Question

ANSYS Fluent

I'm a beginner in the field. Now I am confused about some settings.
I'm doing a analysis job to compare the difference between a car with rear wing and the other one without rear wing.

First, I don't sure which car (the one with rear wing or the one without rear wing)should have a bigger Cd value, somebody says that cars with rear wings will increase drag as well as downforce, is that right ?

Second, I always get Cl values about -2.xxx, but the values seem to be too large. As I google some similar papers to check the results, their Cl values are about -0.2xxx.

So, plz help me to check if there are any mistakes settings that need to be corrected.Thanks!

There are my settings below: (I change these settings only, the others are default)

-I use 1:1 2D cars, created by SpaceClaim. I set up the cars with 4875mm 1780mm 1435mm, So should I change the unit in Ansys Workbench into mm ?

-viscous model: k-e

-inlet 30 m/s (velocity inlet)

-reference values:
Area : 2.09 m^2 (frontal area of the car)
Depth:1.58 m (car width)
length:1.78 m (car length)
velocity:30 m/s (should I give the velocity again here? I have given velocity inlet 30 m/s already)

after these I run 500 iterations

Answer 1

first of all
when rear wings are present, the value of drag increases,m and so does the downforce ( due to tht lift produced in downwards direction)
hence with the presence of rear wings, the Cd should increase as well

now, for calculating CL values
for the given data
V = 30 m/s
A = 2.09 m^2
d = 1.58 m
l = 1.78 m

now, these input values are completely fine but before fingding Cd , Cl values from fluent we have to define the mesh grid around this car
this grid needs to be atleast 5 times bigger than the car on all sides in order to remove edge effects ( except for the surface touching the tyres of the car which can be ket at the location o the tyres and considered to be the road)

hence for the mesh
the length is l = 10*1.78 + 1.78 = 19.58 m
width w = 11*1.58 = 17.38 m
height = h = 6*(A/d) = 6*2.09/1.58 = 7.936708860 m

the tyres of the car have to be touching th elower surface of the msh control volume
the six surface of this cuboid mesh surface will have the following boundary conditions

inlet surface, exit surface, v = 30 m/s
top surface anbd the side two surfaces = v = 30 m/s
bottom surface, v = 0 m/s ( No slip condition)

and
bno slip condition has to applied on the car surface as well
after this, the mesh quality should be checked to be above 95%
after this meshing one can find the value of Cl and Cd after running multiple iterations
useful meshes are tetragonal (regular) and tetragonal and trigonal (mixed)