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-   -   loss in force (http://www.cfd-online.com/Forums/main/2642-loss-force.html)

chris September 28, 2000 02:04

loss in force
 
I calculated for test-purposes a very simple 2d-problem: a channel with length l and symmetry, velocity-inlet and an outflow. I used the standard-wall-model and k-eps. The y+ are between 150 and 300 and ok. Now the problem: If I take the shear-force at a part of the wall (the flow is developed) I get 3.3 (dimensionless) The other way would be to calculate it by hand:

F_res=rho*c1**2-rho*c2**2+p1*A1-p2*A2

with c1=c2 and A1=A2, I get: F_res=A*(p1-p2)

If I sum up the area-weighted pressure at the positions 1 and 2 I only get 2.6 as the resulting force. It seems, I lost about 0.7..where are they ?

John C. Chien September 28, 2000 08:56

Re: loss in force
 
(1). Check the equation first, where did you get this equation?

chris September 28, 2000 17:13

Re: loss in force
 
ok. error in the equation, sorry. It should be rho*ci**2*Ai (!!), and if I sum it up in the correct integral (!) form, i get the correct result...

John C. Chien September 28, 2000 17:18

Re: loss in force
 
(1). Congratulation! (2). I wasn't so sure when I answered your question. So, You should say, you found your own errors.


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