How fast can we swim with a dolphin like propeller?

[VladZenin]

Some regularities and rules in dolphin swimming and dolphin model efficiency are:
· A dolphin body oscillates in the vertical plane;
· The frequency of the propulsive cycle (oscillatory frequency) increases linearly with swimming speed;
· Peak-to-peak oscillatory amplitude remains constant with respect to swimming speed;
· Mean peak-to-peak amplitude ranged from 0.02 to 0.06 body length at the rostrum and 0.17 to 0.25 body length at the fluke tip; (1)
· Dolphins use a large amplitude gait for brief (1 – 5 s) acceleration. At that time oscillatory frequency is >0,43´speed (1,5 to >3 Hz); (2)
· Dolphins use medium-amplitude gait for cruising (1 s to >1 min). At that time oscillatory frequency is 0,43´speed (0,5 – 3 Hz);(2)
· The efficiency of the model depends on a band M width, a wave’s length, amplitude of oscillating of active points and the oscillatory frequency of the active points. It is easy to see the model shifts on one wave’s length per one stroke. So if the wave’s length is 2 m and the oscillatory frequency is 5 Hz the speed of movement of the model will be 10 m/s or 36 km/h. If the wave’s length will be 4 m and the oscillatory frequency will remain the same (5 Hz) the speed will be 72 km/h.

(1)Frank E. Fish, John E. Peacock, and James Rohr. Phase Relationship Between Body Components of Odontocete Cetaceans in Relation to Stability and Propulsive Mechanisms

(2)Randolph C. Scrovan, T. M. Williams, P. S. Berry, P. W. Moore and R. W. Davis (1999). The Diving Physiology of Bottlenose Dolphins (Tursiops Truncatus)

[kach22i]

What you have written is just above my understanding level. However is it fair to say that the oscillatory frequency is the key to speed?

Could one shead all mechanical devices when copying the Dolphin? That is to say could we create a sound engine to excite adjacent waters and have unlimited speed by increasing frequency beyond mechanical limits?

I am not talking about off shoots of the Stirling engine as in the links below:

http://www.newstarget.com/001930.html


http://solstice.crest.org/renewables.../msg00469.html

What I am taking about is vibrating the ships skin/hull to quiver through the water. The quiver will of course generate sound, but that is not it's goal. The goal would be to excite the surrounding waters with a pressure difference that results in forward motion.

There is a mechanical part to this which I was hoping to free us from. I'm just spewing off ideas and some ignorance here, part of the creative process. If anything sticks maybe we can build from it and develop some new ideas just for fun.

[JonathanCole]

Vlad,
What is the actual range of wave lengths utilized by dolphins? Do you know?

An interesting question would be, does a dolphin at any time exceed its hull speed as calculated by the formula? Or put another way, is there something going on with this sinusoidal propulsion that suggests as-yet-unrecognized factors controlling hull speed. Does viscosity or laminar flow change characteristics under the force of a travelling wave as opposed to pushing a static shape through the fluid medium??

[VladZenin]

The flexibility of the dolphin body utilized just one wavelength, which approximates its body length. I gave an example of the wavelength 2 m because this is a dolphin body length. A dolphin makes a few strokes per second and achieves the average speed of about 35 km/h. The coincidence of these results is evidence that the model is made in accordance with dolphin locomotion.
To reach its maximum speed quickly dolphins use at any time a large amplitude gait for brief (1 – 5 s) acceleration. At that time oscillatory frequency is >0,43´speed (1,5 to >3 Hz). Then dolphins use medium-amplitude gait for cruising (1 s to >1 min). At that time oscillatory frequency is 0,43´speed (0,5 – 3 Hz)(2). This formulas are right for Bottlenose Dolphins(2). They will be different for other marine creatures and models. Formulas for specific model can be find out after testing of this model.

[Leo Lazauskas]

Some further references, Vlad:

T. Y. Wu, "Hydromechanics of Swimming Propulsion. Part 1. Swimming of a Two-dimensional Flexible Plate at Variable Forward Speeds in an Inviscid Fluid", J. Fluid Mechanics, vol. 46, pp. 337-355, 1971.

