comment on 1st-gen Porsche 911/912 lift

[freebeard]

The site I wanted to share is:
TechArt wind tunnel testing on pinterest. The smoke flow visualization clearly demonstrates that the air does not follow the 911's aft-body contour at all.

Looks like it's failing to paste. It should look like this before you click Submit:

The site I wanted to share is:[U*L="http://*URL*"]http://*URL*[/U*L]
TechArt wind tunnel testing on pinterest. The smoke flow visualization clearly demonstrates that the air does not follow the 911's aft-body contour at all.

Can you paste into the post directly? That will tell you if the paste is failing. Then click the link button and use 'x' for the url. Then replace the 'x' twice, once in the quotes and also between the brackets.

MY GTI wing is buried soooo deep. I might be able to get to it in a week.

[JulianEdgar]

Originally Posted by aerohead

1) on attached flow: page 114, ' The C-Pillar vortices are so strong that flow remains attached over almost all the sloping back, despite the large angle of inclination.'

That's right: did you actually read what you wrote in the quote?

The flow remains attached...

Not that 'the flow only looks like it remains attached because of what the tufts show - but they're not right', but the flow is attached.

So, let's go back to the Porsche - initially, the flow is largely attached, as indicated by the tufts. The placing of the spoiler reduces flow attachment ie increases separation. And the placing of the spoiler reduced lift. Therefore, lift was reduced by increasing separation. It seriously isn't hard!

[JulianEdgar]

Originally Posted by aerohead

1) Tufts demonstrate phenomena limited to the boundary region, whereas only smoke flow visualization can accurately represent attached/ separated flow.

Tufts demonstrate what is happening on the vehicle surface, and since we're talking about pressure drag, it's only on the vehicle surface that we're actually interested in what is going on.

Originally Posted by aerohead

2) No one will dispute your Magnehelic readings. If you want to come over to the 'States' and measure Spirit, come on. You will basically find positive pressure acting fore and aft, and 'neutral',or zero lift. That's all I'm claiming for the 'template.'

I am always puzzled by why people want me to come and do their testing for them. All the testing I do is:

• low in cost or even free
• done on normal roads
• done quickly and easily
• requires almost no technical knowledge

But I need to come and do it for you?!

As for no one disputing my surface pressure measurements, I am sure you will find a way of doing so very soon, when they show what you are saying is quite untrue.

How's that 'positive pressure acting fore and aft' going, then? Just as I'd expect, look at all those low pressures over those top curves.



Stand by 'Aerohead' saying that pressure testing has become invalid, just as tuft testing now apparently has!

[kach22i]

Aerohead:

Smoke flow images of the 911 would really help.

I have posted such images in the Random Wind Tunnel Images thread.

What I recall is that on the coupe detachment happens at the top of the rear window where there is a 3/4" step down and a slot vent for the cabin.

On the Targa I have found no such image and assume a similar detachment at the trailing edge of the targa bar that has about 1/2" step down at rear window.

In the many searches I have done over the years I have found no documentation to support my personal hypothesis that the powerful cooling fan drawing air down through the rear deck lid alters the airflow and pulls the entire air flow over the car down sooner.

If I recall correctly threads in Pelican Parts revolving over substituting an electric fan for the belt driven one, a 35 hp motor equal to that powering a Prius would be required. Such a power demand could not be supplied by a alternator/generator and would require the weight of batteries.

To put the power of a 35 HP fan into perspective, my 11 foot long 6 foot wide hovercraft is 35 HP, it can carry about 400 lbs and go about 45 mph over still water, 65 mph over flat ice.

The fan on the hovercraft is 30 inch diameter or about 2.5 times the size of the Porsche cooling fan so it does push/pull a lot more air.

I cannot offer up data to compare the two, just suggesting that to ignore the draw power of a 911 cooling fan is to ignore a powerful physical force.

[Piotrsko]

Fwiw, someone * years ago dyno'd a type 2 engine fan/alternator combo on a drag car and found it used about 15 hp IIRC. On the ranger, currents to idle with the PS pump and alternator calculate to be about 10 hp so I can see consuming 10 with cooling alone.

*HOT VW magazine perhaps?

I agree the porsche fan could be altering local flow but 35 horse to run it seems a bit steep.

[kach22i]

Originally Posted by Piotrsko

Fwiw, someone * years ago dyno'd a type 2 engine fan/alternator combo on a drag car and found it used about 15 hp IIRC. On the ranger, currents to idle with the PS pump and alternator calculate to be about 10 hp so I can see consuming 10 with cooling alone.

*HOT VW magazine perhaps?

I agree the porsche fan could be altering local flow but 35 horse to run it seems a bit steep.

Well, let's put this in context with the additional information you have provided.

15hp on a Type-2 engine (1.1 to 2.0 L) that is half the size of the Porsche engine (2.4 to 3.6 liters).

I am to understand via the track racing folks that the 5-blade cooling fan on my. 2.7L consumes 5 hp less than than the later and more powerful 11-blade fan.

15 +15 + 5 = 35 hp

Sketchbook math, but it adds up.

[kach22i]

Originally Posted by JulianEdgar

Tufts demonstrate what is happening on the vehicle surface, and since we're talking about pressure drag, it's only on the vehicle surface that we're actually interested in what is going on......

Pressure drag?

Why would we limit ourselves to that?

A vortex may be a simple planar pressure difference at the surface, and it's true expression as a force of power forms after.

As I understand it, drag causing vortexes can be delayed by rake fences and fins (vertical stabilizer wings), but they are similar to tentacles that hold the vehicle back like a parachute.

If we look only at tufts or numbers at the surface we miss the bigger picture.

[freebeard]

...they are similar to tentacles that hold the vehicle back like a parachute.

I think of myriad little fingers clutching anywhere they can get purchase, but tentacles are cool.

Calculating power drawn is one step removed from cubic feet per minute. I understand that a 1600 Type I moves 1500cfm. That volume (plus thermal expansion) is available at the difusser area for active management of the airflow.

Tufts vs streamlines: The best tools are Schielren photography and advanced CFD (using OpenVDB and Micropolygon Displacement.

[California98Civic]

Originally Posted by kach22i

Pressure drag?

Why would we limit ourselves to that?

A vortex may be a simple planar pressure difference at the surface, and it's true expression as a force of power forms after. ...

Yes. That is my understanding as well, though you said it better. The tufts and pressure readings tell a real story, but not the entire story of drag.

Originally Posted by freebeard

... Tufts vs streamlines: The best tools are Schielren photography and advanced CFD (using OpenVDB and Micropolygon Displacement.

I think you meant Schlieren imaging?

[freebeard]

Same link. I corrected the spelling of my search [Scheilren]. ...incorrectly.

I easily.