The power of three
The number of windmills in the UK and France has noticeably increased in recent years. These generators all have three blades to capture energy from the wind. I was once an aeronautical engineer and studied aerodynamics, and there seems to me to be no reason not to add more blades to windmills to 鈥渃atch鈥� more energy from the same wind; more power without proportionately more windmills. In propeller-driven aircraft, as engines became more powerful, the number of blades was progressively increased from two, first to three, then four, five and even six, to convert an engine鈥檚 increased power to propulsive thrust. For the reverse objective, why not do the same with modern windmills? Why only three blades? What aerodynamic and other criteria are applied in determining the optimum number of blades?
鈥� In simple terms, a wind turbine rotor with few blades produces less torque but spins faster than a rotor with many blades. So the power output is similar in both cases.
In theory, turbines extract the most energy from wind (estimated to be 59 per cent of the wind鈥檚 total energy) when the wind speed ahead of the turbine is exactly three times the wind speed in its wake. Because each blade on a turbine rotor extracts energy and slows the wind by a certain amount per revolution, the multi-bladed rotor must rotate more slowly than one with fewer blades to maintain an optimum wind-speed ratio.
Advertisement
If a rotor has a very high number of blades, aerodynamic interference between them will reduce power. However, it will produce very good torque at low wind speeds, hence the use of the many-bladed design for powering water pumps in rural areas.
But for generating electricity, high speed and low torque are favoured: generators typically require high rotation speed, and low torque means that shafts and gearboxes can be of lighter construction. So a small number of blades is preferable. But because single-blade and two-blade rotors suffer harmonic problems 鈥� dangerous vibrations at certain resonant speeds 鈥� and are displeasing to the eye, three-bladed rotors are the ideal choice. They are well-balanced, aesthetically pleasing and offer a high enough speed of rotation. Further blades add to manufacturing costs and, if designed to maintain a high rotation speed, would have to be very slender and, as a consequence, structurally inefficient.
Justin Harding
Engineer, Wind Power and Developing Technologies Group
Melbourne, South Australia
The eyes have it
When I go swimming I wear goggles to help me see in the water. Obviously, I don鈥檛 need them in air. On a recent trip to Honduras I encountered Anableps anableps, a fish that floats with its bulbous, protruding eyes half in, half out of the water. How can it see in air and water simultaneously?
鈥� In humans light is focused onto the photosensitive retina by both the cornea and the lens. The cornea is the major refractive surface. Because the refractive indices of water and the fluid inside the eye, the aqueous humour, are similar, when we put our heads underwater, the refractive power of the cornea is lost and everything becomes unfocused as we become long-sighted. For this reason most animals living underwater, such as fish, have spherical lenses, which are more refractive than ours, to produce a perfect image of the underwater world. The lenses of animals living in air, such as humans, are relatively flat.
A. anableps, known as the four-eyed fish although it only has two eyes, has a problem because it lives at the water surface with half of its eye in water and half in air. Therefore, light coming through the air from above the animal will go through a cornea that refracts the light, while underwater light will hit the submerged cornea, which is optically ineffective. So if its lens were spherical, as in other fish, light from one direction would be out of focus. Despite this, the eyes of A. anableps form a perfect image of objects both in air and underwater. It can simultaneously see a potential airborne predator and a tasty underwater snack.
This remarkable feat is achieved by having an elliptical lens positioned so that light from underwater traverses the long axis of the lens while light from above goes through the shorter axis. So airborne light, which encounters a highly refractive cornea, is focused less by the lens, while underwater images, which impinge on an optically neutral cornea, are focused to a much greater degree by the lens.
Ron Douglas and Yasser Yousaf
Department of optometry and visual science
City University, London, UK
鈥� A. anableps is sometimes called A. tetraophthalmus because it appears to have four eyes. It really has only two, but each has two pupils. It has an above-water pupil and a below-water pupil, separated by a line of pigment on its cornea. In terrestrial animals the cornea, or front window of the eye, is as important as the lens in focusing light onto the retina. But looking through water cancels the refractive power of the cornea and humans become very long-sighted underwater unless they are wearing an air space in front of the cornea, preserving its refractive power. This is why we wear diving masks.
Fish, in contrast, have a very strong spherical lens that is capable of forming a good underwater image. But Anableps has an oval lens inside its eye. The lesser diameter of this oval is aligned with its upper 鈥� terrestrial 鈥� pupil and produces an image on its lower retina from above the water. The greater, and therefore stronger, diameter of the lens is aligned with its underwater pupil and again produces a sharp image, this time on its upper retina.
The European kingfisher Alcedo atthis, which dives into pools and rivers, has only one pupil but has a similar oval lens. It has two different maculae 鈥� detailed-vision regions 鈥� in each eye and uses one macula for above the water and the other below.
There is a good diagram of A. anableps鈥檚 eye at
Malcolm Le May
Ophthalmic surgeon
Forres, Grampian, UK
This week鈥檚 question
Mole holes
As autumn progresses my garden is again blessed with a trail of molehills linked by networks of tunnels barely below the surface. This prompts a host of questions: how big is the average mole鈥檚 tunnel network? Is it constantly developing the network and do areas become redundant? How far does the average mole tunnel in its lifetime? And if moles are fiercely solitary, do individual networks overlap? If not, how do they find each other to ensure future mole generations?
Alan Rowe
Insch, Aberdeenshire, UK