1,721,107 research outputs found
Nonlinear forces on a horizontal cylinder beneath waves
No full textMeasurements of forces experienced by a submerged horizontal cylinder with its axis parallel to the crests in deep-water waves reveal nonlinear components with frequencies up to three times the fundamental wave frequency. The dominant nonlinear contribution to the loading is a t the third order in the wave amplitude, and, for Keulegan4arpenter numbers approaching 2, its magnitude was found to be as much as one-half that of the inertia force. It is suggested that the third-order force is associated with circulation generated by steady streaming in the oscillatory boundary layer on the cylinder. At higher Keulegan-Carpenter numbers, form drag becomes increasingly important, and velocity measurements close to the cylinder show the rapid development of the wake. Observations of nonlinear features of the transmitted waves are also discussed
Morison inertia coefficients in orbital flow
No full textInertia coefficients for use in Morison’s equation in conditions of orbital flow are significantly lower than those appropriate to planar oscillatory flow. Circulation around the cylinder resulting from asymmetrical shedding of vorticity generates a lift which opposes the conventional inertia force. The magnitude of the lift and the effect of Reynolds number are discussed on theoretical grounds and with reference to force and velocity measurements in orbital flow. Consideration of the lift leads to the formulation of a modified Morison equation, which is appropriate to all circumstances of wave loading on a cylinder when the incident velocity vector is not collinear with the incident acceleration vector
Loading on a cylinder in uniform oscillatory flow: Part II - elliptical orbital flow
No full textLoading on a smooth circular cylinder at high Reynolds numbers in elliptical orbital flow has been studied by driving the cylinder around elliptical paths through water initially at rest. This experiment reproduces some of the essential features of the flow around horizontal cylinders beneath waves, though like the planar oscillatory flow simulation for vertical cylinders, some important differences remain. Results are presented for Reynolds numbers in the range 70,000 to 222,000, Keulegan Carpenter numbers between 6 and 20, and ellipticities between zero (planar oscillatory flow) and unity (circular orbital flow). In almost all cases both drag and inertia forces diminish with increasing ellipticity. The change in the total loading is associated mainly with a reduction in the inertia coefficient, ascribed to the generation of circulation around the cylinder. Comparisons with measurements on a horizontal or vertical cylinder in waves in similar conditions suggest (possibly owing to the absence of any current) that the effect of ellipticity is rather stronger in tests in oscillatory uniform flow than in wave flows.<br/
Loading on a cylinder in uniform oscillatory flow: Part 1- Planar oscillatory flow
No full textLoading on a smooth circular cylinder at high Reynolds numbers in planar oscillatory flow has been studied by driving the cylinder with simple harmonic motion through water initially at rest. The development of the experiment, capable also of arbitrary two dimensional oscillatory motion, is described with particular attention to the steps taken to eliminate undesirable mechanical vibrations. Results are presented over the range of conditions, Reynolds numbers in the range 20,000 to 280,000 and Keulegan Carpenter numbers from 6 to 20. For the lower Keulegan Carpenter numbers, drag and inertia coefficients follow a reasonable extension of Sarpkaya's U-tube data. Comparison with large scale wave experiments highlights the effects of differences between planar oscillatory flow and wave-induced flow. In some respects, however, such as the magnitude of local transverse forces, agreement between measurements in these different conditions are in surprising agreement.<br/
Orbital flow around a circular cylinder:part 2- attached flow at larger amplitudes
No full textA time-stepping numerical model of uniform circular orbital flow around a cylinder provides results which are compared with the steady-state predictions of a boundary-layer solution by Riley. At small amplitudes of motion excellent agreement is found in most respects, but in the numerical model the outer recirculating flow and related components of loading do not reach a steady state after any finite time. At a Stokes parameter ? of 500, the boundary-layer approach remains reasonably accurate for amplitudes of motion up to about 8 % of the cylinder diameter; for amplitudes up to twice this at the same value of ? the flow remains largely attached. The strength of the outer recirculating flow is enhanced by nonlinear interactions, but the computed nonlinear loading exceeds that observed in experiments. Flow visualization shows a three-dimensional structure in the flow, and it is argued that this has an important effect on the loading that cannot yet be predicted. A computed force component at a frequency of about 30 % of that of the ambient flow is related to the retrogressive motion of vortex structures around the cylinder.<br/
