Downforce- which explanation is correct?

[Per]

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?

I have no experience with HVAC systems, but I would assume they work at slightly different Reynolds numbers than an F1 car. In any case I don't see why slowing air down would have to lead to turbulence by definition.

Lowest pressure happens at the throat because the air which is flowing through there needs to be flowing at a higher speed than in front and behind it (conservation of massflow). As hollus explained very well, this means the air molecules have more of their energy in kinetic energy along the flow, leaving less energy to bump into the floor (=static pressure).

The confusion might be that I am saying the diffuser is slowing the air down, while it is common knowledge that the diffuser is there to help air accelerate. Both are true, the air is accelerated under the floor and then decelerated again in the diffuser itself. The diffuser brings the air back to more or less ambient speed and pressure in a gradual way, which greatly helps the efficiency of the floor.

[godlameroso]

Because slowing down air without a corresponding pressure increase = eddy formation. It's the fastest way that air can fill itself with itself again. Since our air is nice and pressurized it hates vaccuums and will always find the most economical way to prevent it. Ever seen a rocket take off?

In f1 I'm almost positive the air at the end of the rear diffuser is almost as fast as at the kink. In fact I bet if we had a new more recent cfd of the under side of an F1 car you'd see the pressure drop again just at the end of the diffuser though not as much as in the kink.

[bhall II]

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

Dirty air is as much about the diffuser as it is the rear wing: underbody air flow contributes most of the turbulence, and the low-pressure wake left by the rear wing contributes most of the upwash.

Go back to hollus' comments about how air flow is accelerated. The same principle applies here; it just takes on a different form.



Where air flow over the low-pressure side of an airfoil is accelerated as infill to the low-pressure wake, or vacuum, left by the airfoil's leading edge, underbody air flow is accelerated as infill to the low-pressure wake left by the diffuser and the rear wing - the rest of the car, too, for that matter.

Take away the rear wing, and you'll have removed but a single component of dirty air, one that some folks surmise is actually beneficial...

[...]

One of the most significant findings [from studies organized by the Overtaking Working Group] was that the rear wing is a very important device in characterising the wake that a car generates.

"You would think that upwash from the rear wing is bad," [Paddy] Lowe said. "The upwash is strong, but a very strong inwash at ground level is also driven by the rear wing. That inwash brings new high-energy air in at ground level. If you took the rear wing off altogether you would lose that effect and the wake would be a lot worse."

[...]

Personally, I think that's just a weird way of describing force-enhancing edge vortices.

In f1 I'm almost positive the air at the end of the rear diffuser is almost as fast as at the kink.

Perhaps. As I said, diffusers are too small to truly return underbody air flow to ambient conditions. Because of that, pressure recovery takes forever. Hence, dirty air...



One of the reasons for elaborate rear brake ducts these days is to create a larger area for recovery that's filled as much as possible by underbody air flow and not "overbody" air flow...

[Per]

Because slowing down air without a corresponding pressure increase = eddy formation.

Who said there was no pressure increase? Look at bhall's CFD picture of the floor on page one. The lowest pressure is at the kink, then it increases as you go further downstream.

[godlameroso]

I agree, I suppose the main take away is the observation of reality is betrayed by the langauge used to describe it. There's a relationship between air and a moving surface, fine. We can use mathematical equations to describe that relationship, and that's excellent for simulations, but ultimately, you are observing phenomena. Ultimately this(f1) is such a special case environment that to say it can be explained neatly by Newton and Bernouli is perhaps stretching it. The principles are sound don't get me wrong, but can they neatly explain everything that's happening with these cars? I would say sure, but there's things that they can't really explain because there are other factors influencing these principles. So the reality becomes that you have to adopt many different lines of thought, and in that soup of different ideas is what actually works.

[olefud]

Does the fact that the Bernoulli equation has a limiting condition of an inviscid fluid affect the discussion?

[godlameroso]

Probably, I keep saying that you can't ignore the viscous properties of air because it's a variable you can control. However small, air does increase in kinematic viscosity with temperature, which is an interesting phenomenon in it's own right.

For example, take the coke bottle shape, not what happens to air flowing over it, but air passing through the side pods and radiators, and the engine, and all the other things that need cooling and air flow, what happens to all that now hot air? Why nozzle it out, and then diffuse it front of the rear diffuser? Teams are allowed to place the exits in many other places, but all teams have converged on the nozzle diffuser side pod exit. Why?

[J.A.W.]

For example, take the coke bottle shape, not what happens to air flowing over it, but air passing through the side pods and radiators, and the engine, and all the other things that need cooling and air flow, what happens to all that now hot air? Why nozzle it out, and then diffuse it front of the rear diffuser? Teams are allowed to place the exits in many other places, but all teams have converged on the nozzle diffuser side pod exit. Why?

'Meredith effect'?

To utilize the potential efficiency retrieval of otherwise waste heat - via drag reduction aero-ducts..

[McMrocks]

Final question. Is the explanation of lift via Newton and downwash flawed because it doesn't include the force from pressure differences? Ground effect if the wikipedia is right is caused by Bernoulli and pressure differences. The upwash is even smaller if the wing gets closer to the ground.

If we want to explain downforce completely with all aspects we have to use the pressure differences* thing, right?

* Pressure difference is also caused by air being pulled upwards thus this explanation can includes both downforce by upwash and downforce due to Bernoulli/Venturi

[Powy]

Not sure if it can contribute to the discussion but here is a lecture by Doug McLean, a retired Boeing Technical Fellow, about Common Misconceptions in Aerodynamics. He also has a book called "Understanding Aerodynamics: Arguing from the Real Physics".

[godlameroso]

Final question. Is the explanation of lift via Newton and downwash flawed because it doesn't include the force from pressure differences? Ground effect if the wikipedia is right is caused by Bernoulli and pressure differences. The upwash is even smaller if the wing gets closer to the ground.

If we want to explain downforce completely with all aspects we have to use the pressure differences* thing, right?

* Pressure difference is also caused by air being pulled upwards thus this explanation can includes both downforce by upwash and downforce due to Bernoulli/Venturi

Of course! This is at the root of all aerodynamics, everything that you can consider lift is the difference in pressure acting on a body or surface as it displaces itself though a fluid through relative motion, this is the most elementary and physical way of understanding it. Motion being relative in the sense that either the fluid or the surface itself is moving in a temporal vector in relation to a fixed point. But it's not quite so simple as there are many different variables that affect the pressure distribution, heat, velocity, and most importantly geometry of the surface, because the surface geometry directly correlates with the aerodynamic geometry (ie vortecies, eddy currents, laminar flow, wash, wake, turbulence). Furthermore, every single one of these variables are interdependent on speed, trajectory(or vectors) heat, and surface geometry.

All that's left is to put what is evident into practice, and that's what kills you, you can learn from what others have observed to see what works, but in the end you're simply trying to exploit nature for your own ends. There's always an artistic element to dealing with nature in a practical and empirical way. I don't know if you're familiar with KIVA, there's some really interesting things you can do with that program, espeically for modeling things like turbulence and small eddies. Some people are expecting big gains as more detailed meshes are developed, big money in this, very closely guarded secrets.