Pressure thrust?

[Christ]

Dave - these guys have a valid argument about relocating drag... I'd like to read your rebuttal.

[Christ]

Originally Posted by DaveBirkenstock

Hi everyone... it's great to see a spirited discussion of this phenomenon, thank you for the interest.

The one thing not clearly identified on that blimp image is the suction inlet, just upstream of the concave tailcone. By adding energy in the form of suction, the mechanism can generate what Fabio Goldschmied called "fuselage self-propulsion."

Goldschmied proved a significant reduction in total power used for streamlined bodies (his baseline body was the rigid blimp, USS Akron) by exploiting pressure thrust.

As you know, separated airflow drag over the rear of a car makes things much worse than a streamlined blimp shape, but you might be interested in the computed drag coefficients from a CFD run I had done:



The vehicle was a generic sedan and the Cd numbers are as shown. The geometry was not best-case though, the intake area was too small and the concave stagnation zone is too big. When the whole car is optimized, the Cd number should go below 0.1

But just building an airfoil can yield a Cd of .05 or less...

So where is the benefit to what you claim?
Shape Effects on Drag

[theunchosen]

The pressure contour map is the same as what an airfoil is(like if you put an airfoil next to those two cd maps you'd see the flow tendencies over the back of the concavity is the exact same smooth hugging without any complex vacuum or suction schemes.)

The only advantage I can see is you can kind of create the shape you want and then apply this technique for "free"(using air intake or some other naturally occuring engine process(by natural I mean something that it does under normal circumstances)).

Other than that I'm sticking with my foil-prism for my custom build CF sketches.

[Christ]

Originally Posted by theunchosen

The pressure contour map is the same as what an airfoil is(like if you put an airfoil next to those two cd maps you'd see the flow tendencies over the back of the concavity is the exact same smooth hugging without any complex vacuum or suction schemes.)

The only advantage I can see is you can kind of create the shape you want and then apply this technique for "free"(using air intake or some other naturally occuring engine process(by natural I mean something that it does under normal circumstances)).

Other than that I'm sticking with my foil-prism for my custom build CF sketches.

But this would screw with volumetric efficiency, since the engine would have to work harder to draw in the air it needs. So you're still trading efficiency.

Looking at this from both sides, I'm thinking that it's more like a novelty aero-experiment than something viable for right now.

Dave has openly stated that the gains are there to be had, and said that an aero shape was not a valid opponent to this "design"... yet just stated that the achieved Cd of the capsule, which can only be achieved with help, is still higher than the airfoil.

By the way guys, the vacuum needs to occur in the concave area for this to work... that would mean there would have to be a pressure vent somewhere else... like at the tip of the tail cone.

Saying that you're re-issuing drag to another area is a false statement.

The efficiency loss comes from the necessity to drive a vacuum pump to keep the boundary layer attached at the concave region. The idea that the flow slows down so dramatically to stay attached there is what causes the pressure to rise in that area compared to other areas.

[DaveBirkenstock]

Originally Posted by theunchosen

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

Great, thanks for asking & having an open mind... some folks tell me this is "impossible" (despite the successful flight, wind tunnel and CFD tests) and move on.

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

Yup, there is a list of technical papers at the bottom of my site Pressure Thrust

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)?

It depends.. for the image at the bottom of page 2 of this thread, I think the pressure was around 1200 pascals. Should be within the reach of a fairly simple utility blower. That was with the car going 60-65MPH (I forget which).

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

The suction keeps the flow attached across the strong pressure gradient of the concavity. That stagnation & the concave shape that creates it generate the aerodynamic pressure thrust.

I'm sure there is a way to describe this in terms of circulation theory but I'm just not that smart. :-]

-Dave B

[theunchosen]

Originally Posted by Christ

But this would screw with volumetric efficiency, since the engine would have to work harder to draw in the air it needs. So you're still trading efficiency.

Looking at this from both sides, I'm thinking that it's more like a novelty aero-experiment than something viable for right now.

Dave has openly stated that the gains are there to be had, and said that an aero shape was not a valid opponent to this "design"... yet just stated that the achieved Cd of the capsule, which can only be achieved with help, is still higher than the airfoil.

By the way guys, the vacuum needs to occur in the concave area for this to work... that would mean there would have to be a pressure vent somewhere else... like at the tip of the tail cone.

Saying that you're re-issuing drag to another area is a false statement.

The efficiency loss comes from the necessity to drive a vacuum pump to keep the boundary layer attached at the concave region. The idea that the flow slows down so dramatically to stay attached there is what causes the pressure to rise in that area compared to other areas.

Yes its the efficiency being lost in the pumping, I don't think either of us disagree. What Wink and I are suggesting is that the pumping is going to be exactly equal to the amount of force you create.

