On Wave Force Coefficient Variability

1987 ◽  
Vol 109 (4) ◽  
pp. 295-306 ◽  
Author(s):  
J. H. Nath
Keyword(s):  
Phase Angle ◽  
Vortex Shedding ◽  
Vertical Cylinder ◽  
Wave Force ◽  
Maximum Force ◽  
Periodic Waves ◽  
Force Coefficient ◽  
Ambient Flow ◽  
Smooth Cylinder ◽  
Phase Differences

Wave force coefficient variability for cylinders, from wave to wave in a train of periodic waves, has been shown to be dependent on the phase of the force record relative to the ambient flow. The phase varies due to vortex shedding, but the maximum force is approximately constant as seen from this work and the work of other investigators. Thus, the maximum force coefficient is tightly organized according to the Keulegan-Carpenter number and scatter is seen in the phase angle versus Keulegan-Carpenter number. On the other hand, both Cd and Cm have scatter due to these phase differences from wave to wave. For unknown reasons, even when averaged over several wave cycles there is scatter in the results for Cd and Cm. This investigation shows that the maximum force coefficients for a heavily roughened vertical cylinder are tightly arranged according to the Keulegan-Carpenter number and the period parameter. Furthermore, the phase angle is similarly much more organized than for the smooth cylinder.

scholarly journals MARINE ROUGHENED CYLINDER WAVE FORCE COEFFICIENTS

1984 ◽  
Vol 1 (19) ◽  
pp. 182
Author(s):  
John H. Nath
Keyword(s):  
Reynolds Number ◽  
Laboratory Testing ◽  
Wave Force ◽  
Periodic Waves ◽  
Transfer Coefficients ◽  
Cylinder Diameter ◽  
Force Transfer ◽  
Smooth Cylinder ◽  
One Year ◽  
Very High

Steel cylinders were submerged on a platform in the South Pass region of the Gulf of Mexico for one year to accumulate biofouling for later laboratory testing to determine wave force transfer coefficients. They were positioned at -55, -140, and -190 feet below the still water surface. Laboratory tests comprised steady tow up to Reynolds number cd 7x10^, and periodic waves up to Reynolds number of 1.6x10 and Keulegan-Carpenter number up to 25. The force transfer coefficients for the -55 cylinder were about equal to those for a sand roughened cylinder with relative cylinder roughness, e/D, of .03, where e is the height of the equivalent sand roughness size and D is the smooth cylinder diameter. The drag coefficient for very high Keulegan-Carpenter number, or steady tow, is about 1.0 if the effective cylinder diameter is taken into account, for the rougher cylinders.

Experimentally Determined Stability Parameters of a Subsonic Cascade Oscillating Near Stall

1978 ◽  
Vol 100 (1) ◽  
pp. 111-120 ◽  
Author(s):  
F. O. Carta ◽  
A. O. St. Hilaire
Keyword(s):  
Phase Angle ◽  
Incidence Angle ◽  
Minor Effect ◽  
Significant Finding ◽  
Force Coefficient ◽  
Stability Parameters ◽  
Free System ◽  
Pressure Transducers ◽  
Moment Coefficient ◽  
The Stability

Tests were performed on a linear cascade of airfoils oscillating in pitch about their midchords at frequencies up to 17 cps, at free-stream velocities up to 200 ft/s, and at interblade phase angles of 0 deg and 45 deg, under conditions of high aerodynamic loading. The measured data included unsteady time histories from chordwise pressure transducers and from chordwise hot films. Unsteady normal force coefficient, moment coefficient, and aerodynamic work per cycle of oscillation were obtained from integrals of the pressure data, and indications of the nature and extent of the separation phenomenon were obtained from an analysis of the hot-film response data. The most significant finding of this investigation is that a change in interblade phase angle from 0 deg to 45 deg radically alters the character of the unsteady blade loading (which governs its motion in a free system) from stable to unstable. Furthermore, the stability or instability is governed primarily by the phase angle of the pressure distribution (relative to the blade motion) over the forward 10–15 percent of the blade chord. Reduced frequency and mean incidence angle changes were found to have a relatively minor effect on stability for the range of parameters tested.

An experimental study on the inline wave force on a truncated vertical cylinder

2015 ◽  
Vol 15 (1) ◽  
pp. 39-52 ◽  
Author(s):  
Hong Xiong ◽  
Jianmin Yang ◽  
Xinliang Tian
Keyword(s):  
Experimental Study ◽  
Vertical Cylinder ◽  
Wave Force

Wave Force Coefficient Correlation Based on Wake Volume Scaling

1994 ◽  
Vol 116 (2) ◽  
pp. 97-101
Author(s):  
T. E. Horton ◽  
M. J. Feifarek ◽  
H. Golestanian
Keyword(s):  
Boundary Layer ◽  
Dimensionless Parameter ◽  
Wave Force ◽  
Hydrodynamic Drag ◽  
Half Cycle ◽  
Square Root ◽  
Force Coefficient ◽  
Coefficient Corrélation ◽  
Flow Through ◽  
Flow Drag

A correlation of the hydrodynamic drag force on a cylinder for a periodic motion is demonstrated. The correlation indicates the dependance of the unsteady flow drag coefficient on the wake volume parameter. This parameter is a measure of the volume of flow through the boundary layer and into the wake in a half-cycle. For a laminar boundary layer, this dimensionless parameter is proportional to the Keulegan-Carpenter number and inversely proportional to the square root of the Reynolds number. Using wake volume scaling, drag coefficients were effectively collapsed into a single curve.

