# Calculating Drag

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 November 16, 2003, 19:43 Calculating Drag #1 Ajay Rao Guest   Posts: n/a 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 ccc Guest   Posts: n/a 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 Ajay Rao Guest   Posts: n/a 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 Fonzy Guest   Posts: n/a You should fix if you want to name your stress tensor "sigma" or "tau"...

 November 19, 2003, 12:55 Re: Calculating Drag #5 ccc Guest   Posts: n/a 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 Ajay Rao Guest   Posts: n/a 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 ccc Guest   Posts: n/a 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 New Member   PSB Join Date: Nov 2009 Posts: 5 Rep Power: 7 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 New Member   alessio Join Date: Nov 2009 Posts: 5 Rep Power: 7 Hi guys, i'm tring to calculate the drag force coefficient in a 3-d 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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