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Old   September 4, 2011, 08:47
Default Drag coefficient comparison
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Mohammed
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Hi there guys,
I am working on improving a motorhome's aerodynamics. I have been doing 2D simulations of simple shapes to find what modifications would be best aerodynamically. Ive started with a simple rectangle, a rectangle with the top right corner curved and a rectangle with the rear top corner curved. the rectangle gave a high Cd which is understandable. the curved front encouraged the flow to attach to the body reducing the area of turbulence and therefore reduced the Cd by around 27%. however curving the rear whilst keeping the front square actually gave a higher drag coefficient than the original rectangle shape and i can not figure out why. the square front didn't allow the flow to attach, but i dont see how the curved rear would worsen the Cd. I have attached some contour plots. If any one has any idea i would be grateful!
Thanks,
Mo
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File Type: png rectangle 3 velocity.png (54.4 KB, 15 views)
File Type: png rectangle 3 pressure.png (45.5 KB, 10 views)
File Type: png rectangle 2 velocity.png (46.6 KB, 11 views)
File Type: jpg rectangle 1 velocity.jpg (38.8 KB, 12 views)
File Type: png rectangle 1 pressure.png (55.0 KB, 6 views)
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Old   September 5, 2011, 04:28
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Hi Mohammed,

This is known effect. For instance, rounding of car back could increase the aerodynamic drag (see image attached).

I think this happens because of additional pressure drop due to "centrifugial force" on the curvilinear part: perhaps in the case of the sharp corner the flow leaves the surface immediately and there is no pressure drop due to curvilinear motion, so the drag is lower. This might be the explanation, although I am not sure it is correct.

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Old   September 5, 2011, 06:52
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Mohammed
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Hey Truffaldino
Thanks for the reply, you have been a great help!
Mo
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Old   September 5, 2011, 18:37
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Martin Hegedus
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In addition to what truffaldino wrote, there *might* be a little more going on involving the separation bubble which occurs when the front is flat.

When the back side is rounded the reversed flow (which travels up the back) may increase the size of the separation bubble on the top side. This would decrease the pressure on the back side of the motorhome. All this stuff is very nonlinear, so it is just a thought.

If this is the case, then putting a horizontal lip on the back side and adding a fillet so the flow comes up the back side and flows backward might help reduce the size of the separation bubble on the top.

But, your simulation is 2D. So 3D will be much difference and probably not as severe.
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