Numerical approach on drag reduction for automotive vehicles

Mateus Brizotti, Patrick Rambaud and Florian Visentin


TURBULENCE MODEL and WALL TREATMENT
Table of contents



Turbulence Model
 
    We used the SST K-Omega models to simulate the flows around the car. The K-Omega model is a two-equation model that is an alternative to the K-Epsilon model. The transport equations solved are for the turbulent kinetic energy k and a quantity called ω, which is defined as the dissipation rate per unit turbulent kinetic energy ( ω~ε/k).
The advantage of the K-Omega model over the K-Epsilon model is its improved performance for boundary layers under adverse pressure gradients. However, The biggest disadvantage of the K-Omega model, in its original form, is that boundary layer computations are very sensitive to the values of
ω in the free stream.
In an attempt to address this shortcoming, Menter introduced modifications to the K-Omega models and his model is called the SST (Shear-Stress Transport) K-Omega model. The SST model has seen fairly wide application in aerodynamics problems, where viscous flows are typically well resolved and turbulence models are generally applied throughout the boundary layer.


Wall Treatment


    Three types of wall treatment are provided with StarCCM+, although all three might not always be available because they depend on the turbulence model used:
  • The high-y+ wall treatment implies the wall-function-type approach in which it is assumed that the near-wall cell lies within the logarithmic region of the boundary layer (y+~30).
  • The low-y+ wall treatment is suitable only for low-Reynolds number turbulence models in which it is assumed that the viscous sublayer is properly resolved (y+=1 or lesser).
  • The all-y+ wall treatment is a hybrid treatment that attempts to emulate the high-y+ wall treatment for coarse meshes and the low-y+ wall treatment for fine meshes.

       We used the third wall treatment (ally+) for our simulation.


Unsteady Case

        For the non-stationnary simulation, we  used  the  2nd order implicit unsteady model and we set the time step as 1.0E-4 s.