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 Rangarajan August 22, 2005 08:12

screens in wind tunnel

Hi every body,

I am studying on Climatic wind tunnel where i have to design a duct.In this outlet velocity at nozzle end is to be uniform velocity .I have to implement in screens on the duct. At present its 2d base analysis. For screens, an porous jump to be implemented.In 2d ,whether porous jump is to be mention as line or face ( width = thickness)? and also how to calculate the pressure drop calculation for screens.

Regards Ranga

 razvan August 22, 2005 13:55

Re: screens in wind tunnel

1) Porous jump in 2D is always a LINE and in 3d is a FACE.

2) If you would have been more curious you would have found in Fluent's Documentation a fast, approximate method for calculating the C0 and C1 coefficients you need to input for porous jump boundary condition. The 1st is the inertial resistance of the screen and the 2nd is the viscous resistance. All nice and easy. The only problem is that the method is highly inaccurate (I tried to use it a short time ago and for my soul's peace verified the results using another approach: not even close!!).

So how can you do it? Simple: take a SINGLE hole of your screen and using symmetries all around, construct a test flow domain with a proper mesh (it is better if you can obtain y+=1, results will be more accurate) and simulate flow conditions for several velocities beyond minimum and maximum you expect to obtain for the wind tunnel imposing constant static pressure (101325 Pa) downstream and a velocity inlet upstream (if you can consider incompressible flow) or a pressure inlet (for compressible flow). Then calculate the total pressure drop for each case and plot vs. inlet velocity. You shold obtain a parabolic curve. For this curve, construct a interpolation polynome of this form:

a*vel^2+b*vel=0

a=C0, b=C1. Now there is another problem: as you will notice for the example given in the documenation, C1 coefficient is NEGATIVE!!!, which means the viscous contribution for total resistance of their porous media is NEGATIVE , which is obviously nonsense! I'm not sure they even noticed this, but one thing is true: you definitely cannot input a negative C1 in Fluent, this will always return error! This problem could appear in your case too, but don't be worried, increase the value of C0 just enough to cover C1's missing.

Now you will ask: can I trust this method? I compared the results obtained with experimental data and the agreement was very good.

Tip: if can mesh it with y+=1, use one of the low Reynolds models, not the Enhanced Wall Treatment!

And also don't worry about the time, I did all this in two days, for 4 different types of screens.

Best wishes, Razvan

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