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  1. #1
    Contributing Member loudes13's Avatar
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    Default Tunnel Throat Design

    I'm new to aero, so please extend some patience to my ignorance.

    I own a Prince LSR P1 car. I am scratching my head on the tunnel design.







    The previous owner told me that Pat Prince added the teeth before throat to add turbulence for less downforce. I'm a newb to aero and how diffusers work, so is this correct?

    I will be running Hillclimbs and tight tracks like Blackhawk and Autobahn, so I'm more concerned with downforce over drag.

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    Underneith areo-trays/diffusers/tunnels- uses Bernoulli's Principle-"conservation of energy".If you speed up the flow, the pressure decreases[more downforce]-therfore if you slow the flow the pressure increases[less downforce]. The tray/tunnel traps the airflow and the diffusser increases the velocity. The equations are avail. in any good Physics book. Since the "teeth" create turbulance they should decrease the velocity and reduce the down force, but since they are decreasing TOTAL velocity they are probably decreasing TOTAL downforce!! But I am No areo expert. I just can't see them having a local effect.

    Have fun,Ted K.

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    Contributing Member DaveW's Avatar
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    Default I am no aero expert, but...

    I have seen teeth like these used to increase local velocity, generate vortices, and help the flow stay attached to the surface (underside of the tunnel). Keeping the flow attached results in more downforce since more of the tunnel area is effectively used. If flow separates, turbulence and stagnation occurs, causing a loss of downforce and a possible increase in drag.

    In addition, as Steve Lathrop said in post # 5 below, if the airflow is unstable and unpredictable, the handling will be similar, and possibly dangerous, or at least, unpleasant.

    On my undertray (Citation F2000) I accomplish good, stable, airflow by using small wing sections at the end of the undertray to accelerate the air at the exit, keeping the air attached all the way to the end of the "winglets." Photos of bottom rear of undertray below.
    Last edited by DaveW; 10.21.14 at 10:38 AM. Reason: added photos and middle paragraph
    Dave Weitzenhof

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    Contributing Member provamo's Avatar
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    sometimes i think we give all these aero tweaks too much thought

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    Quote Originally Posted by provamo View Post
    sometimes i think we give all these aero tweaks too much thought

    A better way to look at "aero tweaks" is that if they are not done correctly they can turn a decent race car into a monster to drive and possibly make it dangerous.

    An average FC can produce down force approaching 50% the weight of the car at speeds the car is reasonably expected to reach. But even if it was only 25%, that is a lot and it can have a very dramatic effect for or against enhanced performance. When you tune a car with ARBs, springs and ride height adjustements, you are changing the loads on the wheels by 25 pounds, maybe more maybe less. Aero can easily exceed the mechanical adjustments one might make. This is even true for a FF with "no wings".

    The fact that someone took the time to put shark's teeth in the tunnels shows that something needed to be done to fix a problem. My guess is that the tunnels made enough down force to influence the handling and that influence was not predictable or desireable. The structure is a little rough for what it is trying to do.

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    Contributing Member loudes13's Avatar
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    Keep the responses coming.

    Wouldn't the better solution for lowering rear downforce be to run a smaller wing, less AOA, or less gurney?

    I also suspect this was done because runoffs was at road america.

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    Senior Member SEComposites's Avatar
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    Vortex generators like these are used to trip the air into a turbulent state. Turbulent air flow stays attached to steeper tunnel and wing angles producing more downforce. Turbulent flow is of course more draggy than laminar flow. In my experience adding vg's to stimulate flow highlights a flaw in the design of the part. That said I've seem them used successfully on some cars with the benefit of very high quality CFD and wind tunnel testing. I'd be more inclined to play with the tunnel angles on your car rather than keep those vg's.

    It's difficult from that photo to see exactly what's going on, but I'd also be inclined to make a blanking plate to fill in the area that has the vg's in it so you'd end up with a more conventional flat floor forward of the rear tunnels.

