Vortex Generators 2

[sprintcar]

Trying to improve lift on a race wing at slow speeds - 90 to 125 mph range. Any hints / formulas / websites on using vortex generators? There are commercial (plastic) ones available, but location relative to leading edge is unclear. We have a 3M tape that will adhere whatever we can build to the aluminum surface.
Thanks for the help!

 

[Haf]

Under certain conditions, vortex generators have been shown to decrease drag, although I don't know about the effect on lift. The geometry of the vortex gernerators is important, and is primarily determined by Reynolds number. Try doing a literature search on vortex generators and see what you come up with. I think I might be able to dig up a couple of references if you can't find any. Let me know.

 

[lorenzo73]

Hi! I hope you refer to increase downforce and not lift for a car!

First you have to consider the chord lenght of the wing (distance from leading to trailing edge).
Once it's specified, you can calculate Reynolds number.
assuming costant and equal to 1.2 kg/m3 the density of air, and the car's velocity parameter known, you can calculate:

Re=density of air * V * L / viscosity of air

where V=car's velocity (in m/s or feet per second)
L = chord lenght
viscosity = 1.8e-5 Ns/m2

sorry, you will have to convert meters to feet...(i don't have here the convertor)

this parameter is useful to evaluate in what range of flow you will find yourself (prevalent laminar or turbolent?)

For your specified case...a downforce wing has its critical side on the lower part of the wing (the contrary of lifting airfoil).
The most difficult thing to do is placing in a very good position the vortex generator or turbolator.

This has to be placed on a point of adverse Pressure gradient (where there is a positive slope of pressure).
To intercept this point, you would have to study the velocity distribution on the wing.
The strip must be placed for optimal resolution just exactly before the transition region of boundary layer, to introduce enough disturbance to cause transition into turbulent state.

Another thing to keep in mind is that if you place the vortex generator forward on the wing – you will avoid bubbles (bad separation) at higher downf coeff (but this creates bubbles al lower downf coeff)
When it’s RWD – no effect on high downf coeff, but in lower.

I've seen the strip placed normally to 70% chord.
But Attention!! This has to have a minimum high to be effective, sufficient to cause transition, but NOT THICKER than boundary layer, to avoid additional drag (keep in mind that normally, a turbolent boundary layer is some mm high).

If your interest is instead only to increase downforce and not consider additional drag, the best place to fix the turbolator is on trailing edge, creating the gurney flap.

I think vortex of plastic is quite good for your case.


I hope this will be useful to you...
if you will have more questions (especially on gurneys), ask!
Hi!
Lo

 

[sprintcar]

Thanks!!
The overall length (Front to back) is 60" or 1.524 meter. Width is the same according to rules. There is a panel on each side and we can add 'rudder strips' front to rear to help straighten airflow.
Velocity = 125 mph = 56 m/sec
We already use a gurney lip - 1" tall. I typically set it to be vertical when the wing is in race position.
The wing is typically run at 17 degrees from horizontal - The research I did earlier indicated the profile had a stall angle of 9 degrees. I'd like to run it flatter - about 10-12 degrees to cut drag.
The plastic Vortex Generators were from large truck applications where they were used on the back of the cab or trailer. The leading edge is about 1/4" tall and I wonder if this is too much?
I tried a few small aluminum fins under the wing about 18" back from the leading edge. You could see the change in airflow by the traces left from track dirt, but I couldn't tell you if it made a difference.

 

[sprintcar]

good converter calculator site

http://microimg.com/science/velocity_equiv.html

 

[lorenzo73]

ok, understood!
compliments!! your wing is very flat!...
the Re in your case are approx 5689601.13792, not so laminar. in this range there is a turbolence, not so strong but not so positive too!
the theoritical idea under turbolators are simple (if you are interested):
when there is a separation bubble, this can be of 2 types:
"short" - if it's just behind the leading edge;
"long" - if it starts far behind the l.e.
in the 1' case there isn't an alteration on P surface distribution (in respect on a condition in which a bubble donesn't exists), in the 2' yes, and also strong.

if a bubble have a delayed reattchment or worst a not complete reattchment, there will be a very negative effect for downforce coeffs!!!
this will 'cause (in this case) at a separated laminar flow from wing. the b layer will leave the surface in a tangential direction, creating a turbolent wedge with increased drag (high levels) due to the great height of turbolent particles.

a bubble will reattaches only if there will be a turbulent flow inside (natural or artificially created = your case!!).
before this will happens, better it will be for drag levels.

your pratical case is the 1': "short".
this, because the most critical part for a downforce wing, is the area just behind the leading edge on lower surface (where it grows the strong adverse pressure gradient).

concluding, the things i can tell you for your pratical case are:
1- keep the surface of the wing the more cleaned as possible (clean it with a water jet if you run in a off-road condition or (as you have said) there is dust).
this will prevent an easier separation due to surface roughness (at Re < than 0,3 Mach the flow is very sensitive at this parameter).
2-place the turbolator just behind the rounded ledge region on lower surface (18 inches seems perhaps to me too much ...try a little nearer...10 or 11).
3-place (as you have already done!) a gurney (very good the vertical position...because this lead to an acceleration of flow form lower to upper surface, just right to the vertical tip, creating a rotational motion of flow. remember that this must be placed ALWAYS vertical to FLOW DIRECTION !!!).
this is the best method to increase downforce but at a medium to high drag cost.
this gurney (for a wing of chord = 70 cm), usually is high 1-2 cm.
the panels (vertical endplates) are also useful to force the flow just over and under the wing, creating a sort of infinite wing effect.

but remember also that increasing the alpha (angle of attack) will lead to an increase of drag, not the contrary.
you can only try to reduce it...
with turbolator!!!
i understand your doubt when say that you can't see if there is an improvement...this i think it would be clear only in a wind tunnel.....!!!!

