Ideal design for underbody venturi?

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ConsFW
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Ideal design for underbody venturi?

Post by ConsFW » Mon Jul 24, 2006 10:41 pm

Hi guys,

I've been wondering, what would be the ideal shape for the underbody of a car, without all those rules? Could someone explain how in detail how ground effect works? I know the basics of it, but would like some more in-depth understanding.

After searching the internet, I could only find pictures and explanations of wings and aerodynamics on the top of the car, never any pictures or explanations of the bottom (other than generic, vague statements and descriptions). Anyone have pictures of the bottom of a car that utilizes ground effect or some venturi?

Thanks! :wink:

Tom
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Post by Tom » Mon Jul 24, 2006 11:16 pm

The bottom would be dead flat by my understanding, or it might have a wing underneith with ground effects skirts, depending what downforce/drag ideals you can acheive on top.
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captainmorgan
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Post by captainmorgan » Mon Jul 24, 2006 11:47 pm

flat with a rear upward sloping diffuser. side skirts to keep flow longitudinal. Maybe 'venturi tunnels,' but i dont know exactly how they work.

http://www.jbskyline.net/R34/GTR/Showro ... Bild36.php

shows the front and rear diffusers for the old Skyline R34

http://images.google.com/imgres?imgurl= ... D%26sa%3DG

show the same for a 360 Modena. Those actually might be venturis, I'm not sure what the precise definition is.

basrawi
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Post by basrawi » Tue Jul 25, 2006 1:02 am

In racing cars, a designer's aim is not for increased lift but for increased downforce, allowing greater cornering speeds. (By the 1970s 'wings', or inverted aerofoils, were routinely used in the design of racing cars to increase downforce, but this is not ground effect.) This kind of ground effect is easily illustrated by taking a tarp out on a windy day an holding it close to the ground, it can be observed that when close enough to the ground the tarp will suddenly be sucked towards the ground.

However, substantial further downforce is available by understanding the ground to be part of the aerodynamic system in question. The basic idea is to create an area of low pressure underneath the car, so that the higher pressure above the car will apply a downward force. Naturally, to maximize the force one wants the maximal area at the minimal pressure. Racing car designers have achieved low pressure in two ways: first, by using a fan to push air out of the cavity; second, to design the underside of the car so that large amounts of incoming air are accelerated through a narrow slot between the car and the ground, lowering pressure by Bernoulli's principle. FIA regulations disallow ground effects in Formula One. 1982 was the last year in formula 1 with ground effect cars.

and the great example for a ground effect car is 1978 louts 79
M Basrawi

ConsFW
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Post by ConsFW » Tue Jul 25, 2006 1:12 am

Thanks for the responses. I have studied the principles behind the venturi tube and can see how it is applied to cars, but I would like something more indepth than just "it lowers the air pressure." Some pictures may help, or a more detailed discussion perhaps?

R1ceboy32
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Post by R1ceboy32 » Tue Jul 25, 2006 1:22 am

have fun =p
http://home.earthlink.net/~mmc1919/venturi.html
you also have to remember that the physics behind this principle will be different for air since it is compressible. I forget by how much though. It might be inverted.
*edit
i was going to type up this nice reply but basrawi essentially beat me to the punch =p welcome to the forums basrawi
Last edited by R1ceboy32 on Tue Jul 25, 2006 1:35 am, edited 1 time in total.

ConsFW
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Post by ConsFW » Tue Jul 25, 2006 1:35 am

Yes, thanks for the link but I've actually already studied that in my fluid dynamics class in college. I'm looking for an actual design-oriented discussion of underbody fluid dynamics (like how to optimize the shape, what to look out for), and perhaps some discussion of the "little" things that are not mentioned in typical text books. There is very little discussion in my textbooks devoted specifically to ground effects. For example, how does the aerodynamics of the venturi tube change when one of the surfaces is moving while the other one is stationary?

basrawi
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Post by basrawi » Tue Jul 25, 2006 1:46 am

Image

As shown above the underbody of the lotus79 looks like upside down aircraft wing (in red colour), when the air flows under the car it gets accelerated creating a low pressure area as a result the Car will literally be sucked to the ground. The yellow area are the side skirts, they prevent any air to escape from the sides of the car.
M Basrawi

ConsFW
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Post by ConsFW » Tue Jul 25, 2006 2:08 am

Great picture! But what happens when you are not constrained by an open-wheel form? Do you have any pictures of Lemans-type car underbodies?

Here's one of the things I've been thinking about. If you shape the car like an upside down wing like this :

(back) ```---..._____/ (front)

In the front you have a high pressure zone beneath the nose as the air hits it, like when you put your hand out the window of a car and tilt it upwards. This should produce an upward force, locally, but because of the overall shape of the wing, it should overall provide downforce. So on the one hand, the overall shape is good for downforce, but at the front there's a high pressure area that works to lift the front of the car. What would be ideal here? Wouldn't the high pressure at the nose destabilize the car?
Last edited by ConsFW on Tue Jul 25, 2006 2:48 pm, edited 1 time in total.

