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Old   February 14, 2006, 12:57
Default Force on body by velocity compared to force_y@...
  #1
Stefan
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Dear posters, it would really help me if anyone could give me a hint with this:

in order to calculate the Force on a bluff body normaly one uses force_y@bluffBodyName. This shuould calculate the y component of Integral(sigma dot normal)*dS

this integral is equal to

areaInt((-p+2*Dynamic Viscosity*(Velocity u.Gradient X))*Normal X)@bluffBodyName + areaInt(Dynamic Viscosity*(Velocity u.Gradient Y+Velocity v.Gradient X)*-Normal Y)@bluffBodyName + areaInt(Dynamic Viscosity*(Velocity u.Gradient Z+Velocity w.Gradient X)*-Normal Z)@bluffBodyName

But they are not equal! Question: Why?

(I need to calculate the Integral this way, because in my application I need to direcly acsess the memory of the solver with Fortran routines and map forces from a deformed to a reference configuration)

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Old   February 14, 2006, 13:08
Default Re: Force on body by velocity compared to force_y@
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opaque
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Dear Stefan,

I assume you are using a laminar model because otherwise you cannot assume that the shear force is visc*(grad vel + gradT vel).. Recall that the turbulence models use wall functions, and the expression for wall shear is C * (Vel Node - Vel Wall) or something in that direction..

Good luck, Opaque..

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Old   February 15, 2006, 10:27
Default Re: Force on body by velocity compared to force_y@
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Stefan
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Thank's a lot for your answer.

Yes I am using a laminar model the reynolds number is only 20. no wall function or anything like that is used.

using f_x@cylinder I get 5.56847e-005 [N] and using visc*(grad vel + gradT vel) leads to 5.48238e-005 [N] which is not the same and I need it to be exact.

Really looks like I foregot something. Can you think of anything else that went wrong?

greetings

Stefan
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Old   February 15, 2006, 10:29
Default Re: Force on body by velocity compared to force_y@
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Stefan
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Foregot to say that the fluid is visous and incompressible
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