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Old 12-11-2014, 03:57 PM   #31 (permalink)
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(I wish I could post pics correctly)
Me too! If I get what you're saying, compare to this:


http://www.speedhunters.com/2012/06/for-my-ally-is-the-force/

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Originally Posted by Hypermiler
The best option I see (if it works) is to make a mini boattail, make the entire boattail the sharpest angle of the aero template, without the slow angle change, then use VGs to keep the air attached in the transition from the roof to the boattail.
Radiusing edges works. Especially in cross-winds. You're always in cross-winds. Plus everything aerohead said.

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Without blowing,or suction.there's only so much VGs can do.

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Old 12-12-2014, 05:27 PM   #32 (permalink)
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Post#3

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Originally Posted by Hypermiler1995 View Post
I am using the link in post #3 of this thread as evidence for my theory, but I am not sure about the amount of drag created By the VGs.
*from Julian's article you can see that the VGs increased the drag on the Insight.Hucho came to the same conclusion on on everything he tested.
*There's nothing for the air to reattach to.
*So a boat tail with a trailing edge no more radical than the trailing edge of the Lancer (both down and in) would have a high degree of confidence as far as some drag reduction.It's a known quantity.
*Again,you'd have to calculate the Turbulent Boundary Layer thickness for the location of attachment,and size the VGs accordingly.They need to be just large enough and no larger.
*VGs for the roof may have to be different from those on the sides,depending on the particular vehicle.
*Store-bought VGs aren't going to cut it.
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Old 12-14-2014, 07:27 PM   #33 (permalink)
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Here we go, another reference for the box cavity (which has come up in other threads, but also seems relevant here): J.D. Coon and K.D. Visser "Drag Reduction of a Tractor-Trailer Using Planar Boat Tail Plates" (Google Books) The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains, Volume 1, edited by Rose McCallen, Fred Browand, James Ross, p250

Quotes previous research showing poor results from splitters and vanes, good results for a 'non ventilated cavity', lots of supporting references given to follow up on, including early development, patents, investigations and optimisation by NASA and an investigation on minivans by the well-regarded Hucho, as well as detailed optimisation info for the tractor-trailer case.
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Old 12-14-2014, 10:37 PM   #34 (permalink)
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Another reference from the "FutureCar" project at the University of California, Davis, mentions use of sharp trailing edges and a box cavity (briefly, on page 10) http://www.uctc.net/papers/375.pdf
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Old 12-14-2014, 11:12 PM   #35 (permalink)
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Originally Posted by aerohead View Post
*from Julian's article you can see that the VGs increased the drag on the Insight.Hucho came to the same conclusion on on everything he tested.
*There's nothing for the air to reattach to.
*Store-bought VGs aren't going to cut it.
that is why I said to use the mini boattail, a virtual would be too uncontrolled,
might cut down the low pressure area, but would cause turbulence. The yurastyle neo prius has VGs on the mirrors, but there is far less drag area to fill.

Aren't these store-bought VGs? (though they're the only ones i found that seemed to work.)
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Old 12-15-2014, 06:30 PM   #36 (permalink)
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tail/VGs

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Originally Posted by Hypermiler1995 View Post
that is why I said to use the mini boattail, a virtual would be too uncontrolled,
might cut down the low pressure area, but would cause turbulence. The yurastyle neo prius has VGs on the mirrors, but there is far less drag area to fill.

Aren't these store-bought VGs? (though they're the only ones i found that seemed to work.)
*Yes,I caught the part about the mini boat tail.And I was trying to emphasize the importance of where the trailing edges terminate to guarantee reattachment WITH the VGs.
*With respect to the VGs,I understand that Mitsubishi designed the units used on the Lancer.
*VGs on mirrors are usually the result wind noise discovered in an acoustic wind tunnel.The VGs can alter the noise signature of the mirror and may not be intended for drag reduction.Again,with a mirror,there would be no structure for separated air to reattach to,just like the semi-trailers.
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Old 12-15-2014, 10:30 PM   #37 (permalink)
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Madact -- It's worth noting that the first linked article includes the Englar material that went behind a paywall, starting on page 277.
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Old 12-15-2014, 11:16 PM   #38 (permalink)
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Looking closely at the results from those makes me wonder if there'd be merit to a 'double box' arrangement: On the outer edge (or at a very small offset), have a very shallow box, either straight or angled in, with the conventional 'offset cavity' inside it (blue). The shallow outer box (or 'lip' I guess) might even allow a splitter (red) to be more effective, though my money would be on the inner box cavity.

Goes against the KISS principle of course but I'm thinking if rounded edges is bad, sharp 90 degree edges is better, maybe sharper-than-90-degree edges may be better still? Unfortunately I'm not in possession of a large truck fitted with precision instrumentation to try it out

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Old 12-15-2014, 11:41 PM   #39 (permalink)
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I don't have a good picture handy, but my understanding is that the box cavity needs to touch the Template line, then you get little recirculating eddies on it's outer faces.

Subdividing might reduce the scale of the turbulence in the wake, or not.
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Old 12-16-2014, 01:56 AM   #40 (permalink)
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Originally Posted by freebeard View Post
I don't have a good picture handy, but my understanding is that the box cavity needs to touch the Template line, then you get little recirculating eddies on it's outer faces.
I'd agree with the latter part of the statement (about the stable eddies), but the "Template" (dare I speak it? ) may not be, shall we say, precisely universally applicable. I believe the base-pressure box cavities that have been found optimal all end well inside where the 'Template' would put them. With the of straight 'offset' cavities on the trucks, IIRC, it was about 0.7m in from the edge and 1m long, the 'full width' straight box also showed drag reduction (nothing remotely Template-like there), and with the 'full width' cavity with panels pointing in the good angles were in the 15-20 degree range - and that's even given a pretty sharp transition. The study on the mercedes A class hatchback also ended up with a box cavity depth of 10cm, with a fairly hefty inset.

<rant>
The template is a very nice rule of thumb which works well for attached (vs. almost-attached) turbulent flow and kammback construction, but it's only optimal / close to optimal for a limited set of circumstances. When you stray outside those, and get things like stable eddy structures which differ from the kammback case (as in this case), or laminar flow (good luck with that ), or cavitation (not a problem for us, dealing with land vehicles), or transsonic flow (not a problem for most of us) it becomes merely an optimal solution for someone else's problem. By way of analogy, in a free stream, a teardrop is better than a flat-nosed cylinder sometimes and better than a double cone sometimes and better than a rounded wedge making 2/3 of the length on the leading edge with a concave tail making 1/3 of the length on the trailing edge sometimes - but there are situations where the teardrop loses to each of these...
</rant>

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