another idea

robherc,

Air molecules are always moving individually at the speed of sound thereabouts, and constantly crashing and colliding into each other. In stagnant air they are statistically moving in every conceivable direction, so that the pressure is static, and there is no flow. If they pass into a venturi inlet, they are deflected towards the centerline by the venturi walls. Of course, there are other molecules on the other side of the centerline that are being deflected the other way, and these all interact so that statistically more of them are moving towards the throat than any other direction. At the throat, most are moving in the flow direction, almost none are moving against the flow, and very few are able to strike the wall except at very oblique angles. The sum of all these motions is still at the same potential as the original static condition, but now are converted into ram pressure along the axis, and correspondingly less pressure is exerted against the walls.

Any holes into the duct would allow atmospheric pressure to bleed into the low apparent pressure inside, and would tend to frustrate the flow. In the old carburetors this principle was used to draw fuel into the air stream. It is clear that it would be impossible to get the flow to go faster than sound through the throat because that would mean the molecules had aquired some additional velocity.

 

Robherc,

To elaborate a little on that last post, the characteristic shape of the venturi is due in part because the 'disturbance' created by the venturi wall has to have time to propagate into the bulk of the flow. If the angles are too steep, then this is not enough time for this 'equalizing', and if the walls are too gentle you are simply getting diminishing returns. At some point the ideal venturi will satisfy the design needs for a set of conditions of speed, temp, density and scale.

 

OK, that works for me for an "ideal" venturi, but I don't think the counter-weighted louvers in my drawing would have any significant impact there due to the following:

1. They are effectively "one-way" valves in that they will swing open, under enough force, to let water out, but would strike against stops (either built into the duct, or the louver before them) if they tried to open inwards...os no significant airflow would be admitted there.

2. I intentionally drew them at the very highest wall-pressure position in your duct...where many/most air AND water molecules will be striking the wall of the venturi for the first time, so no collimating effect has yet been enacted on them. This means that the air-pressure inside the duct will be slightly HIGHER than 1 atmosphere here, and that any water in the duct will be exerting the most possible force to open the louvers & drain itself here.

I haven't physically tested these statements, so they are open to argument, but I'm quite sure of them nonetheless.


Also, on a similar topic, won't your "bent venturi" duct suffer dramatic efficiency losses due to having to redirect & collimate the flow into not one, but 4 directions sequentially, relative to the craft? (Vertical [up], then horizontal

, then vertical [down, through the turbine], then horizontal again [right, out of the duct])​

 

robherc,

I think you still have a misconception of the flow dynamics in a venturi. The fluid doesn't try and sqeeze through a small hole (the throat), it sees nothing but a low pressure region ahead of it, and wants to expand into it. There is never a point where the pressure is higher than atmospheric.

The entire problem of water ingestion is not very important because it would be easier to place the duct high enough to avoid the problem, and also because venturis are notorious atomizers of liquids. That water will go through like a fine mist, and actually might add to the propelling force since it is more dense than air.

As for the bent shape, all I show is the characteristic shape, it could be stretched to be more gentle. Surely there will be some forces developed, but since the duct is symetrical fore and aft, these forces are mostly cancelled.

Just remember the basic principle of lift, that a small pulling force can generate a large lifting force. Some people might consider that a mechanical amplifier. All we are trying to do is to bend, twist and redirect our flow to insist that the forces work for us and not against us.

 

I understand most of your theories here, but I don't think your bent venturi, unless it is stretched to an unweildy size, will have the "low pressure region ahead" effect on the fluid stream. The fluid (air) will still have inertia when it comes into contact with the first bend, and part of that intertia will still be exerted as >1Atm pressure against the duct wall. The eventuality of this will be entropy in the form of heat and fluid-friction (wear) imparted to your duct.

Even in your "perfect venturi" concept, I don't believe you will achieve an escape from SOME drag due to entropy & fluid friction, otherwise the airflow behind would delaminate, and be accelerated to the extent that the venturi would develop its own thrust equal to T=MS^2 where M=mass of air flowing through the venturi, and S=speed of sound. Menwhile, you would also be generating drag (vacuum wake) behind the venturi equal to D=Pi(S) where Pi=the pressure-incidence of the vacuum, and S=the surface area (cross-sectional) of the delaminated-flow area behind your venturi (the difference between the throat area, and the total area of the venturi, by cross-section).

