Gordon Murray Automotive T.50 'rear fan'

[freebeard]

What I'm fishing for is a description that contradicts aerohead's.

I know it isn't easy. Luigi Colani had a grasp of fluidics that I appreciate but couldn't explain.

[JulianEdgar]

Originally Posted by freebeard

What I'm fishing for is a description that contradicts aerohead's.

I'll be blunt - Aerohead's description of how diffusers works is so ridiculous that no reference is ever going to mention his theories.

If you really want to know - and boy, am I giving you the benefit of the doubt - diffusers work like this. (And I am giving the normal workings of a diffuser, not odd ones where you suck away boundary layers, etc.)

1. They have attached flow. That attached flow, as it curves upwards, develops a low pressure.

2. That low pressure helps pull the car downwards.

3. That low pressure also helps the airflow under the car*, so giving high speed flow (and so lower pressure under a big area, so pulling the car even more downwards). *This is sometimes called 'diffuser pumping'.

4. The wake is made smaller as the airflow is directed partly upwards - just as on the upper surfaces, a fastback shape directs the air downwards. So drag can also be reduced.

5. On motorsport cars, the diffuser is much steeper and flow separates down the edges (and strakes), creating vortices. These just add to everything from 1-3, making downforce stronger but at the expense of a lot of drag.

6. There is a lot about diffusers that is not currently understood, eg diffusers work differently in drag and lift reduction on squarebacks versus notchbacks and fastbacks. The current lit suggests taking care in using diffusers for drag reduction on squarebacks.

As far as I can ascertain, Aerohead believes that lift/downforce comes from separated flow, so he assumes the diffuser is always in separated flow and it's wake pressures doing the work. But of course pressures from attached flow can be much greater than wake pressures, so no one would think that way. (Unless you'd read only a 1987 book where separation was common, of course.)

Seriously, Freebeard, if you want to learn about the topic, get away from the PC and read a book. Barnard is cheap as dirt, he is a beautiful writer and he has no major errors I have ever found.

[AeroMcAeroFace]

Originally Posted by JulianEdgar

I'll be blunt - Aerohead's description of how diffusers works is so ridiculous that no reference is ever going to mention his theories.

As far as I can ascertain, Aerohead believes that lift/downforce comes from separated flow, so he assumes the diffuser is always in separated flow and it's wake pressures doing the work. But of course pressures from attached flow can be much greater than wake pressures, so no one would think that way. (Unless you'd read only a 1987 book where separation was common, of course.)

There is a difference here between low angles used in drag reduction and high angles used for downforce:

at low angles "under-body diffuser is all about generating a pressure recovery to increase the base pressure"

"An under-body diffuser requires attached flow to enable pressure recovery"

But that is in a PhD thesis, so in aeroheads mind is completely invalid.

But in high angles, there is separation, which would lower base pressure, I don't know about backwards flow, but there is definitely separation. But the angle at which separation occurs is influenced (as always) by what occurs upstream and vorticity. And separation angle can be anywhere between 6 degrees and 30 degrees.

[aerohead]

Originally Posted by JulianEdgar

I guess this is why so much misinformation has been disseminated here.

To state the bleeding obvious - it is wrong information that you are spreading. Outright wrong. No nuance here - wrong.

How you say a diffuser works is simply quite wrong. So what do you do about it?

Nothing.

1) Science doesn't concern itself with 'wrongness.'
2) There's no such thing as too much information.
3) If certain evidence disagrees with a premise, that premise needs to be examined.
4) I beg you to illustrate where I said anything at all.
5) I simply reached into my archive and presented a reference to visual information which would qualify for ' hey, look at this- isn't it interesting ?'

[aerohead]

1) Nature abhors a vacuum.
2) Any existing pressure differential creates a dynamic in which a 'high' will attempt to 'telegraph', 'communicate', with a 'low.'
3) The, what is typically considered, 'low' base pressure behind a vehicle, CAN be of higher pressure than elsewhere on a vehicle. An example would be a hatchback car with the hatch propped open, to allow the transport of an oversize item.
4) With the hatch open to base pressure, and a driver's side window rolled down, it's normal for engine exhaust to travel forwards through the cabin and asphyxiate the motorists with carbon monoxide, due to a low pressure caused by the A-Pillars.
5) A tunnel-effects ( ground-effects ) venturi , underneath and ahead of a diffuser, could certainly impart a suction peak ( that's what a venturi does ) sufficient enough to induce counter-flow beneath the Lamborghini. It's a physical imperative for this to happen. The orientation of the tufts would suggest that this is the case, with respect to this particular vehicle. Whether it agrees with a textbook or not.
Exceptio probat regulam. Just sayin'

[JulianEdgar]

Originally Posted by aerohead

1) Science doesn't concern itself with 'wrongness.'
2) There's no such thing as too much information.
3) If certain evidence disagrees with a premise, that premise needs to be examined.
4) I beg you to illustrate where I said anything at all.
5) I simply reached into my archive and presented a reference to visual information which would qualify for ' hey, look at this- isn't it interesting ?'

