Air dam - aerodynamics

[Fluido]

Few questions about air dam(flexibile vertical plate on front bumper)

1. What is main purpose of air dam in car industry, reduce drag or reduce lift(increase downforce)?

2. How dam reduce drag if : a)increase frontal area, b)increase stagnation pressure at the front of car, c)cause high
turbulent air behind dam, d) decrease air speed under the car(if gap is zero)?

3. Air dam or flat undertray, does air dam has sense when car has flat smooth undertray?

4. Is better to put dam all the way to the road or must have some gap to work?

5. Make sense to install front dam and side skirts with zero gap(zero lekage) and leave back open to achieve maximum suction under the car?

Please dont use simple jurnalist explanation, I want scientific proves how air dam works, maybe some CFD-study or wind tunnel testing or so...

[Tommy Cookers]

I have no contribution to make .....
but to thank you for calling this an air dam - (and not calling it a spoiler)

please can we do the same for the thing at the rear ? - the upstandy ducktaily thing (that's also not a spoiler)

[Vyssion]

Hi mate, I'll do my best to explain from what I know:

1. What is main purpose of air dam in car industry, reduce drag or reduce lift(increase downforce)?

They were originally added as "anti-lift" devices back in like 1960 or something because as top speeds of cars increased in the past, the drag reduction activities of the designers led to overall car shapes which would could actually generate lift. It mainly affected closed-wheel racing (as opposed to open-wheel racing) because their shape more naturally is one of lift inducing.

Simon McBeath has a bit of a summary on them in his book "Competition Car Aerodynamics", but long story short, increasing airdam depth roughly increases downforce and decreases drag (to a point, obviously -- and the downforce gain is biased towards the front of the car; i.e. shifts aero-centre forward).

So simply, adding an airdam will reduce lift (and if you design it well, add downforce) and reduce drag.

The Hancha Group did some work on a Stock 1990-1997 Miata showing airdams vs. splitters here

2. How dam reduce drag if : a)increase frontal area, b)increase stagnation pressure at the front of car, c)cause high
turbulent air behind dam, d) decrease air speed under the car(if gap is zero)?

The effect on the air is that less air passes below the car, and more air is moved around the sides of the car vs. having no airdam. There's going to be an area of recirculation directly behind the airdam itself, of course, and the stagnation point on the front of the car is going to be slightly lower down than without the airdam. Both of these things means that more air is also going to go over the top of the bonnet than without the dam too.

You will usually see a much lower static pressure below the front half of the car with an airdam that will "suck" the car onto the road more, and then as the air moves to the rear half of the car, it can actually reverse force direction and "lift" the rear; giving rise to an overall downforce increase, but a further forward effect on aero-centre.

Regarding drag, the additional surface area at the front of the car will create more drag, for sure. But the increased region of stagnation will decrease drag over the forward portion of the bonnet, and due to this plus whats happening under the car with downforce, the "messiness" created by the underfloor and wheels etc is "neater", which also can reduce drag. It's a balancing act, but in general, you should see a slight reduction in overall drag when you sum up all the deltas.

3. Air dam or flat undertray, does air dam has sense when car has flat smooth undertray?

Because the airdam is kind of "neatening up the flow" under the car and also just reducing the amount of air that flows under a car -- which has all those messy pipes and brackets and components etc -- it stands to reason that you would expect drag to reduce just cause there's less "air" to impinge and react on ugly surfaces. If, however, you already have a smooth bottomed car, then you've reduced the amount of aerodynamic interaction already, and so you might find you don't get as much of a drag delta by adding an air dam -- and you might even just add drag outright given the effect of the forward stagnation pressure on the dam itself.

4. Is better to put dam all the way to the road or must have some gap to work?

It has to have some amount of gap to function correctly else you start to get into flow choking and just parachuting yourself into the air.

5. Make sense to install front dam and side skirts with zero gap(zero lekage) and leave back open to achieve maximum suction under the car?

See above answer, but reducing the amount of air which can bleed into your underfloor is always a good thing to do -- either by side skirts or using outwaching vortices.


