Extracting Drag Force
 

Hello all,

I am currently simulating 2 Ahmed bodies in 2D travelling in a platoon. My aim is to extract a drag force from each vehicle in the platoon and calculate the drag co-efficient and compare the results at different inter-vehicle distances and speeds.

I have calculated the drag co-efficient of a single Ahmed body to be 0.36 using the function calculator in Post to calculate the force in the X direction and then calculate the co-efficient myself.
When I have simulated 2 bodies in platoon I have been getting a negative force for the lead vehicle and and positive larger force for the trailing vehicle. I checked the pressure and velocity contours and I couldn't see anything wrong with them. I have attached some screenshots to show both contours along with the drag force the function calculator is giving me

If anyone could suggest a way to fix this or explain what I have done is wrong that would be extremely helpful