Developments of stream-function wave theory
No full textRecent work on the problem of the periodic wave of permanent form has revealed some unexpected characteristics which are not predicted by any of the wave theories in engineering use. Of these, the stream-function wave theory is the most accurate, and it is shown in this paper by means of a reformulated method of application to be capable of predicting correctly the behaviour of steep and near-breaking waves. Errors in stream-function wave-theory tables are assessed and found to be particularly significant for very steep waves, when crest particle velocities are under-estimated by 25% or more. As a development of the modified method of application, an approximate stream-function wave theory is presented. It permits a solution to be obtained for given wave conditions in any depth of water but requires much more modest computer resources than the full stream-function theory.<br/
On frequency-focusing unidirectional waves
No full textPhase convergence to within lOin mechanically generated waves was achieved at a specified point in a wave flume by frequency focusing. Owing to the presence of nonlinear waves, the crest elevation reaches a maximum somewhat further from the waveboard, where also breaking occurs if the waves are of sufficient size. The critical amplitude for breaking was not in close agreement with previous measurements when normalized with respect to the central wave number of mechanically generated waves. It is suggested however that the limiting conditions are related to the phase speed near the focus point, where the wave group propagates with a considerable degree of coherence not present in a linear model. Predictions of a time-dependent nonlinear numerical model of Baldock and Swan (1994) are found to be in good agreement with the behavior of the crest in this region
Orbital flow around a circular cylinder. Part 1. Steady streaming in non-uniform conditions
No full textThis work is concerned with the source of an important component of nonlinear loading on a horizontal cylinder beneath waves that is not present in conventional diffraction calculations. Earlier measurements (Chaplin 1984b) have suggested that circulation induced by steady streaming around the cylinder may be responsible for loading which in some cases reduces the perceived inertia force by 50%. The present work is aimed at studying the steady streaming around a cylinder in general non-uniform orbital flow, and determining whether in the particular case of wave-induced flow it could be related quantitatively to the loading.The steady outer flow has been obtained numerically for cases where the steady streaming does not have a reversal, and for cases where a weak reversal is compatible with a uniform outer circulation. It is found that the outer circulation is closely related to the mean streaming velocity around the cylinder at the outer edge of the shear-wave layer. Results for conditions corresponding to previous measurements of force on a horizontal cylinder beneath waves suggest that separation, turbulence, transient effects and organized three-dimensional instabilities should also be considered.<br/
Vortex excited vibration of a vertical circular cylinder in waves
No full textThis study deals with some of the nonlinear phenomena of the vortex-excited vibration of a circular cylinder in waves. Laboratory experiments have been performed to study the dynamic transverse response of a vertical circular cylinder in regular waves. The test cylinder was pivoted at its base and supported flexibly by springs at its top. The movement of the test cylinder in the inline direction was restricted. The most remarkable result is the appearance of two types of peak in the amplitude of the vortex-excited vibration of the cylinder produced by perfect resonance coupled with waves and by vortex coupling. In the case of steady flow, perfect resonance appears in the locked-in range, but in waves, it appears only close to ratios of wave frequency fw to the natural frequency fnw of the cylinder in still water in the sequence of fw/fnw = 1/2, 1/3, 1/4,…; elsewhere vortex coupling may occur for light damping in which the oscillation frequency is not a simple multiple of wave frequency.<br/
Use of a rectangular liquid-filled bottom-mounted distensible device to harness nearshore wave power
No full textThe present work aims to find out the applicability of the bulge concept used in the Anaconda (Chaplin et al., 2011) on a possible seabed version. The tested model is a long rectangular duct covered with an elastic membrane and placed on the seabed, parallel to wave propagation. The tension in the membrane and its submergence can be varied by pressurising the system. Two configurations were tested: one with closed ends and one with a Power Take-Off system. Measurements were made of pressure in the duct, membrane displacement and capture width and are compared with a theoretical 1D model.</p
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