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location of separation point

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Old   August 25, 2018, 08:44
Default location of separation point
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A. Min
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Hi foamers

I want to find the exact location of separation point in flow around cylinder.
According to fluid mechanics books, I should find the point (on the cylinder surface) in which shear stress is zero.(is it correct?)

I calculate the xy component of stress tensor as:

Code:
List<symmTensor> Tau00;
List<scalar> Tau0;

Tau00 = (tauS+tauP).boundaryField()[cylinder];

forAll(Tau00,i)
{
      Tau0[i] = Tau00[i].xy();
}
the tau_xy - teta diagram is:



While streamlines show that separation is happened in teta=150:



Could you please tell me what the problem is?
Thanks
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Old   August 26, 2018, 07:07
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Santiago Lopez Castano
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Not only shear, but when the pressure gradient along the surface inflects from adverse to positive...
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Old   August 26, 2018, 11:30
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Michael Alletto
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I think tau0.xy gives you the shear stress in the case of a flat plate which is parallel to the x-axis.


To get the shear stress in an arbitrarily defined surface in 3d the steps to perform are the following:


1) Project the shear stress tensor on the surface -> this gives you the vector of the force acting on the surface taus



2) Compute the dot product of taus with the vector pointing parallel to the surface -> this gives you the shear stress
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Old   August 27, 2018, 05:12
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Quote:
Originally Posted by mAlletto View Post
I think tau0.xy gives you the shear stress in the case of a flat plate which is parallel to the x-axis.


To get the shear stress in an arbitrarily defined surface in 3d the steps to perform are the following:


1) Project the shear stress tensor on the surface -> this gives you the vector of the force acting on the surface taus



2) Compute the dot product of taus with the vector pointing parallel to the surface -> this gives you the shear stress
Dear Michael

Thanks for your reply.

I rotated the stress tensor from xy to r-teta coordinates. I think this is correct, isn't it?
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Old   August 27, 2018, 09:36
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yes this should be correct
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