Cylinder In Regular Waves Research Articles

Estimation of inertia forces on a horizontal circular cylinder in regular and irregular waves at low Keulegan-Carpenter numbers

Inertia forces acting on a horizontal circular cylinder in regular and irregular waves significantly decrease at low Keulegan-Carpenter numbers ( Kc). The rapid reduction of inertia force is caused by a circulating flow around the cylinder. In order to clarify the mechanism of the occurrence of the circulation, flow visualizations around a horizontal circular cylinder in regular waves and a circular cylinder moving along a circular orbit were carried out. Simple formulae to estimate the circulation strength and the inertia force reduction in regular waves were deduced from a starting vortex model with reference to the results of the flow visualizations. The prediction method was applied to the cylinder inclined to wave crests. The estimated inertia coefficients in regular waves are in good agreement with the experimental results at Kc less than 2.5. The inertia force reduction in irregular waves was also estimated by considering the circulation effect for each half wave period which was calculated by the prediction method in regular waves. The estimated results are in good agreement with the experimental results.

Second-order wave diffraction by a vertical cylinder

The main difficulty in second-order diffraction analysis stems from the contribution of the second-order potential, which obeys an inhomogeneous free-surface boundary condition. In some applications it is sufficient to know the second-order hydro-dynamic force, which can be calculated without explicitly evaluating this second-order potential. This technique cannot however be used for calculating other quantities such as the hydrodynamic pressure at any point, the sectional force and bending moment in the cylinder or the free-surface elevation due to the second-order effects. This paper provides a detailed analysis of the second-order diffraction problem of a uniform vertical circular cylinder in regular waves. This furnishes results not only on the cylinder surface, but also on the free surface, and in principle in the fluid domain surrounding the body. The analysis may help to throw some light on the physical interpretation of the second-order theory and its mathematical description. Moreover, this information is intended to complement the development of general numerical methods for arbitrary bodies.

Resonant and non-resonant behaviour of a flexibly mounted cylinder in waves

Previous research has indicated that the fluid-structure interactions of slender members in offshore structures are extremely sensitive to the ratio of wave frequency to the natural frequency of the structure, ƒw/ƒNw. This paper presents experimental measurements of forces and response displacements from a spring-mounted vertical cylinder in regular waves for a constant surface Keulegan-Carpenter number, SKC, equal to 8 and frequency ratios in the range 0·3≤ ƒw/ƒNw≤0·8. Peak values of force coefficients and response amplitudes are observed to occur when the frequency ratio f. If,. is an integer submultiple. Further peaks may also occur for ƒw/ƒNw =1/[(SKC/2π) + 1] and ƒw/ƒNw =1/[(SKC/2π) + 2] due to coupling between the in-line force and transverse response. Reasonable agreement is found between the experimental resonant force and response displacement loci with a simulation based on a finite-difference time-stepping solution of the non-linear equations of motion.

A comparison of complete and approximate solutions for second-order diffraction loads on arrays of vertical ciŕcular cylinders

This paper presents a comparison between two theoretical methods for computing the second-order diffraction loads on arrays of bottom-mounted, surface-piercing vertical circular cylinders in regular waves. One method presents a complete solution for the second-order hydrodynamic loads on the cylinder array via a numerical integration over the mean fluid free-surface. The other method is based on a large spacing approximation between the array members and involves the solution of a set of equivalent isolated body problems to obtain estimates for the second-order hydrodynamic loads. Numerical results for a pair of cylinders indicate very good agreement between the two methods at center-to-center spacing of both three and five radii, indicating that the approximate method may be sufficient to compute hydrodynamic interference effects to the second-order in many practical engineering situations.

Wave Forces Acting on a Moving Cylinder

Abstract Wave forces acting on a surging circular cylinder in regular waves are measured in a wave tank. Experiments are carried on various combinations of the amplitude of a wave and a surge, and the phase difference between them. The results of experiments are compared with those of calculations. The Morison equation extended to the case of relative motion is employed to calculate the forces on the cylinder. The inertia, added mass and drag coefficients used in the calculations are obtained from wave force measurements on a fixed cylinder and forced-surge tests in still water.

Wave Forces Acting on Horizontally Submerged Cylinders in Regular Waves at Low Kc Number

Wave forces acting on circular, square and diamond cylinders and a flat plate horizontally submerged with their axises parallel to the regular wave crests are experimentally investigated.A rapid reduction of the wave forces acting on the cylinders at low Kc number region is found to be caused by a circulating flow round the cylinder which is created by viscous wake. Flow visualization demonstrates that much larger separated vortices than those in plane oscillatory flow are created in regular waves. It is found that the vortex shedding patterns in regular wave is similar to those in plane oscillatory flow at the corresponding Kc number if a new Kc, number in terms of the circumference of the orbital motion of a fluid particle in waves instead of the diameter is defined.