I'm sorry, but I really can't seem to wrap my mind around that working....sounds too much like "quantum physics" to me...and I've developed on SEVERAL theories that disprove most "quantum" properties from being anything truly exceptional from the world of "normal" physics...when viewed from the proper frame of reference.

 

That said, I'm not negating the benefits achievable by collimating the airflow in a venturi...but I think we should exercise some caution in exactly how much we expect to gain; and realize that there will be losses incurred...we ALWAYS have to "pay the piper" in the end.

I believe that the venturi design will give us the improvement of a denser airflow over the turbine, with MINIMAL increased drag. Hopefully to the point of improving our Lift/Drag ratio of efficiency overall. I just don't want to get too excited and start expecting greater returns than are truly possible.

 

robherc,

There are losses in all machines, it is a question of keeping the losses less than the gains. Try turning an internal cumbustion engine over by hand, and you might wonder how it could ever work at 5000 RPM! Remember the L1011 aircraft with the engine in the tail, it had a tunnel like inlet that made two 90 deg bends, one after the other. The duct was 8 feet in diameter, and the air went through it at 550 MPH. We are not suggesting that we should go that fast, we are looking at maybe 100 MPH vehicle speed tops. The main purpose of the complex ductwork is to position the forces so they don't fight us.

 

You're right, I just was starting to think that your descriptions of the physics surrounding the venturi duct as approaching a religious perspective
...Wanted to make sure we weren't going to try to harness quantum dynamics on a wind-powered boat, that's all. hehee

Anywise, we'll still just have to wait & see what the actual efficiency results of your bent venturi duct are in your first test. Meanwhile, I think I'm going to work on designs for a venturi-ducted wind turbine to power my next home (I'm buying land on an undeveloped section of beach)...so I guess I have gained useable knowledge from this debate, though the bent-duct would gain me nothing, as mine'll be a fixed-position turbine.

 

robherc,

You are correct, a fixed system has no use for the twisted duct, a straight thru would be better.

I have attached a concept dwg of how I would approach the layout of a vessel using the ducted tech.

I have some other types of forgotten wind generator ideas you might be interested in for a fixed setup.

 

Cool Old Invention...

robherc,

This idea was one done by an Austrian inventor named Oppholtzer. He built a huge model with scoops about 1 meter across that ran for months and months. However the politics of power generation in hydro-electric rich Austria killed it.

 

robherc,

This is my version of that last idea. A ducted Oppholzer turbine, on a swivel base to always face into the wind.

 

A very interesting concept...and well thought out. I like his idea immensely, and your duct should improve on its efficiency a bit.
Unfortunately, however, I think it would end up having a higher cost (and amount of airspace used) per KWh than a venturi ducted turbine, in addition to having more exposed moving parts.

I see nothing truly "wrong" with the idea, just don't see it as the most practical for my own purposes. (and I've been in love with ducted turbines since I was a child, truth be told )

 

robherc,

The design I show is for a masthead generator, but the same idea could work in other forms. The advantage of this approach is there is no adverse thrust, as the driving force is always purely axial (to the duct or axis to the wind). This arrangement could be made very tall without such immense towers used on conventional windgens. It is like two huge ferris wheels, where the wind strikes only the top. The recirculating end could actually be sunk below ground level.

 

robherc,

As an engineer 40+ years experience, I never worry about the cost until the final concept is in place. Do you know what it costs to make a can of CocaCola?.....$10,000,000.......for the first one, $5,000,000 for the second one, $3,333,333 for the third....etc, etc.

 

LOL, as a strictly self-funded engineer with a fixed income, I ALWAYS worry about the cost before I even start building a prototype.
As the case may be, I can build turbine blisks fairly well with the tools, equipment, and experience I already have. Unfortunately, though, I don't have any experience making parts for a vertical-axis turbine (even if it's turned 90degrees) adn a few of the parts I'd need would take some experimentation to get their strength right. So, I think I'll stick with the concepts that I'm actually qualified to work with