Can anyone give me some good pointers for non-reflecting BC's for 3D unsteady euler flow, I'm currently using extrapolation in a Dual-Time finite volume solver with Roe's flux differencing scheme.

The boundary conditions have to be prescribed on the characteristics of the flow (or alternatively the characteristics equations of the flow have to be solved at the boundary). Let me give you some references, and if you have any specific question, just post it here.

Abarbanel et al. 1991, J. Fluid Mech. 225, 557

Givoli, 1991, J. Comput. Phys, 1 (review paper)

Engquist and Majda, 1977, Maths Comput 31, 629

see also all the references therein. Extrapolation does not always work, only for the outgoing characteristics of the flow. For the incomming characteristics of the flow, exact values have to be prescribed at the outer boundary (or also the fast incoming characteristics can be set to zero to avoid incoming noise and numerical oscillations).

I hope this helps.

Patrick