[Karthik98]

Hello everyone,
I’m studying the wall roughness effects on a wall using LES turbulence model.
In Ansys Fluent 2024 R1, 3 new method is introduced for LES viscous model to change the effect of wall roughness and they are:
Kader Blending Wall Functions
Harmonic Blending Wall Functions Based on r+
Werner and Wengle Wall Treatment
Out of these 3 methods, we can give physical roughness only for Harmonic Blending Wall Functions Based on r+
For wall roughness effects in turbulent wall-bounded flows, we have three distinct roughness regimes based on a nondimensional roughness height K_s^+.
Hydrodynamically smooth when (K_s^+≤2.25)
Transitional when ( 2.25〖<K〗_s^+≤2.25)
Fully rough when (K_s^+>90)
Here to find out K_s^+, we have the formula:
K_s^+=ρK_s u^*/μ
Where,
ρ= Density of the fluid
K_s= Physical wall roughness height
μ= Viscosity of the fluid
u^*=C_μ^(1/4) k^(1/2)
C_μ value is given a default value in k-ϵ viscous model. Whereas it’s not mentioned in LES turbulence model.
My query is how to obtain the u^* value from Ansys Fluent, so that I can identify the distinct roughness regime by substituting the physical wall roughness height and other parameters

[LuckyTran]

The difference between u* and u+ is the factor of C_mu^(1/4)

[Karthik98]

Thank you Lucky for your response,
I am still not having good clarity because from the web resources I found that the Friction velocity term u* does not depend on u+ in LES turbulence model.
This relation goes well with k-ϵ and in k-ω turbulence models and c_μ is defined in those models.
I also found the regular formula to find the u* in terms of shear stress and density.
u^*= √(τ_w/ρ)
Where,
τ_w= Wall shear stress
ρ= Density
And similarly for u+
u^+=u/u^*
Where,
u= Velocity magnitude

Can I use the above equation to find the k_s^+ , and to identify the roughness regimes?
Is this the correct approach for the LES model?