Quote:
Originally Posted by RedDevil
I'm confused about the wheel fairings/spoilers on the car in the upper picture.
They are tapered along a straight center line, which would be great if there were nothing else around there.
But then there is the car body, pushing the air aside at the front and pulling it back in at the back. Imho the center line should follow the direction of air flow, or even slightly more slanted than that to help push away or draw in the air.
So I would think the fairings ahead of the front wheels should need strong 'toe in', and on the rear wheels some 'toe out' would be beneficial.
By directing the air under the tail that could rise slightly steeper, allowing for a shorter and lighter tail with no added drag.
I also wonder why they are that long. The wing profiles in the second picture suggest that causes unnecessary drag.
I'd shape them like this:
Excuse me for my poor drawing skills - just wanted to show my idea.
Said the aero layman... sorry if I have missed the obvious. |
Red Devil,these are all good questions and observations,and I'll attempt a reasonable and rational explanation,as I'm certainly no expert when it come to such things.
*The premise of the simple canoe fairings,is that the front airdam is shearing most of the air away from the underside,and the flow which remains is traveling for the most part,longitudinally towards the rear.
*The smooth underside is a 'wall' with the wheels erupting out of it,contributing interference drag from these Cd 0.488 rotating wheels with 'MOONs' on both sides.
*The 'wall' affects the boundary layer thickness around the fairings,and by the time the horseshoe annulus at max. cross section displaces to the tail of the fairing,the semi-circular cross section of the boundary layer is quite thick geometrically by comparison,significantly altering the effect of the fairings performance.
*Hoerner reported that due to this interaction,it was beneficial to lengthen not only the aft-body of the fairing,but also the forebody,with minimum drag occuring for fineness ratios between Length/Height = 10 and 15.
*A marginal fillet at the fairing/wall intersection also was found to be efficacious in reducing drag (as you see on all aircraft wing roots).
*Since underbody drag can constitute 50% of total drag on a low-drag car,the fairings are critical.
*The long fineness provides minimum pressure spikes and respects the peculiar boundary layer conditions present.
*The symmetry of the fairings provides uniform accelerations,velocities,and pressures,which help prevent transverse contamination of the boundary layer which might trigger vortex formation.(This is not to say that in this simplistic 'model' of the 'Template' underbody that flow is actually as is theorized,but to know would require actual wind tunnel testing and verification which is beyond the scope of a simple 'one-size-fits-all' solution).
*Historically,very fine results have been achieved with 'straight centerline fairings,as we see today with Cambridge University's CUER eco racer of Cd 0.11.
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*Looking at your illustration,I suspect that there's a chance for separation to occur on the leeward face of the lead fairing.I can't prove it,but I'm certain that this surface,without suction, would be be beyond burble point and into full stall.
*As to the asymmetry of the tail fairing,again,the steep contour appears to invite separation,as well as asymmetrical opposing pressure regimes which could induce vorticity as the flow fields attempt to re-combine.
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*Neil Blanchard has Goro Tamai's 'The Leading Edge' and I hope that at some point,Neil may be able to shed some light on these fairing issues.
*In the past,many World Solar Challenge university teams granted access to GM's Hughes Electronics' VSAERO CFD program,ran the simulations without wheels as they are too difficult to model mathematically in a 3-D flow domain.
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*With the looming CAFE standards and such,and production vehicle Cds approaching 0.20,I think there will be more and more interest in these underbody issues.
*That's about all I can think of for now.We should dedicate a thread to this issue,where all the members can have an opportunity to contribute.
*Another year is passing and Santa Claus has failed again,to deliver EcoModder's full-scale wind tunnel.
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