hydrodynamics

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hydrodynamics, a branch of physics concerned with pressures and behaviour of fluids, and is an important consideration in naval architecture. Its origins may be traced to the middle of the 18th century when Euler formalized earlier work of Bernoulli, Newton and d'Alembert into the study of what Bernoulli referred to ‘hyrodynamics’. Its, largely empirical, application to the design of ships was pioneered by the English engineer, naval architect, and mathematician, William Froude (1810–79). When he discovered that the wavemaking resistance (or ‘residuary’ resistance as it was then called) of scale models of ships towed at scale speeds through water along a towing tank varied in accordance with their full-size prototypes, he was able to propound his Law of Comparison, also known as the Law of Mechanical Similitude. He also showed that the behaviour of models in waves represented their full scale behaviour, some of which he was able to measure on ships at sea. This demonstrated that scale models could be used to determine in advance the seagoing and performance characteristics of a design in various sea conditions. This was an important breakthrough in naval architecture as running model tests is less time-consuming and less costly than building the ship first and making the required modifications afterwards.

Study of the movements of liquids around bodies immersed in them also resulted in the discovery of what came to be termed the streamlined form, a body which presents the lowest resistance to a liquid moving through it. For example, if a floating rectangular box is towed through water, or is anchored and the water is allowed to flow past, a wave will build up against the front face of the box, forming an area of high pressure. The disturbed water then flows round both sides and underneath the box, producing friction drag over its surface. Behind the flat rear end the water flow endeavours to close in upon itself. It cannot do this because the change in body slope is too great for the flow to adhere. Pressure recovery therefore breaks down, as does the flow, forcing a series of eddies and ‘dead’ flow to occur which results in a disturbed wake for some distance downstream. This lack of pressure recovery creates a suction on the aft face of the box which in turn acts as a significant source of resistance. This, when added to the high pressure resistance on the forward face and the friction over the wetted surface of the box, creates a considerable amount of drag as is shown in diagram 1.

If a streamlined form is adopted for the body, its resistance will be very much reduced. Developed in the first 30 years of the 20th century, streamlined forms were necessary in the quest for speed with aircraft of the time. These unstreamlined vehicles had very high so-called form and parasitic resistance (the former from the bluff shapes used for the fuselage, wheels and supports etc and the latter from the many struts, bracing wires and various protuberances) and this, coupled with the limited power-to-weight ratios of the engines of the time, limited speed. Streamlined forms and smooth structures were therefore adopted, resistance dropped dramatically and speeds increased as a result. The streamlined form is teardrop-shaped, as in diagram 2, whose main purpose is to prevent (or at least minimize) the eddying, separated and ‘dead’ flow seen astern of the box in diagram 1. This it does by a nose designed to split the flow and allow it to accelerate smoothly to the thickest part of the body. At this point the pressures on the body are low and must carefully recover to their original undisturbed values by the time the end of the body is reached if separation is to be avoided. The aft body is therefore carefully shaped to give gentle slopes which the decelerating flow can follow without separating. By so doing form resistance is significantly reduced. By going further and smoothing the body, and minimizing all protuberances, parasite drag is reduced and the overall resistance, composed largely of frictional forces, is considerably reduced.

It might be thought that such a streamlined form would be eminently suitable for the design of a ship's hull, for like a fish it seems ideal for offering the minimum resistance to the ship's progress. During the 18th and 19th centuries shipbuilding did in fact copy a modified ‘cod's head and mackerel tail’ form of hull below the waterline in different types of sailing ships and in yachts.

However, considerations other than the quest for speed in commercial ship design often make a fully streamlined hull impractical; but its principal features are still used in design. For maximum cargo capacity and stowage, the ideal shape for the hull of a cargo ship is a box, but, as has been shown, such a shape suffers from excessive drag. For large cargo carriers, running at comparatively low speeds, a bow can be added to a ‘box’ which, while not quite as rounded as that of a streamlined body, has similar features and may be quite bluff. Considerable care is taken in the design of the aft body to minimize or eliminate eddying or dead, separated, flow and this part of the hull will be elongated for the same reasons as its counterpart in the streamlined body. As a result, the propeller(s) act efficiently in well-behaved flow. The bluff shape of the forebody may be enhanced by the addition of another streamlined body below the waterline: the bulbous bow. This reduces drag at or near service speed by reducing bow waves and improving the flow (and hence pressures) over the forebody.

All vessels moving on the sea ‘squat’ or settle in the water. This comprises a bodily sinkage and trim by the bow at low speeds, changing to a bodily rise and trim by the stern at high speeds when the vessel, if it is able to move fast enough, is said to be planing. This is aided by the fact that water is virtually incompressible so that the planing vessel behaves like a small flat stone being skimmed. As much of the hull is out of the water, drag is considerably reduced with the result that very high speeds can be obtained. When only early steam propulsion was available, its weight in relation to the power it developed, that is the power/weight ratio, rendered it impracticable to drive a boat fast enough for it to do this, and it was only when the development of the internal combustion engine and lightweight boat construction produced a more advantageous power/weight ratio that boats could be made to plane.

To make a vessel plane, the old concept of a fast hull with a fine entrance forward, narrow beam, and a fine run to the stern, so as to make as few waves as possible, was discarded. Instead, a new type of hull was designed on the principle of a water sledge which would be able to rise onto the surface at high speed, with only its after part, rudder, and propeller in the water. The design was found to need a broad flat stern to prevent squatting at speed, and sharply V'd bow sections. At velocities of 40 knots and over, water can be treated as though it were almost solid. To assist boats to start planing, many hydroplanes, as they were called, were built in the early days with one, two, or more steps on the underside of their hulls, on which they were designed to ride like a sledge.

The hydrofoil is also based on the same principle, having a series of curved vanes or hydro- fins which are fitted to a leg attached to each side of the hull forward, and similar vanes to another pair of legs fitted aft. As soon as the speed reaches a certain point the vessel rides up until its hull is completely clear of the water, and so runs on only the hydrofins.

Sailing vessels can plane too, though it is essential to have one of light construction with a V'd bow sections and a flat bottom at the stern together with a highly efficient rig to supply the necessary drive. Although some light-displacement yachts and some multihulls can sail downwind with short bursts of speed in a state bordering on planing, true planing—riding along the surface at high speed for considerable periods—is normally achieved only by certain classes of high-performance dinghies.

Making use of this property of water the hovercraft is able to lift itself just clear of the surface by maintaining a cushion of air at low pressure which is more or less imprisoned within the skirt surrounding the vessel's bottom. Here the water acts almost like a solid surface, and only a small amount of surface water is dissipated in spray while the vehicle is hovering.

See also wave line theory.

hy·dro·dy·nam·ics / ˌhīdrōdīˈnamiks/ • pl. n. [treated as sing.] the branch of science concerned with forces acting on or exerted by fluids (esp. liquids). DERIVATIVES: hy·dro·dy·nam·ic adj. hy·dro·dy·nam·i·cal / -ˈnamikəl/ adj. hy·dro·dy·nam·i·cist / -ˈnamisist/ n.

hydrodynamics see mechanics .