Car (AUDI) External Flow CFD Simulation, ANSYS Fluent Training

Project Description

This project is going to study an incompressible isothermal air flow adjacent to an Audi-A4-2017 car by ANSYS Fluent software. The geometry is a 2-meter F-35 plane inside a 100-meter wind tunnel. Also, the maximum speed of 77.76 m/s, which is quite common in urban use, is selected for the testing, and the drag force value is obtained.

Car Geometry & Mesh

The 3-D geometry of the model is designed using Design Modeler software. The present model includes a rectangular cubic computational domain with dimensions of 20 m * 10 m * 5 m for airflow, in which an AUDI vehicle is located. The following figure shows a view of the geometry.

The meshing of the model has been done using ANSYS Meshing software and the mesh type is unstructured. The element number is 4950697, and the meshing accuracy is higher in the vicinity of the vehicle surfaces. The following figure shows the mesh.

Car External Airflow CFD Simulation

To simulate the present model, several assumptions are considered:

• We perform a pressure-based solver.
• The simulation is steady.
• The gravity effect on the fluid is ignored.

A summary of the defining steps of the problem and its solution is given in the following table:

Models (car)
k-epsilon	Viscous model
Standard	k-epsilon model
Standard wall function	near wall treatment
Boundary conditions (car)
Velocity Inlet	Inlet
77.76 m.s-1	velocity magnitude
Pressure Outlet			Outlet
0 Pascal	gauge pressure
Wall	Wall of AUDI
stationary wall	wall motion
Solution Methods (car)
PISO			Pressure-velocity coupling
Second order	pressure		Spatial discretization
Second order upwind	momentum
First order upwind	turbulent kinetic energy
First order upwind	turbulent dissipation rate
Initialization (car)
Standard	Initialization method
0 Pa	gauge pressure
77.76 m.s-1	x-velocity
0 m.s-1	y-velocity , z-velocity

Boundary Condition

Results & Discussion

After the solution has been converged, the results could be observed through post-processing. Meanwhile, as an assurance of an excellent convergence, the drag value was monitored during the solution iterations. In this study, the solution was a converged one when the drag force reached a constant value, and the residuals were below 10^-5 values. As the results show, most drag forces are due to pressure forces, and the share of frictional forces in drag is low. The highest amount of pressure has occurred in the front and back of the car, which has occurred in these areas due to the minimum velocity contour. This and the velocity vectors indicate that there is flow separation at the front and rear of the vehicle. This separation is more concentrated in the front of the car and is wider in the back of the car, which can be attributed to the broader geometry in the back of the car.

The pressure on the surface of the car body is almost evenly distributed, but in areas with a greater angle to the direction of flow, this pressure is higher. The more perpendicular the surface is to the flow, the greater the pressure on it. Also, the pressure on the sharp edges, especially on the front wheel and the edge of the front window, is negative, and the speed in these areas has increased, which can be attributed to the rapid change in the angle of flow due to the change in fracture. The Shear force contours also show that the flow separation has started when these contours have changed sign.

Results & Discussion

Velocity and Static Pressure Contours and Streamlines are drawn in the figure below:

The drag force obtained in this problem is 1684.74 Newtons. The convergence diagram of the drag force is shown in this Figure:

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.