riemann invariants in 1D flows). The BCs are imposed on the incoming characteristics only and the outgoing characteristics are set to some computed numerical values propagating outward. THe best is to set the (fast) incoming characteristics to zero.

I hope that helps.