Trains and tunnels

Tim

HI,

I'm wondering if anyone can help me answer a query... I'm working on some CFD code in f95 that should simulate the movement of a train through a tunnel.

I'm using the MacCormack scheme with a source term simulating the tunnel and train interaction using a variable area duct and solving the Euler eqns, applying density and velocity bc's at the inlet and atmospheric pressure at the outlet.

I thought that the code was fairly simple but when it runs I get some very nonphysical pressure oscillations at the points where the train nose and tail are. I shouldn't get these as the area change is sinusoidal and not discontinuous (they shouldn't exist at all I think) but I'm not sure if I've gone wrong somewhere.

I've read in Hirsch that artificial viscosity terms are needed to smooth out oscillatory behaviour but my simulation is purely subsonic so I haven't implemented them... is this the root of my evil?

Help!

Thanks, Tim Aplin Dept of Aeronautics, Imperial College, London