Second-Order Diffraction Loads on Plural Vertical Cylinder With Arbitrary Cross Sections

1987 ◽  
Vol 109 (4) ◽  
pp. 314-319
Author(s):  
K. Masuda ◽  
W. Kato ◽  
H. Ishizuka
Keyword(s):  
Boundary Value Problem ◽  
Cross Sections ◽  
Present Method ◽  
Boundary Value ◽  
Vertical Cylinder ◽  
Wave Force ◽  
Second Order ◽  
Wave Forces ◽  
Order Diffraction ◽  
Rectangular Cylinders

The purpose of the present study is development of a powerful numerical method for calculating second-order diffraction loads on plural vertical cylinder with arbitrary cross sections. According to the present method, second-order wave force can be obtained from a linear radiation potential without solving second-order boundary value problem. The boundary value problem for the radiation potential is solved with the hybrid boundary element method. The computations for circular and rectangular cylinders were carried out and compared with the experiments. In addition, second-order wave forces on twin circular cylinder are calculated with the present method.

Vortex Shedding from Smooth and Roughened Cylinders in Cross-Flow near a Plane Surface

1979 ◽  
Vol 30 (1) ◽  
pp. 305-321 ◽  
Author(s):  
G. Buresti ◽  
A. Lanciotti
Keyword(s):  
Vortex Shedding ◽  
Plane Surface ◽  
Cross Flow ◽  
Reynolds Numbers ◽  
Cylinder Surface ◽  
Hot Wire ◽  
Cylinder Axis ◽  
Supercritical Regime ◽  
Subcritical Regime ◽  
Smooth Cylinder

SummaryThe characteristics of the flow field around a circular cylinder in cross-flow placed at various distances from a plane, parallel both to the flow and to the cylinder axis, were analysed using a hot wire anemometer. Experiments were performed in a wind tunnel with Reynolds numbers ranging from 0.85×105 to 3×105. The spectra of the hot wire signals were obtained using a Fast Fourier Transform technique programmed on a PDP 11/40 computer. As regards a smooth cylinder, the main features of the vortex shedding mechanism in the subcritical regime remained unaltered for distances from the plane greater than approximately 0.4 diameters; in particular the Strouhal frequency did not show any significant variation relative to the typical value for an isolated cylinder. As for lower values of the distance from the plane, the regular vortex shedding disappeared and the hot wire spectra showed typical turbulent features. The possibility of obtaining supercritical conditions by roughening the cylinder surface was confirmed together with the importance of the Reynolds number based on the typical roughness size, Rk, in the evaluation of the flow regime around the cylinder. In the case of roughened cylinders, and with values of Rk below-350, the regular vortex shedding disappeared at a distance from the plane smaller than 0.3 diameters. This fact suggests that, at least in part of the supercritical regime, the influence of the plane can be smaller than in the subcritical regime.

Numerical Simulation of Vortex Shedding and Lock-in Characteristics for a Thin Cambered Blade

2007 ◽  
Vol 129 (10) ◽  
pp. 1297-1305 ◽  
Author(s):  
Baoshan Zhu ◽  
Jun Lei ◽  
Shuliang Cao
Keyword(s):  
Numerical Simulation ◽  
Vortex Shedding ◽  
Forced Oscillation ◽  
Tvd Scheme ◽  
Convective Term ◽  
Variation Diminishing ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Lock In ◽  
Phase Differences

In this paper, vortex-shedding patterns and lock-in characteristics that vortex-shedding frequency synchronizes with the natural frequency of a thin cambered blade were numerically investigated. The numerical simulation was based on solving the vorticity-stream function equations with the fourth-order Runge–Kutta scheme in time and the Chakravaythy–Oscher total variation diminishing (TVD) scheme was used to discretize the convective term. The vortex-shedding patterns for different blade attack angles were simulated. In order to confirm whether the vortex shedding would induce blade self-oscillation, numerical simulation was also carried out for blade in a forced oscillation. By changing the pitching frequency and amplitude, the occurrence of lock-in at certain attack angles was determined. Inside the lock-in zone, phase differences between the blade’s pitching displacement and the torque acting on the blade were used to infer the probability of the blade self-oscillation.

scholarly journals Key features of wave energy

2012 ◽  
Vol 370 (1959) ◽  
pp. 425-438 ◽  
Author(s):  
R. C. T. Rainey
Keyword(s):  
Wave Energy ◽  
Small Body ◽  
Fluid Velocity ◽  
Wave Force ◽  
Maximum Force ◽  
Wind Speeds ◽  
Tidal Turbine ◽  
Low Wind Speeds ◽  
The Cost ◽  
Multi Body

For a weak point source or dipole, or a small body operating as either, we show that the power from a wave energy converter (WEC) is the product of the particle velocity in the waves, and the wave force (suitably defined). There is a thus a strong analogy with a wind or tidal turbine, where the power is the product of the fluid velocity through the turbine, and the force on it. As a first approximation, the cost of a structure is controlled by the force it has to carry, which governs its strength, and the distance it has to be carried, which governs its size. Thus, WECs are at a disadvantage compared with wind and tidal turbines because the fluid velocities are lower, and hence the forces are higher. On the other hand, the distances involved are lower. As with turbines, the implication is also that a WEC must make the most of its force-carrying ability—ideally, to carry its maximum force all the time, the ‘100% sweating WEC’. It must be able to limit the wave force on it in larger waves, ultimately becoming near-transparent to them in the survival condition—just like a turbine in extreme conditions, which can stop and feather its blades. A turbine of any force rating can achieve its maximum force in low wind speeds, if its diameter is sufficiently large. This is not possible with a simple monopole or dipole WEC, however, because of the ‘ nλ /2 π ’ capture width limits. To achieve reasonable ‘sweating’ in typical wave climates, the force is limited to about 1 MN for a monopole device, or 2 MN for a dipole. The conclusion is that the future of wave energy is in devices that are not simple monopoles or dipoles, but multi-body devices or other shapes equivalent to arrays.

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