Aerodynamics for Cooling Systems of Turbo Car - Fuel Economy, Hypermiling, EcoModding News and Forum

wildman10 · 05-12-2016, 01:55 PM

I'm heavily involved in adding a turbo to a normally-aspirated car and have been designing the cooling systems from scratch, with particular emphasis on aerodynamics. Although the car, a Mazda RX8, is not noted for ecomodding I've tried to use its principles in my work.

Write-Up on cooling system aerodynamics

I wrote the write-up here to explain my thoughts on cooling system aerodynamics. If you have any feedback on my write-up then please comment away.

Exhausting Air Flows Into Wheel Wells

I've started to look again at the grey area that is the effect of exhausting cooling air into wheel wells, particularly those at the front, as I may be adding a gearbox oil cooler where ducting cooling air into a front wheel well is an option (from the lack of any alternative I'm already retaining the oil coolers ahead of the front wheel wells and exhausting cooling air into them).

I think the issue is very important as the wheels are supposedly responsible for up to 33% of drag, most notably at higher speeds, from generating a complex series of vortices and turbulent flows.

I've found very little on air flows within wheel wells in general in several hours of searches let alone on adding air from cooling flows. From what I knew and have found out, air flow in and around wheel wells is very complex, depends on many factors and is very hard to model. Furthermore, some unexpected factors such as the groove between tyre tread and sidewall has significant impact. In general, air in the wheel wells have a negative Cp (AFAIK talk of 'higher' pressures in the wheel well to explain fender/wing vents refers to their values relative to those above the fender/wing not pressures in absolute terms) that would help pull air through a radiator and adding more air to be exhausted from the wells will create more drag in many forms.

My gut feeling is that I should avoid dumping more air into the front wells as that will create more drag than dumping that air underneath the body.

Does anyone have anything to help educate me here?

freebeard · 05-12-2016, 02:41 PM

I'm too much of a knucklehead to be able to copy a Google link instead of re-downloading the PDF, but here are your search terms:

Session-3-1-Jeff-Howell-Summary-of-university-dindings-Wheel-Wheelarch-drag
Numerical simulation of the flow around the wheel within wheelhouse - Sasan_Sarmast_Jan2010

You could do worse than mimic Porsche:

http://www.bombaydigital.com/boxster/projects/radiator/

wildman10 · 05-12-2016, 03:00 PM

Thanks. I'll look at those later. I tried to post the write up as a file attachment but apparently it was too large; perhaps it's the way I tried.

freebeard · 05-12-2016, 05:46 PM

There is a button for that.

Try breaking it into chapters spread over multiple posts? How many words?

wildman10 · 05-12-2016, 06:50 PM

When I saw how much larger the pdf file size was than the attachment limit I dismissed the thought of splitting it. If opening up the link is difficult I'll break the original Word doc into smaller units for multiple pdfs that I'll add separately.

Again, thanks for your references. I'd seen all 3 before my post: the first confirmed my view of the general importance of wheel wells to drag; the second is pretty hard going and worthy of further discussion below; the third describes the fit of an extra oil cooler/radiator in the bumper and I can see little, if anything, on the oil coolers ahead of the front wheels.

The second reference, the numerical simulation paper, provides confirmation of the negative Cps in the wheel well (indeed, the image I posted comes from this) and shows the difficulties of getting CFD to get even close to values measured experimentally. It also allows me to use a bit of imagination to see how adding air into the wheel well will affect the distribution. For example, the vortex diagrams give me an idea of how the extra air will be thrown out.

I can, though, see nothing on the effectiveness of various locations for cooling air. I'll carry on looking.

freebeard · 05-13-2016, 12:57 AM

The pinnacle, gaining thrust from the heat exchanger was accomplished on the P-51 Mustang. Here's a local search: Google Search Results - EcoModder.com

wildman10 · 05-13-2016, 10:05 AM

That is the favourite example for the design of a radiator duct taking advantage of the Meredith effect. However, we should be a little careful of what we take from that design as the Mustang was more efficient and faster with wing-mounted radiators in place of the guns, as exemplified by those used in the Reno and Cleveland pylon races.

jamesqf · 05-13-2016, 02:01 PM

Quote:

Originally Posted by wildman10

...the Mustang was more efficient and faster with wing-mounted radiators in place of the guns...

But not nearly as much fun :-)

Seriously, though, I wonder how much of that is just due to eliminating weight & drag from the guns.

wildman10 · 05-13-2016, 03:27 PM

I suppose that depends on which side of the engine you were.

The weight saving of having no guns is irrelevant because all the P51 racers had them removed too. Of course, that doesn't apply to the originals as a fighter is useless without guns and even the photo recce carried them (IIRC one US recce pilot was credited with 13 kills). Incidentally, the USAAF used a large number of UK-built recce aircraft, notably Spitfires and Mosquitos, that did not have guns as they were practically immune from the Luftwaffe until the Me262 jet came along in late 1944; many visitors to the American Air Museum at Duxford in the UK are perplexed when they see the easily-recognisable Spitfire and Mosquito outlined in the glass panels with an opaque profile of every US aircraft lost operating from the UK in WWII.