Pressure thrust?

[winkosmosis]

Sounds like trying to drive a sailboat by setting up a fan behind the sail.

[DaveBirkenstock]

Originally Posted by winkosmosis

Sounds like trying to drive a sailboat by setting up a fan behind the sail.

That's a common reaction, especially among fluid types who have learned over decades that laminar flow is the only way to increase aero efficiency (this explains the persistent interest in LFC despite serious operational deal-breakers). Their gut tells 'em that the added complexity and energy demands of the suction system can't possibly be more efficient than a streamlined body.

The net benefit can be shown by a modified version of the MPG calculator that MetroMPG put up in another thread. Even with the added power for the suction pump and for a design as poor as the one I posted earlier, the modified car consumes less energy at high speed.

A design that doesn't suffer the problems I pointed to will have a lower Cd figure and may require less suction power, too.

Skin friction drag is still there and is as real as ever. This drag can be offset by pressure thrust on the rear of the body at the cost of the suction power. The question becomes which setup is more efficient?

Does this help it feel less like snake-oil?

If not, where does it fall apart? I really am eager to learn where things break down.

-Dave B

[aerohead]

Sorry,I'm late to the party.On the closing pages of Paul Valkenburg's Race Car Aerodynamics I believe,he showed a Corvette with an engine-driven suction-slot at the base of the rear windscreen.By applying suction,they were able to pull the flow onto the rear deck without separation,cutting drag,without the longer roofline so loved by classical aerodynamicists.The bummer was that the power requirements to run the pump exceeded the power reduction attributed to the drag reduction.Is this germain to the issue of thrust? Has there been a breakthrough in pump design? Or is it a function of where the air is harvested? Help?

[DaveBirkenstock]

Originally Posted by aerohead

The bummer was that the power requirements to run the pump exceeded the power reduction attributed to the drag reduction.Is this germain to the issue of thrust? Has there been a breakthrough in pump design? Or is it a function of where the air is harvested? Help?

I'm not familiar with that book or that project. It makes me wonder what their goal was and what the geometry looked like.

Without a concave structure downstream of the suction inlet, no aerodynamic thrust will be created & if you're going to spend the energy anyway, why not get some thrust as return on that investment?

As for the suction pump, the first flight test was in the 1940s. They proved over a 40% increase in efficiency using a centrifugal compressor as the suction pump. I'm sure current pumps/fans are better.

Are "sucker cars" like the Chaparral 2J and/or Brabham BT46B in that book? I sometimes tell people this works kindof like they did only instead of powered downforce it's powered pressure recovery.

-Dave B

[winkosmosis]

Originally Posted by DaveBirkenstock

That's a common reaction, especially among fluid types who have learned over decades that laminar flow is the only way to increase aero efficiency (this explains the persistent interest in LFC despite serious operational deal-breakers). Their gut tells 'em that the added complexity and energy demands of the suction system can't possibly be more efficient than a streamlined body.

The net benefit can be shown by a modified version of the MPG calculator that MetroMPG put up in another thread. Even with the added power for the suction pump and for a design as poor as the one I posted earlier, the modified car consumes less energy at high speed.

A design that doesn't suffer the problems I pointed to will have a lower Cd figure and may require less suction power, too.

Skin friction drag is still there and is as real as ever. This drag can be offset by pressure thrust on the rear of the body at the cost of the suction power. The question becomes which setup is more efficient?

Does this help it feel less like snake-oil?

If not, where does it fall apart? I really am eager to learn where things break down.

-Dave B

Are you saying that the purpose of the design is to blow air into the low pressure area at the rear of the vehicle to increase pressure from the normal vacuum? That's fine with me.

But that isn't shown in any of the posts so far... the concept seems to rely on the shape of the tailcone

[winkosmosis]

BTW, a wing doesn't work through the Bernoulli effect. That's a myth perpetuated by grade school textbooks and even pilot training. The pressure difference between the top and bottom of a wing travelling perpendicular to its plane is very low, if it exists at all. A wing develops lift by being positioned at an angle to the flow.

[theunchosen]

I'm in that fluids group so I have questions.

Has this been tested on something today and can I see pictures of all of its components?

