Vortex generators in diffusers
 

Hi, has anyone tried the Vortex generators or airtabs as if they were a diffuser?

In theory, they should push the air upwards, enhancing the effect of the diffuser, not?

Yes, but...

I can't find the picture[s] I've seen of the underside of a modern race car, but vortex generators generally are used to add energy to the flow to encourage reattachment to the body. The race cars (I saw) use them behind the front wheels at the beginning of underfloor tunnels. On a diffuser they'd be at the front of it, but given the nature of the airflow in the plenum between the underbody and the ground, fences are what are used, to segregate and straighten the flow.

Maybe not what you were asking for, but something said reminded me of this image below from page-2 of the link.

Most interesting car aerodynamically - 2016
https://ecomodder.com/forum/showthre...6-a-34854.html
http://www.mulsannescorner.com/AudiR...st2016-SC3.JPG

Original source:
Mulsanne's Corner: Race Car Engineering Le Mans 2016
http://www.mulsannescorner.com/Kolle...ce2016-SC1.JPG
http://www.mulsannescorner.com/Toyot...ne2016-SC1.JPG

Might as well out myself...that site looks familiar :)

Has anyone tried something like this?

https://www.sigmaautomotive.com/perf...kz/image12.jpg

Site: sigmaautomotive

That's what I was looking for evidence of in that race car but could not find.

No, I have not seen that done before.

Is there any point to putting those at the very end when there is nothing to reattach to?

There is just a wake behind the car.

And in this white car's situation, not even a plane before it, just some very confused air.

From the clip flanges there seems to be a belly pan piece missing.

I think it can improve because it can make the air move up where there is less pressure and thus gain more downforce. And reduce wake area.

The same happens with the airtabs installed on the sides of the trailers.

And how race cars have diffusers, they should not need this.

The picture suggests but doesn't prove that someone has.
Those aren't vortex generators, those are (wimpy) fences.

It looks like they're all pointing straight back, aligned with the car's direction of travel. But for those to generate vortices, they need to present surface area to the flow i.e. be placed at an angle to the flow. So, depending on what the flow field looks like back there, they may or may not be vortex generators.

But, I have several questions:
1) Does generating a vortex or several vortices there mean they'll migrate upward, against gravity?
2) What effect or interaction will that vortex/those vortices have on the large vortices formed naturally and shed by the car's C-pillar?
3) How many times can one say "vortices" before it starts to sound weird?

It's hard to tell from that angle but it looks like the two sides are set at a slight angle. Just not enough— compare to the V-shaped Airtabs.

I don't think anything can act at right angles to the airflow. Any effect will propagate downstream.

If you want to wrap air around an edge, you need the Coanda effect.

https://ecomodder.com/forum/member-f...70-imagesa.jpg

Else, cold plasma actuators. Both require energy.

The last two posts ask some very good questions.

Aerospaceweb.org | Ask Us - Wing Vortex Devices
http://www.aerospaceweb.org/question.../generator.jpg

May 15th 2019
BAE Systems 3D printing enables first ever flight using supersonically blown air
https://3dprintingindustry.com/news/...wn-air-155452/
https://3dprintingindustry.com/wp-co...-thumbnail.png
The thing to perhaps keep in frame of mind is the squeezed high velocity air trapped under the car once liberated from it's confines will slow and expand and increase in pressure very rapidly, hence causing trailing drag behind the car.

What ever one can do to delay or mitigate this disruptive transition should help lower aerodynamic drag.

I just don't see these tiny shark fin devices located where they are having much of a chance to do that but welcome viewing any CFD or wind tunnel images anyone can come up with.

Not a right angle, necessarily, but an acute one--there has to be low enough pressure on the trailing side of the VG, since that's what induces the vortex.

