Downforce- which explanation is correct?

[McMrocks]

Hi guys, i hope this is the correct place for the thread. Before you say that this is off-topic please notice that the post is aimed at the question how downforce is created or how downforce can be explained on F1 cars. I'm currently studying automotive engineering at a University but all the explanations of the fluid dynamics Proffesors couldn't help me.


The lift theory (for aircrafts) we all learn in school goes:
The air moves faster over the top of the wing and therefor the pressure on top of the wing is lower than the pressure beneath it.

This explanation is based on the believe that the air above and below the wing will meet at the trailing edge at the same time. It is called the equal transit theory.
Yet- this is wrong. You can't simply assume that the air molecules have to meet again at the same time. And they don't(0:45) :


This is why NASA says that this is the incorrect lift theory: https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
I, for my part, believe that NASA is a credible source.

Other websites say that this theory is flawed too: http://www.allstar.fiu.edu/aero/airflylvl3.htm

The usual claim is that when the air separates at the leading edge, the part that goes over the top must converge at the trailing edge with the part that goes under the bottom. This is the so-called "principle of equal transit times".

As discussed by Gale Craig (Stop Abusing Bernoulli! How Airplanes Really Fly., Regenerative Press, Anderson, Indiana, 1997), let us assume that this argument were true. The average speeds of the air over and under the wing are easily determined because we can measure the distances and thus the speeds can be calculated. From Bernoulli’s principle, we can then determine the pressure forces and thus lift. If we do a simple calculation we would find that in order to generate the required lift for a typical small airplane, the distance over the top of the wing must be about 50% longer than under the bottom. Figure 1 shows what such an airfoil would look like. Now, imagine what a Boeing 747 wing would have to look like!

please have a look on the airfoil in the link. It is obvious that no aircraft has an airfoil like this.
Also:

If we look at the wing of a typical small plane, which has a top surface that is 1.5 - 2.5% longer than the bottom, we discover that a Cessna 172 would have to fly at over 400 mph to generate enough lift. Clearly, something in this description of lift is flawed.

Not good.

However after this theory is deemed wrong the link has another explanation for lift:

The lift of a wing is equal to the rate of change in momentum of the air it is diverting down. Momentum is the product of mass and velocity. The lift of a wing is proportional to the amount of air diverted down per second times the downward velocity of that air. Its that simple. (Here we have used an alternate form of Newton’s second law that relates the acceleration of an object to its mass and to the force on it; F=ma) For more lift the wing can either divert more air (mass) or increase its downward velocity. This downward velocity behind the wing is called "downwash". Figure 5 shows how the downwash appears to the pilot (or in a wind tunnel). The figure also shows how the downwash appears to an observer on the ground watching the wing go by. To the pilot the air is coming off the wing at roughly the angle of attack. To the observer on the ground, if he or she could see the air, it would be coming off the wing almost vertically. The greater the angle of attack, the greater the vertical velocity. Likewise, for the same angle of attack, the greater the speed of the wing the greater the vertical velocity. Both the increase in the speed and the increase of the angle of attack increase the length of the vertical arrow. It is this vertical velocity that gives the wing lift.

The source once more: http://www.allstar.fiu.edu/aero/airflylvl3.htm
Saying that it is the downwash that generates lift.

And Peter Prodromou agrees. At 1:25 "...the wings purpose is to divert the flow down."



BUT
It all goes wrong when it comes to ground effect. The link above says in the last paragraph (really worth a read on a test day) that ground effect exist because the ground minimises the upwashed air ahead of a wing which causes negative lift (downforce) on airplanes. The link also says that the upper surface is much more important for lift because the coanda effect is very powerful and allows to divert a lot of air above the wing down.

And now we have a look on F1 cars where everything is upside down.

First of all the diffuser: If we want to create as much upwash as possible it makes less sense to drive as low as possible to the ground. The incorrect lift theory makes much more sense on this. It makes sense that the low pressure beneath the floor and the high pressure above it creates downforce and not the upwash behind the floor.

Also the front wing:
why would you run it so close to the ground? It diminishs the amount of air than can be diverted upwards. And as we know it is the outside surface that is important for the upwash.


The questions are: Is the explanation of lift based on upwash and downwash just as flawed as the explanation of lift with the equal transit theory? Is downforce on F1 cars not rather the product of different pressures above and below the wing?

And last but not least: What is the correct lift theory in your opinion?

Have a nice (testing) day and enjoy reading the links above. It is worth it

[Andres125sx]

Very interesting thread, I´m just a fan of aerodynamics as I fly rc planes and watch F1 for some decades, so aerodynamics has always interested me. But I´m not aeronautical engineer so my knowledge is very limited and can´t add anything useful to the thread. I actually had no idea about this, to me this is surprising

But I´m asking myself if it may be possible both theories are correct so you can create lift both ways, and it´s just that wings take more advantage of diverting airflow while floors take more advantage of creating different pressures


Also for planes, lift is not created exclusively by different air speeds due to different lenghts of the top and bottom surfaces, increasing angle of attack also create lift regardless of lenghts and air speed on each surface (aerobatic planes with symmetrical wings), and also motor thurst, due to angle of attack, create a vector pushing upward wich becomes obvious when aerobatic planes do a hover hanging on the prop, but that same force doesn´t start when it´s looking 90 degrees up, but when angle of attack is anything positive, so in take off when a plane roll up and increase AoA, both the engine thrust and increased pressure in the bottom surface of the wing add some lift to the ecuation. Than may explain the reason a Cessna take off at a much more reasonable speed than those 400mph wich only take into account different air speed between surfaces.

