Visual quiz

[jakobnev]

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[JulianEdgar]

Originally Posted by jakobnev

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Yes, the winner! I’ll post the full graph tomorrow (my time).

[JulianEdgar]

Here's the original:



As I said, the vertical scale variation seems to me to be a bit exaggerated - but that's what they published.

Note the suction peak at the header rail of the windscreen, but how that occurs over only a small area. If we were shade the low pressure area of the graph across the roof and rear hatch, we end up with the majority of lift happening between positions 30 and 45.

You can be pretty confident the fairly large rear spoiler added to the turbo (and later 944) versions increased the hatch pressures quite a lot.

The full graph is also interesting in that it gives us front and rear undercar pressures. (I edited these out inthe 'quiz' graph.) So position 1 is slightly less than atmospheric pressure (ie under front bumper) and positions 46-50 are all at wake pressure.

The odd front pressures that confused people are apparently just the result of the bumper/intake shape.

[M_a_t_t]

Is there any significance in the fact that it is a log scale (appears to be atleast) in the x axis?

[freebeard]

I was fooled (again!) by that.

The station are unwrapped from about 270° on the car. As we can now see from Permalink #43.

[aerohead]

Originally Posted by JulianEdgar

Here's the original:



As I said, the vertical scale variation seems to me to be a bit exaggerated - but that's what they published.

Note the suction peak at the header rail of the windscreen, but how that occurs over only a small area. If we were shade the low pressure area of the graph across the roof and rear hatch, we end up with the majority of lift happening between positions 30 and 45.

You can be pretty confident the fairly large rear spoiler added to the turbo (and later 944) versions increased the hatch pressures quite a lot.

The full graph is also interesting in that it gives us front and rear undercar pressures. (I edited these out inthe 'quiz' graph.) So position 1 is slightly less than atmospheric pressure (ie under front bumper) and positions 46-50 are all at wake pressure.

The odd front pressures that confused people are apparently just the result of the bumper/intake shape.

There's an online photograph of a red 924, shot in the wind tunnel, at automobilesport.com, which reveals the boundary layer leaving the roof at the top of the backlight, ahead of the backlight contour, leaving the separation void which the spoiler will later address.

[JulianEdgar]

Originally Posted by aerohead

There's an online photograph of a red 924, shot in the wind tunnel, at automobilesport.com, which reveals the boundary layer leaving the roof at the top of the backlight, ahead of the backlight contour, leaving the separation void which the spoiler will later address.

Your interpretation doesn't match mine:



Same distance between smoke stream and body at front of car and rear. Smoke in wake clearly shows how separation doesn't occur until end of hatch.

[aerohead]

Originally Posted by JulianEdgar

Your interpretation doesn't match mine:



Same distance between smoke stream and body at front of car and rear. Smoke in wake clearly shows how separation doesn't occur until end of hatch.

1) No. 3 smoke filament from the floor makes it into the boundary layer over the roof.
2) The roof contour does not follow the local streamline.
3) The roof has produced too much of a pressure rise.
4) The pressure rise has decelerated to flow.
5) The decelerated flow has lost momentum.
6) Now there's not enough momentum transfer to the lowest stratum of the TBL adjacent to the body's surface boundary.
7) Without a reduction of the pressure the TBL must separate and does.
8) The separation is inducing a weak pair of attached longitudinal vortices.
9) The vortices are inducing a weak downwash.
10) The downwash is holding the flow attached down the centerline..
11) The wake is 'smaller' but overall total drag is higher than if the roof was 'streamlined', on account of the induced vortex-drag.
12) Vortex drag is the highest form of drag.
13) Later addition of the spoiler will allow the flow to reattach onto the spoiler's raised height.
14) The modified flow will force a higher velocity.
15) The higher velocity will produce Dr. Thomas Wolf''s ' stronger negative pressure gradient.'
16) The stronger negative pressure gradient will re-invigorate the TBL.
17) The TBL will now flow over the separation bubble created by the spoiler.
18) When the flow does separate at the spoiler, it will be at a higher static pressure.
19) Lift will be reduced.
20) Drag will be reduced.
21) Again, attached flow due to downwash is not 'attached flow.'
22) Downwash is an artifact of separation, as is the vortices.

[jakobnev]

I'll leave this here for everyone for now:

https://www.youtube.com/watch?v=JcxOub0E4aA

I have to go to sleep now, but I'll have questions about aeroheads terminology tomorrow.

[JulianEdgar]

Originally Posted by aerohead

1) No. 3 smoke filament from the floor makes it into the boundary layer over the roof.
2) The roof contour does not follow the local streamline.
3) The roof has produced too much of a pressure rise.
4) The pressure rise has decelerated to flow.
5) The decelerated flow has lost momentum.
6) Now there's not enough momentum transfer to the lowest stratum of the TBL adjacent to the body's surface boundary.
7) Without a reduction of the pressure the TBL must separate and does.
8) The separation is inducing a weak pair of attached longitudinal vortices.
9) The vortices are inducing a weak downwash.
10) The downwash is holding the flow attached down the centerline..
11) The wake is 'smaller' but overall total drag is higher than if the roof was 'streamlined', on account of the induced vortex-drag.
12) Vortex drag is the highest form of drag.
13) Later addition of the spoiler will allow the flow to reattach onto the spoiler's raised height.
14) The modified flow will force a higher velocity.
15) The higher velocity will produce Dr. Thomas Wolf''s ' stronger negative pressure gradient.'
16) The stronger negative pressure gradient will re-invigorate the TBL.
17) The TBL will now flow over the separation bubble created by the spoiler.
18) When the flow does separate at the spoiler, it will be at a higher static pressure.
19) Lift will be reduced.
20) Drag will be reduced.
21) Again, attached flow due to downwash is not 'attached flow.'
22) Downwash is an artifact of separation, as is the vortices.

The normal mix of correct theory, incorrect theory, irrelevancies, very clearly no experience of on-road testing of a shape anything like this, extrapolated even weirder theory - and so on.

For people who actually want to learn, just look at the pressure chart and the flow pattern, and draw some obvious conclusions about the relationships.






And as for the rear spoiler, just mentally place one at the back of the car (say, the same angle as the windscreen but only 1/5th as high) and then look at what the windscreen did to the pressure of the attached flow in front of it.