One last thing to keep in mind is your car's cooling. By creating a region of lower static pressure behind the airdam (and below the engine) you actually can potentially increase your airflow "through" your engine. Lots of cars we drive every day seem to have a big hole at teh front to allow air "in" to the engine bay, but don't really do much to help get that hot air that's gone through the radiator "out"... If you can promote more airflow through the engine, then your rads can work better, and your engine can run cooler... which means that you can trade that off and run a smaller air intake (which is also a source of drag btw!) and save some weight too. However, this would require you to test / simulate this to make sure you still have enough overall cooling!! You also shouldn't forget about brake cooling too, but yeah... it's all interlinked

Hope this helps

[Fluido]

It has to have some amount of gap to function correctly else you start to get into flow choking and just parachuting yourself into the air.

Thanks for detail answer...

It seems there is two completly opposite way to produce downforce:
1. stop airflow with dam and skirts like NASCAR
or
2. accelerate airflow like F1,Aston martin valkyrie, wings, venturi tunnels etc


Isnt better to 100% seal front with air dam and sides with skirts and leave back open, isnt that produce maximum negative pressure ?Because dam is not here to accelarate airflow...

Look at this,dam and side skirts slide at the road surface

[Vyssion]

Thanks for detail answer...

It seems there is two completly opposite way to produce downforce:
1. stop airflow with dam and skirts like NASCAR
or
2. accelerate airflow like F1,Aston martin valkyrie, wings, venturi tunnels etc

Isnt better to 100% seal front with air dam and sides with skirts and leave back open, isnt that produce maximum negative pressure ?Because dam is not here to accelarate airflow...

Look at this,dam and side skirts slide at the road surface
https://i.redd.it/x8n2preg1yt01.jpg

Can't really tell if you're being sarcastic or not about "the detailed answer".

Regardless, I don't know the difference between fully blocking the entire airflow to the ground vs. allowing a relatively small amount of air to pass through it. Although in that image you linked, you can see some quite large mesh holes allowing at least some air to flow through; so it might be more than just a gain in isolation; e.g. back to my point about cooling -- they may theoretically get a gain by blocking entirely, but at the cost of terrible cooling, or something else.

I've never been involved in the design of a closed-wheel racing car, so beyond me making an educated guess, I'm not sure off the top of my head.

[Fluido]

Can't really tell if you're being sarcastic or not about "the detailed answer".

Regardless, I don't know the difference between fully blocking the entire airflow to the ground vs. allowing a relatively small amount of air to pass through it. Although in that image you linked, you can see some quite large mesh holes allowing at least some air to flow through; so it might be more than just a gain in isolation; e.g. back to my point about cooling -- they may theoretically get a gain by blocking entirely, but at the cost of terrible cooling, or something else.

I've never been involved in the design of a closed-wheel racing car, so beyond me making an educated guess, I'm not sure off the top of my head.

I am not sarcastic.

Air that enter this mesh holes dont exit under the floor,because floor is covered with flat plates.
Why do you think that leave small amount of air under the dam is good?

I think that air dam dont make sense at flat smooth undertray, if goal is reduce drag.
They probably reduce drag but only at rough uncoverd undertray.

[Vyssion]

Air that enter this mesh holes dont exit under the floor,because floor is covered with flat plates.

Nascar cars still have underbody components and features -- you can see in the images below that there's quite a lot of "opening area" around the engine and transmission.

Why do you think that leave small amount of air under the dam is good?

Cooling, as I said above:

One last thing to keep in mind is your car's cooling. By creating a region of lower static pressure behind the airdam (and below the engine) you actually can potentially increase your airflow "through" your engine. Lots of cars we drive every day seem to have a big hole at teh front to allow air "in" to the engine bay, but don't really do much to help get that hot air that's gone through the radiator "out"... If you can promote more airflow through the engine, then your rads can work better, and your engine can run cooler... which means that you can trade that off and run a smaller air intake (which is also a source of drag btw!) and save some weight too. However, this would require you to test / simulate this to make sure you still have enough overall cooling!! You also shouldn't forget about brake cooling too, but yeah... it's all interlinked

It's really bad engineering to just design a component without considering the effect on the whole system. Sure -- sealing off the entire underfloor may give you "max downforceeeee" but if your engine overheats in 3 minutes of racing, you're not gonna win.

I think that air dam dont make sense at flat smooth undertray, if goal is reduce drag.
They probably reduce drag but only at rough uncoverd undertray.

