[minervac96]

Good morning,
I'm doing a CFD analysis of Tesla cybertruck in 2d,I have the student edition of Ansys fluent,I did the mesh and refined with face sizing on the back of the car where there it starts the wake and used edge sizing and inflation to refine the mesh near the car.The problem is that when i run the calcul on FLUENT i have convergence but i get a CD of 0,25 and CL of 1,5( really big i think).
I'm using k-omega with inlet velocity of 50 km/h,bottom inlet V=0 km/h ,pressure outlet, no slip conditions on the car walls and shear=0 on the top.
As intensity i put 1 % and viscosity ratio 0.1.My professor suggested me to use these conditions but now is not answering me and i'm in the panic.For refernce values i took are =heigth of the car 1,49 m and width=2m,length=5,8m
Thanks you

[vinerm]

In Reference Values, you also need to provide density and velocity as per the free-stream. Furthermore, use larger values for intensity and viscosity ratio. Use 5% and 5 or 10. 1 is a the smallest value for viscosity ratio. Value below this does not have a relevance.

[minervac96]

I tried with bigger values of turbolence as you told me but keep giving me the same results

[vinerm]

Turbulence value at the inlet will not have a drastic effect. More important are reference values for the coefficients. Do note that reference values do not affect the solution. So, you don't need to rerun the simulation. Just change reference values for density and velocity and then determine the coefficients.

[minervac96]

Originally Posted by vinerm

Turbulence value at the inlet will not have a drastic effect. More important are reference values for the coefficients. Do note that reference values do not affect the solution. So, you don't need to rerun the simulation. Just change reference values for density and velocity and then determine the coefficients.

I had already changed,density is 1.225 kg/m3 and velocity 13.89 m/s but nothing change

[vinerm]

Then you need to look at the setup. Are the boundary conditions correct? Is the mesh fine enough to resolve the boundary layer, since you are using ? And what do you mean by bottom inlet velocity?

[minervac96]

Originally Posted by vinerm

Then you need to look at the setup. Are the boundary conditions correct? Is the mesh fine enough to resolve the boundary layer, since you are using ? And what do you mean by bottom inlet velocity?

I sent you a picture attached.For the top i put wall and shear stress=0,the walls of the cars i put no slip

[vinerm]

Which boundary is bottom? Do you mean the ground? Ground must be no-slip wall.

[minervac96]

Originally Posted by vinerm

Which boundary is bottom? Do you mean the ground? Ground must be no-slip wall.

Yes is the ground,could you tell me if i have to change other things?And i have to set the ground as no slip like the walls of the car?

[vinerm]

Ideally, since the reference of frame changes from inertial to vehicle, the ground should also be given a velocity towards the rear of the vehicle. And it has to be a moving wall at 13.89 m/s. However, this won't have much effect. So, you can either use no-slip wall or moving wall at 13.89 m/s in the opposite direction of vehicle's motion. But not a velocity inlet.

[minervac96]

Originally Posted by vinerm

Ideally, since the reference of frame changes from inertial to vehicle, the ground should also be given a velocity towards the rear of the vehicle. And it has to be a moving wall at 13.89 m/s. However, this won't have much effect. So, you can either use no-slip wall or moving wall at 13.89 m/s in the opposite direction of vehicle's motion. But not a velocity inlet.

The Cd is of 0.45 and CL of -0.92,do you think is good?or CL is too big?Here attached some results

[vinerm]

I cannot comment on the values of drag and lift coefficient. Better to compare against some experimentally determined values for same or similar body.

[minervac96]

Originally Posted by vinerm

I cannot comment on the values of drag and lift coefficient. Better to compare against some experimentally determined values for same or similar body.

Is normal that CL is negative,because i knew that should be positive and small?Thanks you very much

[vinerm]

For vehicles, it must be negative, else vehicles will fly-off the road. However, the negative or positive does not really matter here. Lift coefficient is not even calculated for road vehicles, only drag coefficient is determined.

[minervac96]

Originally Posted by vinerm

For vehicles, it must be negative, else vehicles will fly-off the road. However, the negative or positive does not really matter here. Lift coefficient is not even calculated for road vehicles, only drag coefficient is determined.

Thanks you very much