Does the shape of a car produce lift or downforce?

[Incanus]

I have a few questions related to downforce in cars. Please forgive any fundamental misunderstanding of aerodynamics.

• Does the shape of a car, say a Volkswagen Golf, driving on a road produce lift or downforce?
• Is the Coanda Effect at play there?
• How does the height of the car above the ground affect the lift or downforce?

Assuming a car produces lift:

• Can the basic shape (i.e. airfoil) of the car be manipulated to produce downforce, perhaps by changing the angle of attack (e.g. lowering the front of the car)?
• When the air hits the front of the car, is it not redirected to a smaller area underneath the car, causing it to accelerate? Is this an example of the Venturi effect, causing downforce?

Any clarification is much appreciated, thanks!

 

[jedishrfu]

Welcome to PF!

Is this homework or personal interest?

I found this on Wikipedia which may answer your questions:

http://en.m.wikipedia.org/wiki/Downforce

 

[Incanus]

I've read a bunch of the related Wikipedia articles, but I feel like I'm not understanding all the variables and how they affect each other. I understand racing cars can produce downforce as a result of wings, diffusers, etc. However, Wikipedia claims, for example, that almost all cars produce lift (citation needed) and I don't understand why that would be the case (both in terms of the physics - wouldn't the Venturi effect cause downforce? - and in terms of design - why did racing engineers have such a hard time adding downforce to cars in the 70s when there are so many ways to do so?). This is for personal interest, of course.

 

[rcgldr]

Most street cars produce a small amount of lift because of the shape of the upper body. When the air hits the front of the car, it's deflected in all directions, and the part that is deflected under typically experiences an acceleration that increases pressure for at least part of the way under the car. Racing cars use splitters and/or air dams to prevent air from flowing directly underneath a car, so the increased pressure is mostly in front of the underbody, and the area under the car and behind the splitter and/or air dam has decreased pressure.

Formula 1 cars started adding wings for downforce back in 1968, to cars that were otherwise similar to the non-downforce Formula 1 cars of 1967. As the years went by, the car bodies themselves were modified to improve downforce with methods such as diffusers, and relying on a low front wing to scoop the air away before it gets under the car. Formula 1 cars use skidboards under their bodies, while Indy Racing League cars use channeling which converges inwards for venturi effect and further decreases pressure under the race car (this is not allowed in Formula 1 by the rules). For a while, Formula 1 cars used movable side panels on the side that slid on the pavement to futher decrease pressure under the car, but this was banned (due to risk of sudden loss of downforce, if the side panels failed). Some of the CanAm cars (or maybe just the Chaparral) used fans at the rear to exhaust air from under the car body. Red Bull Formula 1 cars from a few years ago were orienting the exhaust flow so that it assisted the difusser, but this was later banned by the rules.

 

[CWatters]

Air flow over a car..
http://4.bp.blogspot.com/-iWBdkG9VyKM/T979hRIfJ8I/AAAAAAAAHrQ/_6UtUkW08iA/s1600/Historical+SLs+in+the+wind+tunnel+(5).jpg
Air flow over a wing..
http://thecfiguy.com/student/aerodynamics/airflow.jpg

Look similar?

 

[cjl]

Most cars produce lift, since the air accelerates as it flows over the car, creating a region of low pressure. In addition, the underside of most cars is pretty rough, and not streamlined at all. This tends to slow down air under the car, preventing much of a venturi effect due to the flow restriction there. High-performance cars often have a smooth underside with a diffuser at the rear of the car, which promotes high speed flow under the car and can generate some downforce that way. They often also have a flat or even upturned rear, which helps generate a higher pressure region at the rear of the car and negate some of the lift generated by the roof (it's nearly unavoidable that the roof of the car will generate some lift, due to the shape required to fit people and allow for visibility).

 

[rcgldr]

Most cars produce lift, since the air accelerates as it flows over the car, creating a region of low pressure. In addition, the underside of most cars is pretty rough, and not streamlined at all. This tends to slow down air under the car, preventing much of a venturi effect due to the flow restriction there. High-performance cars often have a smooth underside with a diffuser at the rear of the car, which promotes high speed flow under the car and can generate some downforce that way. They often also have a flat or even upturned rear, which helps generate a higher pressure region at the rear of the car and negate some of the lift generated by the roof (it's nearly unavoidable that the roof of the car will generate some lift, due to the shape required to fit people and allow for visibility).

It should be noted that the acceleration and decleration of air in this quote is with respect to the car.

Almost all race cars reduce and/or slow down the air flow under a car (relative to the car) using a wing, splitter, and/or air dam. The goal here is to prevent the air from flowing directly to underneath the car. The pressure in front and/or above the wing, splitter, and/or air dam is increased, but the restricted flow past the wing, splitter, and/or air dam results in reduced pressure. The upwards taper at the rear end of a race car (diffuser section), provides a path for the air to return to ambient pressure in order to maintain the low pressure under the car.

The main exception to this is (was? the rules may have changed) is Indy Racing League Car, where whatever air flow that does manage to flow under the front wing is horizontally converged to provide an additional venturi effect. This is not allowed in Formula 1, since they use flat bottoms called skid boards. In the case of Formula 1, the front wings may be a bit lower than that of Indy Racing League cars, to reduce the flow under the car more than what occurs with IRL cars. However, the rules for Formula 1 raised the minimum height for front wings in 2014, so I don't know if Formula 1 front wings are still lower than IRL front wings.

 

[cjl]

It should be noted that the acceleration and decleration of air in this quote is with respect to the car.

Almost all race cars reduce and/or slow down the air flow under a car (relative to the car) using a wing, splitter, and/or air dam. The goal here is to prevent the air from flowing directly to underneath the car. The pressure in front and/or above the wing, splitter, and/or air dam is increased, but the restricted flow past the wing, splitter, and/or air dam results in reduced pressure. The upwards taper at the rear end of a race car (diffuser section), provides a path for the air to return to ambient pressure in order to maintain the low pressure under the car.

Well, kind of. They do try to restrict the amount of air that can flow under the car, in large part to prevent a high pressure region from forming under the front of the car, but they absolutely do not try to restrict the velocity, and flow that does end up going under the car tends to accelerate (relative to the car). The diffuser returns the air to ambient pressure precisely because it increases the flowpath area for the air flowing under the car, allowing it to slow back down to freestream speed and pressure (relative to the car).

I agree with the rest of what you said though.