Pulsatile flow in curved pipes

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Abstract
The characteristics of some flows that occur when fluid is driven through a curved tube are disclosed for an imposed pressure gradient of pulsatile nature, varying sinusoidally with time about a non-zero mean. The fully developed motion depends on three parameters, a traditional Dean number D, a frequencyrelated parameter β and a secondary Reynolds number Rs, it being assumed that the pipe's radius of curvature is much greater than its cross-sectional dimensions. The theoretical description of the flow field is extended from the steady and purely oscillatory limits hitherto studied to all the key situations arising when Rs is of order unity and one of the other parameters β or D takes a large or small value. During this analysis, which in certain cases involves the interactions between steady boundary layers and Stokes layers, a number of pulsatile motions are revealed and the manner in which at high frequencies the secondary motion can change its direction, from inward ‘centrifuging’ to outward, is also explained. Two further illustrations of pulsating motions, stemming from the steady limit, produce an alternative mode of transition from steady boundary-layer flow to the boundary-layer flows occurring when Rs ∼ 1. The study, which mainly deals with the flow in an arbitrary cross-section, lays down a formal basis for deriving the fundamental attributes of many physical situations, some of which are expressible in terms of crucial modifications to, or combinations of, flow problems whose properties are already appreciated.

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