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Problem in compiling property(density) UDF

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Old   October 31, 2008, 05:18
Default Problem in compiling property(density) UDF
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Hi Fluent Users,

I have great difficulty in figuring out my problem.

i had written a property udf for density which follows isentropic law:

p/rho^gamma= constant; actually my problem is to predict compressibility effects in incompressible flow through obstruction type flow meters.

so the compressible fluid in incompressible flow follows a isentropic law and it's udf is:

#include "udf.h" DEFINE_PROPERTY(cell_density,cell,thread)


real density; real pressure = C_P(cell,thread); density = pow((pressure/80756.35),1.0/1.4); return density;


value 80756.35 is equal to initial pressure/initial denisty^1.4

101325/1.176^1.4=80756.35 during the flow from inlet to outlet density has to vary with respect to pressure variation following isentropic law

libudf was successfully built

loaded successfully

when i am trying to initialise the solution from inlet

it is showing fatal error access voilation

please help me anybody my project thesis is depends on this problem.

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Old   October 31, 2008, 06:23
Default Re: Problem in compiling property(density) UDF
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What is the value for pressure you are initializing? If it is zero your udf will return a density of zero as well which might be the reason for the error.

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Old   October 31, 2008, 15:25
Default Re: Problem in compiling property(density) UDF
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Try this and let me know if it helps. while initializing the solution, do not select inlet from the drop-down list, instead type in all the quantities for inlet velocity, temp.... etc. for initializing the solution and then initialize.
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Old   October 31, 2008, 16:06
Default Re: Problem in compiling property(density) UDF
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Hi Rajesh,

Thank you for the suggestion and problem is that i know only gauge initial pressure to initialise the solution

one of my friend suggested me that

I should for all zones in the drop down box or simple have to click on the init button

otherwise trying to solve with constant density for 10^-3 convergence and then compiling density udf for further solution by which it will have sufficient pressure field to calculate per cell basis density from available pressure field

Please suggest me if u understand anything

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