DaveKillens
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Post by DaveKillens » Tue Jul 25, 2006 2:11 am

Have you ever vacuumed the floor? It's basically the same, just done with racing in mind.
The idea is to have a chamber (underbody) that is sealed on the front, sides, and maybe even the rear. Then you suck the air out. In some older, radical racing cars (notably the Can Am Chaparral 2J of 1970 and the 1978 Brabahm BT46B) they had a fan at the rear to suck out the air from the underbody.

http://www.petelyons.com/Photo%20Galler ... ucker.html


Now some numbers. If you can get an underbody of just five feet by ten feet, and achieve just a reduction of one PSI, you get a downforce of 7,200 Lbs.
But they were too radical, too dominant over conventional race cars of their era, and found themselves out of favor.
The fans went the way of the dodo, but the idea was too strong to die. So some later series of race cars evolved where they were still closed off at the front and sides (the famous sliding side skirts) but the rear was a huge venturi at the rear to suck as much air out as possible.
The current Formula One car has severe regulations to avoid ground effects. Sliding skirts are banned, ride height of the chassis is controlled, and most of the underbody is fixed in dimension and flat. But at the rear, where the regulations allow, the designers put in diffusers to suck as much air out as possible. The high nose and all those bargeboards and turning vanes are there to carefully control how much air enters the underbody. The sidepods are sculpted to have a low pressure zone along the side of the sidepods, and the underbody flat tray is sharp, to discourage the flow of higher pressure air from the sides flowing under the body.
Ground effects are very much alive and well in racing, the engineers just have more restrictions and have to use every trick to eke out as much downforce from the chassis as possible.

ConsFW
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Post by ConsFW » Tue Jul 25, 2006 5:55 am

I've seen that fancar, it's really cool, but that's more active aerodynamics. I'm referring to passive aerodynamics. I understand how pressure, surface area etc work; I've studied fluid dynamics in college. I was hoping someone here could provide more than just the generic, basic concepts that I already understand... Maybe shed some light on the question I posed in my previous post? Another question I have is (not having any real world experience in aerodynamics) how do aerodynamicists optimize the aerodynamic forms? Do they just eyeball a design, test, crunch the numbers based on that design, revise, repeat?

scarbs
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Post by scarbs » Tue Jul 25, 2006 10:16 am

The ideal configuration is to make the car as much like a wing as possible with endplates sealed to the ground to retain the low pressure under the car.
In this respect the Lotus 88 and Ligier JS19 were the closest to the ideal layout, where by a full length tunnel was created.

Image

Taking this to an extreme tunnels starting from the front edge of the car to the rear edge exclude interference from the wheels. The biggest issue with this set up would be the packing of the rear suspension and drive shafts to allow as taller tunnel as possible, probably bringing the rear wheels forward would be better aerodynamically compromising the mechanical set up. Also making a skirt seal along such a long length would be difficult, the shape of the car above and beside (in the case of a closed wheel car) wood be relatively unimportant, aside form perhaps devices to create low pressure along the outside of the skirt and also gurneys above the trailing edge to help pull the air out from inside the tunnel.

Image

As a wing car (lets use that term rather than ground effect) has its centre of pressure move forward at speed mechanical set up such as third spring to maintain ride height (or active suspension)

Feeding such a huge and efficient tunnel would require the ramp at the forward edge of the floor, this as you pointed out would create a local high pressure (and hence lift), but as the total flow through the device would be that much better, the global effect is more downforce.

ConsFW
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Post by ConsFW » Tue Jul 25, 2006 7:37 pm

Thank you scarbs, great post! In the picture, the ramp angle is shallow and quite low. What happens when the ramp at the front of the car is increased? There should be more air flowing to the underside of the car, and the local pressure at the front should be increased, correct? Also, what happens to the stability of the car with this wing car configuration? If the car hits a large bump and the front end lifts, could this drastically reduce downforce and possibly cause a blow-over at high speeds? With the ramp angle at the front and increased local pressure, will this configuration cause instability? Why are lemans style cars all extremely low nosed?

scarbs
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Post by scarbs » Wed Jul 26, 2006 9:27 am

What happens when the ramp at the front of the car is increased?
Its case of no too much not too little…..The inlet needs to be configured to feed the tunnels at the cars chosen speed, too little and it will stall, to much and there will be lift at the front,
If the car hits a large bump and the front end lifts, could this drastically reduce downforce and possibly cause a blow-over at high speeds?
Yes due to the large surface area of the floor a blow over is likely, but wing cars are a little better (for a given fsurface area) than a flat bottom.
Also wing cars tend to porpoise, that it when the aero pushes the down on the ground , then soemthing causes the underbody to lose downforce (either choking or a skirt losing its seal) the car rises back up on its suspension and then sinks down again as the dowforce returns. This is one of the reason 80s F1 wing cars had such hard springs.
Why are lemans style cars all extremely low nosed?
The visible section of a Le Mans cars nose is at floor level underneath the body work the area under the nose rises up to form its own diffuser, this creates front downforce. Check out http://www.mulsannescorner.com/

furnik28
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Post by furnik28 » Wed Jul 26, 2006 10:08 pm

scarbs you can use your design with as like the enzo ferrari has.
rok