The paper presents velocity measurements, using particle image velocimetry, as well as a reconstruction of hydrodynamic pressures for the analysis of fast ships. Stereoscopic PIV measurements with a towed underwater PIV system are conducted during towing tank tests to obtain the velocity field in the bow region of a fast ship at speeds up to Fr=0.8. While the model is kept at a fixed trim and sinkage, multi-plane PIV measurements with a total of 68 measurement planes are conducted to reconstruct a volumetric representation of the time-averaged velocity field in the bow region. The obtained velocity field is subsequently used for a volumetric description of the time-averaged hydrodynamic pressure field. In addition to these captive runs, forced oscillation tests are conducted. During these tests, the flow field is recorded in three successive planes to obtain a local phase-averaged description of the velocity and its gradients for the reconstruction of the phase-averaged hydrodynamic pressure field. The postprocessing procedure for the pressure reconstruction, including the solution of the Poisson equation, is implemented into the open-source CFD package OpenFOAM. For the detection of the free surface and the ship hull, an automated procedure is presented. Experimental results are finally compared to results from numerical simulations. Results show that the PIV method is capable of capturing the flow characteristics in the bow region of a fast ship. In addition, it can be used together with the pressure Poisson equation to obtain the hydrodynamic pressure field. However, large out-of-plane velocities require a large dynamic range, which limits the resolution of local effects close to the ship hull.

For Offshore Floating Photovoltaics (OFPV) applications, thin-film PV panels on lightweight floating support structures gain increasing scientific and commercial interest. Over the past years, several different concepts of thin-film OFPV have been proposed, with the common denominator of floating mattress or blanket-like support structures with very little draft in the order of centimeters compared to their width and length in the order of several tens to hundreds of meters. Mostly made from polymer foam materials, these floating support structures are more flexible than the conventional Very Large Floating Structures (VLFS) investigated in 1990s. The flexibility of a floating structure is expressed by the characteristic length derived from the ratio of the structural bending stiffness and the hydrostatic stiffness of the support. For conventional VLFS, this characteristic length is usually longer than the dominant wavelength of the ocean waves, resulting in only moderate structural deflections of the order of 1/10 of the wave height and the total thickness of the structure. The newly proposed structures have characteristic lengths of less than the wavelength of ocean waves. This allows the structures to move with the waves and follow the wave elevation like a floating blanket. Therefore, these structures are classified as Very Flexible Floating Structures (VFFS). Despite the growing interest in VFFS, little is still known about their hydroelastic deformation and their influence on the surrounding wave field. To start the experimental VFFS research at Delft University of Technology, Digital Image Correlation (DIC) measurements were carried out in this study to investigate the vertical deflection of a VFFS at model scale in a small towing. The model’s characteristic length was 1/3 of the shortest wavelength and it was tested in long-crested regular longitudinal waves. The wavelength varied between 1/10 and 1/5 of the structure length. The measurements showed that the structure indeed mostly followed the wave elevation and revealed 3D effects across the structure, which require deeper investigation into wave scattering of VFFS.

Very flexible floating structures have been proposed for offshore floating photovoltaics installation. Characterized by having structural lengths much longer than wavelengths, small thickness, and low bending stiffness, these structures are prone to large vertical deflections and strong hydroelastic interactions. Experimental information on these structures is scarce. In this study, we employed digital image correlation (DIC) to investigate the hydroelastic interaction of a flexible floating sheet with a length-to-height ratio of 1,000 in regular long-crested head waves. The wavelength was one-tenth and one-fifth of the structure length, with a wave steepness of 0.04. The repeatability of wave conditions and measurement results was demonstrated, and measurement errors were quantified. Surface elevations showed that the sheet followed a local wave elevation in long waves. In shorter waves, strong hydroelastic interactions led to wave lengthening underneath the floating structure and three-dimensional (3D) effects across the structure width. Wave lengthening agreed well with prediction from the hydroelastic dispersion relation. Observed 3D effects necessitate further research into the possible influence of viscoelastic effects. It was shown that the DIC technique is suitable to measure flexible floating structures in waves with low error and good repeatability. Experimental data are publicly available.

Flexible floating structures received increasing attention in recent years as support structures for floating offshore solar installations and other forms of oceans space utilization. An early example for such structures was the Mega-Float structure proposed as floating airport runway for Tokyo Bay. More recent examples can be found in the large inland floating solar parks where interconnected pontoons form a flexible floating structure. The common denominator of these structures is their small height compared to their length and width resulting in low bending stiffness in the vertical direction. Structural length being much longer than the wavelength and low bending stiffness result in large vertical deflections of the floating structures and strong hydroelastic interaction with the waves. Similar behavior can be observed for sloshing mitigation measures with flexible membranes. In this study, we investigated the wave structure interaction of a floating flexible sheet with a length to height ratio of 1000 in regular long-crested head waves in the small towing tank of Delft University of Technology. Wavelength was varied between 1/20 and 1/5 of structure length with wave steepness in the range of 0.02 to 0.05. Digital Image Correlation (DIC) was used to measure the surface elevation of the entire structure and wave elevation was measured in three different locations to provide reference data. The results show that the floating sheet mainly followed the local wave elevation and a reduction of motion amplitude was observed over the length of the structure. Further, the results reveal 3D effects of different elevation amplitude across the width of the sheet, which suggests strong interaction with the waves.

The present study describes the application of the particle image velocimetry (PIV) technique for the reconstruction of hydrodynamic pressures and loads on a ship model from measured velocity fields during towing tank tests. As an alternative to conventional pressure and force measurement techniques the method simultaneously pictures the velocity field and captures the dynamic aspect of the flow. The presented measurements are conducted in the transom region of a generic hull of a planing vessel which is equipped with an interceptor to create a stagnating flow, associated with a high pressure peak. The flow close to the hull is captured with an underwater stereoscopic PIV system and the pressure peak in front of the interceptor is reconstructed from time-averaged velocity fields. Results show the effect of different interceptor heights on the pressure distribution in the center-plane of the model. Further, a 3D flow field is reconstructed from scanning PIV measurements to analyze the lift reduction due to the finite span of the interceptor. The spatial variation of the measurement uncertainty is analyzed and propagated to the pressure field uncertainty and the potential of the method is further evaluated by comparison with numerical results from steady Reynolds Averaged Navier-Stokes (RANS) simulations.