udf

hamid1 · December 14, 2011, 08:38

Hi all

about UDF, when do need to use C_P(c,t) and when F_P(c,t), also the other parameters like U_P(c,t) and F_P(c,t) and so on....

dmoroian · December 16, 2011, 07:42

C_P(t,c) gives the pressure value in the centroid of the cell.
F_P(t,f) gives the pressure value in the centroid of the face.
I would recommend to read the manual regarding these macros. The face values are not available in the entire domain for all variables!

hamid1 · December 16, 2011, 08:19

thanks for the reply, iv actually read already but get confused a bit,
im wondering if the following is correct for calculating pressure at outlet:

DEFINE_EXECUTE_AT_END(network_calculation)
{

FILE *flow_rate,*update,*velocity;

Domain *d;

real force=0.;
real NV_VEC(A);
face_t f;
Thread *t;
d = Get_Domain(1);
t = Lookup_Thread(d,2);

PF = 0.0;
area = 0.0;
force = 0.0;

begin_f_loop(f,t)
{
F_AREA (A,f,t);
force+=F_P(f,t)*NV_MAG(A);
area +=NV_MAG(A);
PF= force/area;
}
end_f_loop(f,t)
}

thank you

dmoroian · December 16, 2011, 09:07

It looks ok if the outlet zone id is 2.

hamid1 · December 17, 2011, 04:37

thanks for reply,
but the point is, for example when i measure the area in solid work it is a bit different from the area+=NV_MAG(A) computed through this code in fluent,
that is why it makes me feel doubt about how correct the code i use is..
please let me know
thanks so much again...

dmoroian · December 17, 2011, 09:42

That's easy: Solid Works or any other CAD software keeps the surface as a parametric object (meaning there is an equation describing the surface), whereas fluent keeps the same surface as a triangular surface (meaning a discrete form). The discrete form will always include a discretization error based on the size of the discretization, so there is no surprise that you get a difference. This difference, between the CAD and CFD software should decrease if you have a finer mesh.

December 14, 2011, 08:38	udf	#1
hamid1 Senior Member hamid Join Date: Nov 2010 Posts: 185 Rep Power: 15	Hi all about UDF, when do need to use C_P(c,t) and when F_P(c,t), also the other parameters like U_P(c,t) and F_P(c,t) and so on....

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December 16, 2011, 07:42		#2
dmoroian Senior Member Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 20	C_P(t,c) gives the pressure value in the centroid of the cell. F_P(t,f) gives the pressure value in the centroid of the face. I would recommend to read the manual regarding these macros. The face values are not available in the entire domain for all variables!

December 16, 2011, 08:19		#3
hamid1 Senior Member hamid Join Date: Nov 2010 Posts: 185 Rep Power: 15	thanks for the reply, iv actually read already but get confused a bit, im wondering if the following is correct for calculating pressure at outlet: DEFINE_EXECUTE_AT_END(network_calculation) { FILE flow_rate,update,velocity; Domain d; real force=0.; real NV_VEC(A); face_t f; Thread t; d = Get_Domain(1); t = Lookup_Thread(d,2); PF = 0.0; area = 0.0; force = 0.0; begin_f_loop(f,t) { F_AREA (A,f,t); force+=F_P(f,t)NV_MAG(A); area +=NV_MAG(A); PF= force/area; } end_f_loop(f,t) } thank you

December 16, 2011, 09:07		#4
dmoroian Senior Member Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 20	It looks ok if the outlet zone id is 2.

December 17, 2011, 04:37		#5
hamid1 Senior Member hamid Join Date: Nov 2010 Posts: 185 Rep Power: 15	thanks for reply, but the point is, for example when i measure the area in solid work it is a bit different from the area+=NV_MAG(A) computed through this code in fluent, that is why it makes me feel doubt about how correct the code i use is.. please let me know thanks so much again...

December 17, 2011, 09:42		#6
dmoroian Senior Member Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 20	That's easy: Solid Works or any other CAD software keeps the surface as a parametric object (meaning there is an equation describing the surface), whereas fluent keeps the same surface as a triangular surface (meaning a discrete form). The discrete form will always include a discretization error based on the size of the discretization, so there is no surprise that you get a difference. This difference, between the CAD and CFD software should decrease if you have a finer mesh.