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-   -   1/7th power law (https://www.cfd-online.com/Forums/fluent/97977-1-7th-power-law.html)

 Gamb1t February 29, 2012 17:01

1/7th power law

I am trying to work out static pressure distribution across an orifice. i have plotted it and noticed that since the upstream pipe length is not long enough the flow is not becoming fully developed(turbulent).

I looked at how it was meant to be modelled and it says inlet boundary condition with 1/7th power-law was used. i know this has something to do with User defined functions which i am unfamiliar with. Do i have to do some programming or something? or is it a setting within fluent.
thank you for any help.

 Gamb1t February 29, 2012 20:51

I have found a 1/7th power law but when i interpret i get the following error: line 9: parse error.
I think it means the line DEFINE_PROFILE(x_velocity,t,i) but i dont know why that is since i copied and pasted it from the UDF manual. i have changed the value for YMAX and UMEAN. Any help would be gladly appreciated. Thank you.

/*#include "udf.h"*/

#define YMIN 0.0 /* constants */
#define YMAX 0.05
#define UMEAN 20.0
#define B 1./7.
#define DELOVRH 0.5

DEFINE_PROFILE(x_velocity,t,i)
{
real y, del, h, x[ND_ND], ufree; /* variable declarations */
face_t f;

h = YMAX - YMIN;
del = DELOVRH*h;
ufree = UMEAN*(B+1.);

begin_f_loop(f,t)
{
F_CENTROID(x,f,t);
y = x[1];

if (y <= del)
F_PROFILE(f,t,i) = ufree*pow(y/del,B);
else
F_PROFILE(f,t,i) = ufree*pow((h-y)/del,B);
}
end_f_loop(f,t)
}

 ghost82 March 1, 2012 01:38

Hi,
uncomment the first line #include uds.h.
Take in mind that this udf will work for 2d axisimmetric problem.

Daniele

 Gamb1t March 1, 2012 14:09

Thanks for the reply Danielle. It worked fine. Since the model is axisymetric, does it mean YMAX is the actual diameter or half of it as it is modelled.
Thank you

 Gamb1t March 1, 2012 20:03

Axial velocity with parabolic profile

Is there a udf that i can use to specify an inlet boundary condition for laminar flow that will give axial velocity with parabolic profile.
Thanks.

 ghost82 March 2, 2012 07:33

Quote:
 Originally Posted by Gamb1t (Post 347141) Thanks for the reply Danielle. It worked fine. Since the model is axisymetric, does it mean YMAX is the actual diameter or half of it as it is modelled. Thank you
Hi, YMAX refers to your axisymmetric grid so it's half diameter.

Daniele

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