Flow over upper surfaces and lift
I forgot I had this shot, taken where I was looking down a little on the tufted Jaguar.
Notice there's no separation on the upper surfaces at all from nose to tail, except for a tiny amount at the base of the rear windscreen. https://i.postimg.cc/MGMswJkg/DSC-0065.jpg Measured pressures: https://i.postimg.cc/T1CjmtMr/B-2a.jpg Once you get a feel for it, it's one area in car aero where, if there's attached flow, you can be pretty confident where there will be low and high pressures - and also make a good guess at their relative values (on the upper surfaces, anyway). Incidentally, look at how low the pressures are above and below the car, compared to the wake pressure. |
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. http://2.bp.blogspot.com/_ddqtkOiADu...nasa+image.jpg 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. http://greggfraley.com/wp-content/up...eo-960x540.jpg 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. https://ecomodder.com/forum/member-c...coming-off.jpg |
no separation
Quote:
* The roofline is kinda 'fast.' * Tufts around the C-Pillars implicate vorticity. * Which are confirmed by the CFD image on page 86. |
Quote:
Quote:
|
Quote:
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:
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! |
Quote:
|
https://ecomodder.com/forum/member-c...coming-off.jpg
Quote:
Quote:
Quote:
Quote:
Quote:
I'd love to see some action pics - but if they look like this, I'd say there's very little you can do to improve things in the pictured areas. Honda did a great job. Edit: compare with the Honda Legend I used to own: https://i.postimg.cc/hPTvnSGw/Figure-6-65a.jpg |
Well we are definitely comparing our readings of the tea leaves here, so... You point out that "Honda did a good job," and I'd agree. The tufts surprised me and then discussing it here, looking closely again at the rear window angle, and reading in Hucho demonstrated that my coupe has an approximation of the 22° or 23° angle Hucho's description suggest is often highly effective for the notchbacks. That and a ton of other optimization.
I think that your Legend shows a similar pattern. Look at how the top tufts, toward the sides on the glass are angled inward. This would be the meeting of the flow from the top and the sides meeting. It would be where those large rotating vortices begin. Lemme gamble f on a guess for a second: you had the Sedan? |
All times are GMT -4. The time now is 08:07 AM. |
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com