Numerical approach
on drag reduction for automotive vehicles

Mateus
Brizotti, Patrick Rambaud and Florian Visentin

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.

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.

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