Vehicle Aerodynamics
I carried over simulation of air flow over a flat plate for turbulent boundary conditions and found the drag acting on the plate to be 2.1 N(approx.). I wish to extend the problem to the analysis of flow over a car's roof. It is understood that for a car's sunroof (of the same dimensions) the drag acting is 2.1N but can I infer from this that when the sunroof is open the drag force acting on the car increases by 2.1 N ?

What I mean is that I read from somewhere that Cd of a car increases from 0.32 to 0.41 when sunroof is opened. Calculations with the analytical formulas using these values shows an increment of over 100N in drag force. Really confused??

As far as I know, flat plate drag predictions do not correlate with the drag on a car's roof. The only aspect that may correlate is skin friction drag, which is by no means the whole story. In order to properly estimate the drag of a car, the flow over the entire car and a large surrounding volume must be solved.
If you could provide more detailed information about the problem, I may be able to guide you a little more precisely. Zachariah 
The problem is that I wish to find the effects of car's sunroof being open or closed on fuel economy and therefore I tried to find the drag acting on a car with and without sunroof. Is it possible by analysing drag only on sunroof?

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porous wall
I have a question of how can I simulate the cylinder with porous wall and solve fore concentration gradients, and what is the best software for.
thank you in advance I dont know if its the correct place to post this!! 
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Yes, it'll massively change your wake, thus the forces acting on your car.

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