# Airflow Past an Automobile

Airflow past an automobile at 55 miles per hour is simulated. The finite
element mesh was generated with an
automatic mesh generation system developed at the AHPCRC by Andrew A. Johnson.
The model is quite detailed and contains wheels, recessed headlights, and
a spoiler. This tetrahedral element mesh contains 447,180 nodes and
2,801,488 elements. At each time step, a system of equations with
1,638,389 unknowns is solved using matrix-free iterations with
GMRES search technique.

The incompressible flow, with a modified Smagorinsky turbulence model, was
computed under two flow conditions. The first simulation modeled road
conditions.
Here, we applied the free stream velocity to both the inflow
boundary and the
road, and we forced the wheels to spin (a rotational flow field was imposed
on the tires). Zero velocity was imposed on the rest of the automobile.
This flow condition yielded a drag coefficient of 0.455.

The second simulation modeled wind tunnel conditions. Here, we applied the
free stream velocity on just the inflow boundary, and applied slip conditions
along the road. Zero velocity was imposed on both the tires and the
automobile. This flow condition yielded a drag coefficient of 0.354. The
actual drag coefficient of a Saturn SL2 automobile under wind tunnel
conditions is 0.343.

Shown in the first image is the pressure distribution on the automobile
(red being high pressure and blue being low pressure).

Shown in the next images are streamlines around the automobile
(the streamline colors represent the magnitude of the velocity).

The results here were computed on the Cray T3D for half of the domain
since the geometry is symmetric. This solution has also been computed
on the Thinking Machines CM-5 for the full domain, and on the
Silicon Graphics parallel ONYX for half of the domain.

## You can find more information about this simulation
here and also
here.