Wave field measurements of regular wave-monopile interaction using Free-Surface Synthetic Schlieren

Spatio-temporal wave patterns due to wave field-structure interaction can be very complex to measure and analyze when using (intrusive) point probes. Free-surface field measurements can offer much needed insight in this domain. Nevertheless, to the best of the authors' knowledge, these methods are rarely used in experimental offshore engineering and research. In these fields, typical domain sizes are at least in the order of several m(2), whereas (optical) free-surface field measurements are often not performed in domains with dimensions larger than roughly 0.5 x 0.5 m(2). In the current work, the optical free-surface measurement technique named Free-Surface Synthetic Schlieren (FS-SS) is applied to measure the interaction between an incident wave field and a surface piercing cylinder, or monopile, in a domain of several m2 for the first time. The FS-SS method is validated for wave fields with wavelengths lambda << L to lambda >> L, where L is the domain size in the direction of wave propagation. It is found that the incorporation of an additional water level measurement improves the agreement between intrusive wave height meters and the FS-SS measurement for large wavelengths (lambda/L > 0.5) as compared to assuming a zero mean free-surface. Wave field-monopile interaction is measured for two values of D/lambda:D/lambda = 0.1 and D/lambda = 0.2, where D is the monopile diameter. For the case D/lambda = 0.2 the interaction wave field is analyzed by subtracting the measured wave field in the absence of a structure, from the measured interaction wave fields. The measured difference wave field reveals many interaction phenomena such as locations of amplification, both near the monopile and further away, a wake that has certain similarities with a Kelvin wake, and a circular small wavelength diffraction pattern. Additionally, the embedding of the measurements in a wave breaking regime map is presented. In this map, the applicability for certain wave conditions can be clearly visualized. It is concluded that the FS-SS method, including the proposed improvement using additional sensor data, is a useful addition to the toolbox of hydraulic engineers and researchers, and that especially the measured locations of wave amplification in the far field will not be easily detected using (arrays of) point probes.