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    Contributing Member Mike Devins's Avatar
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    Who had the car before you - was this Matt's old car?

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    Contributing Member DaveW's Avatar
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    Quote Originally Posted by loudes13 View Post
    Keep the responses coming.

    Wouldn't the better solution for lowering rear downforce be to run a smaller wing, less AOA, or less gurney?

    I also suspect this was done because runoffs was at road america.
    Those tabs, IMO, would create more downforce by keeping the flow attached, not less. So, for Road America, for the same downforce, less wing would be necessary, resulting in a net reduction in drag, since wings normally have a higher drag-to-downforce ratio than undertrays.
    Dave Weitzenhof

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    Fallen Friend nulrich's Avatar
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    If there was any real CFD (meaning proper detail and analysis, which is very unlikely at this level) or wind tunnel or track testing then I'd defer to data.

    However, I doubt very much these "shark teeth" increased downforce. They represent a very substantial portion of the tunnel cross-section and, unlike similar devices on the rear of an undertray or on a rear wing, they aren't acting in free air, the flow is constrained on all four sides (by the ground and the tunnel).

    It looks to me like they will simply choke the flow through the tunnel, reducing downforce and increasing drag.

    Given the shape of the tunnel from what I can see in the photos it will also be extremely sensitive to pitch angle, likely stalling in certain car attitudes. As Steve suggested, they may have installed these shark teeth to reduce that sensitivity, which they did by making the tunnel fairly inefficient at all times. The better approach would be to design a proper tunnel. Look at some of the tunnel designs on GTP cars before the flat bottom regulations or CART Indy cars.

    Intuitively it doesn't look like anywhere near an optimal solution, but the designer would have much more comprehensive information (I assume). Why don't you contact him?

    Nathan

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    Contributing Member DaveW's Avatar
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    Nathan,

    Maybe the result is somewhere in between what you said and I said - from our perspectives, we'll never know for sure. I'd guess though, that these were certainly not part of the original design. As Steve said, they were added in an attempt to fix a problem.
    Last edited by DaveW; 10.21.14 at 4:21 PM. Reason: Added credit to Steve
    Dave Weitzenhof

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    Contributing Member provamo's Avatar
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    Quote Originally Posted by DaveW View Post
    Nathan,

    Maybe the result is somewhere in between what you said and I said - from our perspectives, we'll never know for sure. I'd guess though, that these were certainly not part of the original design. They were added in an attempt to fix a problem.
    too bad tunnel time is so expensive

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    Contributing Member Rick Kean's Avatar
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    Initially I assumed these vortex generators were located downstream of the tunnel throat to add some energy and delay separation. Now I realize that they are upstream, and suggest you talk to Pat.
    "You GO Now"

    Rick

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    It's a ruse, or u can scratch your head with it, .......... old NHRA circle track trick ....... put something on the car and the next weekend everyone has it on their car. ..... I'd say the air comb slows down the thing and downforce ...... less efficient, dunno, need to get a leaf blower and steam stream .....

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    Contributing Member loudes13's Avatar
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    Quote Originally Posted by Mike Devins View Post
    Who had the car before you - was this Matt's old car?
    Mike, I'm glad you saw this. This is Mat Cutter's old Prince LSR.

    These teeth are about 6" in front of the throat (which momentarily matches the rest of the floor).

    I could make a blanking plate to reduce the angle going to throat pretty easily.

    Anyone think these 'vortex generators' might be helpful at very low speeds <70mph?

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    use wide cord rear and front wings for low speeds, can keep the tilt in the efficient range, think piper cub, it takes off at 60 mph me thinks, don't know about vortex but if it did would only be x percent more/less downforce ............

    geez, think dirt track outlaw top wing!!!! forgot that one .....

    perhaps long cord (long n short) would be more appropriate since we may refer to the span as "wide"
    Last edited by Modo; 10.21.14 at 4:55 PM.

