CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Calculating Drag

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 16, 2003, 19:43
Default 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]);

  Reply With Quote

Old   November 17, 2003, 03:15
Default 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.
  Reply With Quote

Old   November 17, 2003, 10:13
Default 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
  Reply With Quote

Old   November 18, 2003, 11:19
Default Re: Calculating Drag
  #4
Fonzy
Guest
 
Posts: n/a
You should fix if you want to name your stress tensor "sigma" or "tau"...
  Reply With Quote

Old   November 19, 2003, 12:55
Default 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.

  Reply With Quote

Old   November 19, 2003, 21:51
Default 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
  Reply With Quote

Old   November 20, 2003, 10:30
Default 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
  Reply With Quote

Old   November 30, 2009, 02:28
Default
  #8
psb
New Member
 
PSB
Join Date: Nov 2009
Posts: 5
Rep Power: 17
psb is on a distinguished road
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.
psb is offline   Reply With Quote

Old   February 15, 2010, 10:15
Default
  #9
New Member
 
alessio
Join Date: Nov 2009
Posts: 5
Rep Power: 17
alessios123 is on a distinguished road
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

alessios123 is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
calculating drag and lift in fluent morteza08 FLUENT 4 December 13, 2012 16:34
Reference Area for Calculating Lift and Drag Coefficient of 3D wing mahbub03 Main CFD Forum 3 May 19, 2011 01:21
calculating drag at low Re no over FX airfoil M. Essuri FLUENT 1 November 1, 2006 16:55
Which kind of pressure use calculating Drag? Fer Main CFD Forum 5 January 27, 2006 13:32
How to update polyPatchbs localPoints liu OpenFOAM Running, Solving & CFD 6 December 30, 2005 18:27


All times are GMT -4. The time now is 14:59.