Second order diffraction forces on a vertical cylinder in regular waves

Reexamen de la theorie de Lighthill et Molin et derivee des resultats definitifs. Expressions analytiques pour obtenir la force au second ordre sur un cylindre vertical perce en surface

Drift of an articulated cylinder in regular waves

Potential flow theory is used to investigate the wave induced harmonic response and the mean drift of an articulated column in regular waves. The mean drift horizontal force is evaluated by means of the Stokes expansion to second order in wave steepness. Analyses based on both near field and far field formulations are shown to give identical expressions, provided that the second-order forces at the intersection between column and seabed are included in the near field approach. The latter have not been considered in previous studies concerned with drift forces on floating bodies. It is shown that the drift forces on a column, although of second order, can excite piech responses of first order: this is because articulated columns are designed to have a low natural frequency in the tilt mode, relative to wave frequencies. Comparison of the theoretical results with experimental data, from a model tested in regular waves, suggests reasonable agreement for the drift forces over a range of frequencies and two wave heights.

Wave Force Tests on Vertical and Inclined Cylinders

A wave tank test was performed on fixed vertical and inclined cylinders in which forces on the cylinder in regular waves were measured. The hydrodynamic coefficients are computed based on the measured local forces on the cylinder, Morison equation and stream function theory. The inertia and drag forces are shown as functions of the Keulegan‐Carpenter number. The lift coefficients and lift force frequencies are presented as functions of the Keulegan‐Carpenter number. The coefficients obtained locally are shown to generally apply to the entire cylinder based on the respective local KC values.

The Effect of Speed of Advance upon Wave Force acting on a Vertical Circular Cylinder

Hydrodynamic forces acting on a vertical circular cylinder moving forward with a constant velocity and making forced oscillations are measured. Wave forces on the moving cylinder in regular waves are also measured. The purpose of this investigation is improving the accuracy in estimation of the motion of an ocean structure in waves, and the reduced velocity and Keulegan-Carpenter number chosen in these experiments are larger than those in exper-iments for the structural vibration induced by the vortex.The measured force is compared with that with no advance velocity.The conclusions obtained are as follows;1) Steady drag coefficient CDY0 of a cylinder forced to sway, nondimensionalized by foward velocity U and swaying velocity X, does not depend on KC number but depend slightly on the advance velocity U.2) Oscillating drag coefficient CDX1 of the cylinder does not depend on U and decrease with the decrease of Kc number at Kc<30.3) Steady drag coefficient CD0 of a cylinder forced to surge does not depend on Kc number but depend slightly on U.4) Oscillating drag coefficient CD1 of the cylinder does not depend on U and decrease with the decrease of Kc number at small Kc number.5) Steady drag coefficient CDW0 of a cylinder in regular waves decreases with the increase of U.6) Oscillating drag coefficient CDW1 is smaller than that with no advance velocity and is slightly smaller than that in steady flow.7) Inertia coefficient CM is almost equal to that with no advance velocity

Wave Forces exerted on Submerged Circular Cylinders fixed in Regular Waves

Wave forces exerted on circular cylinders in regular waves are investigated. The circular cylinders are fixed horizontally below the water surface.Experimental results are compared with values calculated by linear theory and the other data obtained differently.Main conclusions obtained are as follows :1) In case of small period parameters, inertia coefficients obtained by experiments show good agreement with theoretical values calculated by linear theory. But for large period parameters, the experimental values are much smaller than the theoretical ones.2) For large period parameters, drag coefficients derived in the present investigation approximately coincide with those obtained in experiments on hydrodynamic forces acting on circular cylinders in oscillatory flow. But there is much difference between them in case of not so large period parameters.

On the Unstable Swaying Motions of a Moored Cylinder in Regular Waves

The present paper deals with the nonlinear swaying motions of a moored circular cylinder in regular waves. With the consideration of the higher order forces caused by the interaction of the cylinder and waves, the Mathieu equation is introduced as the equation of swaying motion. Therefore it is expected that an unstable swaying motion occurs when the ratio of the natural period of a moored system to wave period becomes certain values. The experiments were carried out to confirm the possibilities of the unstable swaying motions of the moored circular cylinder in regular waves, and the results obtained by the experiments show qualitatively good agreement with the theoretical prediction. Further investigations are necessary as to the quantitative estimation of stability limit in this phenomenon.

Transverse Forces on Vertical Cylinders in Waves

The results of a laboratory investigation into the development of forces on a vertical circular cylinder in regular waves are presented. The force transverse to the direction of wave propagation is examined in detail and its relation to the shedding of vortices is considered. It found that the Keulegan-Carpenter number is important in determining whether lift forces will be developed and at what frequency compared with the wave frequency. The unsteady pressure distribution round the cylinder at various depths has been measured and the sectional forces derived from these distributions. The flow in waves has been compared with that round a circular cylinder oscillating in still water.