  17. #17
    Contributing Member Rick Kean's Avatar
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    Quote Originally Posted by Modo View Post
    .......... old NHRA circle track trick .......
    Cheers!!
    "You GO Now"

    Rick

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    Contributing Member EYERACE's Avatar
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    Default like eye no anything

    maybe not so weird............

    get all your shop fans and put them as close to the floor as you can......get large torn open cardboard boxes and duct tape to make baffles that you direct from the fan body tops to your car's front.....maybe get a friend to hold a leaf blower in about the same place as the fans.....also......rig your shop vac with not only a nice long intake but also a long tube/flex-tube in the exhaust and put a narrowed tip on the end..........get some smoke bombs......light the smoke bombs and put them close to your shop vac intake [don't let them get sucked inside the shop vac!!!]..........now hold the smoking exhaust of the shop vac near the front of the car and watch the flow around/under the car

    sure it won't be a wind speed of 70 miles an hour but maybe you get a rough idea or two

    or at least some seriously burning eyes

  19. #19
    Senior Member Neil_Roberts's Avatar
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    Examples of well developed ground effect tunnels are 1989-1999 CART cars and the last couple of Formula Atlantic cars. Emulate one of those and you will probably like the results.

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    Contributing Member Jnovak's Avatar
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    Quote Originally Posted by Neil_Roberts View Post
    Examples of well developed ground effect tunnels are 1989-1999 CART cars and the last couple of Formula Atlantic cars. Emulate one of those and you will probably like the results.
    Where would you get such examples?
    Thanks ... Jay Novak
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    It's hard to comment on your setup without any pictures of it, but the devices which trip a laminar boundary layer into turbulence are usually quite small. Example:
    http://en.wikipedia.org/wiki/Wind_tu...aight_wing.jpg

    If you have large shark fins, they are probably there to smooth over sharp points in your aero map at specific rideheights and roll/yaw conditions. That or they're just an exagerated attempt at a trip strip.
    -Robert

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    Contributing Member loudes13's Avatar
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    Last night I spoke with Mat Cutter. When Pat Prince designed the tunnels he included these teeth as vortex generators to excite the air going into the diffuser. I understand the theory, but think it's flawed at anything more than parking lot speeds.

    My problem is do I sacrifice my tight budget to rework the floor P1 or P2 legal or leave it as is for a while? I can also eliminate the teeth and add an air fence for the time being.

  23. #23
    Contributing Member Jnovak's Avatar
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    Quote Originally Posted by loudes13 View Post
    Last night I spoke with Mat Cutter. When Pat Prince designed the tunnels he included these teeth as vortex generators to excite the air going into the diffuser. I understand the theory, but think it's flawed at anything more than parking lot speeds.

    My problem is do I sacrifice my tight budget to rework the floor P1 or P2 legal or leave it as is for a while? I can also eliminate the teeth and add an air fence for the time being.
    If the car has tunnels then it is most likely legal to race in P1. That would. Be the low cost solution. Alternatively you could cover up the tunnels and race in P2. I doubt that this would be too expensive and could probably be done with sheet aluminum or flat pieces of fiberglass.
    Thanks ... Jay Novak
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    Contributing Member loudes13's Avatar
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    I will autox it this weekend. I will spray some wd40 on there. If I get a visible pattern, I'll take some pics. I'll also try some rub strip air fences in the area and take pics of those patterns.

    Wish me luck!

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    Contributing Member provamo's Avatar
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    auto-x?

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    Contributing Member loudes13's Avatar
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    Parking lot autocross.

    Here's a pic looking forward from the rear.



    Looking back at RR:


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    Contributing Member loudes13's Avatar
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    Left side throat/choke point:



    Left side looking rear:




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    Contributing Member loudes13's Avatar
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    Looks like I can eliminate the choke point. I also don't like the splitter design. I was hoping this would be a little more plug and play, but I see potential.

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    Contributing Member provamo's Avatar
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    i just do not see that under-aero at your auto-x is important......how much time do you spend at , lets say, 55 mph?

