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Ajay Rao November 16, 2003 19:43

Calculating Drag
The following code I am using to calculate the Drag is way off ! Could any one point out the mistake . Thanks in advance -Ajay



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) ;


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) ;



sigmazz=C_MU_L(c,t)*2*(C_DWDZ(c,t))- C_P(c,t);


/*compute the force vector at each cell centroid */

/* area vector points inwards hence a= -a */






printf(" Drag force = %2.20e ",force_vector[0]);

ccc November 17, 2003 03:15

Re: Calculating Drag
I think the values used to compute the force on the wall should be the ones located on the face centroid.

Ajay Rao November 17, 2003 10:13

Re: Calculating Drag
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

Fonzy November 18, 2003 11:19

Re: Calculating Drag
You should fix if you want to name your stress tensor "sigma" or "tau"...:)

ccc November 19, 2003 12:55

Re: Calculating Drag
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.

Ajay Rao November 19, 2003 21:51

Re: Calculating Drag
It would be nice to hear what the FLUENT ppl have to say , please keep me posted , Thanks a lot -Ajay

ccc November 20, 2003 10:30

Re: Calculating Drag
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

psb November 30, 2009 02:28

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.

alessios123 February 15, 2010 10:15

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.
Thank to all

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