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

What does “Mean Wall Shear Stress” mean?

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 16, 2009, 09:15
Question What does “Mean Wall Shear Stress” mean?
  #1
New Member
 
Join Date: Jun 2009
Posts: 11
Rep Power: 16
dfytanid is on a distinguished road
Hello,
I am running an unsteady simulation and I have a question about unsteady wall statistics.

Having already activated the data sampling from the iteration menu I use the Unsteady wall statistics and more specificly the Mean wall shear stress option from Display/Contours../ .
And now the question is: How does fluent calculate this “Mean” values??
Is it the mean from the magnitudes of the shear vectors or
the magnitude of the mean shear vector?? (Please see the following equations)

Mean WSS=\frac{\sum|\vec{WSS}|_{t}}{number of time steps} or

Mean WSS=\frac{|\sum\vec{WSS}_{t}|}{number of time steps}


In some cases it is the same but in my case I have a variable velocity inlet (with positive and negative values) and thus the Shear Vector change direction.

Thank you for your answers,
Dim
dfytanid is offline   Reply With Quote

Old   October 14, 2016, 10:19
Default hey
  #2
New Member
 
armin amindari
Join Date: Oct 2013
Posts: 3
Rep Power: 12
amindari is on a distinguished road
HEY dim
have you found the answer?
amindari is offline   Reply With Quote

Old   October 14, 2016, 14:16
Default
  #3
New Member
 
Join Date: Jun 2009
Posts: 11
Rep Power: 16
dfytanid is on a distinguished road
Quote:
Originally Posted by amindari View Post
HEY dim
have you found the answer?
Yeap. What Fluent calculates is the average wss magnitude.
The magnitude of the average wall shear stress vector has to be calculated either using a UDF or in a post-processing tool(.e.g Post-CFD, tecplot etc).
regards,
Dimitrios
dfytanid is offline   Reply With Quote

Old   October 17, 2016, 02:05
Default
  #4
Senior Member
 
Paritosh Vasava
Join Date: Oct 2012
Location: Lappeenranta, Finland
Posts: 732
Rep Power: 22
vasava will become famous soon enough
Are you calculating OSI? If so, it would be interesting to know your approach.
vasava is offline   Reply With Quote

Old   November 6, 2016, 21:19
Wink
  #5
New Member
 
Join Date: Jun 2009
Posts: 11
Rep Power: 16
dfytanid is on a distinguished road
Quote:
Originally Posted by vasava View Post
Are you calculating OSI? If so, it would be interesting to know your approach.
I am really sorry for my late answer.
I was traveling and I missed this one.

Well... over the years I have tried both approaches (use of udfs/UDMs vs tecplot/matlab) and I would say that I really prefer the first one, since this way you can avoid problems related to the need of saving huge amount of data over time to avoid introducing errors in the osi/rrt calculations.

Unfortunately, I don't have the code I used while I was working on biofluids because I have moved to another institution but it shouldn't be very different from the following:


Code:
#include "udf.h"          /**** mandatory
#include "math.h"       /**** probably not necessary but I always include it 
#include "storage.h"    /**** you need it
#include "sg_udms.h"  /**** you will need this one too


#define domain_ID 1    /***** put here your domain ID (fluid??)
#define zone_ID 7      /****** put here your zone ID (WALL??)



/**** initialize our UDMS
DEFINE_INIT(init,domain) 
{ 
    Thread *c_thread; 
    cell_t c; 
 
    thread_loop_c(c_thread,domain) 
    { 
        begin_c_loop(c, c_thread) 
        { 
            C_UDMI(c,c_thread,0)= 0;   /* cumulative wss magnitude 
            C_UDMI(c,c_thread,1)= 0;    /* cumulative x-component of wss
            C_UDMI(c,c_thread,2)= 0;    /* cumulative y-component of wss
            C_UDMI(c,c_thread,3)= 0;   /* cumulative z-component of wss
            C_UDMI(c,c_thread,4)= 0;    /* osi
        } 
        end_c_loop(c, c_thread) 
    } 
}



/**** calculate OSI
DEFINE_EXECUTE_AT_END(OSI) 
{ 
    Domain *domain; 
    real AREA; 
    face_t f; 
    real A[ND_ND]; 
    cell_t c, c0; 
    Thread *t,*t0, *c_thread; 
    real BEDSHEAR[ND_ND]; 
 
    domain = Get_Domain(domain_ID); 
    t = Lookup_Thread(domain,zone_ID); 
 
    begin_f_loop(f, t) 
    { 
        F_AREA(A,f,t); 
       AREA = NV_MAG(A); 
        NV_V(BEDSHEAR,=,F_STORAGE_R_N3V(f,t, SV_WALL_SHEAR)); 
        c0 = F_C0(f,t); 
        t0 = THREAD_T0(t); 
        C_UDMI(c0,t0,0) += NV_MAG(BEDSHEAR)/area; 
        C_UDMI(c0,t0,1) += -BEDSHEAR[0]/area; 
        C_UDMI(c0,t0,2) += -BEDSHEAR[1]/area; 
        C_UDMI(c0,t0,3) += -BEDSHEAR[2]/area; 
        C_UDMI(c0,t0,4) = 0.5*(1.-sqrt(pow(C_UDMI(c0,t0,1), 2) + pow(C_UDMI(c0,t0,2),2) + pow(C_UDMI(c0,t0,3), 2))/C_UDMI(c0,t0,0)); 
    }
    end_f_loop(f,t) 
}
You can also find more info in the post by khuti from where I found pieces of the code above.

I hope it works for you too.

some more tips:
1. You may want to exclude the initial period(s) from the osi computation as the results will be affected by you initial conditions (u=v=q=p=0??).
2. Keep monitoring OSI/ RRT statistics (max, min, average) to make sure that your results are period independent.

Best regards,
Dimitrios
dfytanid 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
Wall Shear in User Fortran Allan CFX 0 March 26, 2008 06:29
Errors running allwmake in OpenFOAM141dev with WM_COMPILE_OPTION%3ddebug unoder OpenFOAM Installation 11 January 30, 2008 21:30
Calculating average wall shear? Joe CFX 3 March 17, 2007 10:42
Multicomponent fluid Andrea CFX 2 October 11, 2004 06:12
What is the detail definition of wall shear stress zjm FLUENT 0 January 2, 2002 08:43


All times are GMT -4. The time now is 08:27.