Quote:
Originally Posted by JulianEdgar
I've just spent a few hours immersed in my aero textbooks and SAE papers.
One of the things that Aerohead has said that I have found confusing, and thought probably misleading, is that (and I hope I paraphrase him correctly):
Attached flow, as indicated by tufts, on steeply inclined rear surfaces (eg the Porsche 911) is caused by the downwash of trailing vortices. Therefore, what you see as attached flow through tufting, isn't really attached flow.
I found that a bit odd, because on all the cars I have tufted (I've been tufting whole cars on the road since 1988), tuft patterns have always been really good indicators of what is actually happening. That is, I don't think I have ever seen something in a tuft pattern that didn't quite add up. The presence of trailing vortices, for example, can be indicated by diagonal tufts on angled pillars (front or back). Separated and attached flow have always been quite obvious.
The reason I went to my references was to explore this idea in more detail - that apparently attached tufts really aren't, if they're showing attachment on steeply inclined rear surfaces through the action of trailing vortices.
SAE 2011-01-1075 (Mayer, W. and Wickern, G., "The New Audi A6/A7 Family - Aerodynamic Development of Different Body Types on One Platform," SAE Int. J. Passeng. Cars – Mech. Syst. 4(1):197-206, SAE Technical paper 2011-01-0175, 2011) has a good description of the indicative trailing vortices associated with squareback, fastback and notchback shapes.
Interestingly I had intended in my book quoting sections from this SAE paper on these different body shapes. In fact I did so in draft form but Dick Barnard wasn't happy with this. Instead we developed the bullet points on Pages 17-18, which are much more general. (I assume that Dick thought the SAE paper text was too prescriptive.)
However, the SAE paper does indicate the strong impact that trailing vortices have on flow attachment.
But it is a quote that I used in my book that I think is most relevant. It was something that Adrian Gaylard wrote to me when reviewing a draft chapter. It was in reference to the effective back-light angle (EBLA), that refers to the angle between the trailing edge of the roof and the trailing edge of the trunk (boot). He said:
Typically, for lowest drag, the EBLA is around 12 degrees. As this increases, so does drag, until around 30 degrees where the rear-pillar vortices burst and the rear flow fully separates. With an effective backlight angle approaching 30 degrees, it’s often better to separate it as the drag can be lower for a fully separated rear flow, compared to one where rear pillar vortices are keeping the rear screen flow attached on a high screen angle.
So:
1) He refers to flow being attached to rear inclined surfaces through the action of the trailing vortices. Therefore, I think in this situation the tufts are showing attached flow, and can be referred to in that manner.
and
2) He suggests that drag can be higher when there is attached flow down a steep rear backlight than when it is separated. Because of its angle, I'd suggest that attached flow down that rear screen would not only be causing increased drag, but also increased lift.
So in the context of that, the Porsche pics again.
Attached flow, possibly caused by the downwash of trailing vortices off the pillars:
Ducktail spoiler fitted, causing separated flow and reducing lift:
In this case, separating the flow reduced lift. Lift was not caused by early separation; it was reduced by creating earlier separation. I think that is often the case on modern cars that have attached flow right to the end of the body.
But the point that flow attachment on steeply inclined rear surfaces can be caused by the action of trailing vortices, while mentioned in Adrian's quote, is something I should have more heavily emphasized. |
1) As to EBLA on prismatic bodies, the 'optimum' angle for drag reduction is a function of the aft-body length, compared to the overall length of the vehicle, represented as a percentage. Hucho gets into this on page 153. For a vehicle with the lo/l= 0.45,the lowest drag is achieved at 22-degrees downslope angle. So it's very conditional.( from Buchheim et al.)
2) For EBLA on 'curved-roof' vehicles, on page 154, Buchheim et al., found that for a vehicle of lo/l= 0.336, the optimum angle for low drag was 22-degrees.
3) For the Audi 100-III, the drag minimum was found at a backlight-to-boot angle of 17-degrees. (page 156,157)
4) Wunibald Kamm recommended that one never exceed 10-degrees. That would never pass muster.
5) The comment about 'vortex burst' at 30-degrees
is correct only for simple prismatic bodies investigated,as the Ahmed body.
6) No one has made a simple relationship for 'curved-roof' vehicles.
#Very important to separation on a cambered roof, is the angle as a function of the length of the aft-body itself. An inspection of W.A. Mair's boat-tail demonstrates a fine 'template' for an aft-body. Separation occurs beyond 22-degrees downslope, but it's extremely important to get to the angle after a very gentle transition. Just like Buchheim's Flow body. I limited the 'template' downslope to a maximum of 22-degrees on account of Mair's research. Buchheim's 'Flow' body uses a maximum 22.5-degrees. It's a fine 'template.'
You can also use a clock face for a 'template',derived from the known separation line on any sphere beyond critical Reynolds number. Your roof apex is at 12:00 high,and flow separates around 4-5 seconds after. If you scale this contour up to the size of the vehicle, it's an equally good and 'proven' architecture.
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At http:// images.the samba.com/vw/gallery/pix/525654.jpg there's an equally dangerous tuft study photograph of a VW Beetle candidate car, which also would imply 'attached' aft-body flow all the way to the license plate.
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Online, there's some full color smoke-flow images of the Cd 0.217, Volkhart V2 Sagitta. It's also based upon the 1938 AVA 'Lange' car,which inspired the Porsches. The lowest smoke filament coming off the back of the roof would better represent what actually happens on the 911 Carrera.
PS
* , putting a protractor to the bottom of the lowest smoke filament coming off the roof measures 22-degrees.
* For the 1966 VW Beetle, it's 18-degrees.
* The 2001 VW NEW BEETLE RS1 used two spoilers to get the rear angles down to 14.5-degree, but they could only salvage Cd 0.38 out of it.
* The Killer Audi TT had to resort to a pop-up spoiler in an effort to get some air to make contact.
* The 2013 VW XL1, and its 'template' roof contour needs no spoiler for it's 100-mph top speed (124-mph unlimited).