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Help Needed - Lift and Drag for a Flat Plate, Normal to Flow
Posted 07.04.2014, 17:15 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.3a, Version 4.4 8 Replies
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I am following the method described in the Flow Past a Cylinder Tutorial. I am evaluating the drag force over the boundary integral of my flat plate. I then compute C_d = - 2*reacf(u)/(spf.rho*V^2*height).
Can someone tell me where I might be going wrong? I have attached my COMSOL model for your reference.
v/r
Connor
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-2*reacf(u) / (spf.rho* (Re*spf.nu/chord)^2 *height) / (2*height+2*chord)
That is equivalent to the following:
(Reaction Force in U) / ( 1/2 Density * U0^2 * Height ) / (Perimeter)
The perimeter term comes from the line integral. You can see that in my second line integral which integrates the #1... It gives the perimeter back as an answer. The units now come back as unitless, as they should (I think). I also made the height a parameter and included it everywhere, instead of carrying the 0.1. the domain is now 10 chords wide and 10 heights tall.
The new value I get is 0.86, which is getting closer to where you want to be, I think. Anyone else have some thoughts?
-TCL
P.S. The updated model file is attached. But I think I am 4.4... hmmm, didn't think that one through.
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Does the moving wall have the same velocity component as the free stream velocity: Re*spf.nu/ chord in the x-hat direction?
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I set the wall condition for the top and bottom of the domain to 'moving' at a velocity of ( Re*spf.nu/chord ) and
added a separate no-slip condition for the flat plate's boundary.
I am still getting a different value for C_d: 6.19649
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It seems that it isn't necessary to divide by the perimeter within the integral evaluation which is strange as the line integral of '1' over the surface yields the perimeter. I have attached an excel spreadsheet that shows my data thus far. The values labeled "COMSOL - C_d" were returned by the equation for C_d divided by the perimeter whereas the "COMSOL w/ Perim" values are the aforementioned values multiplied by the perimeter.
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-reacf(w) [N] / ( 1/2 * spf.rho * (Re*spf.nu/Diam)^2 * (pi*Diam^2/4) ) (1)
0.37155
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I'm trying to plot the lift and drag force on a rotating cylinder in a uniform stream (2D) for low Re=100. I've been successfull with calculating/plot the coefficient for the non-rotating cylinder which is well in order with literature.
For the non-rotating cylinder I've used the non-slip boundary condition on the cylinder and the expression: -reacf(v)*2/(spf.rho*U_mean^2*d) for the lift coefficient. Where d is the cylinder diameter and U_mean is the free stream velocity.
Now, when I set the boundary to "sliding wall" and set a rotation velocity the calculated lift force C_L=-reacf(v)*2/(spf.rho*U_mean^2*d)=0. Why is that? please help me asap
thanking you
susobhan
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