Today I’d like to introduce my first Biomimetic project to you. It is about a floating turbine, which could be used to generate electrical energy while swimming in a river or other streams. I’ll follow my own pathway that I used several years ago, with the exception that some sources are now available for everyone. The series of articles will consist of three parts:
Part II: Aerodynamics of Penguins
I hope you willl enjoy this small trip over the weeks and get something out of it. Maybe you’ll find something in addition to improve the concept or you get another idea. If this is the case, feel free to share your ideas or get in contact with us/me.
And now here we go: Part II
The last time I presented you the flight of albatross and how it converts kinetic energy into potential energy and vice versa. This was to show you some possibilities of creatively using an instinctive mechanism of an animal to stabilize a turbine in a stream.
Today I would like to show you the inspiration for the shape of such a “floating turbine”. Of course it needs to be highly aerodynamic. This means that the body of a turbine should have the lowest drag coefficient (cw value) as possible. Why? You may ask… The turbine needs also a generator and nearly all the resistivity against the stream should be located there to increase the efficiency (more about that in Part III).
But what is the best inspiration you could choose for an object that will be working completely under water? A fish? A dolphin? Nope, in this case it is a bird. A penguin! More exactly, the Pygoscelis adeliae.
The following video will give you an imagination of the penguins themselves. There are no comments included but I hope you can enjoy it like as I do.
Before I continue to talk about our little bird, I would like to show you some numbers:
- Cw values of cars: 0.25 – 0.5
- Cw value of submarines: ~ 0.1
- Cw value of highly optimized spindle: 0.02
The Cw values represent different effects on resistivity of a body in a fluid, based on its shape. It is obvious that current car models are still far from achieving good resistivity values. Here are some examples:
- a usual Porsche 911 has a Cw value of about 0.3.
- Mercedes built a car with a Cw value of 0.19.
From the available information, these are the lowest values I could find.[2,3]
Let us now take a look at submarines. Usually submarines are shaped like torpedoes, the classic streamlined bodies that everybody heard about. This shape seems appropriate for an “U-boat” but it is not reaching the minimum drag values.
Of Form and Shape
Now, I would like to take a look at the spindle shaped body with its awesome low drag coefficient. How can this absolute low resistivity against streams be achieved? A shape should be created, that prevents the stream of being interrupted. By developing the existing design step by step in order to keep the stream flow continuous, the Cw value will lower. So you have different stages in your design that will help you to reach this. Every time a stream gets interrupted it leads into small vortexes/whirls, which causes highly increased friction (increased Cw value). Stage 1 of the design process would be a thin tip that simply splits the water on the front side. This leads to stage 2 where the thickness of the body along the streamline is increased. The following stage 3 is slightly thinner, which is connection to a much thicker main body part, stage 4. In the latest stage 5, the thickness decreases till the end. All these stages are effecting the stream directly, causing it to develop the velocity around the body in separate steps, which are increasing the stability of the stream around the whole body. 
And now… Watch this!
Did you see that? Yep, you’re right! That is exactly that shape I was talking about. Here are the actual numbers of our penguin friend.
Cw value Pygoscelis adeliae while swimming: 0.05 
Just to understand the Cw value in a more physical experience. The Cw value of 0.05 means that a body has the same resistivity against a flow as a 2€ coin thrown in water, facing forward the thin side!
Surprisingly, the value is not as low as the one of a manmade highly optimized shape. The reason is that the Cw value of a penguin is measured while swimming, which is a dynamic process. The optimized shape, on the contrary, is only recorded in a static state, for easier comparison with established technology. Nevertheless, the Pygoscelis adeliae is chosen as a champion of low drag movement for the design of my floating turbine.