Diffuser Confusion

[mep]

http://www.racecar-engineering.com/tech ... odynamics/

[vonk]

http://www.racecar-engineering.com/tech ... odynamics/

Sorry for the belated response.

The linked article is well written, presenting in popular terminology what many seem to have accepted as gospel. It sounds plausible, but I (blasphemously ) believe it to be factually incorrect.

The article claims that the diffuser is a device “creating an area of low pressure”, that “draws the air out from under the car” and that it “literally sucks the car onto the track”. This implies that a diffuser is an active vacuum pump. It is well known that pumping requires work [(pressure difference)x(volume flow rate)]. A passive diffuser, if properly dimensioned, can modify flow conditions according to Bernoulli’s equations. If it is to diffuse accelerated low pressure flow, the acceleration must have occurred first by some other mechanism.

The article suggests wing aerodynamics as the theory behind flow acceleration under the car. An F1 car is hardly a wing. Instead, it uses a horizontal splitter plate to purposely minimize body shape influence on the flow entering under the car.

Discussing Fig 2, the author overlooks the fact that the pressure coefficient represents the pressure difference between the local and free stream pressures. Accordingly, the darkest blue represents the least difference from ambient pressure with everything else being greater. This is not surprising since Fig 2 shows a bell mouth at the far left that creates a ram pressure increase at the inlet to the underside of the car (evidenced by the red lip on the extreme left). To avoid this, F1 cars use a horizontal splitter plate.

[hollus]

Cause and effect, effect and cause... everything affects everything in air flow around and under the car.
Why can't it be the diffuser, a low pressure area, what is partly accelerating that flow under the car? That air is still ahead of the diffuser, of course, but the speed of sound is much, much higher than the speed of the car, so there is nothing against parts of the car affecting the air flow upstream.
The car can act as an air pump, but the diffuser not? What is so different between the diffuser and the rest of the car's surfaces?

[vonk]

Why can't it be the diffuser, a low pressure area, what is partly accelerating that flow under the car?

Because for the diffuser to provide a “low pressure area” it would have to be a pump. That requires added energy. Thus, the blown diffusers emerged. But that’s not what this thread was originally about.

It will be interesting to see what happens after the new exhaust regulations take effect.

[hollus]

So there is no low pressure in the volume enclosed by the diffuser and the track?

[Just_a_fan]

Why can't it be the diffuser, a low pressure area, what is partly accelerating that flow under the car?

Because for the diffuser to provide a “low pressure area” it would have to be a pump. That requires added energy. Thus, the blown diffusers emerged. But that’s not what this thread was originally about.

It will be interesting to see what happens after the new exhaust regulations take effect.

The diffuser links the underbody flow to the low pressure regions created by the beam wing, the main rear wing and the rear wheels. The car creates a low pressure zone behind it - that's partly what causes drag. The diffuser uses this low pressure zone to draw air from the underside. It makes the process efficient by easing the velocity change between underfloor and behind-car flows. The car is the pump, not the diffuser per se.

Run any car with a flat bottom and nose-down rake and you'll create downforce. Add a diffuser and you'll make more downforce and less drag. That's what the thing does.

[Pierce89]

http://www.racecar-engineering.com/tech ... odynamics/

Sorry for the belated response.

The linked article is well written, presenting in popular terminology what many seem to have accepted as gospel. It sounds plausible, but I (blasphemously ) believe it to be factually incorrect.

The article claims that the diffuser is a device “creating an area of low pressure”, that “draws the air out from under the car” and that it “literally sucks the car onto the track”. This implies that a diffuser is an active vacuum pump. It is well known that pumping requires work [(pressure difference)x(volume flow rate)]. A passive diffuser, if properly dimensioned, can modify flow conditions according to Bernoulli’s equations. If it is to diffuse accelerated low pressure flow, the acceleration must have occurred first by some other mechanism.

The article suggests wing aerodynamics as the theory behind flow acceleration under the car. An F1 car is hardly a wing. Instead, it uses a horizontal splitter plate to purposely minimize body shape influence on the flow entering under the car.

