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1/7th power law

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Old   February 29, 2012, 17:01
Default 1/7th power law
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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.
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Old   February 29, 2012, 20:51
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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)
}
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Old   March 1, 2012, 01:38
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Hi,
uncomment the first line #include uds.h.
Take in mind that this udf will work for 2d axisimmetric problem.

Daniele
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Old   March 1, 2012, 14:09
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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
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Old   March 1, 2012, 20:03
Default Axial velocity with parabolic profile
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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.
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Old   March 2, 2012, 07:33
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Quote:
Originally Posted by Gamb1t View Post
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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