T. Y. Wu, "Hydromechanics of Swimming Propulsion. Part 2. Some Optimum Shape Problems", J. Fluid Mechanics, vol. 46, pp. 521-544, 1971.

M. J. Lighthill, "Aquatic Animal Propulsion of High Hydromechanical Efficiency", J. Fluid Mechanics, vol. 44, pp. 265-301, 1970.

I have many other references but not much time every day to type them up. Hope you don't mind receiving them in dribs and drabs.

Leo.

[VladZenin]

Leo,
Thank you for references.

[markdrela]

Originally Posted by VladZenin

· The frequency of the propulsive cycle (oscillatory frequency) increases linearly with swimming speed;
· Peak-to-peak oscillatory amplitude remains constant with respect to swimming speed;

This implies that the dolphin's tail has the same S-shaped path geometry (same velocity triangle shapes) regardless of speed. In terms of propeller terminology, the tail is operating at the same advance ratio at all speeds. This is perhaps not too surprising. Any given prop wants to operate at one specific advance ratio to maintain maximum efficiency, and I expect the dolphin's tail works the same way.

The dolphin tail's flexibility could possibly shift the optimum advance ratio with thrust level, sort of like auto-variable pitch on a prop, but its tough to say how much. The observed constant frequency/speed ratio suggests that this particular effect of flexibility is small.

[jehardiman]

People, there is nothing new going on here.

Fish/dolphin swiming is a well understood hydrodynamic principle. You need to stop looking the the obvious manifestations of frequency and amplitude and start looking at the celerity of the generated vortex. By controling the instantanious wavelength of it's oscillation to something slightly greater than the vortex celerity, the animinal is able to be "pushed and pulled" along by the pressure gradients the vortex pairs generate. Just wagging back and forth with a fixed wavelength along the body will not work (see the original ROBOTUNA tests). The way it is mechanically accomplished (at much lower effiency) is to have a static fin of differential stiffness that increases wavelength/amplitude for the same transverse force along it's length. Look at a cuttlefish fin vice a dive fin for example of how this differs.

Now why can't we make a machine to do this? We can, if you want to have massively embedded sensors with massive parallel processing and a very large number of bi-directional pull-pull joints able to generate a large number of wavelengths for any given frequency of oscillation. Conversely, you could design a fixed speed mechanical system using an elastic membrane supported by rays of differential stiffness. Neither of these two systems, given todays technology, can match the existing bio-mechanical effectivness of a fish/dolphin.

[kach22i]

Originally Posted by jehardiman

Neither of these two systems, given todays technology, can match the existing bio-mechanical effectivness of a fish/dolphin.

Santa already knows this.

[VladZenin]

Originally Posted by kach22i

What you have written is just above my understanding level. However is it fair to say that the oscillatory frequency is the key to speed?

Could one shead all mechanical devices when copying the Dolphin? That is to say could we create a sound engine to excite adjacent waters and have unlimited speed by increasing frequency beyond mechanical limits?

I am not talking about off shoots of the Stirling engine as in the links below:

http://www.newstarget.com/001930.html


http://solstice.crest.org/renewables.../msg00469.html

What I am taking about is vibrating the ships skin/hull to quiver through the water. The quiver will of course generate sound, but that is not it's goal. The goal would be to excite the surrounding waters with a pressure difference that results in forward motion.

There is a mechanical part to this which I was hoping to free us from. I'm just spewing off ideas and some ignorance here, part of the creative process. If anything sticks maybe we can build from it and develop some new ideas just for fun.

Yes, the oscillatory frequency is the key to speed? You can see 5 oscillations per second is enough for speed about 36 km/h if the wave’s length is 2 m.
Dolphins average speed is about 35 km/h. The coincidence of these results (wave's length is 2 m) is evidence that the model is made in accordance with dolphin locomotion. It is known the efficiency of the screw propeller is improved, if we can increase the propeller diameter and turn it more slowly. With this propulsion device designer can realize an old dream “to increase the propeller diameter and turn it more slowly”. A few (3 -5) strokes per second allow a dolphin to achieve a top speed about 50 km/hour. Artificial propulsion device can easily to break dolphin records. There are no particular limitations on the size and shape of the artificial acting body. 5 oscillations per second are not a mechanical limit too.

