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Old   January 11, 2006, 02:32
Default pressure drop
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Hi I am getting very low pressure drop in flow analysis of duct which is not possible in real case. can anybody tell what could be the reason. Thanking you

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Old   January 11, 2006, 02:52
Default Re: pressure drop
Hubert Janocha
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total pressure drop or static pressure drop ?

in case of total pressure drop you have to take into considerations the in and outflow area. (see Bernouli-Equation)

in case of static pressure drop, possibly you have much too small y+ values when using a k-eps-turb. model.

Possibly your case (suppose steady state) has not fully converged. Check the input and output mass flow (Continuum equation)

In General have to give much more information about your calculation. Otherwise nobody will answer to your questions.

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Old   January 11, 2006, 10:25
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Very true, it could be any number of things, from mesh, to boundary conditions to specifying the wrong fluid properties!

its a bit like saying, "I have a car, it's a BMW, its not running, why is this?!"
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Old   January 13, 2006, 07:49
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Sir The problem is 3d,steady, turbulent with porous media. Air is the fluid medium. I have given the cell length 3.5. solution is getting converged. But when i tried with y+ switch on, solution getting diverged. Thanking you

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Old   January 15, 2006, 01:59
Default Re: pressure drop
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First: I would like to invite all the participants in this forum to STOP using the term "Static pressure" from any discussion related to Fluid Dynamics, Static pressure is developed in non moving fluids. For the exact meaning of this ugly term, all are invited to consult the reference book written by George Batchelor "An introduction to Fluid Dynamics" and to save your time, go directly to page 14 Equation 1.3.11 and the following comment. The correct terminology is Equilibrium Pressure or Thermodynamic Pressure whichever you prefer.

Second: Grab a pencil, draw a 2D duct of the shape you like (Just for simplicity), draw an integral element(from wall to wall), take a force balance and after a simple mathematical manipulation you will find that the pressure drop along that element is a function of the shear stress at the boundaries Now how the wall shear stress is calculated, all CFD programmes that I am familiar with use the wall functions to do this calculation, in plain language, if your Y+ values are out of bounds, then your calculated pressure drop is not the correct one

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