Attached flow on inclined rears, and trailing vortices

[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.

[j-c-c]

So I wonder, in the pics above, was the engine running at a typical RPM for the windspeed used, ie, was the grille engine cooling air part of the results seen?

Meaning somewhat, GIGO.

[JulianEdgar]

Originally Posted by j-c-c

So I wonder, in the pics above, was the engine running at a typical RPM for the windspeed used, ie, was the grille engine cooling air part of the results seen?

Meaning somewhat, GIGO.

Interesting point. But they are official Porsche pics, so they were obviously happy with them at the time.

[aerohead]

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).

[aerohead]

At:
forums.pelicanparts.com/porsche-911-technical-forum/740377-anyone-have-911-cfd-analysis.html
there are some additional wind tunnel tuft study images of the 911 Carrera, shot from a higher vantage point.
There is clearly separation behind the backwards facing step at the top of the recessed backlight, where vents are positioned. An inverted pyramidal area of disturbed tufts extend halfway down the glass, illustrating the reversed flow. This area is sloped at 26-degrees to the horizon, three degrees steeper than what can support attached flow.
The angling of the C-Pillar tufts also suggest high pressure body side flow directed towards the low pressure, fast-moving flow above the roof, rolling up into the tornadic vortices as these two regions' attempt at pressure equalization fails at the intersection.
Further down the rear surface, the tuft orientation is 'corrected', presumably,now caught in the vortex-induced fastback downwash, strongest of all body forms.
According to Hucho, page 281,in his Second Edition, by simply lofting the 911's rear contour ,up to the thickness of the flow separation,you'd kill both the lift and drag, and better than with the spoiler.(page 175). This WOULD, however, alter the 'look' of the car, and potential for a ' risk to sales.' ( page 188).
It would take until after 1994,and the end of the air-cooled 911s, before Porsche would begin this lofting of the rear body contour. Parked side-by-side, the aft-bodies of the early and current 911s would show little resemblance. A good thing!

[JulianEdgar]

Originally Posted by aerohead

At:
forums.pelicanparts.com/porsche-911-technical-forum/740377-anyone-have-911-cfd-analysis.html
there are some additional wind tunnel tuft study images of the 911 Carrera, shot from a higher vantage point.
There is clearly separation behind the backwards facing step at the top of the recessed backlight, where vents are positioned. An inverted pyramidal area of disturbed tufts extend halfway down the glass, illustrating the reversed flow. This area is sloped at 26-degrees to the horizon, three degrees steeper than what can support attached flow.
The angling of the C-Pillar tufts also suggest high pressure body side flow directed towards the low pressure, fast-moving flow above the roof, rolling up into the tornadic vortices as these two regions' attempt at pressure equalization fails at the intersection.
Further down the rear surface, the tuft orientation is 'corrected', presumably,now caught in the vortex-induced fastback downwash, strongest of all body forms.
According to Hucho, page 281,in his Second Edition, by simply lofting the 911's rear contour ,up to the thickness of the flow separation,you'd kill both the lift and drag, and better than with the spoiler.(page 175). This WOULD, however, alter the 'look' of the car, and potential for a ' risk to sales.' ( page 188).
It would take until after 1994,and the end of the air-cooled 911s, before Porsche would begin this lofting of the rear body contour. Parked side-by-side, the aft-bodies of the early and current 911s would show little resemblance. A good thing!

Well, your reference starts off with a picture that I commissioned from an artist (Dave Heinrich), drawn on the basis of wool tuft testing that I carried out on the road in 2001 - see https://autospeed.com.au/cms/a_1065/article. So thanks for the compliment. (However, it is of a different 911 model, so it doesn't match the rest of the pics in your cited thread.)

So let's look at Anyone have 911 CFD Analysis - Pelican Parts Forums, the thread you cited.

The pics show, without the spoiler, a separation bubble on the upper part of the slightly recessed rear glass, with flow attachment then re-occurring.

With the spoiler fitted, the separation is much more widespread on the rear glass, and flow attachment does not re-occur.

We don't need to read all your theories to see exactly what is happening: the spoiler creates flow separation that reduces lift, exactly as would occur over any lifting shape.

That's the reality, that is obvious to any observer looking at the story of the tufts - and I encourage people to go look for themselves at these pics.

(And, as you often do, you're misquoting Hucho. Neither page reference supports what you claim of it.)

[JulianEdgar]

Originally Posted by aerohead

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.

[snipped - massive amount of theory]

Thanks, but I'll go based on what I can directly measure on the road (attached/separated flow, lift/downforce, surface pressures) and what professional aerodynamacists have told me.

The days of trying to make up rules that are then extrapolated to all cars are long gone. We can now just measure what is actually happening on our own cars, each with their unique shape. And measure that in real atmospheric conditions, on real roads - and all at low cost.

Seeing what is really happening - not what 'Aerohead' theorises will happen.

[aerohead]

Originally Posted by JulianEdgar

Well, your reference starts off with a picture that I commissioned from an artist (Dave Heinrich), drawn on the basis of wool tuft testing that I carried out on the road in 2001 - see https://autospeed.com.au/cms/a_1065/article. So thanks for the compliment. (However, it is of a different 911 model, so it doesn't match the rest of the pics in your cited thread.)

So let's look at Anyone have 911 CFD Analysis - Pelican Parts Forums, the thread you cited.

The pics show, without the spoiler, a separation bubble on the upper part of the slightly recessed rear glass, with flow attachment then re-occurring.

With the spoiler fitted, the separation is much more widespread on the rear glass, and flow attachment does not re-occur.

We don't need to read all your theories to see exactly what is happening: the spoiler creates flow separation that reduces lift, exactly as would occur over any lifting shape.