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    Contributing Member loudes13's Avatar
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    Quote Originally Posted by provamo View Post
    i just do not see that under-aero at your auto-x is important......how much time do you spend at , lets say, 55 mph?
    Yes and no. Others have been seeing real gains with diffuser advancements, but I also run on traditional tracks.

    This season's track events are pretty much over here in northern IL, so this weekend's autox is one of the few events I have to get a feel for a new to me car. I also really enjoy the intense challenge of autox.

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    Quote Originally Posted by Jnovak View Post
    Where would you get such examples?
    There's a few of those cars at the Speedway museum at IMS. It has been a few years since I have been there, but I remember squating way down to get a look up under the back of one. Didn't see any teeth...
    Lola: When four springs just aren't enough.

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    Senior Member Neil_Roberts's Avatar
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    Quote Originally Posted by Jnovak View Post
    Where would you get such examples?
    "Ask the Man Who Owns One"

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    Contributing Member provamo's Avatar
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    Quote Originally Posted by Offcamber1 View Post
    There's a few of those cars at the Speedway museum at IMS. It has been a few years since I have been there, but I remember squating way down to get a look up under the back of one. Didn't see any teeth...
    at one time the PORSCHE Speedway car featured "teeth" at its leading edge

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    Jay,
    Next time you are in St Louis, try to visit Can-Am Cars Ltd. They have several ground effects Indy cars as well as well as several dozen undertrays stacked against one wall. Fun place to spend a couple of hours.
    Marty

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    Contributing Member Jnovak's Avatar
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    Quote Originally Posted by Marty Nygard View Post
    Jay,
    Next time you are in St Louis, try to visit Can-Am Cars Ltd. They have several ground effects Indy cars as well as well as several dozen undertrays stacked against one wall. Fun place to spend a couple of hours.
    Marty
    Thanks a ton Marty.
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    Banned Modo's Avatar
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    Hmmmmmmm, unlimited auto-x or prehaps A mod Solo II .....,....... can use evacuation fan like the old 956 sucker car and the other one, modest 25 Horse fan motor and appropriate blade,, at zero to 65-70 probably can't hurt yourself should the fan not suck!!!! at least clean the parking lot. LOL .....

    maybe just the other car (Hall???), 956 may have just had the twin turbos, step on that HP!!

    Below Jim - makes sense, but could be a local club event, yea didn't even like testing my FC in the u of md parking lot, cold tires can go a long way and first goes up to 80 mph so can't do much, semi-warm tire temps running in a circle at careful speeds, maybe can sense some superb stick per camber-weight changes, maybe not ........well if u can afford skid pad testing that's what u do
    Last edited by Modo; 10.23.14 at 2:04 PM.

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    Contributing Member Jim Garry's Avatar
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    Sucker fans were outlawed in Solo about 3 decades ago.

    Also, A and B Mod cars go a long way once they spin. If a fan gave out suddenly in an A Mod car at 60 mph or more, something bad could happen. Hit something solid or a course worker.
    Jim


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    doesn't go any farther than other cars that spin at 60mph.

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    Contributing Member provamo's Avatar
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    Quote Originally Posted by kjchristopher View Post


    doesn't go any farther than other cars that spin at 60mph.
    wings, skirts, vacuum..............................HOW could that thing spin out? LOL

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    Contributing Member Jim Garry's Avatar
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    Quote Originally Posted by kjchristopher View Post

    doesn't go any farther than other cars that spin at 60mph.
    I worded my statement too quickly and poorly. My apologies.

    A & B accelerate faster and get to higher speeds than other cars. Therefore in a given spot on a course where the vast majority of cars may be at 50 or below, A and B will be at 60 and above. There may be fixed objects that are safe at 50 but not above. We can say that the course should be altered but realistically some sites do not allow this.

    Thus it is less safe to use a device that further increases speed if that device can more easily fail than a fixed aero device.
    Jim


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