﻿ Environment: Wave data

# Environment: Wave data

### Number of wave trains

You can define a number of different wave trains; the overall sea conditions are the superposition of all the wave trains. In most cases a single wave train is sufficient, but multiple wave trains can be used for more complex cases, such as a crossing sea with locally-generated waves in one direction and distant storm-generated swell in a different direction.

Each wave train is given a name and a direction. A wave train can be one of the following: a regular wave (with a choice of wave theory), a random wave (with a choice of spectrum), given by time history or given explicitly by a list of wave components.

### Simulation time origin

The simulation time origin allows you to control the period of time that the dynamic simulation covers. It defines the global time that corresponds to simulation time $t{=}0$, so changing the simulation time origin allows you to shift the period of global time that is simulated. Altering the simulation time origin shifts the simulation time relative to all of the wave trains; you can also time shift an individual wave train by altering its wave time origin. See the dynamic analysis topic for details of the time frames used in OrcaFlex.

### Kinematic stretching method

Kinematic stretching is the process of extending linear Airy wave theory to predict fluid velocity and acceleration (kinematics) at points above the mean water level. OrcaFlex offers a choice of three stretching methods: vertical, Wheeler, and extrapolation.

 Note: Random waves are modelled by combining a number of linear Airy waves, so kinematic stretching also applies to random waves.

The horizontal velocity wave preview graph can be used to see the effect of the different kinematic stretching methods.

## Data for a wave train

The following data are common to all wave types.

### Wave direction

The direction in which the wave progresses, measured positive anti-clockwise from the global X-axis when viewed from above. So, for example, 0 degrees means a wave travelling in the positive $X$-direction, and 90° means a wave travelling in the positive $Y$-direction.

If you are using a directional spreading spectrum then the wave direction represents the principal direction.

The direction of the first wave train is the primary direction; this determines the grid with which the wire-frame sea surface is drawn and the wave directions drawn with the environment axes.

### Wave type

Each wave train can be one of the following types:

For regular waves we recommend the Dean wave – this is a nonlinear wave theory using a Fourier approximation method, suitable for all regular waves. The Airy wave theory is a simple linear wave theory that is only suitable for small waves. The cnoidal wave theory is only suitable for long waves in shallow water. The Stokes' 5th wave theory is only suitable for short waves in deep water.

If the specified wave is not suitable for the selected wave theory, OrcaFlex will give a warning or may report that the wave calculation has failed. If this happens please check that the wave theory selected is suitable.

### Wave origin and wave time origin

Each wave train has its own spatial origin and time origin. The spatial origin is specified relative to the global origin and the time origin is specified relative to the global time origin.

The wave train's data specify the wave train relative to its own origins, so you can shift a given wave train in space or time, independently of the other wave trains, by adjusting its origins.

For a regular wave train the wave time origin is the time at which a wave crest passes the wave origin. You can therefore use the origins to arrange that a wave crest passes a particular point at a particular time during the simulation.

For a random wave train, the phases of the wave components that make up the wave train are randomly distributed, but they are fixed relative to the wave time origin. You can therefore adjust the wave time origin to arrange that the simulation covers a particular timeslice of the random wave train. This can be useful for two purposes:

• You may want to select a particularly significant event in the wave train, such as a large wave. OrcaFlex makes this easily available from the waves preview page of the environment data form.
• You may wish to repeat a series of runs, with the same wave train data but different random phases for all the wave components. This can be done more easily by keeping the same wave train data, and giving randomly-chosen wave time origins for each run: statistically, the two methods are equivalent.