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Old 05-29-2020, 05:13 PM   #5 (permalink)
JulianEdgar
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Quote:
Originally Posted by California98Civic View Post
The tufts and the pressure readings visualize what is happening on the surface of the car. Add a third visualization. Wind tunnel smoke trails can help tell the story of what is happening in the area above the tufts.

Wind tunnel smoke trail pics are usually taken for promotion and they love to maneuver the wand to create bowing arches of gentle beauty. But the EM thread of wind tunnel and smoke trails has some shots that tell a messier story of the roof.

On this somewhat older Mercedes red dramatically highlights the vortices beginning to grow at the windshield header.


On this Caddy (more like your jag), the trail in the flow also begins to show vortices and breakup just behind the windshield header. It is less dramatic but you can see the growing disturbance of the smoke begin just behind the windshield header.


In this light, your pressure readings confirm the possibility that vortices are beginning to develop across your jag's roof just behind the windshield header. Your pressure readings show that pressure begins to return before reaching the point of greatest roof camber (from -105 to -95 ) and continues to climb after (‐50, -10, +30). That story of rising pressure in the flow is associated with the story of growing vorticity seen in the smoke trails.

And all the while the tufts would be pasted flat even across parts of the very messy rear window and trunk. Take for example my tufts from my Civic. They look smoother than expected after 45 mph 7 years ago when I took this shot. The vortices off the c pillars and the downwash from the roof are doubtlessly more turbulent than on the sides or the roof but the tufts can barely tell that story.

I think you have your logic reversed. Literally, the only thing that affects drag/thrust, lift/downforce are pressures acting on the surface of the car's body.

Nothing else.

And, as you can see from the example I have given, tuft behaviour and pressures correlate pretty well.

Smoke and tufts are just means of visualizing flow - and from that, estimating surface body pressures.

Without you giving examples that show both pressures and tuft or smoke behaviour, they unfortunately don't show much.

On Page 80 of my book I show smoke streamlines (Mercedes wind tunnel) and measured pressures (by me on the road) for two further cars. Again the correlation is much as you'd expect.

Quote:
Originally Posted by California98Civic View Post
In this light, your pressure readings confirm the possibility that vortices are beginning to develop across your jag's roof just behind the windshield header. Your pressure readings show that pressure begins to return before reaching the point of greatest roof camber (from -105 to -95 ) and continues to climb after (‐50, -10, +30). That story of rising pressure in the flow is associated with the story of growing vorticity seen in the smoke trails.
The pressure on the Jaguar is lowest at the sharpest curve (windscreen header) and higher (ie less low!) as the curve flattens. The positive pressure at the tail is caused by the kick-up. None of that has to do with imaginary vortices on the roof. The pressures are as measured, and they reflect the wool tuft pattern of attached flow (and the little separation at the base of the rear windscreen).

The Mercedes pic looks suspiciously like a model. In full size, I am aware of no cars that have separation from the windscreen header - but that sort of premature separation is common in models where the Reynolds numbers are wrong.

Your Caddy pic just shows a thickening boundary layer - just what you'd expect. Nothing to do with vortices.

And I am sorry, but taking a pic of wool tufts with the car stopped is pointless. That tells you nothing!
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