You asked this quesiton before, and I replied to the question in my original reply:

Because the airdam is kind of "neatening up the flow" under the car and also just reducing the amount of air that flows under a car -- which has all those messy pipes and brackets and components etc -- it stands to reason that you would expect drag to reduce just cause there's less "air" to impinge and react on ugly surfaces. If, however, you already have a smooth bottomed car, then you've reduced the amount of aerodynamic interaction already, and so you might find you don't get as much of a drag delta by adding an air dam -- and you might even just add drag outright given the effect of the forward stagnation pressure on the dam itself.

[Fluido]

It's really bad engineering to just design a component without considering the effect on the whole system. Sure -- sealing off the entire underfloor may give you "max downforceeeee" but if your engine overheats in 3 minutes of racing, you're not gonna win.

Do you think that 100% sealing with dam and skirts can achieve more downforce than with tunnels/diffuser like F1 do?
Becasue this is completly opposite way to get downforce,ona stops flow as much as possible and one accelerate as much as possible..

[Just_a_fan]

It's really bad engineering to just design a component without considering the effect on the whole system. Sure -- sealing off the entire underfloor may give you "max downforceeeee" but if your engine overheats in 3 minutes of racing, you're not gonna win.

Do you think that 100% sealing with dam and skirts can achieve more downforce than with tunnels/diffuser like F1 do?
Becasue this is completly opposite way to get downforce,ona stops flow as much as possible and one accelerate as much as possible..

The cars shown so far do not 100% seal the body to the floor anyway - they all have open wheel wells and thus lots of dirty air will be sucked in through the wheel well and under the car. Rather makes me wonder why they do it the way they do other than it's within some limit set by the rules.

A properly managed flow under the car, with underbody surfaces contoured correctly is going to make more downforce than a crude dam and skirts affair that relies solely on the base suction to create a "vacuum" under the car. I'd bet that the simple dam and skirts shown here:



Is pretty poor at making downforce, even with the barn door ducktail device it's running to increase base suction.

Also, looking at the flipped NASCARs show in another post, it looks like the radiator package vents in to the underfloor region behind the air dam. No doubt that helps cooling a lot when drafting nose to tail as they do.

[MrGooch]

Do you think that 100% sealing with dam and skirts can achieve more downforce than with tunnels/diffuser like F1 do?
Becasue this is completly opposite way to get downforce,ona stops flow as much as possible and one accelerate as much as possible..

It's a very different beast, obviously, but this is largely how funny car dragsters work. The car punches a huge hole in the air as it travels down the dragstrip at 250+ mph. The skirts and air dam seal off the bottom of the car and the low pressure zone behind is 'pulled' or extended forward underneath the body. I've seen it called 'hollow body' aero although I don't think that's a common name.

They key to this configuration is generating a big low pressure bubble behind the car (hence the quite tall funny car wedge shape) and creating a smooth transition from the bubble at the back to underneath the car. Traditional stock cars lack any kind of real underbody aero at the rear, hence why they still produce lower downforce numbers and benefit from pack racing. The low pressure region behind the car is stuck there until it is filled by another car.

edit: For OP - Some very basic simulation of air dams/skirts/diffusers. http://dpcars.net/, click on the second car in the list (D8) and go to Page 1.

[Just_a_fan]

Do you think that 100% sealing with dam and skirts can achieve more downforce than with tunnels/diffuser like F1 do?
Becasue this is completly opposite way to get downforce,ona stops flow as much as possible and one accelerate as much as possible..

It's a very different beast, obviously, but this is largely how funny car dragsters work. The car punches a huge hole in the air as it travels down the dragstrip at 250+ mph. The skirts and air dam seal off the bottom of the car and the low pressure zone behind is 'pulled' or extended forward underneath the body. I've seen it called 'hollow body' aero although I don't think that's a common name.

They key to this configuration is generating a big low pressure bubble behind the car (hence the quite tall funny car wedge shape) and creating a smooth transition from the bubble at the back to underneath the car. Traditional stock cars lack any kind of real underbody aero at the rear, hence why they still produce lower downforce numbers and benefit from pack racing. The low pressure region behind the car is stuck there until it is filled by another car.

Isn't that about keeping the centre of pressure as far to the rear as possible? Doing that gives you a nice stable car that wants to go in a straight line. And running the front bib on the floor helps to prevent a blow over at speed too, of course.