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zonexo April 5, 2006 12:00

Strange oscillating velocity

i'm running a simulation past an airfoil like body using my own 2d fvm fractional step cartesian ns code. hence, the body will be represented as steps.

my inlet velocity is either u=1 or slowly increasing from u=0.01 to u=1 in 100 time steps.

when i run at re=100, the u velocity along y=0 (center line) is approx 1 until it nears the body, where it starts to decrease. that's perfectly logical.

however, the strange thing is when run at re=2000, u changes from u~1 (far field upstream) ~0.9 ~0.95 ~0.93 ~0.94 to ~0.8 ~0.84 ~0.76 ~0.8 ~0.6 ~0.4 (just next to body). this answer seems a bit illogical...

my scheme has rhie chow interpolation. moreover, body is about 8 units from upstream so that should be enough right?

Can someone explain to me what's happening? thanks a lot!

ag April 5, 2006 12:40

Re: Strange oscillating velocity
Sounds like a boundary condition effect to me. Depending on the quality of the BC you may need to move the outer boundary much further out. It is not uncommon to see outer boundaries at 50 to 100 chord lengths from the airfoil. I personally would not go less than about 20 (and then only if I had to).

zonexo April 6, 2006 11:38

Re: Strange oscillating velocity

i've tried lengthening both the upstream as well as the downstream distance. unfortunately, the result is the same.

however, when i increase my grid to 256x128, the solution looks better, ie, the oscillation becomes smaller and the u velocity decreases as it approaches the body. but i wonder if i had really solved the problem.

btw, for a FVM fractional step non-staggered scheme, there is usually a need to define the cell center and face velocity(outward normal).

so for my inlet velocity, i've defined u_center=1, u face east=1 and u face west=-1 for first column of cells. is this reasonable? of course, this will result in a non-solenoidal field. i wonder if this is the cause of the problem. i've no idea how to generate a solenoidal field. it'll be great if someone can direct me some references.


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