That's the reality, that is obvious to any observer looking at the story of the tufts - and I encourage people to go look for themselves at these pics.

(And, as you often do, you're misquoting Hucho. Neither page reference supports what you claim of it.)

1) The tufts do not represent the actual flow conditions, they are an artifact of the induced downwash, created by the longitudinal vortices, which the tufts fail to represent.
2) The TBL on the 911 is already compromised, half way between the top of the backlight and roof apex. The 911's rear contour happens too early, and its downslope is too steep to support attached flow.
3) Since the backlight is at 26-degrees, there's no chance of reattachment. What your viewing is a tuft alignment due to the downwash already in place.
4) The duckbill spoiler is attempting to 'reach' up,out of the separated flow where the inviscid flow can attack it. It falls short, however the vortex-induced downwash is attacking it, aiding engine cooling, lift, and a tiny bit of drag.
5) Your concept of a 'lifting shape' is not supported by science. You know from your own experiments that, any major lift on the 911 would be ahead of the windshield header.
6) If you'll continue your studies, you'll learn that the 'lift' that you're interested in is caused by the region of separation-induced low pressure acting over the top majority of the Porsche's aft-body.
7) You won't understand this concept until you investigate Bernoulli's Theorem. Ask any of your four favorite aerodynamicists.
8) Any 'observer' who's passed a course in Fluid Mechanics would object to your explanation of the spoiler's function.
9) As to Hucho, all I can do is, list his quotes which are germane to this issue. I cannot comprehend them for anyone else.

[aerohead]

Originally Posted by JulianEdgar

Thanks, but I'll go based on what I can directly measure on the road (attached/separated flow, lift/downforce, surface pressures) and what professional aerodynamacists have told me.

The days of trying to make up rules that are then extrapolated to all cars are long gone. We can now just measure what is actually happening on our own cars, each with their unique shape. And measure that in real atmospheric conditions, on real roads - and all at low cost.

Seeing what is really happening - not what 'Aerohead' theorises will happen.

1) Over the past weeks there's been an underlying theme running through your correspondence.
2) If you've conducted measurements on the road with instrumentation, we are to take that as gospel.
3) If 'real' scientists, who came before you conducted tests with instrumentation, under clinical conditions, and published the results, all that is to be completely discounted.
4) Hucho's text is a compilation of results from empirical testing, conducted with laboratory-grade instrumentation, dating back to 1922.
5) My citations have to do with quotes, tables, or charts published by Hucho, Schlichting, Von Mises,Prandtl, Kamm-Fachsenfeld, Hoerner, Tamai, Abbott and Von Doenhoff, and others, whom I presume published the materials so that no individual would ever have to duplicate efforts, wasting time accomplishing redundant operations.
6) I don't know why you bought books, but I bought mine to profit from. I certainly am not in a position to replicate 98-years worth of research. The books have been a rich treasure trove of knowledge.
7) There is nothing 'theoretical' about anything I've posted. Everything was proven long ago.
8) You may be confusing 'hypothesis' for 'theory.' Only after a hypothesis is proven may it be used for theory. Students are introduced to 'theory' so they'll understand the underlying logic of formulae they may use on a day-to day basis. Like the' Theory of Gurgling Stomachs.' It's meal time, go eat something.

[JulianEdgar]

I am sorry, but that's all just so much rubbish.

Originally Posted by aerohead

1) The tufts do not represent the actual flow conditions, they are an artifact of the induced downwash, created by the longitudinal vortices, which the tufts fail to represent.

Translation: the tufts show what is happening. You don't like it, so the tufts must actually be showing something else.

Originally Posted by aerohead

2) The TBL on the 911 is already compromised, half way between the top of the backlight and roof apex. The 911's rear contour happens too early, and its downslope is too steep to support attached flow.

Translation: here is my theory, so what the tufts actually show must be wrong.

Originally Posted by aerohead

3) Since the backlight is at 26-degrees, there's no chance of reattachment. What your viewing is a tuft alignment due to the downwash already in place.

Translation: here is my theory, so what the tufts actually show must be wrong.

Originally Posted by aerohead

4) The duckbill spoiler is attempting to 'reach' up,out of the separated flow where the inviscid flow can attack it. It falls short, however the vortex-induced downwash is attacking it, aiding engine cooling, lift, and a tiny bit of drag.

Translation: here is my bizarre theory of how spoilers work.

Originally Posted by aerohead

5) Your concept of a 'lifting shape' is not supported by science.

Translation: here is my even more bizarre theory, based apparently on the notion aircraft wings don't exist.

Originally Posted by aerohead

You know from your own experiments that, any major lift on the 911 would be ahead of the windshield header.

No, I know from measuring actual pressures on real cars (you could try it some time) that would definitely not be the case.

Originally Posted by aerohead

6) If you'll continue your studies, you'll learn that the 'lift' that you're interested in is caused by the region of separation-induced low pressure acting over the top majority of the Porsche's aft-body.

Translation: more of my bizarre theory, one not supported by textbooks from Barnard, Scibor-Rylski, et al.

Originally Posted by aerohead

7) You won't understand this concept until you investigate Bernoulli's Theorem. Ask any of your four favorite aerodynamicists.

Um, they vetted my book. They have already spoken!

Originally Posted by aerohead

8) Any 'observer' who's passed a course in Fluid Mechanics would object to your explanation of the spoiler's function.

I don't think so.

Originally Posted by aerohead

9) As to Hucho, all I can do is, list his quotes which are germane to this issue. I cannot comprehend them for anyone else.

I think you've got used to quoting Hucho incorrectly, as you did above, and few people have bothered checking on the actual quotes. So far, nearly every time I have checked, it's wrong. It's a bit like someone who believes in a flat earth referencing Encyclopedia Britannica pages that mention the word 'earth'.