Sorry if this is too obvious or absurd, as I´ve explained I´m far from an expert here

[timband]

You are confused about the term "ground effect". It refers to something quite different when referring to racing cars than it does when referring to aircraft. See the following Wikipedia articles: https://en.wikipedia.org/wiki/Ground_ef ... ynamics%29 and https://en.wikipedia.org/wiki/Ground_effect_%28cars%29.

The ground effect in racing cars is a tricky thing to understand. My layman's understanding of it is that the ground rushing past underneath the car is pulling air out, and so provided you can restrict the air going in you get an area of low pressure formed under the car; this might be hopelessly wrong, though.

[krisfx]

My understanding is that it's a bit of both in regards to Newton & Bernoulli, which is well documented on the web, I'm a bit rusty from the work we did in uni as I've had little free time to continue learning aerodynamic bits.

Iirc, the reason for lower pressure is that the stream lines on the longer surface are closer together, reducing static pressure. A lot of literature also points out that we get "downwash" from the trailing edge of wings, which means they're exerting a force on the air and there must be a reaction force, pushing the wing up.

I dunno if this is a lot of what you have already said, I skimmed on my lunch, but I'm sure there'll be more in depth from some people on the site!

[McMrocks]

My understanding is that it's a bit of both in regards to Newton & Bernoulli

I guess this is true. However it would be interesting how much % of lift of an aircraft is made by Bernoulli and how much is made by diverting the airflow down

You are confused about the term "ground effect".

That would mean front wings are making use of the ground effect, thus they are so low to the ground. Meaning that it aren't solely front wings but also (front) diffusers

[tuj]

Trust me, they would run the rear end higher if they could, but its a compromise of ride height versus stalling the diffusor at high speeds. At high speeds, the suspension compresses because of the aero loading which chokes the diffusor and it stalls. This was a widespread problem in 2012. The Ferrari in particular, because of its aggressive DRS, would exhibit tail-happy behavior on initial braking / corner entry.

One of the problems is that you design a diffusor for the average high-speed and medium-speed cornering flows, not for the straights. This was why you saw Mercedes use a Double DRS system that routed air to the front wing; this effectively stalled the front wing and allowed more air to flow under the car.

[bhall II]

In context, I don't know that I agree with the above assessment.

At Monza, ride heights have to be set low enough to promote some stall in the diffuser at high speed while maintaining grip at around 130mph as the car pitches, yaws and rolls through the tricky second part of the Ascari chicane. As the DRS is activated on the straight, the stall invoked in the rear wing has to promote a more generalised stall through the beam wing and diffuser and, in so doing, shed the speed-sapping drag that is an inescapable feature of downforce.

That would mean front wings...aren't solely front wings but also (front) diffusers

In fact, they're only diffusers. Save for the rear wing, the entire car is in ground effect and behaves accordingly.

EDIT: Actually, that's probably oversimplified. Let's just say this: the overwhelmingly vast majority of front wing R&D is dedicated to enhancing their diffuser-like properties.

[McMrocks]

The ground effect on cars theory implies that the Bernoulli theory is correct too, as Andreas125sx said, and lift/downforce can be created by pressure differences above and below the body. So the theory deemed wrong isn't so wrong after all?

And shouldn't therefor the rearwing be banned as it creates a lot of upwash and thus dirty air? It is also more sensitive to dirty air than the diffuser which is just creating a pressure difference and thus doesn't rely on clean air being diverted upwards

[Per]

My understanding is that it's a bit of both in regards to Newton & Bernoulli

It's not a bit of both. Both are a different approach to saying the same thing.

Bernouilli's theorem is a great observation of aerodynamic phenomena and is represented in a mighty elegant mathematical formula. It is a great piece of work. But it is not a fundamental description of the physics behind lift. When someone says "the air at one surface moves faster, therefore the pressure is lower", I always ask "why is that so", and most importantly, "how come the air moves faster in the first place?"

For that, Newton comes in to play. Newton's laws also apply to gas particles, i.e. air will only change its speed or direction when a force (pressure differential) is exerted to it. The lower pressure on the upper surface of an aircraft wing is generated due to viscosity of the fluid, and this in turn causes air above and in front of the wing to accelerate.

Bernouilli then observed that faster moving air corresponds to lower static pressure. This is absolutely correct, and it is not contradicting Newton. But Bernouilli is an observation, Newton gives you the explanation.

[Per]

The ground effect on cars theory implies that the Bernoulli theory is correct too, as Andreas125sx said, and lift/downforce can be created by pressure differences above and below the body. So the theory deemed wrong isn't so wrong after all?

