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JulianEdgar · 05-30-2020, 07:35 AM

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.

05-30-2020, 07:35 AM	#1 (permalink)
JulianEdgar Banned Join Date: Nov 2017 Location: Australia Posts: 2,060 Thanks: 107 Thanked 1,605 Times in 1,136 Posts	Attached flow on inclined rears, and trailing vortices 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.