Master or Project Thesis

Motivation

Investigations of the motion behaviour of floating structures in waves require the generation of an "artificial" natural seaway to represent the wave conditions encountered in reality. This is necessary for both wave tank tests and numerical simulations. To ensure an accurate generation of the specified sea state, the statistical properties of the artificially generated seaway have to be compared to the properties of the specified seaway. The statistical properties of a seaway can be represented by its spectral power density distribution, or simplified, its spectrum (Merigaud and Ringwood, 2017). This implies the necessity of estimating the spectrum of the generated seaway.

For long-crested waves, this is usually done by installing a wave probes (real or virtual) and measuring the elevation of the free surface. Based on the obtained data, a 1D spectrum can be estimated and compared to the input spectrum to analyse the generation accuracy.

In the case of short-crested waves, however, the measurement of the directional distribution of the waves is challenging which makes the estimation of a 2D spectrum difficult. In practice, it is often left uncertain whether the generated seaway meets the statistical properties of the original seaway.

Using an array of wave probes to simultaneously measure the free surface elevation at several points in space, gives insights about the directionality of a seaway. There are already approaches in the literature to derive 2D directional wave spectra from such measurements (e.g. Nwogu 1989, Tannuri et al. 2007, Panicker and Borgman 1970). Wave probes are readily available at most experimental wave tanks and easy to implement in the case of numerical simulations. The advantage of wave probe arrays over point measurements lies in the more accurate measurement of wave directionality due to the increased spatial extent of the measured field.

Objective

The objective of the thesis is to evaluate different approaches for estimation of 2D directional wave spectra using data obtained with wave probe arrays based on a numerical or experimental investigation. After evaluating the different approaches in the literature, at least one procedure to estimate directional seaway from probe arrays should be implemented in a Python program or in Matlab. Afterwards, the influence of the probe arrangement (e.g. shape of the arrangement, number of probes) should be investigated with the developed program. Test data could be acquired by conducting either tests at the wave tank of the Institute for Fluid Dynamics and Ship Theory or by running numerical simulations (time-domain 3D BEM or RANS). The work might be expanded regarding seaways of increased complexity (e.g. combinations of wind seas and swell or Non-Gaussian seas of restricted depths) or by comparing the probe array approach with approaches based on surface elevation snapshots (e.g. from X-radar or optical measurements).

• Evaluation of different approaches to derive 2D directional wave spectra

• Development a concept and implementing of a program for estimating of 2D directional wave spectra from wave probe array data

• Conducting verification and validation studies

• Investigation of the influence of array configuration on the calculated 2D directional wave spectra and formulate recommendations to be followed during wave tank tests or in numerical simulations

• (optional) Verify the applicability and consistency of the solution developed for seaways with increasing complexity

• (optional) Compare the results obtained by the implemented approach with surface elevation snapshots (via simulations)

Type and start

Master Thesis or Project Thesis

Start of the project immediately 

Supervisor

Carl Reiner (TUHH)

Literature

A. Mérigaud and J. V. Ringwood. (2018). Free-Surface Time-Series Generation for Wave Energy Applications, in IEEE Journal of Oceanic Engineering, vol. 43, no. 1, pp. 19-35, Jan. 2018, doi: 10.1109/JOE.2017.2691199.

O. Nwogu. (1989). Maximum entropy estimation of directional wave spectra from an array of wave probes, Applied Ocean Research, Volume 11, Issue 4, 1989, Pages 176-182, ISSN 0141-1187, doi.org/10.1016/0141-1187(89)90016-3

E. A.Tannuri, P. C. Mello, J. S. Sales, A. N. Simos, V. L. F. Matos. (2007). Estimation of directional wave spectrum using a wave-probe array. Marine Systems & Ocean Technology. 3. 123-129. 10.1007/BF03449221.

N. N. Panicker, L. E. Borgman. (1970). Directional Spectra from Wave-Gage Arrays. 12th International Conference on Coastal Engineering. September 13-18, 1970. doi.org/10.1061/9780872620285.008