VGs and diffuser
 

If the diffuser is designed for low drag,it will not have any separated flow for a VG to 'solve.'
The 'long' diffuser will be 2.8-degrees
The 'short' diffuser will be 4-degrees
That would be the end of story if designing for low drag.
VGs would only be a drag-inducing excrescence.
For a 'trackable' car,you'd want to follow Ferrari,which has 'valves' to control the diffuser as a drag reducing device.I think Freebeard is touching on that.

tried
 

I,as others,would question their placement.
Typically,a VG is positioned such that is enhances separated flow to reattach onto a trailing panel,downstream of the VG.
Since the VGs pictured are already at the trailing edge of the vehicle,I can't imagine what the expectation would be for their performance.If there was a box-cavity or something back there,it would be a different story.No testing that I know of ever created a reduced wake,'phantom' tail using any type of device such as this.:o

2D vs 3D
 

Hucho mentioned and emphasized that, never has a 2d flow device ever lowered the drag of a 3D body.
And history has shown that a simple low drag body is the best way to achieve low drag.They're not as 'sexy' as a race car,their just faster and more fuel efficient.

I do not question the operation of a diffuser. But in a car without diffuser, put the Vortex generators. For less drag and more downforce?.

He left a DYNA study on vgs and airtabs. And how they are placed in the back of the car.

Dyna

dyna
 

Thanks!
I would say that the results would be valid,only for the specific vehicles tested,exactly how they were configured, and any extrapolating of the performance equivalency to any other vehicle would be dubious.
If their numerical simulation satisfies the full Navier-Stokes equation,then I'm okay with that.
The base pressure of a vehicles wake is established by the local pressure at the 'first' separation line at the rear.The presence of VGs at the separation line would not alter that local pressure.There's no ramp behind the VGs on which the edge vortices induced by the VGs to act.I don't understand the drag reduction mechanism they purport to achieve.
It would have been more proper for MIRA to have conducted,rapid,back-to-back runs,with and without the VGs on the same vehicle,to isolate any variables between the two.I'm quite surprised that they conducted the testing in the manner in which they did.The SAE would not have allowed it.

I think that with the VG happens the same thing as with the publicity of the airtab

http://www.airtab.com/Site/themed-im.../trailflo1.gif

publicity
 

I know that what they promote is implied in the animation.I'm not sure that what actually happens is in agreement with the facts.
Details such as this would require a 'full-scale',full Navier-Stokes equation, numerical analysis,which currently requires about 2-days run time on a supercomputer.
Again,Hucho ran the Volkswagen full-scale climatic wind tunnel for 10-years,and he tested these sorts of things.He never saw anything lower drag ,except for box-cavities and boat tails.
I'm not calling Air-Tabs a 'liar',but I'd want a more rigorous investigation.
They haven't altered the frontal area,nor pressure at the separation line.I'd like an explanation for how they achieve a higher base pressure without some sort of reversed step.

Other example:

https://www.bumpershopuk.com/wp-cont...88184006-6.JPG

Rear undertray of several Toyota. Those two fins has any explanation?

As I mentioned in another thread here recently, those fins are not VGs. Toyota first used them on the 3rd gen Prius in 2010, and stated in the press materials that they improve directional stability--in other words, they are straightening vanes, similar to but not as pronounced as those seen on hypercars. Here's the 2015 Nissan LMP1 Le Mans car:

https://ecomodder.com/forum/member-v...322-162156.jpg

Toyota uses them on the current Prius, Camry, Corolla, Sienna, and Avalon.

Here are the diffuser vanes on the current Nissan Leaf, larger than Toyota's but with the same function:

https://ecomodder.com/forum/member-v...101-162607.jpg

VGs are smaller, since they don't need to protrude beyond the boundary layer, and shorter, since they aren't directing airflow but disrupting it slightly.

explanation
 

I think 'fin' is the proper term for which to describe them.I don't believe they are concealing a body eruption under the car for which they would serve as a drag-reducing fairing.They are very well streamlined,and would not themselves produce any appreciable drag.
I'm not familiar with the particular
cars they belong on,and can only guess that these fins are there simply to direct the flow longitudinally,and prevent any transverse flow at the point of their attachment under the car.Why? I don't know.
It's also possible,that in a crosswind gust,the fins might provide some pressure-biased,directional stability,otherwise lacking without them.The gang of 'strakes' under the tail of the 1987 GM/AeroVironment SunRaycer,solar racer,were used exactly for this purpose.

