A recent windtunnel surprise, Volkhart V2 Sagitta

[KamperBob]

Back in the day with an air-cooled rear engine I have to wonder it detached flow was beneficial and intentional...

[kach22i]

Good point.

Opinions? Duck tail vs. Carrera Tail (aerodynamically speaking) - Rennlist Discussion Forums


When Porsche first put the "ducktail" on the 911 it decreased lift and increased cooling by building up air pressure before the tail and allowing the fan to draw in more air.

http://forums.pelicanparts.com/porsc...to-engine.html


From the above thread:

Originally Posted by Walter_Middie

I'm an aerodynamic engineer and I usually stay out of these discussions because they deteriorate quickly. I'll just say no - cutting slots in the spoiler will not help.

The spoiler works completely differently than a wing. The spoiler spoils and separates the airflow coming off the roof, creating a high pressure area on the back window and top of engine lid where the engine airflow intake is (as shown in your picture). Engine cooling air is increased because the intake is now in a high pressure region, and air is forced into the engine compartment. Cutting the hole shown in blue will decrease the effectiveness of the spoiler and decrease the amount of pressure pushing air into the engine compartment.

[freebeard]

Originally Posted by KamperBob

...I have to wonder it detached flow was beneficial...

Not sure what you're saying here. One could say flow is laminar, attached turbulence or detached. You never see laminar flow in ground effect, detached flow will communicate the base pressure forward and spoil cooling intake.

The Volkhart Saggita's cooling intake is similar in form to the split window bus—flush slots. With the bay window bus the back of the slots is pulled out to make an angled scoop. With the Vanagon it's reversed, with the back pulled in to make a recessed scoop.

Here's the way the cooling is laid out:


Here's the stock bodies air flow:


This appears to be a racer, the stock engine compartment has a floor that separates intake and cooling air from the hot components and exhaust.

If you want to eavesdrop on people dealing with this on a daily driver basis, try:
thesamba.com: Big CC in a bus = running Hot?
thesamba.com: Adding modern scoop for split bus engine bay cooling / pres.
On the 2nd page of that last one, someone talks about a F1 style diffuser to pull hot air out of the vehicle.

The cooling intake is right before the point of separation (hopefully). The diverted air probably 'pulls down' the flow that wants to detach.

[some_other_dave]

Originally Posted by freebeard

This appears to be a racer, the stock engine compartment has a floor that separates intake and cooling air from the hot components and exhaust.

All air-cooled VWs and Porsches that I know of had that separation. Maintaining it is very important in anything but a racer, which is usually going fast enough and has enough air coming in to avoid re-ingesting warm air.

-soD

[KamperBob]

If the bottom rear slope of old bugs where air vents are located are too steep to support attached flow then I wonder why. It occurs to me that suction effect from flow detachment could aid in cooling an air-cooled rear engine. Tufts tied to vent slots should confirm flow direction at speed...

Originally Posted by freebeard

Not sure what you're saying here. One could say flow is laminar, attached turbulence or detached. You never see laminar flow in ground effect, detached flow will communicate the base pressure forward and spoil cooling intake.

The Volkhart Saggita's cooling intake is similar in form to the split window bus—flush slots. With the bay window bus the back of the slots is pulled out to make an angled scoop. With the Vanagon it's reversed, with the back pulled in to make a recessed scoop.

Here's the way the cooling is laid out:


Here's the stock bodies air flow:


This appears to be a racer, the stock engine compartment has a floor that separates intake and cooling air from the hot components and exhaust.

If you want to eavesdrop on people dealing with this on a daily driver basis, try:
thesamba.com: Big CC in a bus = running Hot?
thesamba.com: Adding modern scoop for split bus engine bay cooling / pres.
On the 2nd page of that last one, someone talks about a F1 style diffuser to pull hot air out of the vehicle.

The cooling intake is right before the point of separation (hopefully). The diverted air probably 'pulls down' the flow that wants to detach.

[freebeard]

It occurs to me that suction effect from flow detachment could aid in cooling an air-cooled rear engine.

This effect is easily achieved by propping open the engine compartment lid of either a Type I or II. The *intake plenum* is exposed directly to base pressure. Then people argue endlessly about the varying results. Examples are shown in the Samba threads I linked. I finished reading those this morning.

The aid you seek would come from exposing the exhaust of the cooling ducts and under-car airflow to base pressure. Which is why the diffuser appeals to me. The designed shape of it would be would be critical. I have no clue.

To bring it back on topic, the rounded tail of the V-S could be brought up to date with Coanda nozzles fed with engine cooling air exhaust.


The original 25hp engine didn't move much air, but it's at 5 psi.

[aerohead]

Originally Posted by KamperBob

Back in the day with an air-cooled rear engine I have to wonder it detached flow was beneficial and intentional...

All the air-cooled engines of course have forced-air cooling from the generator shaft.So,in a sense,it doesn't matter where the air comes in from,as engine power is robbed to provide the pressure differential across the oil cooler and cooling fins.
That said,the flow separation,while lowering the base pressure behind,does maintain a downwash,with momentum carrying the airstream down on a collision course with the body.By locating the inlet at the site of the collision,you can harvest this downwash,easing the horsepower requirement of the cooling fan.
In a modern application,the ghost of Ferdinand Porsche might utilize NACA submerged inlets to bring air into the engine bay.
The Lange car form,which Porsche used,according to Hucho,had Cd 0.14-16 in scale-models,so you can imagine how attractive this might seem at a time when 'conventional' cars were Cd 0.6-7.

[Xist]

It is always interesting to see the front skirts. Sometimes the wheels seem too close to be able to turn well.

[aerohead]

Originally Posted by KamperBob

If the bottom rear slope of old bugs where air vents are located are too steep to support attached flow then I wonder why. It occurs to me that suction effect from flow detachment could aid in cooling an air-cooled rear engine. Tufts tied to vent slots should confirm flow direction at speed...

Hucho says that the longitudinal vortices of the Bug's pseudo-Jaray roofline will help induce a high deflection attached flow downwash at the center of the back.This would aid cooling,although the high drag vortices would raise the road load,requiring more power (heat flux) which would have to be dealt with.
In 1936 or so,the Beetle's Cd 0.495 would have been considered quite low compared to contemporary cars.

[Cd]

I found an image showing the flow off the sides of the car :