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Calculating pressure coefficient on surface of an object in ABL simulation 

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July 3, 2019, 23:39 
Calculating pressure coefficient on surface of an object in ABL simulation

#1  
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Faiaz Khaled
Join Date: Oct 2017
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Hello Everyone,
I would like to thank all the experts who leave comments and suggestions on CFD problems on this forum. I am grateful to them. I am trying to reproduce atmospheric boundary layer flow using openfoam and to measure surface pressure coefficients on a simple building placed in the domain. I am using Komega turbulence model since this model is more useful for simple algorithm. I tried to follow the guidelines regarding yplus for Komega model. I want to calculate the pressure coefficient on the surfaces of the building placed on the ground in the computational domain. I will briefly write out the boundary conditions below: Code:
p inlet { type zeroGradient; } outlet { type fixedValue; value uniform 0; } Code:
Quote:
Now, my specific question is, while calculating pressure coefficient of the building surface, do I need to subtract the p recorded on the side near the wall of the domain? I am little confused on this part, as I am trying to match with experimental data. When I subtract the side p value at reference height from the building p value, there is little gap from experimental value. Otherwise, if I only use the surface recorded p (without subtracting the side p value) then I have better match with experimental data. Just to make it clear, to obtain pressure coefficient from 'p' (obtained from probe or from paraview), should I just multiply it by 2 as my reference velocity is 1 m/s? Cp=(pp_ref)/(0.5*rho*U^2) Here, U is 1 m/s. rho is already considered under 'p'. Considering the case I described please let me know, the way I am trying to process pressure coefficient is correct or wrong? Please shed some light on how to obtain the p_ref from the domain I described. Thanks Regards, Faiaz 

July 4, 2019, 11:14 

#2 
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Kmeti Rao
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Hi Faiaz,
As per my knowledge in an incompressible simulation, the pressure is defined as gradient. Therefore, the output pressure is the differential pressure and the reference pressure p_ref can be set to zero. Thus you can directly select the output value of surface pressure given by paraview. As you have already described, don't forget to multiply by density. Krao 

July 5, 2019, 13:58 
How to set or control the reference pressure

#3  
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Faiaz Khaled
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Hello Krao,
thank you for your quick reply. Quote:
I have been reading on different threads regarding the topic. But, the question is how to set or control the reference pressure in OpenFOAM simple algorithm? I have 0 pvalue on the outlet and also collecting values on building surface in terms of pressure/rho. So, do you think I need to subtract anything from the reading on building surface? Please let me know. If I need to subtract which value should I consider? So far, I placed two probes near the walls of the domain at roof height of the building and subtracting average of those p/rho from the readings on the building surface. I dont know if I am confusing myself too much. Best Regards, Faiaz 

July 8, 2019, 06:44 

#4 
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Kmeti Rao
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Hi Faiaz,
I also use zero at the outlet for my propeller simulation. I am not sure about your problem, but in my case I select the surface pressure on the propeller for further calculations. I usually extract surface pressure directly using paraview and multiply it with density. Regards 

July 18, 2019, 13:54 

#5  
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Faiaz Khaled
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Quote:
Thank you for your reply, Krao. I Really appreciate it. .. Seems like you and I are processing the pressure data in the same way.. I was wondering if somebody else has a similar experience on extracting pressure from paraview in openfoam with incompressible case. Just wanted to be sure.. if I have 0 pvalue on the outlet then do I need to to subtract any value from p reading on an object placed in the computational domain. Thanks Regards, Faiaz 

July 19, 2019, 05:18 

#6 
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Hrvoje Jasak
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Hi,
Don't do it like that  please use the forceCoeffs function object, which will use the native data and FVM operators, (eg for the viscous stress) within FOAM. Good luck, Hrv
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July 19, 2019, 12:46 

#7  
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Faiaz Khaled
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Quote:
I read a little about the forceCoeffs function. Seems like people use it to obtain the drag, lift and moment coefficient etc. I can dive more into it to find how to obtain pressure coefficient with that. If I use this function, what is the impact of having the outlet P boundary condition to be 0. But, my another question, was the approach mentioned in the earlier post right or wrong? I am also using probes to collect p values for specific locations of interest. So, the pvalue I am getting from the probes at the end of the converged simulation, is it the final (pressure/rho) value to calculate the pressure coefficient or I need to add/subtract something from that p value? Probably, a more generic question would be if I have a pvalue of 0 at the outlet, then can I consider my reference pressure to be zero and process any pressure in the domain without adding or subtracting anything from it? Thanks Please let me know. Regards, Faiaz 

July 25, 2019, 20:07 

#8  
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Bruno Santos
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Quick answers:
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... although it's still without the hydrostatic pressure, but you could calculate that field too and add it to the expression, to get the total pressure.
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