Discussing Fig 2, the author overlooks the fact that the pressure coefficient represents the pressure difference between the local and free stream pressures. Accordingly, the darkest blue represents the least difference from ambient pressure with everything else being greater. This is not surprising since Fig 2 shows a bell mouth at the far left that creates a ram pressure increase at the inlet to the underside of the car (evidenced by the red lip on the extreme left). To avoid this, F1 cars use a horizontal splitter plate.

about figure 2: What you stated would be true if the the blue area represented a lower but positive cp, but it actually represents a negative cp, so, no it doesn't represent that area being closer to freestream pressure, the blue represents that it's BELOW freestream pressure.
With that figure being a nice post processed cfd result, that means it's based on MATHEMATICS.
So, for you to be correct the whole math and science behind the commercial cfd programs will have to be wrong. No disrespect, but I take the word of the whole world of math and science over your idea.

[Pierce89]

Why can't it be the diffuser, a low pressure area, what is partly accelerating that flow under the car?

Because for the diffuser to provide a “low pressure area” it would have to be a pump. That requires added energy. Thus, the blown diffusers emerged. But that’s not what this thread was originally about.

It will be interesting to see what happens after the new exhaust regulations take effect.

The diffuser links the underbody flow to the low pressure regions created by the beam wing, the main rear wing and the rear wheels. The car creates a low pressure zone behind it - that's partly what causes drag. The diffuser uses this low pressure zone to draw air from the underside. It makes the process efficient by easing the velocity change between underfloor and behind-car flows. The car is the pump, not the diffuser per se.

Run any car with a flat bottom and nose-down rake and you'll create downforce. Add a diffuser and you'll make more downforce and less drag. That's what the thing does.

I think he's already tried to reject this basic premise of F1 aero

[slimjim8201]

I think he's already tried to reject this basic premise of F1 aero

I think the thread title "diffuser confusion" sums up his posts nicely.

[Pierce89]

I think he's already tried to reject this basic premise of F1 aero

I think the thread title "diffuser confusion" sums up his posts nicely.

+1

[vonk]

about figure 2: What you stated would be true if the the blue area represented a lower but positive cp, but it actually represents a negative cp, so, no it doesn't represent that area being closer to freestream pressure, the blue represents that it's BELOW freestream pressure.

You are correct and I was wrong, because……Ah, never mind.

I humbly apologize to the community.

[vonk]

The diffuser links the underbody flow to the low pressure regions created by the beam wing, the main rear wing and the rear wheels. The car creates a low pressure zone behind it - that's partly what causes drag. The diffuser uses this low pressure zone to draw air from the underside. It makes the process efficient by easing the velocity change between underfloor and behind-car flows. The car is the pump, not the diffuser per se.

This puts the finger on my problem, which was to assume that the diffuser exit connects to the car’s turbulent wake at ambient static pressure. Instead, I understand you to say it connects to a sub-ambient pressure region.

Thanks for being the first to call me on this.

[mep]

Vonk I am affright to say it but it seems you still haven't understood how the diffusor works. The beam wing is not the reason for it. Many cars do have diffusors without having a beam wing. The beam wing is just a add-on.

[vonk]

Vonk I am affright to say it but it seems you still haven't understood how the diffusor works. The beam wing is not the reason for it. Many cars do have diffusors without having a beam wing. The beam wing is just a add-on.

OK, then. I’m willing to learn. How does it work without add-ons? Am I at least correct in that it is not a vacuum pump?

[hollus]

The diffuser is using the energy from the rest of the car to create a vacuum, even if it is not powered by itself. The space that was occupied by the car in one moment will have to be occupied by something the moment after the car has passed by, and gases will only be able to fill in this void created behind and under the diffuser at a certain speed. So where the difusser is, one moment before there was no air, and all of a sudden there needs to be a lot of air. Air will have to rush in to fill this space either from under the floor, from the sides, or from behind the car. And it will never arrive quickly enough, so yes, the diffuser by itself, creates a partial vaccum. Like any other part directly in the rear of the car that is not an airfoil.
The wings will just make this partial vacuum even bigger.
By assuring a laminar flow of air under the floor with a smooth transition, the diffuser makes sure that a good part of the filling will be done by air from under the floor, hence (invoke Bernoulli if you want) creating a partial vacuum (but much less of it than in the diffuser) all under the floor.