Those VGs ahead of the tire, I don’t suppose they were aiming for the same effect as a wheel skirt, were they? I also see something similar in the airdam of the new Tacoma to (I guess) direct air around the tires

vanes
 

Another benefit of the vanes,are that of corrugated stiffeners,which allow for a very light panel which is devoid of sagging or flutter,while not adding any appreciable drag.
In a rear,quartering wind gust,the vanes might add a degree of crosswind stability,while also preventing transverse flow contamination of the diffuser.

VGs ahead of the tires
 

I'm guessing that they are just flow guides,and to protect against spanwise flow,to keep the air directed straight back across the wheel.

http://www.airtab.com/Site/themed-im.../trailflo1.gif

Airtabs add frontal area.

https://ecomodder.com/forum/member-f...29-0-large.jpg

A serrated edge should have some similar effect.

The primary purpose of strakes in an LMP1 diffuser is to segregate the flow fields considering the relatively dirty flow outboard near the spinning rear wheel. The secondary function is to generate vortices while in yaw. Some teams cut them short of the diffuser trailing edge or have cutouts and notches in them in order to further promote yaw-induced vorticeis. In LMP1 you're limited to one pair that must be parallel to car CL.

Short of diffuser TE:
http://www.mulsannescorner.com/Acura...ng2009-MF5.jpg

Notch in strake TE:
http://www.mulsannescorner.com/Peuge...it2011-MF3.JPG

http://www.mulsannescorner.com/Rebel...it2011-MF3.JPG

http://www.mulsannescorner.com/OakRa...it2011-MF6.JPG

Holes in strake:
http://www.mulsannescorner.com/Oreca...ns2011-MF1.JPG


In the recent past, when there were few restrictions on their design, they were designed as vortex generators primarily.

http://www.mulsannescorner.com/MkIIIrearundertray.jpg

http://www.mulsannescorner.com/Nissan-CR4.jpg

http://www.mulsannescorner.com/Nissan-CR6.jpg

This I thought was ecomodder, not increasedownforcemodder. One must keep in mind that the basic principle of low drag is to NOT disturb the air you are plunging through. The more you make the air move, the more drag that will be felt. Moving air takes energy, we call that extra energy drag. To that end, having something stick up can only make air move more. If you want to decrease drag, you need to change the basic shape of the thing being pushed through the air.

Absolutely, there's very little to be learned looking at race cars. There is only one quest in racing, that's downforce. Downforce produces drag.

Yes--but, don't throw the baby out with the bathwater; there's a lot of room for improvement in detail optimization without changing the basic shape of the car (or even its look--Mercedes CLA, anyone?). That's 90% of what we do around here, after all.

I would agree with the last two posts except that most race cars address the underside of the vehicle very well aerodynamically speaking, whereas most production cars do not.

Air flowing under the car is very important, just look how Luigi Colani took advantage of it.

luigi colani: aerodynamic race cars
https://www.designboom.com/design/lu...mic-race-cars/
https://www.designboom.com/weblog/im...lanicars06.jpg

https://ecomodder.com/forum/showthre...iew-26432.html
http://www.carstyling.ru/resources/s...harisma_05.jpg

https://www.forocoches.com/foro/show...4212666&page=2
http://www.carstyling.ru/resources/s..._Utah-8_05.jpg
http://www.carstyling.ru/resources/s..._Utah-8_04.jpg

So yes, in general race cars suck as far as aerodynamics go, but to travel safely at speed they have to be sucked down. Cars are not aircraft, and there is something to be learned from any vehicle that travels though air or water!

1991 Colani Corvette Racer
https://www.forocoches.com/foro/show...4212666&page=2
http://www.carstyling.ru/resources/s...type_Racer.jpg

I wouldn't say that at all. Yes, there's one goal in racing, but it's competitive advantage. And that can take the form of decreased lift or decreased drag, or some combination of the two, and all within a (usually very) narrow set of rules.

Stability is also a primary concern of OEMs making passenger cars and trucks, based on how many pages are devoted to it in the various textbooks I've been reading, and stability is directly influenced by lift. Manufacturers have learned a lot from racing cars, and over the past few decades street car designs have reduced both drag and lift. Something like the Prius, which has Cd 0.24 and overall negative lift was unthinkable in the 80s in a production car. Heck, even modern Ferraris have drag coefficients in the 0.35 range with hundreds of pounds of negative lift at speed.