How much vacuum is needed to create a sufficiently low pressure upstream of your concavity(a surface that curves inward is a concavity convex surface bows outward)?

the idea is to slow the air down enough that it "falls" along the curve pushing the car forward?

Or is the aim to generate a swirling eddy current that then pressed against the curve back to front motion and then up into the normal air current?

The amount of suction is going to drastically climb as the unit accelerates, wouldn't that defeat any chance of using this on something designed for ultra-lightweight and do compressors that powerful exist in the market outside of jet engines or maybe power plants?

By decelerating the flow rate(either the vehicles speed through the fluid, the fluids speed over the vehicle or in real applications the speed of the air relative to the car) aren't you just going to relocate the drag to wherever the deceleration occurs? Its not like I can decelerate air without touching it or touching something that touches it(magnetic bottling excluded and even that still creates the same amount of force if you just pushed it)?

If you can decelerate something for less than it costs to do the decelerating why wouldn't you just apply the entire concept in reverse and apply a blower across a convex surface to create "cheap" acceleration, instead of using a round about way of lessening the cost of maintaining acceleration apply direct acceleration(then you could even just route your exhaust over it)?

Just my curiosity

[theunchosen]

Originally Posted by winkosmosis

BTW, a wing doesn't work through the Bernoulli effect. That's a myth perpetuated by grade school textbooks and even pilot training. The pressure difference between the top and bottom of a wing travelling perpendicular to its plane is very low, if it exists at all. A wing develops lift by being positioned at an angle to the flow.

Yeah Bernoulli and. . .what is it elapsed time? are the two biggest "myths" about lift.

It seems kind of obvious that its not as the key ingredient to simple lift equations is "angle of attack." Meanwhile shape sits in the Cd position to determine the amount of work absorbed into countering the drag.

[DaveBirkenstock]

Originally Posted by winkosmosis

Are you saying that the purpose of the design is to blow air into the low pressure area at the rear of the vehicle to increase pressure from the normal vacuum? That's fine with me.

But that isn't shown in any of the posts so far... the concept seems to rely on the shape of the tailcone

Yes, the goal is to prevent the typical low pressure separation bubble behind the vehicle. I added a 'before & after' graphic at the bottom of page 2 that shows a sedan with no suction that suffers drag from this bubble and the same geometry with suction that eliminates the bubble & over half of the momentum deficit.

You're right, the shape of the tailcone is very important. Just like lift from a wing depends on camber, the thrust created by this concave tailcone depends on the nature of that concave curve. In the image on page 2, the suction would have created more thrust if the concave curve was sharper. In that case the total Cd would be lower than what is shown.

-Dave B

[winkosmosis]

Originally Posted by theunchosen

I'm in that fluids group so I have questions.

Has this been tested on something today and can I see pictures of all of its components?

How much vacuum is needed to create a sufficiently low pressure upstream of your concavity(a surface that curves inward is a concavity convex surface bows outward)?

the idea is to slow the air down enough that it "falls" along the curve pushing the car forward?

Or is the aim to generate a swirling eddy current that then pressed against the curve back to front motion and then up into the normal air current?

The amount of suction is going to drastically climb as the unit accelerates, wouldn't that defeat any chance of using this on something designed for ultra-lightweight and do compressors that powerful exist in the market outside of jet engines or maybe power plants?

By decelerating the flow rate(either the vehicles speed through the fluid, the fluids speed over the vehicle or in real applications the speed of the air relative to the car) aren't you just going to relocate the drag to wherever the deceleration occurs? Its not like I can decelerate air without touching it or touching something that touches it(magnetic bottling excluded and even that still creates the same amount of force if you just pushed it)?

If you can decelerate something for less than it costs to do the decelerating why wouldn't you just apply the entire concept in reverse and apply a blower across a convex surface to create "cheap" acceleration, instead of using a round about way of lessening the cost of maintaining acceleration apply direct acceleration(then you could even just route your exhaust over it)?

Just my curiosity

That's how I'm picturing it... If you are decelerating air relative to the vehicle, that means you're pulling that air along, which means you're just relocating the drag. Well said.