[VladZenin]

Originally Posted by jehardiman

People, there is nothing new going on here.

Fish/dolphin swiming is a well understood hydrodynamic principle. You need to stop looking the the obvious manifestations of frequency and amplitude and start looking at the celerity of the generated vortex. By controling the instantanious wavelength of it's oscillation to something slightly greater than the vortex celerity, the animinal is able to be "pushed and pulled" along by the pressure gradients the vortex pairs generate. Just wagging back and forth with a fixed wavelength along the body will not work (see the original ROBOTUNA tests). The way it is mechanically accomplished (at much lower effiency) is to have a static fin of differential stiffness that increases wavelength/amplitude for the same transverse force along it's length. Look at a cuttlefish fin vice a dive fin for example of how this differs.

Now why can't we make a machine to do this? We can, if you want to have massively embedded sensors with massive parallel processing and a very large number of bi-directional pull-pull joints able to generate a large number of wavelengths for any given frequency of oscillation. Conversely, you could design a fixed speed mechanical system using an elastic membrane supported by rays of differential stiffness. Neither of these two systems, given todays technology, can match the existing bio-mechanical effectivness of a fish/dolphin.

Thank you for the active support.

[VladZenin]

Originally Posted by markdrela

This implies that the dolphin's tail has the same S-shaped path geometry (same velocity triangle shapes) regardless of speed. In terms of propeller terminology, the tail is operating at the same advance ratio at all speeds. This is perhaps not too surprising. Any given prop wants to operate at one specific advance ratio to maintain maximum efficiency, and I expect the dolphin's tail works the same way.

The dolphin tail's flexibility could possibly shift the optimum advance ratio with thrust level, sort of like auto-variable pitch on a prop, but its tough to say how much. The observed constant frequency/speed ratio suggests that this particular effect of flexibility is small.

I don’t know why are you talking about tail. A tail itself can’t realize a dolphin achievements. The Forces and Reactions Diagram of the swimming dolphin shows that every part of its body in every moment of the time takes part in pushing a dolphin through the water. That is why the dolphin stroke is so powerful. Suggested propulsion device entirely simulates the dolphin locomotion and allows realizing the most of advantages of the dolphin body propulsion: high efficiency, minimum vulnerability, noise and danger, good starting and stopping abilities and maneuverability.

[jehardiman]

Try this website for a brief discussion of different styles of marine animal swimming and robotic fish.

http://www.nmri.go.jp/eng/khirata/fi...e/index_e.html

[JonathanCole]

What is left out of most of these discussions is the issue of interfacial dynamics. When a solid (like a hull or propellor) is in contact with a liquid (like water), there are quantifiable energy state changes at the interface. The solid (with its particular energy state) somehow has to make the transition to a liquid (with its differing energy state). I studied this phenomenon in the solid/liquid electrical double layer, the underlying premise of EDL supercapacitors. This electrical double layer is several angstroms from the solid surface. The water molecule being a natural dipole, lines up at the surface of a conducting solid, acts as a dielectric, and a capacitor is formed at the interface. See attached diagram.

Investigating the phenomenon you notice that layering is a very common situation at interface boundaries, both on the quantum scale and the macro scale and even the cosmological scale. Things don't change abruptly from solid into liquids or liquids into gas (at least their energy signatures don't). In boat design we run across layering as laminar flow, that tendency for moving fluids to form tightly bound layers around volumes of greater density, like solids.

What the sine wave forms of propulsion, versus rotary propulsion may do differently is to interfere less with the laminar flow of the fluid over the hard, moving surface. A rotational movement is going to shed vortices in a quite dramatic way. These vortices, have multidirectional forces within them, many of which impede rather than propel. A traveling wave generator does not necessarily interrupt the laminar flow which occurs in close proximity to the solid surface and which is also likely to be bound in some sort of viscous coupling to the electrical double layer. Minimizing the disruption of these layered energy transition states may have big advantages in net propulsive thrust.

[VladZenin]

Jehardiman,
Thank you for the http://www.nmri.go.jp/eng/khirata/f...le/index_e.html