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November 16, 2003, 19:43 
Calculating Drag

#1 
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The following code I am using to calculate the Drag is way off ! Could any one point out the mistake . Thanks in advance Ajay
begin_f_loop(face,tf) { c=F_C0(face,tf); /* gets the adjacent cell */ t=THREAD_T0(tf);/* gets the adjacent cell thread */ /* doing this so that i can use C_DUDX(c,t) */ /* F_DUDX notpresentto get derivative at wall */ sigmaxy=C_MU_L(c,t)*(C_DUDY(c,t)+ C_DVDX(c,t)); sigmaxz=C_MU_L(c,t)*(C_DUDZ(c,t)+ C_DWDX(c,t)); sigmaxx=C_MU_L(c,t)*2*(C_DUDX(c,t))  C_P(c,t) ; sigmayx=sigmaxy; sigmayz=C_MU_L(c,t)*(C_DVDZ(c,t)+ C_DWDY(c,t)); sigmayy=C_MU_L(c,t)*2*(C_DVDY(c,t))C_P(c,t) ; sigmazx=sigmaxz; sigmazy=sigmayz; sigmazz=C_MU_L(c,t)*2*(C_DWDZ(c,t)) C_P(c,t); F_AREA(a,face,tf); /*compute the force vector at each cell centroid */ /* area vector points inwards hence a= a */ force_vector[0]=force_vector[0]tauxx*a[0]tauxy*a[1]tauxz*a[2]; force_vector[1]=force_vector[1]tauyx*a[0]tauyy*a[1]tauyz*a[2]; force_vector[2]=force_vector[2]tauzx*a[0]tauzy*a[1]tauzz*a[2]; } end_f_loop(face,tf) printf(" Drag force = %2.20e ",force_vector[0]); 

November 17, 2003, 03:15 
Re: Calculating Drag

#2 
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I think the values used to compute the force on the wall should be the ones located on the face centroid.


November 17, 2003, 10:13 
Re: Calculating Drag

#3 
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Ya True , But fluent does not give access to the derivatives at the wall . They have functions like C_DUDX which gives the derivative at the cell centroid. But they dont have any functions like F_DUDX to give the derivative at the wall . Any idea how to go around this ?
Regards, Ajay 

November 18, 2003, 11:19 
Re: Calculating Drag

#4 
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You should fix if you want to name your stress tensor "sigma" or "tau"...


November 19, 2003, 12:55 
Re: Calculating Drag

#5 
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hi, Ajay
I will attend a Fluent User Conference tomorrow and try to find some help from engineers. A possible way I am considering is to find the stresses by interpolating from the values on the centroid. 

November 19, 2003, 21:51 
Re: Calculating Drag

#6 
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It would be nice to hear what the FLUENT ppl have to say , please keep me posted , Thanks a lot Ajay


November 20, 2003, 10:30 
Re: Calculating Drag

#7 
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hehe, the one I asked said nothing. it seems we have to solve this problem by ourselves.
let us inform the other when new ideas come into mind. good luck 

November 30, 2009, 02:28 

#8 
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PSB
Join Date: Nov 2009
Posts: 5
Rep Power: 8 
Hi Ajay,
I have been facing the same trouble. I have to give a traction free condition at the outlet of a pipe with a porous layer surrounding it. The condition applies to the entire outlet. Its, p+(mew)du/dx=0. Since I do not know how to apply a traction free condition, I am trying to specify a pressure profile at the outlet in terms of (mew)du/dx. However, the same problem persists: derivative is calculated considering cell centroid and pressure profile is hooked at face centroids. If you find a solution to this please let me know. Thank you so much. 

February 15, 2010, 10:15 

#9 
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alessio
Join Date: Nov 2009
Posts: 5
Rep Power: 8 
Hi guys, i'm tring to calculate the drag force coefficient in a 3d case and the fluid is behind a cylinder.
the fluid is: density = 800 kg/m^3 dinamic viscosity = 0.005 kg/ms velocity = 8 m/s the cylinder is: diameter = 13 mm length = 300 mm stiffness modulus = 200000 MPa poisson coefficient = 0.3 to calculate the drag coefficient i push: solve/monitors/force/drag coefficient..... the direction of the vector of force is in parallel to the flow flowing i set the reference value: Area = 0.013*0.3 that is the product for diameter and the length of the cylinder, Length = 0.013 that is the diameter then i set other values: viscosity, velocity, density of fluid as i wrote upforward. i put 90 nodes around the circonference of the cylinder, and 25 along to the length of the cylinder. the algorytm used is LES, because this is a 3d case I used PISO and second order upwind for momentum... the Cd value is incresing but after 0.02 seconds it is still to 0.7. How can i do, please help me. Alessio Thank to all 

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