Down the plughole
Do giant whirlpools ever form in hydroelectric reservoirs where the water runs through the base of the dam? If they do, do they have adverse effects? And if they don鈥檛 form, why not?
鈥� Yes, whirlpools do form where water is drawn from hydroelectric dams. Engineers normally refer to these as vortices and talk about vortex formation, which can be a problem for the operation of the dam鈥檚 hydraulic machinery, such as turbines or pumps.
Vortices affect the distributions of pressure and flow velocity in the approach to the dam, which can reduce the efficiency of the machinery. Vortex formation can cause vibrations, which can result in damage, and in extreme cases may cause cavitation 鈥� formation of vapour bubbles within the water. This occurs at the tips of fast moving blades, like those found in turbines, when the fluid pressure falls below that of the vapour pressure of the water. The bubbles are carried downstream by the flow, and when they enter an area of higher pressure they collapse violently. If the collapse takes place next to a hard surface then it can cause considerable damage.
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Engineers therefore try to develop designs in which vortices don鈥檛 occur, or else they try to suppress them. Physical models are often used to test the performance of designs.
The picture (below) shows an air-core vortex forming at a pump. The bell-mouth of the pump is pointing vertically downwards and creating the vortex that can be seen to the right of it.
Roger Bettess
River & Urban Catchment Group
Wallingford, Oxfordshire, UK
鈥� Giant whirlpools would indeed form at the power intakes of hydroelectric dams if measures were not taken to avoid them, and they would be extremely damaging too, thanks to the violent pressure fluctuations caused by air being sucked into tunnels and turbines.
Immense care is taken in the design of intakes to ensure vortices do not form. The two means by which this is done are to limit the draw-down level of the reservoir, the height between the surface and intake, and to construct anti-vortex structures above the intake opening.
You will know from watching your bath draining away that a vortex only forms when the water level has dropped to a low enough level. Conversely, maintaining a sufficiently high water level prevents a vortex from forming. Unfortunately, limiting the level to which the water in a reservoir can be lowered means that there is always an amount of water that cannot be used to generate power.
So the design and construction of an anti-vortex structure over the intake prevents vortices from forming even at lower reservoir levels, and improves the useful capacity of the reservoir.
The anti-vortex structure is essentially an array of beams constructed above the intake, and dam engineers devote considerable effort to the design and hydraulic modelling of such structures.
Tim Saunders,
Charvil, Berkshire, UK
Markism
I have often wondered what causes the splotches of skin pigmentation that are commonly known as birthmarks? Why do they persist on one area of the skin?
鈥� Birthmarks fall into two major categories: vascular lesions, which are spongy and consist of blood vessels, and non-vascular pigmented lesions.
Vascular lesions are caused by abnormal development of blood vessels in the skin. Although they are usually present at birth, they may also develop later in life. They may also appear suddenly, persist for a while, and then disappear again. This can happen when the body is going through a big physiological change, such as puberty or pregnancy.
As with pigmented birthmarks, there are various different ways in which vascular malformations are presented. The two most common forms are port wine stains and venous plexi 鈥� intricate networks of veins. Port wine stains are pinkish-red and flat, and may darken in colour to a purple colour several years after birth. They are usually found on the face, but may occur anywhere on the body, and they persist for life, although they can be treated by surgery or laser therapy. They may even grow larger and more conspicuous later in life, darkening, thickening or forming vascular bumps. Those that occur on the face may be associated with eye or brain problems. Venous plexi are thin and light blue, and may be flat or raised.
Haemangiomas are a third type of vascular lesion. They may develop after birth and consist of many tiny blood vessels bunched together. Haemangiomas occur in up to 2 per cent of newborns, but as many as 12 per cent of babies develop them by age one. Interestingly, they are more common in girls. Haemangiomas may change in size, and most disappear completely by age 10.
The non-vascular pigmented lesions are also more usually seen in children than in adults and are caused by abnormal melanocyte cells in the skin producing too much of the pigment melanin. It is not clear what the cause of the abnormality is. The number of birthmarks, and their size and degree of pigmentation, depend on the number of pigment cells, their activity and how deep they are within the skin. The location of such birthmarks on the body appears to be random.
Such pigmented birthmarks appear in various different forms. Congenital 鈥渘aevi鈥� are pigmented skin spots, more commonly known as moles. These are usually present from birth, and there is a risk that large moles will become cancerous later in life.
Other types include caf茅-au-lait spots, which usually persist for life and may increase in number; 鈥淢ongolian spots鈥�, which usually appear on the lower back and buttocks and are flat and slate-blue or grey; and macular stains or salmon patches, which are thin, flat, pink patches of discolouration. This last type seem to appear along a baby鈥檚 hairline on the nape of the neck, on the upper eyelids or upper lip, or between the eyebrows. They tend to be present at birth and then disappear without treatment a few years later.
With thanks to Dr Vivek Jain, Skin Specialist, Gujarat, India