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*Wave-General

Purpose

To specify miscellaneous parameters to wave loading.

Theory

Refer to Spectrum Discretisation, Random Seed, Wave Kinematics and Selected Frequencies for further information on this feature.

Keyword Format

Format: A number of optional lines to specify a seed value for the random number generator, the Airy wave kinematics algorithm, the extent of the wave zone, solution options relating to floating body analysis, and selected frequencies that Flexcom must include in the range of solution harmonics.

Line setting the discretisation random seed:

[SEED=Random Seed]

Line containing several optional entries relating to wave kinematics, allowing the user to select the kinematic stretching option, assign scale factors to water particle velocities and accelerations, and specify whether drag loading is to be based on absolute or relative velocities:

[OPTION=Kinematics Option], [WATER_PARTICLE_VEL_SCALE=Velocity Scale Factor], [WATER_PARTICLE_ACC_SCALE=Acceleration Scale Factor], [WATER_PARTICLE_DRAG_VEL=Drag Velocity]

Kinematics Option can be MWL or EXTENDED. If omitted the default of superposition stretching is used. Velocity Scale Factor and Acceleration Scale Factor both default to 1.0. Drag Velocity can be RELATIVE (the default) or ABSOLUTE.

A single line defining wave zone option:

[WAVEZONE=Wavelength Factor]

Wavelength Factor defaults to 0.5 if omitted.

A single line requesting the activation or suppression of frequency-dependent radiation damping forces on floating body via convolution integral:

[FLOAT_CONVOLUTION=Convolution Option]

A single line requesting the activation or suppression of first order wave loads from force RAOs on floating body:

[FLOAT_RAO_FORCES=RAO Option]

A single line requesting the activation or suppression of second order wave loads from force RAOs on floating body:

[FLOAT_QTF_FORCES=QTF Option]

Convolution Option may be NO (the default) or YES. RAO Option may be NO (the default) or YES. QTF Option may be NO (the default) or YES.

A block of two lines defining selected wave frequencies. The second line can be repeated as often as necessary. Defining selected wave frequencies is only valid in the frequency domain.

[FREQUENCIES=SELECTED]

Frequency

A single line defining an offset of the wave origin from Y=0.0, Z=0.0. Default is 0.0, 0.0 if unspecified.

[ORIGIN=Y,Z]

Random Number Seed

Purpose

To specify a seed value for the random number generator that assigns random phases to wave components when discretising a wave spectrum or spectra.

Table Input

Notes

(a)Refer to Random Seed for further information on this feature.

Wave Kinematics

Purpose

To select the kinematic stretching option, assign scale factors to water particle velocities and accelerations, and specify whether drag loading is to be based on absolute or relative velocities:

Table Input

Notes

(a)Refer to Wave Kinematics and Water Particle Velocities and Accelerations for further information on these features.

Wave Zone

Purpose

To specify the extent of the wave zone, used in the computation of water particle velocities and accelerations.

Table Input

Notes

(a)Water particle velocities and accelerations due to the wave field are computed at each element integration point. As these velocities and accelerations decay exponentially with depth, for computational efficiency only the regular waves (or component harmonics in the case of random sea analysis) 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 the specified Wavelength Factor. A value of 0.5 is assumed by default, so any wave 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.

Floating Body - Solution

Purpose

To specify solution options relating to floating body analysis.

Table Input

Notes

(a)Frequency-dependent radiation damping forces are computed via a convolution integral of velocity time history and retardation functions (this is standard modelling procedure for random sea analyses in the time domain). However, there are certain circumstances where a different approach is adopted. For example, Flexcom does not generally compute the damping forces in this manner if the wave loading consists of a single regular wave only, even if frequency-dependent radiation damping data has been specified. Typically the program finds a (constant) radiation damping matrix (by interpolation if necessary) corresponding to the single regular wave frequency, and adds this matrix at an appropriate location on the left-hand side of the equations of motion. However, you have the option to override this default behaviour, and compel Flexcom to perform the convolution calculation, in which case the radiation damping becomes a force on the right-hand side of the equations of motion as before. Refer to Wave Radiation Loads for further information on the theory underlying the coupled analysis technique in the time domain.

Selected Frequencies

Purpose

To specify selected frequencies that Flexcom must include in the range of solution harmonics.

Table Input

Notes

(a)This option is normally used to ensure Flexcom includes a solution at any natural frequency occurring in the range of the analysis harmonics. Refer to Selected Frequencies for further information.

(b)This table is only relevant for frequency domain dynamic analysis.

Wave Origin

Purpose

To specify an offset to be applied to the origin of the wave from Y=0.0, Z=0.0

Table Input