on waves A focus

For model tests, the correct generation of a realistic representation of a natural wave field is vital, as the environmental conditions determine the starting point for all the following analyses of any behaviour of a marine structure. Sanne van Essen, Jule Scharnke & Willemijn Pauw,

I Example of focused wave in MARIN’s demonstration basin

n recent years MARIN faced several research challenges which required advanced wave modelling techniques.

For special applications in-house wave generation software has been developed. This software tool was used for generating a Focusing Wave, with a peak wave height of 1.5 m (15 m at scale of the lifeboat pictured), which surprised the unsuspecting visitors at a recent Open Day at MARIN!

Waves with a long period travel faster than waves with a short period. This phenomenon is used when generating a focusing wave. If the short period waves are made before the long period waves, the long period waves over take the short period waves. Calculated timing of the waves with different periods results in all waves meeting at the so-called focal point. This principle can be used to generate a Focusing Wave, (as shown during the Open Day), but it can also be applied to generate pre defined time series of waves at certain loca tions in the basin (deterministic wave generation).

To be able to make a deterministic wave, a proper understanding of the wave maker flap motions and geometry, the transfer of the flap motions to the waves and the prop- agation of the waves through the basin are required. The deterministic wave generation

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process for a double flap wave maker for example, can be divided into four steps, as illustrated (see also [1]).

1. Definition of the target wave train: The target position in time and space is selected. For example, the position of the focal point of the focusing wave or the location where a ship encounters the wave train at a given time. At this location, the target wave train is designed based on spectral parameters or a wave record.

2. Upstream transformation: The target wave train is transformed upstream to the position of the wave maker, e.g. by means of a linear wave propagation model.

3. Calculation of control signals: The corres- ponding control signals are calculated using adequate transfer functions of the wave generator.

4. Performing the model tests: The control signals are used to generate the specified wave train, which is measured at selected positions in the tank.

Practical application of a Focusing Wave A Focusing Wave contains a series of wave components with different periods and has a clear focusing point. This makes

it particularly suitable to improve the under- standing of a basin’s inherent limitations. The focusing wave technique can be used to determine the hydraulic and electrical transfer function of wave makers, to assess the reflec- tion coefficient of beaches and to investigate the non-linear aspects of transfer functions in shallow water conditions amongst other things.

Considering the Open Day - would there be more applications for this type of wave in model testing? The deterministic wave gen- eration approach will also be used in the new BreaKin JIP (see page 14) to generate different types of (near) breaking waves to investigate associated impact loads. These particularly interesting wave trains will be rerun at a different scale to determine their kinematics at Imperial College. More exam- ples of model testing with focusing waves are presented in [2] and [3].

The Focusing Wave is an example of advan ced basin wave modelling with a deterministic wave generation technique. Other aspects that are the subject of investigation are basin inherent limitations (for instance reflection and its amplification due to shallow water effects) and appropriate wave analysis tech- niques to judge the quality of the generated basin wave (such as directional analysis and non-linear wave crest statistics).

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