The Mechanism Behind Flow Separation

[GenKreton]

Originally Posted by LostCause

Not to beat a dead horse here, but if there are any questions I'm willing to give them a go. Such as: What does viscous mean? What is the free air stream? Where do babies come from?

Hopefully you master this tonight, because tomorrow we derive the Navier-Stokes equation!



juuuuuuust kidding...

- LostCause

Deriving it is pretty easy, it's analyzing it that sucks.

[trebuchet03]

Very nice write up Bonus points for thoroughness And thank you so much for saying:

It’s a huge balancing act that is usually solved empirically (or semi-empirically via CFD).

I've been saying something to that effect since forever that I've been saying something about VGs Slapping them anywhere (or everywhere) is no good - hit or miss with a highly chance of miss.

My brain hurts!!! At least you didn't include math too!

That's what sucks about explaining something as simple as a ribbed piece of plastic... There's a great deal of information one must have a basic understanding of before going into how or why that little piece of plastic does what it does. Alas, it takes a lot of wording to explain this - so a bunch of simple concepts becomes a lot to chew on.

[Whoops]

These posts are full of a lot of great information. My head does hurt, but usually I have to go back over it, once I get the general idea. Thanks, to everyone for their sharing.

[boxchain]

Great writeup! I admit I only skimmed it tho

Originally Posted by LostCause

Hopefully you master this tonight, because tomorrow we derive the Navier-Stokes equation!



- LostCause

That's only part of the derivation. Maybe using x-y-z coordinate system would be easier.

[mattW]

Yeah nice explanation. I liked the qualification of detached flow, I always imagined that the stream of air detached from the car like a ball rolling across it would. The fact that it is stationary or backwards moving air (relative to the car) is useful. That helps me to understand that by taking some of the energy of the faster moving higher up streams the difference in speed is not as great so less air has to move and less force is applied on the vehicle. I'm not sure if I understand the momentum analogy. Did they assume that the high pressure on the front side would equalise with the low pressure on the back side but the surface friction prevents the two pressures from equalising? I'm imagining something like two connected balloons with equal pressures in each (perhaps one at the front and one at the back of the car). The car is effectively pushing air into one of the balloons (the front) leaving less pressure in the other. These pressures can't equalise because the friction between the two balloons slows the transfer of air so it can't 'catch up' to the rate that the car is pushing it. Is that right? You can ignore the balloons that just was to help me consider the two separate areas of pressure...

[MD2000]

Your detailed explanation is great. It does lead me to ask this question.
My Honda Insight like most cars today has a camm back.
What type of CD improvements would one expect if I made a tail section that brought it to a point? I guess the terminology would be boat tail?
Mike

[tasdrouille]

Hucho's book provide an example for this.

The back of the insight has roughly the same shape as the Mercedes C 111 III, though shorter and thicker.



On the attached image, the original shape they started with was approximately one third the lenght from the rear wheel skirt.

They tried different lenghts from 0 to 1.5 meters, beyond which there was no improvement in drag.

The function between boat tail lenght and drag reduction in this case was not linear and went up to a 25% reduction in drag.

They settled for roughly 60 cm which lead to roughly 18% in drag reduction.

[aerohead]

Originally Posted by tasdrouille

Hucho's book provide an example for this.

The back of the insight has roughly the same shape as the Mercedes C 111 III, though shorter and thicker.



On the attached image, the original shape they started with was approximately one third the lenght from the rear wheel skirt.

They tried different lenghts from 0 to 1.5 meters, beyond which there was no improvement in drag.

The function between boat tail lenght and drag reduction in this case was not linear and went up to a 25% reduction in drag.

They settled for roughly 60 cm which lead to roughly 18% in drag reduction.

Just wanted everyone to look at the heat-exchanger outlet at the base of the windshield of the C-111.This is where Kamm did his research and for what he's famous.The radiatior air re-energized the flow over the roof and allowed it to remain attached until he arbitrarily chopped --off the rear.The flow would have otherwise stalled and separated,making the "Kamm-back" a non-starter.The C-111 windshield is also ideal ($3,000 at KIT CAR ).Hucho also says they could have lowered drag even more with extra length,even though graph does not reflect it.Great pic!

[OfficeLinebacker]

Where is the original post?

[cfg83]

OfficeLinebacker -

Originally Posted by OfficeLinebacker

Where is the original post?

Ibid.

CarloSW2