Wallshear stress sngrad() implementation

Mathia1510 · December 8, 2017, 13:27

Hello, I am fairly new in the world of OpenFoam and CFD. I am trying to calculate wallshear stress on a cylinder surface (laminar flow) and this thread: Compute shear stress interFoam told me to use the formula: "wallShear = nu * rho * mesh.boundaryField()[patchID].snGrad()", since now I am only using the standard openFoam utility wallshear stress and it does not give adequate results.

So my question is how do you implement/run "mesh.boundaryField()[patchID].snGrad()" ?? Is this something you have to add in the wallshearstress source code? I am a bit confused if you are supposed to add it in a code or run it somehow. Not really programmed in C before, only matlab.

Help is much appreciated!

piu58 · December 9, 2017, 01:11

An easy and understandable way is using the result in velocity and pressure radial of the cylinder. You may use the sample tool and put in the sampleDict all the radial lines you need.

From this output it is easy to calculate the wall shear stress.

Mathia1510 · December 9, 2017, 13:43

Thank you for your reply! I am not really sure how this is supposed to be done? As wall shear stress involves the derivative, and my cylinder has 300 points which means I have to make 300 lines in my sample dict, which seems quite tedious. My end goal is to get the tangential wall shear stress

piu58 · December 9, 2017, 14:20

> make 300 lines in my sample dict, which seems quite tedious

Would be not a problem for me. I would write a small script which generates them.

December 8, 2017, 13:27	Wallshear stress sngrad() implementation	#1
Mathia1510 New Member Mathia Join Date: Dec 2017 Posts: 7 Rep Power: 8	Hello, I am fairly new in the world of OpenFoam and CFD. I am trying to calculate wallshear stress on a cylinder surface (laminar flow) and this thread: Compute shear stress interFoam told me to use the formula: "wallShear = nu * rho * mesh.boundaryField()[patchID].snGrad()", since now I am only using the standard openFoam utility wallshear stress and it does not give adequate results. So my question is how do you implement/run "mesh.boundaryField()[patchID].snGrad()" ?? Is this something you have to add in the wallshearstress source code? I am a bit confused if you are supposed to add it in a code or run it somehow. Not really programmed in C before, only matlab. Help is much appreciated!

December 9, 2017, 01:11		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	An easy and understandable way is using the result in velocity and pressure radial of the cylinder. You may use the sample tool and put in the sampleDict all the radial lines you need. From this output it is easy to calculate the wall shear stress. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

December 9, 2017, 14:20		#4
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	> make 300 lines in my sample dict, which seems quite tedious Would be not a problem for me. I would write a small script which generates them. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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December 9, 2017, 13:43		#3
Mathia1510 New Member Mathia Join Date: Dec 2017 Posts: 7 Rep Power: 8	Thank you for your reply! I am not really sure how this is supposed to be done? As wall shear stress involves the derivative, and my cylinder has 300 points which means I have to make 300 lines in my sample dict, which seems quite tedious. My end goal is to get the tangential wall shear stress