Effectively what has to happen is some air(to be vacuumed close to the surface like this) has to be accelerated to the speed of the car. If I am going to pull it inside the car in any way its at least got to be moving the same speed, otherwise its going to hit the bodywork and be driven off. Another way to do this same thing would be to have a spoiler out from the edge just a little and angled down not up. It will force the air along this curve but its going to come at huge drag costs.

Also I'm not so convinced its going to do it anyway, at least not as a high pressure area. I'm sticking with the rule of thumb that the inverse(air traveling inside pipe with the exact specifications and air being vacuumed out instead of in) generates high pressure and this can only generate low pressure.

well. . .I take that back. If. . .IF the pump is pulling harder than the relative motion of the object and fluid it would become a high pressure spot. . . but then its like your pulling the car backwards in the form of the suction you are creating not to mention the enormous losses you are suffering to MAKING that suction.

[DaveBirkenstock]

Geeze, I'm at least 3 or 4 posts behind you guys...

I'll try to catch up: The main benefit of this is making a car, airfoil or fuselage more efficient. A typical sedan with a Cd of .34 can be made much more efficient at highway speeds if pressure thrust is used on it's aft end. Fully optimized, the modified car would have a Cd below 0.1, would burn less fuel and would mostly look like it does today.

I'm not sure I'm following the part about the car Cd being higher than the airfoil Cd. An airfoil can enjoy meaningful benefits, too, but because it is much cleaner to start with there is less room for improvement.

The amount of air that needs to be pulled into the car is fairly small & the burden put on the suction system is not too great as to destroy the benefit. There is a synergy at work, where the forward motion of the body, the suction power and the concave geometry prevent drag from forming and create an independent thrust force that all work in concert to make the vehicle more efficient than is possible without this synergy.

The suction system will always lose energy, the thrust force will come with the costs of the strongly adverse pressure gradient on the concave curve, but despite these real & unavoidable losses the whole is still greater than the sum of it's parts.

And because the needed hardware is very well understood and available 'off-the-shelf' the concept can be put directly to work on current production vehicles.

Lastly, the inverse rule of thumb works with all this, too (I think); if the suction pump creates a low pressure area inside the concave part near the aft end, then the outside of that shape will have high pressure (thrust). Right?

Please let me know if I missed a question or important point... or if I'm so tired I'm not making sense any more.

Thanks for your time and consideration... I'm going to bed

[theunchosen]

I think I may not have passed along the idea of the inverse rule quite right.

Essentially the quick and dirty way to discover what a solid will encounter in a moving solution, is to assume that the solid is actually the piping for a fluid. Once you decided where the high pressures would be located inside the pipe you can assume the opposite(if its high on the pipe model that particular outside surface experiences low pressure in the fluid).

So if you take say a conical tail fin and we want to know what the relative pressures are on the outside walls we could look at a nozzle in a pipe. The nozzle accelerates the flow by restricting the flow and the pressure forces the fluid to travel faster out of the nozzle.

In that diagram the sloped walls of the nozzle experience relatively high pressure and therefore if instead the nozzle is the tail piece for a jet traveling through a fluid the exterior walls are faced with a relatively low pressure by the rule of thumb.

so in this example its like a half nozzle in the original pressure contour or a full nozzle in others with a pressure release point in the wall(I think someone said in the middle but I'm not going to say that for sure because its late). So the nozzle with an initially steeper slope will see higher pressures than the conical nozzle(more pressure where the curve begins because its steeper than a straight line cone-nozzle). Around the release points(inversely the suction point) there will be a drop in pressure, it won't approach a low pressure zone but it will be like a teal color rather than navy blue. The reverse image of that is a low pressure surface with a suction point of not-quite-so-low pressure but definitely not neutral or high pressure.

[Otto]

Dave, from your website I notice that thought has been given to using Goldschmied pressure thrust to enhance a tractor propeller airplane, as the aft fuselage is sharply tapered, with annular inlet.

How best to use Goldschmied pressure thrust on a pusher configuration?

[aerohead]

Originally Posted by DaveBirkenstock

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

Dave,the Corvette was a notch-back style roof design and the inlet slot was positioned at the base of the rear (nearly vertical ) backlite.The premise of the research was to see if they could achieve with the notch-back,what otherwise would require a fastback roof,with taper in both plan and elevation.They used an outboard power source for the suction,and in the energy balance,a net loss was demonstrated,although the the car did achieve lower drag.------------------- With respect the Chaparral,that car used a Bombardier two-stroke snow-mobile engine to power the dual high-static-pressure fans for the under-car suction.----------- A side note,is that with Ford's Probe-IV concept car,with rear quarter-panel heat exchangers(radiator and AC condenser),the car achieved 4-mph on level road from the thrust generated by the two electrically-driven cooling fans.Of course,this "thrust" would be lost in the signal-to-noise ratio of the engine's output during normal driving,however it cannot be said that it was not there,at least when the fans were operating.