Spectral Analysis of Nonlinear Drag Forces
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The spectral properties of nonlinear drag forces of random waves on vertical circular cylinders are analyzed in this paper by means of nonlinear spectral analysis. The analysis provides basic parameters for estimation of the characteristic drag forces. Numerical computation is also performed for the investigation of the effects of nonlinearity of the drag forces.The results indicate that the wave drag forces calculated by linear wave theory are larger than those calculated by the third order Stokes wave theory for given waves. The difference between them increases with wave height. The wave drag forces calculated by use of linear approximation are about 5％ smaller than their actual values when measured in the peak values of spectral densities. This will result in a safety problem for the design of offshore structures. Therefore, the nonlinear effect of wave drag forces should be taken into consideration in design and application of important offshore structures.Keywords:
Wave drag
Drag equation
Airy wave theory
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Evaluation of wave induced forces on tubular members of offshore structures is sensitive to the choice of hydrodynamic coefficients involved in the Morison's equation. This study is based on experiments conducted in a wave flume to evaluate the coefficients of drag, inertia and lift for rough as well as inclined cylinders. Within the experimental range it was observed that the magnitude of the drag coefficient is more influenced by the degree of cylinder inclination along the 'in-line' direction than by its roughness. The inertia and lift coefficients however are affected by the cylinder inclination as well as by its roughness at high Reynolds numbers.
Morison equation
Flume
Lift (data mining)
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Laboratory experiments were carried out to research drag and inertia coefficients of a vertical circular cylinder in random waves. The results were compared with those of field experiments and numerical simulations to discuss the characteristics of the random wave force. This comprehensive study showed that the drag and inertia coefficients obtained from the least squares fit on a wave-by- wave basis scattered widely as a result of shedding vortices during the previous wave cycle, the so-called history effect. Coefficients determined by least squares fit of the complete force time series of a random wave record were well ordered as a function of Keulegan- Carpenter number defined by significant orbital displacement or significant wave height and showed good agreement with the values in regular waves. This study enabled the authors to apply the results of extensive studies in regular waves or in harmonic flow to estimate wave forces acting on an ocean structure in random waves.
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This paper presents results from experimental works to investigate wave loading on a vertical circular cylinder in random wave conditions in a wave flume with different water depths . In-line force coefficients (drag and inertia coefficient) are estimated from the measured pressures on cylinder's surface at different elevations along the length of the cylinder. The wave kinematics are estimated by using different wave theories. Methods of max-min and least-squares (simplified by fit on wave-by-wave basis) are applied to determine force coefficients.
Wave flume
Morison equation
Flume
Added mass
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Sea state
Wave height
Significant wave height
Fictitious force
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Citations (1)
Added mass
Inviscid flow
Morison equation
Fictitious force
Oscillation (cell signaling)
Drag equation
Strouhal number
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We measure the wave drag acting on fully submerged spheres as a function of their depth and velocity, with an apparatus that measures only the component of the drag due to the proximity of the free surface. We observe that close to the surface the wave drag is of the order of the hydrodynamic drag. In our range of study, the measured force is more than one order smaller than predictions based on linear response. In order to investigate this discrepancy, we measure the amplitude of the waves at the origin of the wave drag, comparing the measurement with a theoretical model. The model captures the measurements at “large depth” but the wave’s amplitude saturates at “small depth,” an effect that partially accounts for the difference between the predicted and measured wave drag.
Wave drag
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We studied the interaction of a compositional lock-exchange gravity current with a square cylinder at moderate Reynolds numbers using two-dimensional high-resolution simulations. Recent experimental work obtained measurements of the drag and lift for circular and rectangular cylinders. However, a detailed quantitative description of the flowfield was not given. Our objectives were to provide such description and, from it, explain the physical mechanisms behind the values and the time variation of the drag and lift coefficients. Such information is valuable for the design of submarine structures under the potential impact of gravity currents. Our numerical results agreed well with available experimental measurements. The interaction can be divided into impact, transient, and quasisteady-state stages. The shedding of vortices during the impact stage had an important effect on the drag and lift variation with time. For example, for a specific set of parameters, the suction created by these vortices produced about 40–50% of the drag. The maximum drag and the maximum lift amplitude occurred during the initial impact stage, and their minimum values were found when the square cylinder was closer to the bottom wall.
Lift (data mining)
Vortex shedding
Gravity current
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Drag equation
Oscillation (cell signaling)
Vortex shedding
Dimensionless quantity
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Solidity
Morison equation
Added mass
Wave drag
Drag equation
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Citations (102)
A review of the Morison‐equation approach to the evaluation of wave loads on cylindrical members in harmonic flow is presented. The results presented show the most comprehensive picture to date of the Reynolds number‐displacement ratio dependence of the drag and inertia coefficient in the small‐displacement‐ratio range and demonstrate the important effect of the Reynolds number in the case of both smooth and rough cylinders. Results from oscillating‐cylinder tests are compared with wave‐tank results for both regular and random waves. Finally, a wake model applied to the high Reynolds number case is shown to correlate the drag coefficient for both rough and smooth cylinders quite well.
Morison equation
Drag equation
Harmonic
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Citations (8)