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# hydrodynamics

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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.

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Subjects: Maritime History.

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