another vortex generator paper

[freebeard]

Egyptologists assume the Meidum pyramid collapsed as the Bent pyramid was half-complete. But enough going off-topic about pyramids


http://www.buzzfeed.com/daves4/all-ducks-wear-dog-masks-cg3#.hyXQdN3Dw

[cyclopathic]

It is funny how these VG threads degrade after several posts, dog mask, heh.

Back to aero posts: if we take for the basis numbers posted in Mitsubishi article (1.4% Cd reduction and 4.5% in Cl) detecting 1.4% outside of controlled experiment would be difficult. For example on my commute route MPG may vary within 10mpg depending on temperature, lights and traffic situation. 0.7-0.8mpg gain will be lost in the background noise.

I disagree that articles like in OP should be discarded completely. While they cannot claim any numerical accuracy, they still give useful comparison. Even if you can't say by what % A is bigger than B, you can still say that A is bigger than B.

There are numerous credible studies on VG airfoil application which prove that VGs can speed up boundary layer and delay flow separation. With cars having more complex shapes things get more complicated. Finding an optimal location for VGs is fool's errand or it could be.

Cars also need to adhere to aesthetics and safety regulations and should be easily incorporated into manufacturing process. So far the most common VG application is to the rear in attempt to reduce wake trail. I am not convinced they will not be effective in the front but may impose pedestrian crash hazard when applied to bumper, hood or roof leading edge.

But this does not mean that VGs are magical item which only works when placed in right place. There could be still benefits even if they are applied to less than optimal position.

Also last but not least assumptions on point of flow separation could be wrong. Even in relatively simple case (hatchback) you can't just assume it will be roof trailing edge. It could be a gap between the roof and hatch door.

[aerohead]

Originally Posted by cyclopathic

It is funny how these VG threads degrade after several posts, dog mask, heh.

Back to aero posts: if we take for the basis numbers posted in Mitsubishi article (1.4% Cd reduction and 4.5% in Cl) detecting 1.4% outside of controlled experiment would be difficult. For example on my commute route MPG may vary within 10mpg depending on temperature, lights and traffic situation. 0.7-0.8mpg gain will be lost in the background noise.

I disagree that articles like in OP should be discarded completely. While they cannot claim any numerical accuracy, they still give useful comparison. Even if you can't say by what % A is bigger than B, you can still say that A is bigger than B.

There are numerous credible studies on VG airfoil application which prove that VGs can speed up boundary layer and delay flow separation. With cars having more complex shapes things get more complicated. Finding an optimal location for VGs is fool's errand or it could be.

Cars also need to adhere to aesthetics and safety regulations and should be easily incorporated into manufacturing process. So far the most common VG application is to the rear in attempt to reduce wake trail. I am not convinced they will not be effective in the front but may impose pedestrian crash hazard when applied to bumper, hood or roof leading edge.

But this does not mean that VGs are magical item which only works when placed in right place. There could be still benefits even if they are applied to less than optimal position.

Also last but not least assumptions on point of flow separation could be wrong. Even in relatively simple case (hatchback) you can't just assume it will be roof trailing edge. It could be a gap between the roof and hatch door.

We'd want to keep an eye on their model scale and tunnel velocities.Gopal & Senthilkumar for instance,used a 1/15-scale model which would require a 300-mph tunnel speed.The 1st problem is,if they actually used 300-mph air,they'd be into transonic flow,with compressibility issues which don't affect real cars.The 2nd issue is that they use only a 16-mph air speed,which locks them into a laminar boundary layer,producing very high drag coefficient.
Their VGs act as dimples on a golfball,in themselves forcing the TBL transition only in the aftbody,resulting in a measured,90% drag reduction @ 15-degrees yaw,and 20% at 10-degrees yaw.The VGs aren't enhancing an ever-present,existing TBL,their affecting it's 'creation.' All real cars are at full TBL by 20-mph with zero enhancement.
-------------------------------------------------------------------------
Manikandan et al. used a 1/25-scale model TATA Nano in their CFD work,which requires 500-mph air for TBL.They say that using a 'momentum deficit method' renders acceptable accuracy.When you look at the momentum deficit method,it's predicated upon a max. 0.3-MACH airspeed.They'd be using 0.68-MACH.
------------------------------------------------------------------------
In 'Drag Reduction of a Car Using Vortex Generator,' the authors assume a boundary layer thickness based upon a BL thickness formula for a Laminar Boundary Layer (something which doesn't exist for cars).Their Reynolds number is below 500,000.Their highest airspeed is 4.8 m/s.
They report drag coefficients as high a 6.859 for their car,when a classic parachute is Cd 1.35.
-----------------------------------------------------------------------
We just need to careful with these small-scale investigations.

[freebeard]

Originally Posted by cyclopathic

It is funny how these VG threads degrade after several posts, dog mask, heh.

