NA Aerodynamics

Magpie · Postby **Magpie** » Mon Nov 12, 2012 8:02 pm

As previously stated I'm in the process of having some Garage Vary wind deflectors installed, so in an effort to see if they will actually do anything I decided to do some wool tuft testing and then compare them after the wind deflectors are installed. Interestingly the hard top has terrible aerodynamics and very bad turbulence so I may need to put the rear wing higher or fix the hardtop turbulence.

The flow over the hardtop is OK until it starts coming down. Lightyear has a post on youtube regarding his hardtop and it is nothing like mine.

Front air flow (shot with a Contour Plus stuck on the passengers door):

Hardtop air flow (shot with a Contour Plus 2 stuck on the rear wing):

Lightyears hardtop (hopefully he will not be mad at me for posting it)

Hjt · Postby **Hjt** » Mon Nov 12, 2012 8:29 pm

perhaps mounting your wing higher will hit non turbulent air?

Magpie · Postby **Magpie** » Mon Nov 12, 2012 8:38 pm

Hjt wrote:perhaps mounting your wing higher will hit non turbulent air?

That is the plan I think, I just have to order some spacers for the wing, they come in 2.5" increments. The other thing I may consider is vortex generators on the roof line (these are a lot cheaper than the wing bits) but will need a heap of testing to find the right place to put them so in the end economics says go the risers.

lightyear · Postby **lightyear** » Mon Nov 12, 2012 8:50 pm

Is your wing 2d or 3d? A 3d wing can be mounted lower. A 2d wing should be mounted at roof height min. And i am not mad. Your hardtop test has been done many times. And you can appreciate why i went to the trouble of making a fastback.

Magpie · Postby **Magpie** » Mon Nov 12, 2012 8:58 pm

It is a 3D wing, at the moment the wing is about 12.5" off the boot. I tried to find a post on the forum before I made this one but there were no videos, however I have guessed that it was done to death.

I am impressed with all the trouble you have gone to to get it right with your fastback, now people can at least see a before and after video.

zephyrus17 · Postby **zephyrus17** » Mon Nov 12, 2012 10:39 pm

I actually studied a bit of aerodynamics and did some CFD work. If you give me a model, I can easily get you simulations of the car under different wind conditions

plohl · Postby **plohl** » Mon Nov 12, 2012 11:20 pm

zephyrus17 wrote:I actually studied a bit of aerodynamics and did some CFD work. If you give me a model, I can easily get you simulations of the car under different wind conditions

What would you model it in?

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zephyrus17 · Postby **zephyrus17** » Tue Nov 13, 2012 4:36 pm

ANSYS CFX. It's a pretty standard industry program that I have access to in the uni. All I need is the .igs file or an equivalent CAD file.

Magpie · Postby **Magpie** » Tue Nov 13, 2012 5:00 pm

Dammmmm all I need now is a CAD model of my NA with all the intended mods. I appreciate the offer zephyrus17 and I will have to see if the surveyors at work can lend me their laser scanner, however I will not hold my hopes up.

zephyrus17 · Postby **zephyrus17** » Tue Nov 13, 2012 6:47 pm

Heh, no problem! I'll be quite curious as well. But I recall that the drag coefficient with the lights down is around 0.34 and with the lights up 0.45 (most likely wrong)

brendanstacey · Postby **brendanstacey** » Tue Nov 13, 2012 7:56 pm

zephyrus17 wrote:Heh, no problem! I'll be quite curious as well. But I recall that the drag coefficient with the lights down is around 0.34 and with the lights up 0.45 (most likely wrong)

That sounds like a massive increase, but you did say that you're probaly wrong lol. Speaking of aerodynamics, I have an exam on it in 2 days

plohl · Postby **plohl** » Tue Nov 13, 2012 10:08 pm

Yeah - used cfx when i was at uni. Powerful and pretty time consuming to get accurate results. Keen to see some progress!

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zephyrus17 · Postby **zephyrus17** » Tue Nov 13, 2012 10:51 pm

The Wikipedia page for the MX-5 states the drag coefficient as 0.38. This can be assumed to be the top down value. With the lights up, you can expect it to increase a bit. As a reference, F1 cars are around 0.9, the Toyota Prius is around 0.26. Very similar cars are the Nissan 280ZX and Ferrari F50 at 0.38 and 0.37 respectively.

For a small car, it's a pretty terrible drag coefficient, but mostly because of the convertible-ness. Having the top up would definitely reduce drag.