[General] Paraview-Coefficient of Drag Calculation, Frontal area

cfdnoob1210 · October 25, 2022, 01:23

CFD noob here

I am trying to calculate coefficient of drag in Paraview (Ran simulation in Openfoam with Simple Foam solver)

I know the steps
1) Extract surface 2) Generate Surface normal 3) Calculate : p*Normal 4) Integrate it to get the Force.

Then to get the Coeff of Drag(Cd) use Cd=(Drag Force)/(0.5*rho*A*V^2)

My concerns are
A) The Force calculated by above formula is in N or some other units because pressure is in m2/s2.

B) Should i use just the last time step to calculate the result or some other time steps before that and average it out.

C) If there are 2 vehicle (Car-Trailer) will the steps for calculating Cd for the Car-Trailer combination be same as the above mentioned(especially projected area of combination).

Marpole · January 12, 2023, 15:36

In simpleFoam, the pressure is kinematic pressure, i.e. $p_{CFD}=p_{static}/\rho$ . The CFD pressure unit is

.

When you calculate drag coefficient, it then should be $C_d = \int p_{CFD}*N/(0.2V^2A)$ , where density is not needed as it is already included.

You should then monitor the Cd over iterations and use the one when it reaches steady-state but not at other time.

October 25, 2022, 01:23	Paraview-Coefficient of Drag Calculation, Frontal area	#1
cfdnoob1210 New Member ron kenneth Join Date: Oct 2022 Posts: 3 Rep Power: 4	CFD noob here I am trying to calculate coefficient of drag in Paraview (Ran simulation in Openfoam with Simple Foam solver) I know the steps 1) Extract surface 2) Generate Surface normal 3) Calculate : pNormal 4) Integrate it to get the Force. Then to get the Coeff of Drag(Cd) use Cd=(Drag Force)/(0.5rhoAV^2) My concerns are A) The Force calculated by above formula is in N or some other units because pressure is in m2/s2. B) Should i use just the last time step to calculate the result or some other time steps before that and average it out. C) If there are 2 vehicle (Car-Trailer) will the steps for calculating Cd for the Car-Trailer combination be same as the above mentioned(especially projected area of combination).

January 12, 2023, 15:36		#2
Marpole Senior Member Charles Join Date: Aug 2016 Location: Vancouver, Canada Posts: 151 Rep Power: 10	In simpleFoam, the pressure is kinematic pressure, i.e. $p_{CFD}=p_{static}/\rho$ . The CFD pressure unit is . When you calculate drag coefficient, it then should be $C_d = \int p_{CFD}*N/(0.2V^2A)$ , where density is not needed as it is already included. You should then monitor the Cd over iterations and use the one when it reaches steady-state but not at other time. __________________ Charles L.

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