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uplus=f(yplus)

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Old   February 8, 2005, 09:26
Default uplus=f(yplus)
  #1
anne
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Hello,

Does anyone has a UDF file to plot uplus as a function of yplus (calculated at every cells) ?

Regards, Anne.
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Old   February 8, 2005, 10:13
Default Re: uplus=f(yplus)
  #2
Andrew Garrard
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If the forumla is

uplus=f(yplus)

Can you not do this using a Custom Field Function?
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Old   February 8, 2005, 10:21
Default Re: uplus=f(yplus)
  #3
anne
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Hello,

By uplus = f (yplus)

I meant that I want to plot u+ in function of y+

I assume a UDF is a way to define the new variables u+ and y+ (at every mesh point)

and then plot them with the plot xy menu.

Is there an other way to proceed ?

Regards, Anne.
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Old   February 8, 2005, 11:08
Default Re: uplus=f(yplus)
  #4
Andrew Garrard
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You are right that a UDF can create new variables and plot them to every mesh point, but it is a fairly complicated procedure if you have never done it befor, which I am guessing you have not.

A custom filed function allows you to create a new variable in terms of the standard fluent vairables.

Y+ is a standard fluent variable.

So you can create a function of Y+ and give it any name you want, e.g.

my_Uplus = f(Y+)

If you still want to use a UDF, I can't find anything in the manual about extracting teh Y+ value directly from fluent, so you will have to create a macro that pulls out the friction velocity, Yp, density and viscosity terms manually.
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Old   February 8, 2005, 14:01
Default Re: uplus=f(yplus)
  #5
anne
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Yes I have already created UDFs but only for initialization or boundary conditions and not for post treatment.

The solution you propose is not the one I seek for because y+ is calculated in fluent only on the first mesh point, and I want it everywhere.

My problem is that I do not kown how the wall shear stress or the friction velocity is called (if the variable exists) in UDFs (so as x-component velocity defined as C_U). Could you help on that point ?

rho is C_R wall distance is C_WALL_DIST viscosity is C_MU

the friction velocity is calculated in fluent, so I assumed it has a name. do you know it ?

Regards, Anne.
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Old   February 9, 2005, 06:20
Default Re: uplus=f(yplus)
  #6
Andrew Garrard
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I am afraid I don't really know how to continue with this, I don't know about how the Y+ is caulated in fluent. I am sure that it is all in the manual, but you can look though that yourself.

I do have one question for you though. The value Yp is the distance from the wall. If you are plotting Y+ for all your cells, which walls are you going to select as the ones to calculate distance from for the cells in the central areas?
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Old   February 9, 2005, 06:47
Default Re: uplus=f(yplus)
  #7
anne
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Well regarding the postreament I will write an UDF though I am sure it has been done before.

Yes the problem of evaluating the wall distance is not trivial. however a choice is made in the code where wall functions or wall distances are used ion models. so I wanted to get the same variable in the udf in order to have the same information than the one used in the simulation. Yes it can be not physical, but is Fluent perfect ?
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