Water Particle Velocities and Accelerations |
 
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Water Particle Velocities and Accelerations |
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Fluid velocities and accelerations due to the random wave field are computed at element integration points using similar expressions to Regular Airy Wave (Eq.7 - 10), extended for multi-directional waves as appropriate. The modifications to Airy wave theory represented by the Wave Kinematics (Eq.1), the Wave Kinematics (Eq.2) and the Wave Kinematics (Eq.3) are used for random seas also, with superposition stretching again the default.
A computational economy is also applied here without any significant loss of accuracy. As the water particle velocities and accelerations decay exponentially with depth, for computational efficiency only those wave harmonics which are in the wave zone are considered to contribute towards the generation of water particle velocities and accelerations. The wave zone is considered to extend from the mean water line downwards by a distance of one wavelength times a specified Wavelength Factor. A value of 0.5 is assumed by default, so any harmonic whose half-wavelength is less than the distance from the mean water line to the integration point in question is omitted from water particle velocity and acceleration calculations.
Morison’s Equation assumes that the force exerted by unbroken waves on a cylinder can be represented by a linear sum of drag and inertial terms. The formulation is widely established for modelling wave forces on slender offshore structures such as risers and mooring lines. In situations where the body size becomes significant with respect to wavelength, the underlying assumptions become invalid, and the effects of radiation and diffraction must also be considered. So in Flexcom’s internal computations, any harmonic whose half-wavelength is less than the element diameter is omitted from the water particle velocity and acceleration calculations.
•*WAVE-GENERAL is used to specify miscellaneous parameters to wave loading. Specifically, the WAVEZONE= input is used to specify the extent of the wave zone, used in the computation of water particle velocities and accelerations.
•*PRINT is used to request additional printed output to the main output file. Specifically, the OUTPUT=WATER PARTICLE HYDRODYNAMICS option provides detailed output regarding the spatial and temporal distributions of water particle velocity and acceleration.