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Volume average of a scalar?

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Old   January 4, 2011, 13:26
Default Volume average of a scalar?
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Carl
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Hello,

Can anybody help me with this please? I have a flow field with one volume and I want to calculate the volume averaged scalar concentration in a sub-volume.

It is easy to do for the whole volume (report->volume integrals) but how do you create a sub-volume not part of the mesh file? You can only create points, lines and surfaces in the Fluent GUI, any tricks for creating a volume?

I have released the scalar from a sub-volume using a UDF but I'm having difficulty calculating the scalar concentration in other sub-volumes. I do have other models with the volumes decomposed already but its not very efficient if you need to know a volume average somewhere else.

Any help is much appreciated.

Carlos.
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Old   January 21, 2011, 14:18
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Nikolopoulos Aristeidis
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There is not a simple solution to your problem.

However I do this:
- Define a User defined memory (UDM, under fluent-> define->user-defined-> Memory).
- Define as a user Field function the variable you want to average
- Patch UDM to zero for the whom volume
- Define your sub-volume in the Fluent-> Adapt-> Iso-Value or Region or Volume
- Patch the User Field function to the UDM for Registers to Patch -> (Name of volume)

Now your UDM stores the variable inside the defined volume and Zeros in the other places.

Now a Sum in the whole volume will give you the SUM in the sub-volume.
With a few calculations you can get average values.


I hope this helped.
The other way is to do it via UDF
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Old   January 25, 2011, 16:25
Default Sub Volumes
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Bill Wangard
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If your subvolume is a hex, sphere, or cylinder, you can use the adaption tool to create registers to mark the cells of your subvolume.

This is done under the Adapt->Region menu. Combine registers of spheres and cylinders to get complex shapes --- if necessary.

Once you define your registers, do NOT adapt them. Instead, go to the Grid->Separate->Cells menu.

Use the adaption register to separate the volume using the adaption register. This will split off the cells and create the sub-volume.

You can simply go to your Report->Volume integrals menu now and integrate your quantity on the newly created sub volume.

You can merge the volumes back together if necessary.

Regards,
Bill Wangard, Ph.D.
President
Engrana LLC
847-424-2486
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Old   January 26, 2011, 02:30
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Nikolopoulos Aristeidis
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Nice billwangard!!!!!

I didn't know that!
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Old   September 3, 2018, 13:50
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Lisa
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Hi Nikolopoulos,
Thank you for the description. Like Carlos, I am also looking into dividing a solid domain into multiple sub-volumes (between say radius 1m to 2m and 1m<z<2m) in order to get volume average temperatures. Can this be done through a UDF? I found this UDF, however, how can you help me limit the domian to a radii limits and z limits.
/************************************************** ********************
UDF to calculate temperature field function and store in
user-defined memory. Also print min, max, avg temperatures.
************************************************** *********************/
#include "udf.h"
DEFINE_ON_DEMAND(on_demand_calc)
{
Domain *d; /* declare domain pointer since it is not passed as an
argument to the DEFINE macro */
real tavg = 0.;
real tmax = 0.;
real tmin = 0.;
real temp,volume,vol_tot;
Thread *t;
cell_t c;
d = Get_Domain(1); /* Get the domain using ANSYS FLUENT utility */
/* Loop over all cell threads in the domain */
thread_loop_c(t,d)
{
/* Compute max, min, volume-averaged temperature */
/* Loop over all cells */
begin_c_loop(c,t)
{
volume = C_VOLUME(c,t); /* get cell volume */
temp = C_T(c,t); /* get cell temperature */
if (temp < tmin || tmin == 0.) tmin = temp;
if (temp > tmax || tmax == 0.) tmax = temp;
vol_tot += volume;
tavg += temp*volume;
}
end_c_loop(c,t)
tavg /= vol_tot;
printf("\n Tmin = %g Tmax = %g Tavg = %g\n",tmin,tmax,tavg);
/* Compute temperature function and store in user-defined memory*/
/*(location index 0) */
begin_c_loop(c,t)
{s
temp = C_T(c,t);
C_UDMI(c,t,0) = (temp-tmin)/(tmax-tmin);
}
end_c_loop(c,t)
}
}SS
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