The way you test a system is to inject a little noise in the signal and listen for echoes.

In this case it seems to be an echo chamber. http://ecomodder.com/forum/showthrea...tml#post502483

Which has a last post dated 20151222. I'd forgotten it completely, but I went looking for Gray's Garage. Who seems to have been sucked down a memory hole here. *spooky music*

aerohead -- I liked Grays Garage's technique. with the upside down model in a water tunnel, with hydrogen bubbles substituting for smoke.

[cyclopathic]

This is a x-post from Prius C bad highway MPG thread:
=======================================
So got to travel on the same road in very similar conditions (load, temperature, wind, etc). Once again using cruise control to take driver out of consideration.

Computer calculated MPG was 41.4 vs 38.7 last time, this is average over almost an hour (had to abandon test after ~50mi as we ran into construction zone with lower speed limit). 7% improvement.

Difference? A set of EVO vortex generators installed alone the roof trailing edge.

I am building the underbelly panels when it warms up.
==================================

Prius C computer collects the MPG data on monthly basis, and the data collected in past 4 month seem to confirm the trend, though they are too imprecise to use in calculations. Since car gets ~2,300-3,000mi a month, this is result of ~11k worth of miles.

[freebeard]

Prius C bad highway MPG

Searching on that doesn't turn up any obvious thread. Adding 'vortex' doesn't help.

It seems like treating only the top one of the four rear sides can't be too effective. Experiments with blown slots (http://ecomodder.com/forum/showthrea...ots-10972.html, http://ecomodder.com/forum/showthrea...ics-29659.html) suggest

Originally Posted by myself@ [URL="http://ecomodder.com/forum/showthread.php/fairing-well-nasa-truck-aerodynamics-29659.html#post438834"

]permalink #10[/URL]]

That's Englar. Active aerodyamics via pumped air. He found he could effect plus/minus drag and lift by valving the slots on the four rear sides; All on to go, sides off to stop, top off (I think) for lift and bottom off for downforce (or vice versa).

vBulletin doesn't handle links inside quotes very well, I give up.


Different effects for the different sides, all four on for least drag, sides and bottom only for lift; it's not clear what doing the top on only would do.

[aerohead]

This article has been in my photo album here at EcoModder for many years,but the image scale is so small it's been essentially useless.
Copy Pro separated out the two photographs and text as three separate scans to enlarge the image scale.
The article is from POPULAR SCIENCE,Feb. 1988,page 20.



If you've seen 'Airtabs' on big rigs,they are also created by Gary O.Wheeler.
PS The title should have 'generator',not what I typed originally.

[freebeard]

Making little ones out of big ones, eh?

I'm not sure I buy the arrowheads on the graphic streamers. Are they saying the entire trunk-lid is in recirculation? Maybe just a patch at the base of the window?

[aerohead]

Originally Posted by freebeard

Making little ones out of big ones, eh?

I'm not sure I buy the arrowheads on the graphic streamers. Are they saying the entire trunk-lid is in recirculation? Maybe just a patch at the base of the window?

*'public indecency'
*My guess is that they're implying zero reattachment onto the boot,with the wake making it all the way forward to the backlight,without VGs. Then,they get reattachment,with limited counter-flow under the locked-vortex with them.Driving on a dusty road would probably tell us.
*The current Wheeler 'Airtabs' are just a single,non-copulating VG and designed quite a bit different.
One thought about them,is that if the Honda's boundary layer is as thick as that of the Mitsubishi Lancer EVO,then the Wheeler VG is a sub-TBL-thickness VG,whereas those employed on the Lancer are as thick as the TBL.Around 25mm.

[cyclopathic]

Originally Posted by freebeard

Searching on that doesn't turn up any obvious thread. Adding 'vortex' doesn't help.

It seems like treating only the top one of the four rear sides can't be too effective. Experiments with blown slots (http://ecomodder.com/forum/showthrea...ots-10972.html, http://ecomodder.com/forum/showthrea...ics-29659.html) suggest

vBulletin doesn't handle links inside quotes very well, I give up.


Different effects for the different sides, all four on for least drag, sides and bottom only for lift; it's not clear what doing the top on only would do.

Here is the link: http://ecomodder.com/forum/showthrea...eed-33138.html

Prius C already has VGs built into tail lights. With addition on the roof you could say that all rear end above waist line is treated.

I haven't had high expectations on VGs, but they were cheap (set for $12 total) to try. The idea was not to improve car aerodynamics, but to improve performance of existing aero devices on the car. As Prius C has the spoiler alone trailing edge, VGs installed ~12" in front, angled at 22deg seem to improve its performance.

Also I would not read too much into 7%, there are likely some other factors at work. Looks like Prius C had been optimized for lower speeds, so slight decrease in aerodynamic drag could have resulted in much higher MPG gain.