Bernouilli was never wrong (this is also stated in one of the links you posted; read the bit below figure 2 in your second link). The bit where popular theory is very wrong, is where it says "two particles of air that are split at the leading edge need to meet again at the trailing edge, therefore on one surface the air must travel faster". THAT bit is utter bollocks. But Bernouilli never said that.

Statements like "faster moving air corresponds to lower static pressure" and "pressure difference above and below the body corresponds to lift", are very much correct and not in conflict with the rest of the explanation of lift as provided by Newton's Laws. Heck, the latter statement is the very definition of lift!

Again, ask yourself why the air under the floor travels faster. The whole floor including diffuser acts as a venturi channel. Air is forced under the floor where the flow area is small, then the diffuser at the back increases that area to bring the speed back to ambient speed. The high speed under the floor corresponds to low static pressure, because the energy in the fluid (~total pressure) is constant. The more energy 'spent' as kinetic energy, the lower the static pressure.

[godlameroso]

Why then does the lowest pressure of the diffuser happen at the throat and not in the volume? Also if the air is "slowing down" as you say, why are there smooth lines from the flow vis, and not turbulence, like is common when used in conventional HVAC diffusers?

[bhall II]

You can say the whole floor acts like a venturi, because air flow is accelerated relative to freestream conditions (for the most part). But, to be a bit more illustrative of what's really happening, it might be better to say a flat floor acts like a pair of venturis: one at the leading edge and one at the throat, or kinkline, of the diffuser. Between them, viscous losses cause air flow to slow down, or stagnate, which is why mid-floor static pressure is higher.



And the reason why you (usually) don't see any signs of turbulent flow on the diffuser is because they're tiny and inefficient and pressure recovery occurs farther downstream.

[ChrisDanger]

My highly accomplished aerodynamics professor once said (with a wry smile) that the best explanation for lift is that it's a direct result of circulation. I imagine he was referring to the Kutta-Joukowski equation:


where

It has mathematical elegance for sure, and points to a physical truth, but for me fails to provide an intuitive understanding of the underlying physics. I'd always trust maths (and, usually, expert opinion) over my gut, so it might be worth playing around with circulation and seeing what kind of understanding develops, which is something I unfortunately never did.

[hollus]

Per nailed it. It is easy to see high pressure piling up in the high side of an F1 wing, but trickier to understand the acceleration in the low pressure side.

Air tends to move as a block and vacuums are so quickly filled that they never truly develop. Imagine that you are moving your wing through honey. Yes, it is hard to move it, but move it anyways. It will create a vacuum in the low pressure side, because the fluid, honey, likes to stay where it was or to keep moving as it was and cannot move fast enough to fill in the space behind the moving wing (partly below in a F1 rear wing). That vacuum is the low pressure side in air, where air moves fast enough to fill it in until the vacuum is only partial. And trying (and succeeding) to fill that vacuum is what causes the air to accelerate.

Now you could take that partial vacuum as partial density, which makes it easier to understand, or accept that density equalizes fast enough as to be effectively constant at low speeds. In that case, you are left with a Bernoulli like explanation: the air that accelerated to fill in the vacuum has spent part of its internal energy to convert it into net kinetic energy, its molecules are literally bumping around slower as payment for the acceleration (which is perpendicular to your surface and hence does not push it), and so the fluid has less "internal kinetic energy" and makes less pressure on your wing than before. Voila, the low pressure side. This filling in the vacuum explanation works in a plane wing, in a F1 wing, in a diffusor's slope and even to explain the Coanda exhausts.

In my opinion, asking for cause and effect in aero at low speeds is futile. Does the low pressure cause the acceleration or the acceleration cause the low pressure? Well, your wing caused both, it pushed and tried to create vacuum pockets, and the response of a fluid with the properties of air to that disturbance includes both the acceleration and low pressure.

Extra note 1: I like to think of a wing, even in open air, as a Bernoulli-like constriction. The air also accelerated as the wing takes some volume itself. It would push air away, but the air in the second layer away from your wing doesn't respond very fast to being pushed and thus the air in the first layer is trapped between a very hard wing and a somewhat hard second layer of air which never gives in as much as it could (there is a third layer or air behind it that doesn't like to be pushed). It then has no alternative but to accelerate.

Extra note 2: in ground effect you still have the upwash (F1), only that most of it is transmitted to planet earth which logically doesn't move much (it would push against your tires anyways).

[krisfx]

My highly accomplished aerodynamics professor once said (with a wry smile) that the best explanation for lift is that it's a direct result of circulation. I imagine he was referring to the Kutta-Joukowski equation:

http://flow.byu.edu/me412/resources/ima ... lation.gif
where http://patentimages.storage.googleapis. ... 5_0001.png

It has mathematical elegance for sure, and points to a physical truth, but for me fails to provide an intuitive understanding of the underlying physics. I'd always trust maths (and, usually, expert opinion) over my gut, so it might be worth playing around with circulation and seeing what kind of understanding develops, which is something I unfortunately never did.

We studied "Kutta condition" last year, fascinating stuff!