URANS turbulence modelling : equations and definitions

aarohimudholkar · February 5, 2023, 10:22

Hi, I am trying to simulate a turbulent flow over a flat plate using URANS. I have chosen the kwSST with gamma ReTheta models.

Now, I am having some trouble processing the data. I need to calculate the RMS+ quantities v/s the y+ values at say 50% of the length of the plate. I want to know which values STARCCM+ gives, how it calculates them and if the formulae I am using for creating the field functions are ok:

1. how does starccm+ calculate tau_w (WallShearStress)? Because it uses WallShearStress = (0.5*rho*(U^2)*Cf) to calculate Cf (SkinFrictionCoeff)

2. for eddy viscosity models, -rho*(u'v')=mu_t*(Dpar(U,y)). from here, I can get u'v'_bar and it seems to work. But, how do I calculate the other reynolds stresses? u'u', v'v', w'w', u'w', v'w'?
where... u'v' = average quantity of fluctuating reynolds stress components ; mu_t = turbulent viscosity ; Dpar(U,y) = partial derivative of U wrt y, U = velocity in x direction ; rho = fluid density.

3. since URANS already produces time-averaged functions, for RMS+ do i have to do a RMS of these u'v' again? I mean, is it true that from the article i'm referring to: RMS+ = sqrt{<u'v'>} = u'v'(ovtained from URANS)

Any full or partial answers will be appreciated. Thanks in advance!

theindianlel · March 13, 2023, 07:18

For the first question, refer to "Theory>Wall Treatment>Wall Treatment for Flow and Energy" in the user guide.

To the second question, I honestly don't have a clear answer either and I am looking for a better answer. I believe you can get v'v', u'u', w'w' by creating a field variance (Monitor>New Monitor>Field Variance), but I am not sure if it works that way.

February 5, 2023, 10:22	URANS turbulence modelling : equations and definitions	#1
aarohimudholkar New Member Aarohi Join Date: Feb 2023 Posts: 2 Rep Power: 0	Hi, I am trying to simulate a turbulent flow over a flat plate using URANS. I have chosen the kwSST with gamma ReTheta models. Now, I am having some trouble processing the data. I need to calculate the RMS+ quantities v/s the y+ values at say 50% of the length of the plate. I want to know which values STARCCM+ gives, how it calculates them and if the formulae I am using for creating the field functions are ok: 1. how does starccm+ calculate tau_w (WallShearStress)? Because it uses WallShearStress = (0.5rho(U^2)Cf) to calculate Cf (SkinFrictionCoeff) 2. for eddy viscosity models, -rho(u'v')=mu_t*(Dpar(U,y)). from here, I can get u'v'_bar and it seems to work. But, how do I calculate the other reynolds stresses? u'u', v'v', w'w', u'w', v'w'? where... u'v' = average quantity of fluctuating reynolds stress components ; mu_t = turbulent viscosity ; Dpar(U,y) = partial derivative of U wrt y, U = velocity in x direction ; rho = fluid density. 3. since URANS already produces time-averaged functions, for RMS+ do i have to do a RMS of these u'v' again? I mean, is it true that from the article i'm referring to: RMS+ = sqrt{<u'v'>} = u'v'(ovtained from URANS) Any full or partial answers will be appreciated. Thanks in advance!

March 13, 2023, 07:18		#2
theindianlel New Member TheIndianLel Join Date: Mar 2019 Location: Germany Posts: 25 Rep Power: 7	For the first question, refer to "Theory>Wall Treatment>Wall Treatment for Flow and Energy" in the user guide. To the second question, I honestly don't have a clear answer either and I am looking for a better answer. I believe you can get v'v', u'u', w'w' by creating a field variance (Monitor>New Monitor>Field Variance), but I am not sure if it works that way.