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Old   November 17, 2005, 10:39
Default Outlet Boundary Headache
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I'm trying to model flow around a yacht hull, but having very little luck in getting water to flow out of my domain in a sensible way. No matter what I do, CFX insists on putting temporary walls in the outlet, to block inflow.

I am using homogenous model, standard homogenous free surface model, standard air at 25 degrees and water. Two inlets: one 100% water below y=0, and one 100% air above y=0.

Boyancy activated at (0, -9.81, 0), ref density 1.185 (air) and ref location (0,0,0). +Y is up in this model, water surface at y=0.

The outlet is static pressure, with a CEL expression pressure profile:

pOut = -997[kg m^-3]*g*y*step(-y/1[m])

I have tried using the same pOut as t=0 initial guess for the domain pressure, but that doesn't change anything.

Anyone around who's been successful in anything similar?

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Old   November 18, 2005, 11:13
Default Re: Outlet Boundary Headache
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CFX calculates the buoyancy force using (rho - rho_ref). Without subtracting rho_ref, your pressures will all be just slightly higher than the resulting hydrostatic head resulting in inflow.

Try this:

pOut = (Water at 25 C.density - Air at STP.density)*g*y*step(-y/1[m])

Using the actual phase densities also ensures you have "exactly" the density that CFX is pulling out of the database.

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Old   November 18, 2005, 12:32
Default Re: Outlet Boundary Headache
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Thanks a lot!

I actually discovered that a reference location of (0,0,0) was not so smart, since that is exactly on the surface. Putting further up in the air solved the major problem, but your tip also made me loose the weird "dip" in the water surface I was getting at the outlet.

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