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Old   October 30, 2012, 15:00
Default Press. oulet BC issues
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Hey everyone,

I'm trying to run a fairly simple CFD simulation of a box thing in a very basic "windtunnel". My windtunnel is really just a big rectangular domain. Simulation in FLUENT 14.0, mesh made in ANSA 13.2.3.

Mesh appears to be great. Skewness all below 0.9 in both solids and shells, no intersections, no negative volume. Made prisms layers for B.L. growth on windtunnel walls, floor, and ceiling, as well as necessary areas of my test object. Hexa-interior mesh.

I ran a realizable k-epsilon model (Inlet velocity of 70 m/s is a Re=7x10^6). My boundary conditions were simple: velocity inlet for the inlet of the windtunnel, pressure outlet for the outlet of the tunnel, and walls everywhere else (ceiling/floor/sides/object). Converged (residuals 10-4 or so) just fine in 2000 iterations. Y+ values look fine (between 30 and 250 or so), as do velocity/pressure contours. So we're happy!

In real life however, the windtunnel facility is actually an open tunnel, so I changed the sides, and ceiling of the tunnel to pressure outlet conditions to try and simulate the open tunnel scenario. I left the floor as a wall. All hell broke loose.

While most of the residuals look ok, continuity hovered around the 10^-2 or greater area. The velocity contours are a dead giveaway of my CFD ignorance. Velocity accelerates out of the back of the domain in the 300 m/s range (remember velocity inlet is 70 m/s), as well as accelerating out of the top, and sides of the domain.

I guess I don't fully understand the pressure outlet BC. Should I be using something else? like a farfield condition or something? Anyone else had this kind of problem before? or should I just leave the top, and sides of the domain as walls and give up on trying to do an "open tunnel" scenario? I'd a appreciate any advice!!

Thanks!
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