# Vehicle Aerodynamics

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 November 4, 2011, 13:04 Vehicle Aerodynamics #1 New Member   ankit chauhan Join Date: Sep 2011 Posts: 16 Rep Power: 13 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 ?

 November 4, 2011, 13:10 #2 New Member   ankit chauhan Join Date: Sep 2011 Posts: 16 Rep Power: 13 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??

 November 5, 2011, 09:47 #3 Member   Zachariah Swetky Join Date: Aug 2011 Posts: 36 Rep Power: 13 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

 November 6, 2011, 08:58 #4 New Member   ankit chauhan Join Date: Sep 2011 Posts: 16 Rep Power: 13 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?

November 6, 2011, 12:19
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Quote:
 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?
No it's not. You would have to run a simulation of the whole car (well definitely everything downstream of the sunroof and likely upstream too; might be able to get away with not including bottom of car). Internal cavity as well depending on setup.

 November 6, 2011, 12:48 porous wall #6 New Member   navi Join Date: Nov 2011 Posts: 2 Rep Power: 0 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!!

November 7, 2011, 09:20
#7
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Zachariah Swetky
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Quote:
 Originally Posted by ac_ankit 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?
If you are just looking for trends, you may be able to get away with modeling only the dome portion of the car. This would give you an idea of how sunroof size, geometry, and location affect drag, but I would not trust it for the final answer.

January 31, 2014, 07:46
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Meimei Wang
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Quote:
 Originally Posted by swetkyz 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
Why do we need to model the whole car? Because the variation of flow in the roof will largely change the flow in the rear part?
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February 3, 2014, 12:14
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Meimei Wang
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Quote:
 Originally Posted by Anna Tian Why do we need to model the whole car? Because the variation of flow in the roof will largely change the flow in the rear part?
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 February 3, 2014, 14:28 #10 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 460 Rep Power: 16 Yes, it'll massively change your wake, thus the forces acting on your car.

 Tags aerodynamics, car, sunroof