Summary
The intention of this study is to observe, experimentally, the characteristics of the forces in both a sinusoidal and random planar oscillatory flow. In the past, the hydrodynamic loading of cylindrical structures has been investigated using sinusoidal planar oscillatory flow, regular (or periodic) waves and random waves in both laboratory wave basins and ocean test structures. The studies in sinusoidal planar oscillatory flow, in particular, have been successful in obtaining a fundamental empirical knowledge of the flow about a circular cylinder. In addition, these studies offer results which can be compared to analytical and numerical results which exist for limited cases.
The extension to a random planar oscillatory flow is motivated by the fact that the fluid motion in applications of interest is random. That is, for design wave scenarios in offshore applications, either a single wave is selected for extremal design, or a wave spectrum is selected to represent an energy density of wave amplitudes and frequencies. The use of a random planar oscillatory flow in this investigation is motivated by the latter method. The use of a wave spectrum requires either a measured spectrum or theoretical model. A wave spectrum describes the energy content versus frequency of the wave heights in a relatively short-term random sea, as in a hurricane condition. In this study, the nature of the random planar oscillatory flow will be determined with reference to one of several available models for wave spectra available. A wave spectrum proposed by Pierson and Moskowitz termed the Pierson-Moskowitz spectrum is utilized in the investigation. Note that in the context of this study, the fluid flow is planar (or one-dimensional) contrasted with actual fluid motion in the oceans which is two-dimensional (orbital) or three dimensional, depending on the corresponding character of the waves. Consequently, the use of the theoretical models for wave height spectra is designed to incorporate an amplitude and frequency dependence in the planar oscillatory flow which is more characteristic of that encountered in ocean design applications.
The basic theme of this investigation is comparison of the forces in sinusoidal and random oscillating flow. Analytical and numerical methods do not exist to allow a comparison in this fashion. Consequently, modeling of the forces will be based on empirical methods employed by researchers and designers alike. It has been mentioned that the hydrodynamic loading on a circular cylinder is in the form of an inline force, defined as the force parallel to the velocity vector, and a transverse force which is perpendicular to the inline force. For the present study , in which planar oscillatory flow is used, the inline force is taken as in the direction of the one dimensional flow and the transverse force is perpendicular. Even though planar oscillatory flow offers a convenient study of the fluid-structure interaction, the complexity of the fluid flow is not simplified. In 1950, Morison, O’Brien, Johnson and Schaaf proposed what is now termed the Morison equation as an empirical approximation to the inline force on a circular cylinder in their investigation of horizontal forces on vertical piles in waves. The Morison equation, has survived almost four decades of critical assessment, and remains in use in spite of its known inadequacies. Models of the transverse force are not as well established and continue to be developed only after experimental investigations have repeatedly shown the magnitude can compare to the inline force. Methods developed by
previous investigators for reducing the transverse force to ascertain its dependence on the fluid flow will be discussed.
Related Publications: Longoria, R.G., Beaman, J.J. and Miksad, R.W., “An Experimental Investigation of Forces Induced on Cylinders by Random Oscillatory Flow,” Journal of Offshore Mechanics and Arctic Engineering, Vol. 113, November 1991.
Longoria, R.G., Beaman, J.J. and Miksad, R.W., “Drag, Inertia and Transverse Force Coefficients for Random Planar Oscillatory Flow,” International Journal of Offshore and Polar Engineering, Vol.1, No. 3, September 1991.
Longoria, R.G., Beaman, J.J. and Miksad, R.W., “Nonlinear Hydrodynamic Forces in Random Oscillatory Flow” European Offshore Mechanics Symposium, 164.
Longoria, R.J., Miksad, R.W., and Beaman, J.J., “Frequency Domain Analysis of Inline Forces on Circular Cylinders in Random Oscillatory Flow,” Journal of Offshore Mechanics and Arctic Engineering, Vol. 115, pp.23-30, February 1993
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