I'd already made it clear what the incorrect information was, but here it is again. You wrote:

That's my take, at least on supercars. Ferrari has flaps that lower to a less radical angle on the straights, where cornering force isn't as important, then on approach to turns, the flaps lift, increasing the ramp angle, allowing low base pressure to communicate under the diffuser. Online, there are photographs of a Lamborghini configured for this and the tufts indicate forwards airflow down and under the car, heading for the suction peak at the venturi.

Low base pressure doesn't communicate under the diffuser - that's not how diffusers work. It's similar to the idea that you have that lift is caused only by separation on the upper surfaces of cars.

[JulianEdgar]

Originally Posted by aerohead

1) Nature abhors a vacuum.
2) Any existing pressure differential creates a dynamic in which a 'high' will attempt to 'telegraph', 'communicate', with a 'low.'
3) The, what is typically considered, 'low' base pressure behind a vehicle, CAN be of higher pressure than elsewhere on a vehicle. An example would be a hatchback car with the hatch propped open, to allow the transport of an oversize item.
4) With the hatch open to base pressure, and a driver's side window rolled down, it's normal for engine exhaust to travel forwards through the cabin and asphyxiate the motorists with carbon monoxide, due to a low pressure caused by the A-Pillars.
5) A tunnel-effects ( ground-effects ) venturi , underneath and ahead of a diffuser, could certainly impart a suction peak ( that's what a venturi does ) sufficient enough to induce counter-flow beneath the Lamborghini. It's a physical imperative for this to happen. The orientation of the tufts would suggest that this is the case, with respect to this particular vehicle. Whether it agrees with a textbook or not.
Exceptio probat regulam. Just sayin'

I am sorry, but this is just all your theory. Maybe consult some good references on how diffusers work?

[aerohead]

Originally Posted by JulianEdgar

I'd already made it clear what the incorrect information was, but here it is again. You wrote:

That's my take, at least on supercars. Ferrari has flaps that lower to a less radical angle on the straights, where cornering force isn't as important, then on approach to turns, the flaps lift, increasing the ramp angle, allowing low base pressure to communicate under the diffuser. Online, there are photographs of a Lamborghini configured for this and the tufts indicate forwards airflow down and under the car, heading for the suction peak at the venturi.

Low base pressure doesn't communicate under the diffuser - that's not how diffusers work. It's similar to the idea that you have that lift is caused only by separation on the upper surfaces of cars.

I hear what you're saying.
The diffuser is open to a car's base. It's directly linked in both directions. High pressure will always induce flow in the direction of low pressure. Just like with separation.
If underbody flow stalls on a steep diffuser ramp, and base pressure now dominates the immediate region, if that pressure exceeds what is upstream, the air would consequently move in that direction. As in the A-pillar analogy.
There's a reason why the Lamborghini demonstrated reverse-flow. Any explanation would have to include a look at local pressures.

[JulianEdgar]

Originally Posted by AeroMcAeroFace

There is a difference here between low angles used in drag reduction and high angles used for downforce:

at low angles "under-body diffuser is all about generating a pressure recovery to increase the base pressure"

"An under-body diffuser requires attached flow to enable pressure recovery"

But that is in a PhD thesis, so in aeroheads mind is completely invalid.

It's a good (free!) paper for people looking at reducing drag.

[orange4boy]

If you really want to know - and boy, am I giving you the benefit of the doubt - diffusers work like this.

Seriously, Freebeard, if you want to learn about the topic, get away from the PC and read a book

I'll be blunt - Aerohead's description of how diffusers works is so ridiculous that no reference is ever going to mention his theories.

Perfect example of how Aerohead operates here. Is completely wrong, won't refer to references that correct his misapprehensions, continues spreading misinformation.

I guess this is why so much misinformation has been disseminated here.

To state the bleeding obvious - it is wrong information that you are spreading. Outright wrong. No nuance here - wrong.

How you say a diffuser works is simply quite wrong. So what do you do about it?

Nothing.

But that is in a PhD thesis, so in aeroheads mind is completely invalid.

Is anyone else sick of this aggressive, unproductive, alienating, unnecessary content? Who would want to post here and have to deal with this if they post something these people think is wrong?