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(Help) CD error simulating laminar steadystate flow over a sphere [OpenFOAM] 

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June 13, 2017, 10:36 
(Help) CD error simulating laminar steadystate flow over a sphere [OpenFOAM]

#1 
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Join Date: May 2017
Posts: 47
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Hello,
I’m having some trouble simulating laminar steadystate flow over a sphere (OpenFOAM). The solution is converged but I’m having about 15% error on the drag coefficient (CD) for 10 < Re < 100, and about 20% for Re <= 1. Please, see some pictures of my simulation in this website http://imgur.com/a/AxIWE.
Does anyone have any clue what’s wrong, please? Thanks in advance. 

June 13, 2017, 11:16 

#2 
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Uwe Pilz
Join Date: Feb 2017
Location: Leipzig, Germany
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Calculating pressures is one of the harder task in CFD, harder than calculation velocities. The reason: The pressure is calculated in an indirect way (by correction velocities.
The case you simulated is dome by others too. You may compare their results with the experimental findings of Schlichting: Deviations are more normal than absent.
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Uwe Pilz  Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) 

June 13, 2017, 13:45 

#3 
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Filippo Maria Denaro
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You could check your solution compared to the analytical solution for the flow around a sphere at low Re number...
How do you compute the stress at the wall? 

June 14, 2017, 09:32 

#4 
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There's a code that you write in the controlDict file that calculates the viscous ("integral of the shear stress on the area") and pressure drag.


June 21, 2017, 08:39 

#5  
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Quote:
u.jpgw.jpg Observations:
Any idea of what could be wrong in the simulation? Thanks 

June 30, 2017, 09:31 

#6 
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Join Date: May 2017
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I found the problem, I was using a first order interpolation scheme for the advection term (Gauss upwind). So I did some research in the OpenFOAM User Guide and chose the Gauss linearUpwind, which is a second order scheme. And that was the problem indeed! After I did that I'm having a 1,3%error for 20 < Re < 200. Unfortunately, for very low Reynolds numbers (Re = 1 and 0.5) the results are still the same.
p+StreamRe80.jpg 

Tags 
drag, laminar, openfoam, sphere, steady 
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