Groynes in the Lower Elbe estuary in Germany are subject to attack from overflowing long-period primary ship-induced waves. Damage can occur on the crest and lee side of the structure due to the high turbulent overflowing flow velocities. An actual design tool for groynes exposed to overflowing primary ship waves is not yet present. Therefore, experimental research based on physical modelling is performed to establish a relationship between the damages and corresponding overflowing primary ship waves. This can be used as a design tool for groynes exposed to these types of wave attack.

The added drag on a buoyant body traveling in a solid-body rotation flow has been rigorously studied over the last century with G.I. Taylor being the first one to describe it in 1922, a hundred years ago. A recent publication by Duinmeijer (2021) investigated the capacity of free-surface vortices to transport solids as a practical application to avoid solids accumulation in wastewater pump sumps. In that research, the assumption was made that a Taylor column exists upstream and downstream of the solids body, aiding the downward motion in the vortex core. This has however never been experimentally confirmed for free-surface vortex flows. The goal of this research is to experimentally check whether or not the Taylor-column induced drag force is the main mechanism for the downward motion of buoyant particles in a free surface vortex core. An experimental set-up consisting of a 600 x 1000mm (diameter x height) Perspex tank is used, in which controlled vortices can be generated. A novel combination of Laser Doppler Velocimetry (LDV) and Particle Tracking Velocimetry (PTV) is deployed simultaneously to obtain synchronised data of both the flow velocities and particle motion. The LDV device measures the tangential and axial flow components in a small measurement volume. Many point-measurements along a horizontal line through the vortex enable us to create the tangential and axial velocity profiles. A deconvolution process is applied on the LDV data to account for the spatial averaging effect of the LDV measurement volume and the wandering of the vortex core. The PTV system is used to determine the particle location over time, from which the velocity is obtained. For simplicity, only one type of particle - a buoyant sphere of 25 mm in diameter - is considered in this study. To obtain repeatability of the experiments, a particle dropping device is introduced to insert the sphere in the vortex core without entrapping air. A PID control system is added to the set-up to keep the discharge through the system constant. The first part of this research focusses on the performance of the measurement system, where the measurements of the flow characteristics of the free-surface vortex in the absence of a buoyant particle are compared to the results obtained by Duinmeijer (2020), repeating one of the experiments from that thesis without a buoyant particle. The tangential velocities measured with LDV, coincide well with the tangential velocity profiles found during PIV measurements by Duinmeijer. The axial velocity profiles however do not agree. Duinmeijer measured maximum axial velocities around the vortex core radius where in this thesis an axial velocity profile with two maxima is found; one in the vortex centre and one at 60-70% of the vortex core radius. Several additional experiments were performed to rule out possible causes for this difference, from which it is concluded that the axial velocity is non-zero in the vortex centre. The second part of the thesis focusses on the characteristics of the flow in the vortex core in the presence of a buoyant sphere. With the synchronized deployment of the LDV and PTV-system, the flow mechanics and the motion of the buoyant sphere can be simultaneously characterized. Such that the influence of the sphere's presence on the tangential and axial velocity components is quantified. New findings were obtained from these measurements, with the most striking one being a significant difference that is observed in the tangential velocities of the flow just above and below the buoyant sphere. Although this effect is described in theory e.g. by Moore and Saffman and Maxworthy (1970; 1968), it has not been measured before in any lab experiment before and therefore it was not expected to be observed so clearly during the experiments. The clear tangential velocity discrepancy experimentally confirms the presence of the Taylor column above and below the sphere.
The last part of the thesis is dedicated to quantifying the Taylor column induced drag force, derived from the tangential velocity difference of the flow above and below the particle. This difference in tangential velocities induces a pressure difference over the particle leading to a downward pointing force. The magnitude of this force is shown to be 75±17% of the total drag on the particle, making the Taylor column induced drag the main downward transport mechanism for the buoyant sphere in the free-surface vortex core given the conditions used in this experimental set-up.
It is recommended to investigate further the effect of the vortex core radius/particle characteristic length ratio on the Taylor column induced drag. The axial free-surface vortex flow is not radially uniform, with a region of high axial flow around the core radius. This axial flow being pushed into a Ekman layer on the sphere surface is generating the pressure difference over the particle and thus the downward force. It is therefore suspected that vortex core/particle size ratio strongly influences the Taylor column induced drag magnitude and thus be researched further.