Velocity fluctuation

Tasneem · June 9, 2017, 18:28

Hello,

Is there any way for me to find velocity fluctuation of u', v', and w' when I am using RANS? I am running a simulation of a pipe at very high Re.

I read other post about this and they suggested to get it from turbulent kinetic energy, k and assume flow as isotropic and u'= sqrt(2k/3). How about anisotropic? And I know that when we use RANS, it means we created another variables of Reynolds shear stress, Rij. But this variables contain a product of two velocity fluctuations. So I guess there is no way to u', v' and w' individually.

So my question, is it possible to get this value from Star CCM+ software? Or do I need to use LES instead? Can I do LES in Star CCM+?

Somebody help me please.

LuckyTran · June 12, 2017, 00:09

To get u', v', and w'. Just take the square root of the normal stresses, e.g. u' = sqrt(u'u'). However, u'u', v'v', and w'w' are much more meaningful quantities because they are the terms that show up in the momentum equation, i.e. the Reynolds stresses.

Star-CCM can do LES. But LES only solves for the resolved part of the Reynolds stresses. You still have to include the subgrid scale contribution and you would have exactly the same problem as you are having now with RANS. In both cases, you have to use Boussinesq hypothesis or the subgrid scale model itself to figure out the Reynolds stresses.

June 9, 2017, 18:28	Velocity fluctuation	#1
Tasneem New Member At-tasneem Join Date: May 2017 Posts: 3 Rep Power: 9	Hello, Is there any way for me to find velocity fluctuation of u', v', and w' when I am using RANS? I am running a simulation of a pipe at very high Re. I read other post about this and they suggested to get it from turbulent kinetic energy, k and assume flow as isotropic and u'= sqrt(2k/3). How about anisotropic? And I know that when we use RANS, it means we created another variables of Reynolds shear stress, Rij. But this variables contain a product of two velocity fluctuations. So I guess there is no way to u', v' and w' individually. So my question, is it possible to get this value from Star CCM+ software? Or do I need to use LES instead? Can I do LES in Star CCM+? Somebody help me please.

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June 12, 2017, 00:09		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,750 Rep Power: 66	To get u', v', and w'. Just take the square root of the normal stresses, e.g. u' = sqrt(u'u'). However, u'u', v'v', and w'w' are much more meaningful quantities because they are the terms that show up in the momentum equation, i.e. the Reynolds stresses. Star-CCM can do LES. But LES only solves for the resolved part of the Reynolds stresses. You still have to include the subgrid scale contribution and you would have exactly the same problem as you are having now with RANS. In both cases, you have to use Boussinesq hypothesis or the subgrid scale model itself to figure out the Reynolds stresses.