And there's room for improvement, where race cars still have an edge. Look at the nicely ducted radiator outlet on the 1999 Dallara IRL chassis, for instance:

https://ecomodder.com/forum/member-v...322-163001.jpg

Or the flattened control arms in the 2017 IndyCar suspension:

https://ecomodder.com/forum/member-v...627-111004.jpg

Or these wheel discs on the 1990 Lola 500 winner:

https://ecomodder.com/forum/member-v...627-104443.jpg

There's still lots we can emulate on race cars.

Anything that deviates from The Template. Right?

Once you deviate from the ideal (for packaging, cooling, etc.) small-scale features can be used to coax the airflow back into balance.

One of my favorite Colani designs:

https://ecomodder.com/forum/member-f...wu5wo1-500.jpg


* It looks like you posting fell off in 2018. Have you been lurking, or away?

You must have that image on "Speed Dial", I looked for it yesterday but it eluded me.

I will agree with the posters saying do not copy race cars thinking they are low drag designs.

I say we have something to learn from any exercise, which is why I started that Non-Aerodynamic or Un-Aerodynamic car thread.

https://ecomodder.com/forum/member-f...-concepts.html

There's more.

https://ecomodder.com/forum/member-f...-c-form-02.jpg

Have worked for better than 20 years in motorsports aerodynamics and, while a blanket statement, in the world of the wide-eyed, you'd do well to steer away from racing aerodynamics as the road car is a different beast. Yes, there are some takeaways. So I shouldn't have been so dismissive, but in racing we have vastly different parameters that are often arbitrary, such as technical regulations. The technical regulations carve out the car's shape, dictating the aero devices/methods you use, therefore the cars generated by the technical regulations are by no means optimized aerodynamically (they are optimized for the rules set, however), and therefore it is a very bad idea to look up to race cars as the ideal if working in a vacuum. Current LMP1 regulations intentionally design-in aerodynamic inefficiency as a hedge to keeping some semblance of control, over the long term, of lap time. What you see racing around the track is the embodiment of work-arounds to those technical regulations, nothing more, and are certainly not paradigms of aerodynamic efficiency. There isn't a racing series on the planet that plays homage to aerodynamics. They all have strict regulations that attempt to limit the aerodynamicists job in creating the fastest car to the regulations.

And hence why I'm out to sea when it comes to road car aero. 20 years in racing and all this stuff is new to me. Pretty cool actually.

My $.02

Bonneville?

racing series
 

I'd add the Southern California Timing Association venue,at El Mirage Dry Lake,California,Human-powered Submarine competition,National,and,World Solar Challenge, solar racing series, SAE Super Mileage competition,SHELL Eco Marathon,and annual IHPVA speed trials @ Battle Mountain,Nevada.:)

There's a big difference between race cars and road cars. Lets go back 56-52 years about talk about the 1963-1967 Corvette Stingray. The car may have been sleek and beautiful but it had a very serious aerodynamic problem. You see that real pretty front end produced lift at higher speeds. The car was a real handful to drive over 100 miles per hour because the front end got very light due to lift.

Jump ahead to the 1969 Dodge Daytona and 1970 Plymouth Superbird which had Cds in the .28 range. On the track those two cars had a huge advantage at high speed and not because of the sleek nose and the spoiler gaining them down force and aerodynamic advantage, but because of that tall wing in the rear. You see you couldn't spin the car out because as soon as you started to slide the back end out the air would hit the side of the strut holding the rear wing and push you right back to into a neutral air configuration.

vgs on trailers
 

We did some testing of Air-tabs on a 35 foot travel trailer using an aircraft engine blowing air down the side of the trailer and both tufts and smoke. Without the airtabs the tufts or ribbons just snapped around everywhere, tying themselves in knots behind the trailer, and the smoke curled tightly around behind the trailer. With the airtabs about 6 inches from the back of the trailer the tufts/ribbons stood straight out behind the trailer with virtually no flapping, and the smoke went back about 15 feet before starting to swirl. Wind speed was in excess of 60MPH down the side of the trailer. (about 12 feet? from a wall) I'd say they do pretty much what the information from airtabs claims they do. They sure keep the back of the trailer clean.