In part 2 of this series of posts, I will be explaining some of aspects of nature’s design that appeared during the research phase of the Neptuno Submarine. How many fins should a fin propelled submarine feature and how are they arranged? such question can only be answered when we stat to understand the evolution of maritime swimmers and their behavior. Lets begin with: Form, Kinematics and Stiffness.
Finding the Right FORM
Looking at nature for the best Form for a fin propelled submarine can be quite overwhelming at first. Trying to define a specific fin shape for the Neptuno Submarine was no easy task giving into to account the HUGE number of different forms and configurations marine creatures have. Thanks to biology research, a lot has been written on this subject.
Form Specialization – (Illustration: A.Kesel 1997 adapted from P.Webb 1984)
Every maritime biological being has passed through various stages of adaptation in the course of its evolution. Thus optimization to different living conditions eventually led to various forms and characteristics. This illustration from Prof. Dr. A. Kesel shows a comparison chart between fish shapes and their behavior characteristics (read strenghts). On the top we see fishes who can accelerate very quickly,on the left fishes which can maintain very high speeds for long distances and on the right of the chart, fishes that can maneuver very precisely.
How many Fins?
When we look closer to a fish, we see that there are many different fins, and that each one of them has a different function. This seems quite obvious, but understanding their functions can bring key insights for design decisions for future vessels.
Tail Fin: Is the mail propeller for a fish, It has also the function of controlling the direction of the swim.
Pectoral Fin: In most of the fishes, pectoral fins help them control their level (maneuvering up and down) but some coral reef fish have very strong pectoral fins that act as the main propulsion system (i.e. Box Fish).
Dorsal Fin:The Dorsal fin acts like a stability aid, keeping the body from oscillating to much when accelerating.
Pelvic Fin:This fin act like an hydrodynamic parachute help fish to brake and slow down.
Anal Fin:the Anal fin has the same function as the dorsal fin, it is there to keep the ride stable.
Kinematics
the correlation of form and function is clear as you look deeply in to the matter, but there is also one other aspect the greatly influence the performance of swimming, its their body movement.
Most importantly, there is a distinction between the actuating angle of the movements between fish and aquatic mammals. Probably because of their need to come back to the surface for air, aquatic mammals have their fin swinging in the vertical plane, making them more agile in that case. Fish have their bodies swinging in the horizontal plane (there are exceptions like the Plaicefish).
What also varies a lot between species, is the amount of body which swing and the type of curvature that it makes. Basically there are fish that use almost the whole body, and there movement is characterized by a wave that travels through the whole body. Which is often called ondulating movement. One example of this kind of kinematics is found by the eel. What is interesting in these kind of fish is the ability to switch the curve direction of their body movement to swim backwards. These fish are very precise in maneuvering but aren’t that efficient when it comes to speed.
On the other hand, Fish like the Tuna, can achieve enormous speeds underwater (up to almost 100 km/h), by having a partial undulating movement of their bodies which on the end of each swing cause a oscillating movement of the tip of their fin (thus called sub-undulating). Their highly hydrodynamic bodies and stiff/narrow fins make up the perfect combination of high speed swimming for long distances in the many situations when it has to swim in open water looking for food.
By contrast, some fish leaving in the narrow environments like coral reefs have the necessity to maneuver very precisely in very small paths, some times even swim backwards. The box fish for example, uses mainly their pectoral fins for propulsion leaving there caudal fins almost exclusively for steering, there fin Oscillate from side to side, almost as if it were pivoted in one point.
Flexibility X Stiffness
Researchers of the MIT working on this subject of flexible fin propellers have come to many achievements regarding the developments towards a more efficient and quite way of propelling vessels underwater. Their Robotuna project served as a platform for testing different functional aspects aroung the theme.
One particular study caught my attention, which was the study from Michael S. Triantafyllou, Alexandra H. Techet, and Franz S. Hover, entitled: Review of Experimental Work in Biomimetic Foils*. This study tell us a bit about the effects of stiffness in the efficiency of such propulsion systems, drawing the conclusion, that with every different swing frequency/amplitude ratio, a specific stiffness of the fin would bring the best results regarding the consumption of energy. This suggested that there was another variable to be taken into account when designing flexible fin propulsion systems, which is the flexibility of the body of the submarine. * IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 29, NO. 3, JULY 2004
Wraping it up for now
These were some of the aspects that I considered most important and to keep in mind during the next phase, the conceptual phase. On the next post I will be writing about the design strategies I developed after this analysis and I will also tell a bit about the festo pneumatic muscle technology which I used as starting point for my proposal.