for your doubt on the leading edge region...consider that (theoretically speaking) if this is thinner area but with a camber (on entire wing), there will be more efficiency at low speed (your case).

hope this will be interesting for you.

ps: thanks for the site...
Lo

 

[sprintcar]

Thanks Lo!
That helps a lot. Looking at wing profiles, it seemed that the thicker sections were better at slower speeds. Currently the &quot;belly&quot; section is about 5&quot; tall for standard wing, 7&quot; for the slow speed wing, and about 3 1/2 for the high speed (mile long track) wings at about 140mph. If the thin section would be best with more attack angle at slower (<= 125mph) speeds, we can make it that way.
Regarding cleaning the wing - we keep the underside as clean as possible. I cover it with a plastic sheet to keep mud from sticking and smooth the surface. Sounds like polishing it would even be better?
Have you ever considered the effect of dimpling the surface like a golf ball?
Send me your email address if you'd like and I'll send a picture of the race car
showshine@aol.com
Thanks again for helping me understand all this!
Bob

 

[lorenzo73]

good the covering with plastic, but eyes that this sheet doesn't &quot;flap&quot; (move like a flag)...if not, all laminar flow it will be ruined!!

my email is:
LO1800@excite.it

it would be great for me see your race car...thanks!

it's a pleasure for me to help....

damping the surface like a golf ball....that surface impefections (holes) are made essentially to create a larger transition on that typical surface, but only because it travels at very very low speeds and because that body is rotating on flow direction.
a wing has a behaviour very different...

i'm studying instead the application of porous surface for wings at Re values at you are interested (if you want to see my related question, it is a previous item on this forum..thanks to Haf for the material that i'm looking also...)...
if i will discover something useful for your application, i'll tell you immediately!!

Hi again!!
Lo

 

[sprintcar]

Plastic is contact paper - used on shelves - sticks very tightly but can be removed.
The wings can now be powder coated which will help surface finish.

Another question - with the side panels.... what should the minimum height be relative to the belly section to keep the airflow from 'spilling over' when the car is at an angle in the turns?? I figured at least 4&quot;

 

[yak55m]

I put votex generators on my airplane (Yak55M)- reduced stall speed, icreased cruise and no high g buffet during aerobatics. The general rule is to put the gerators just before the maximum thickness of the airfoil where the pressure gradient is still positive. You can get complicated and caculate the lenth of laminar flow and all that but it may not end up being real world. Just before the maximum thickness seems to be what works for everybody. Most times engineering comes down to passed down knowlage of what works anyway...

 

[mrvortex]

I was Dan Gurney's chief designer at AARin the 1970's and have been using VGs on airfoils and automotive applications ever since. I was also the one who introduced Gurney Flaps to the aviation world through Robert H. Liebeck at Douglas, Long Beach. My opinion is that there is rarely if ever any such thing as laminar onset flow availabe for any race car wing. And if there were, you still would never want to use a laminar flow airfoil because form or section drag is negligable for race car wings because of their awful aspect ration required by racing rules. Because of the poor aspect ratio, induced drag due to unwanted 3D effects create huge tip vortices that create the lion's share of the wing drag. (tip vortices alone, consumed 600 HP delivered to the road at 200 mph on our 1972-75 Indy Eagles!) Since induced drag due to negative lift as expressed by tip vortex losses is so dominant, the height of Gurney Flaps and VG's is not a drag consideration. The additional negative lift they may yield simply creates larger unwanted and unavoidable tip vortex losses and that is where the drag comes from. Not from the device itself! You may find that it is better to run a much larger Gurney Flap in conjunction with a lower angle of attack because this may get the wing's leading edge into better airflow. It's buried in the math, but if the main wing operates at a higher percentage of vehicle speed by virtue of it's onset flow being better, then it's induced drag will be less! Whether or not this will work for the top-mounted main wing of your sprint car should be easy to explore experimentally. Just trade some increased Gurney Flap height for some reduced angle of attack and see what happens once you get the same amount of downforce. mrvortex

 

[sprintcar]

Thanks!!
The rules limit the GF to 1&quot; but I tend to stretch it a bit since they don't check it - The new wing design uses rounded corners on the sideboards to reduce the tip vortex effect, so it sounds like we're in the right direction.

Great to 'meet' someone who has made such a significant contribution to motorsports. Email me showshine@aol.com if you'd like and I'll send you a picture of the new design, Keep the wheels on the ground
Bob
showshine@aol.com

 

[vortexman]

sprintcar,

You said initially that you wanted to increase lift, and I can only assume that you also want to limit drag. Are you not getting enough lift now, or do you just need to reduce drag to improve speed? The answer to that question is important, because you don't want to make more lift than you need. I have not read all the other responses, so excuse me if I repeat some things. Generally, vortex generators will only help you out if you have early separation due to laminar flow. I doubt you have laminar flow, so I wouldn't assume that VGs will help. Have you determined that you have separation? I would look at an airfoil section with more camber, and try to get it out into the freestream as much as possible. Chances are that induced drag is your biggest issue, due to the poor aspect ratio. Induced drag is highest when a wing is operated at high coefficient of lift, and the wing must operate at high CL when the velocity is low, which is why you want that thing as far into the free stream as you can get it ( within reason ).

Good luck,


vortexman

 

[sprintcar]

Right - reducing drag would be beneficial, but the gain in the wing (inverted airfoil section) producing downforce for tire traction. Generally the wing is mounted well above the car in free air (except when following another car)

I've also seen a new profile that is pinched in the middle - any ideas?

Thanks Keep the wheels on the ground
Bob
showshine@aol.com