Sine wave propulsion

Hm...
How fast can the fin travel from side to side? If it travels to fast you must get cavitation on the low presure side and that means that the mass in the low pressure jet must become almost zero and therefore the impuls drop dramatically.

Why are fish fins foilshaped?

There is something fishy here

 

I have been testing with up to 50 Hz without any cavitation. As the low pressure side change to pressure side every cycle the cavitation has no time to build up.

 

I invite you to make a very simple experiment. Take a hand Fan and move it in front of your face, only 5 degrees on each side. The air you feel is the jet stream the tail is producing.

 

I think it is worth pointing out that Tom Speer's critique is related to the ducted jet aspect of Kjell's device.

Another question is about the parasitic losses of the the water flowing through the duct. When a flipper moves in a duct it does not simply push the water straight back but sideways, as well. This may increase the friction/parasitic losses within the duct itself.

Another interesting question is since water is basically not compressible except perhaps for the gases within it, can you even talk in terms of a low pressure and hi pressure side? Wouldn't you have to describe the forces in terms of force vectors with varying or opposing directions?

Another point worth making is that Kjells device is not really the kind of sine wave propulsion device that originally inspired this thread, because the blade/fin is stiff. Although any point on a non-flexing fin will describe a sine wave once it is in motion, it does not reflect the way creatures generate travelling waves. The propulsor body part in a marine animals flexes in the sine wave shape. Whether tail&fin, body&fin or wings, the flex factor is always there. This is what Vlad was trying to explain in other threads related to this topic. I suspect (as Vlad does) that this travelling wave generator that marine creatures utilize has an especially advantageous affinity for the water medium. I think that it has to do with the way laminar flow works and in water there are issues of surface electronics/chemistry at the interface of the propulsor and the liquid.
See:
http://en.wikipedia.org/wiki/Electrical_double_layer
http://en.wikipedia.org/wiki/Laminar_flow
http://en.wikipedia.org/wiki/Turbulence
However to my knowledge, and in relation to interfacial electrostatic forces, no work has been done to quantify what happens to the interfacial electronic forces if the solid is moving through the solution.

 

The tail in my Jet device is flexible. And it is not pushing the water it is accelerating a big amount of water without to increase the output pressure. The tail can bee operated with sine wave motion. The sin wave motion is the less effective way to move the tail.

 

My apologies Kjell, in the pictures the fin appears to be made of metal. What is it made of?

Also if it is flexible and flaps on a shaft, then it would seem that the fin itself would form the shape of a section of the wave that it generates. The travelling wave shape would then be a function of the rate of flapping and the stiffness and length of the fin. Or am I misunderstanding the way your fin is actuated?

 

You are right.
One of the most important details is the shape and the flexibility of the tail, and the way it is moving.

 

Fish vs. penguin

The Hobie Mirage drive is based upon the action of a penguins wings (filppers)as it "flies" underwater. The flexibility of the Mirage propulsors supposedly approximates a screw propeller of proper pitch and diameter as it swings through its 180 degree arc.
That got me to thinking about a similar propulsion set up using pedals to drive a fish tail shaped fin back and forth. Would a fish tail be more effective than a penguins wings...?
Any thoughts on the subject?

 

The Hobie Mirage drive is a very clever devise. Do to the 180 degree arc the Mirage drive is more efficient than a penguins wings. A fish tail can only make one sweep at the time and the frequency has to bee double than the Mirage drive to bee able to compete.

 

Thrust generation of fins

Found this fotage of flow around a fish fin.
Notice that the thrust is generated by the water flowing from the high pressure side around the aft edge of the fin. In addition to this there exsists a reversed von Kármán street behind the fin.
See ref: Lauder et al

 

So if 180 degrees is more efficient than a fish tail-type of propeller, it would be logical to assume that twice the 180 degrees (360) would be almost as twice efficient, (allowing for losses)? 360 degrees being rotary movement...

 

Twice the 180 degrees does not means twice efficient,
The main difference between the Mirage fin and a tail fin is that the Mirage fin is making a circular movement and the tail fin is making a linear movement. The water speed is the same on total area on the fin tail and the water speed is different on all parts of the Mirage fin.
The advance of the 180 degree movement is that the cavitation has no time to build up and the AoA can bee more than a propeller. The same happens with a fast moving tail fin.

 

This seems to be an interesting article: Anderson, J. M., Streitlien, K., Barrett, D. S. and Triantafyllou, M. S. (1998). Oscillating foils of high propulsive efficiency. J. Fluid Mech. 360,41–72.

Have any of you read it? Is it good? I had hoped to read it tomorrow (at the library) but now work has made it impossible but I will read it as soon as I can.

How efficient is a fin? I have found numbers around 40 % in the articles on fish that I have read on the net.
How efficient is a propeller?

 

Mother nature..

I was part of a project 8 years or so ago, that developed a propulsion system based on the path of tail of a whale through the water, which is a bit similar to a sinus. The propulsion consisted of a 2 vertival wheels with a number of blades (naca profiles) in between. The position of these blades was continiously adapted to follow that tails movements. See http://www.nap.edu/openbook/0309058791/html/946.html or google for whale tail propulsion.

 

This is a really excellent resource that puts the entire issue into an internally self consistent technological and scientific language and framework. The device described here is very similar to some advanced wind turbines that I have seen. Martijn_vE, what role did you play in this work? Do you mean to say vertical axis wheels? Have vessels been built using this concept?