Calculating forces on a non-closed surface
 

Hello everyone,

I'm having a bit of trouble determining the correct way of calculating the net force on one surface of an object with a flow field on only one side of the surface (i.e. I'm calculating the net force on a non-closed surface that only has pressure data for one side).

I am modeling the external aerodynamics of a vehicle in an incompressible airflow at 22m/s. The upper and lower surfaces of the vehicle in the real world are separate from each other (obviously in FLUENT the car body is a closed surface) and thus I need to determine the net force on the upper surface of the car so that the design requirements of the latches holding the upper surface can be defined.

The problem I run into is this: For FLUENT to calculate the correct force on a non-closed surface that does not have a flow field on one side, it needs a correct reference pressure that it will use for the side of the surface that the flow is not modeled for (i.e. the inside of the car). I have tried using the volume averaged total pressure of the flow volume, but using this value gives me a lift force on the top half of the car of 3000N! Let me first say that this car has already been built and run at up to 36m/s and the top half has yet to come off, and I'm completely sure the current latches are not capable of restraining 3kN, let alone whatever force it would be at 36m/s. So clearly this is wrong.

I've also tried using an area-averaged value of the total pressure on the car surfaces (which is actually calculated using FLUENTs area-weighted average of static pressure on the car surface - I confirmed this by writing a udf to calculate the total pressure in the near wall cells on the surface (Ptot=Pstat+1/2*density*v^2), and it's results matched flawlessly with the FLUENTs static pressure on a surface...so does anyone know what FLUENTs Total pressure on a surface is calculating?), but the result it gives me on the lower half of the car is that it is actually producing thrust, not drag, so that obviously can't be correct either.

So, does anyone have suggestions for what reference pressure to use or know of another way to calculate accurate net forces on non-closed surfaces (like a UDF, I just don't know how to extract the viscous forces from within a UDF)?

Thanks in advance,

Scott