This dissertation investigates the mechanisms of ventilation inception on surface-piercing hydrofoils under quasi-static variations of the angle of attack (AoA). Two hydrofoil geometries, a Semi- Ogive profile with a blunt trailing edge and a streamlined NACA 0010-34, were tested in a towing tank across a range of Froude numbers (Fnh) and aspect ratios (AR). The study introduces a novel experimental methodology, quasi-static testing, which eliminates inertial effects associated with acceleration by maintaining constant Fnh while varying AoA. This approach contrasts with traditional ”static tests” found in the literature. A new flow regime map in the α−Fnh parametric space was developed, tracing distinct ventilation inception boundaries for the two hydrofoils and offering a more detailed representation of transitions between flow regimes. The findings challenge the validity of previous methodologies, which assumed that ventilation inception boundaries were dominated by stall angles and fixed AoA, overlooking the dynamics of ventilation inception. At low Fnh, leading-edge (LE) ventilation appears as the dominant mechanism, slightly enhancing lift by stabilizing flow on the suction surface. As Fnh increases, transitions to Rayleigh-Taylor (RT) instabilities become evident, particularly at moderate Reynolds numbers. At high Fnh (≥ 2.5), RT instabilities prevail, with trailing-edge (TE) effects becoming significant for the Semi-Ogive hydrofoil. In contrast, the streamlined NACA 0010-34 primarily exhibits RT-driven mechanisms. The results demonstrate that this new methodology yields precise and repeatable inception boundaries, representing a significant improvement over historical techniques. Notably, contrary to prior assumptions, the AoA at which ventilation inception occurs exceeds 15◦ and is no longer constant across various Fnh. Additionally, trailing-edge geometry was found to influence ventilation inception, particularly at higher AR, as evidenced by the diverging trends observed between the two hydrofoil profiles. Furthermore, the experimental results align well with semi-empirical models for lift and drag coefficients at Fnh ≥ 1.0, reinforcing the robustness of the findings. These contributions provide more in-depth insights into ventilation inception dynamics and offer valuable guidance for the design of hydrofoils in marine applications.

The capability to maintain position in different operational scenarios is crucial for every offshore vessel, including semisubmersible vessels, which are traditionally designed to operate in deep water conditions. With the transition to new sources of energy production, such as wind turbines, the necessity for these vessels to operate in shallower waters is increasing. This aspect necessitates identifying the impact of such conditions on the vessel’s capacity to maintain its position. This thesis focuses on identifying the governing factor in the reduction of thruster efficiency and, therefore, evaluating the thrust delivered by the thruster to the semisubmersible vessel when operating near the seabed. Specifically, it is relevant to identify which parameters, or combinations, induce an additional reduction in the effective thrust propellers deliver to the vessel in reduced clearance beneath the thruster, thereby affecting the vessel station-keeping capability. The primary goal of this work is to estimate the reduction in the delivered thrust when a thruster operates in proximity to the seabed. Therefore, it is crucial to identify the sources of thrust reduction for a semisubmersible vessel based on the available literature and previous tests on similar vessels. Specifically, the interactions between hull, current, and thrusters have been identified as the most impacting effects in reducing thruster efficiency. Therefore, it was possible to estimate the impact of each of these mentioned types of thruster interactions both with analytical solutions and the suggested formulation from the register class (e.g. ABS). The results indicate that the interaction between the thruster outflow and the hull, combined with the impact of the current on this condition, is strongly modified when compared with deep-water conditions. Approximately 50 computational-fluid-dynamics (CFD) steady-state simulations have been performed to investigate the current effect and the vessel’s geometrical characteristics under different current speed with reduced under-thruster clearance. These simulations were conducted across three different water depths at a single vessel draft, combined with four distinct beam-side current speeds. The primary focus is on evaluating the flow development beneath the hull and comparing it with the deep water scenario. The analysis revealed that the proximity of the seabed partly influences the flow deviation. Contrarily, the flow velocity, and consequently, the frictional resistance, predominantly affects the thruster’s performance. A significant finding is that the thruster’s delivered thrust could decrease by up to 53%, with respect to open water conditions, when operating in a current of 1 knot with 6 meters of under-thruster clearance. These results confirm the hypothesis regarding the impact of shallow water conditions on delivered thrust. The obtained data highlights the importance of accounting for additional thrust loss in situations of reduced under-thruster clearance, as this aspect was not previously considered for dynamic positioning operations. The investigated topic covers a broader scope than the presented results. However, this thesis marks the starting point for future research. It emphasizes the significance of studying the impact on thruster performance under reduced under-thrust clearance conditions and provides the thrust reduction obtained for the specific case of beam-side current condition and the thruster operating in the direction of the downstream floater. Furthermore, the obtained uncertainty indicates that the used CFD approach (steady-state) needs a more comprehensive analysis involving time-consuming numerical solutions to address the vortex shedding phenomena, especially for high current values.

The microbubble content is considered one of the most important fluid characteristics influencing cavitation inception and dynamics. However, the effects of microbubbles on cavitation are not yet precisely understood. The current work experimentally assesses the influence of microbubbles on cavitation around the runner of a hydraulic turbine model in a hydraulic test facility. Microbubbles are measured upstream of the modelusingIPI, while cavitation is observed visually. IPI measurements are performed under different DO concentrations, thereby changing the microbubble content in the facility, at different runner speeds. The results show an increase in microbubble sizes and concentrations for higher DO
concentrations. The sizes and concentrations of microbubbles increase accordingly with decreasing σ. This increase is not only the result of a pressure reduction at the measurement position but also the appearance of a cavitating vortex at the outlet of the runner, causing additional bubbles measured at the measurement position. These effects are highlighted once again after the onset of gap cavitation, showing the influence of cavitation on the measured microbubble content as no resorber is located in the test facility. A decrease in the microbubble concentration is observed after the onset of gap cavitation for increasing DO concentrations, as oxygen diffuses into cavitation bubble. Moreover, at low values of σ, a vacuum is applied to the system, resulting in a faster decrease in the DO concentration
at a higher initial DO concentration, resulting in a decrease of the microbubble content. A delay in the onset of gap cavitation is observed at lower DO concentrations due to the availability of fewer and smaller microbubbles in the facility. After the onset of cavitation, the development of gap cavitation is significantly altered by a change in the DO concentration. It is shown that gap cavitation covers a larger area of the runner at high DO concentrations, most likely caused by an increase in the vapour volume fraction at the outlet of the labyrinth seal, thereby increasing the number of vapour bubbles around the gap of the runner

Introduction - Hydrofoils are often used for high-performance craft. They enable the hull to rise out of the water and reduce drag. A recurring problem is ventilation, this is the presence of a cavity filled with ambient air on the lifting surface. Ventilation causes the lift of the hydrofoil to decrease drastically. A lot of research has been done on ventilation behaviour. Ventilation remains a stochastic phenomenon to this day. The same operational conditions do not yield the same flow behaviour, a correlation between the pressure on the lifting surface and ventilation behaviour might exist. This research is an intermediate step towards finding a correlation between pressure on the surface of a foil and ventilation, making ventilation repeatable. The main goal of this research is to find out if a full surface pressure reconstruction can aid the search for a correlation between pressure and ventilation.

Method - The test geometry is a NACA0012 surface-piercing hydrofoil with pressure measured using 2 rows of 15 pressure tappings. Extension pieces are used to obtain the span-wise pressure distribution while keeping the submerged span constant. The test program has three sets: all vertical runs (Set 1), high roll angle runs (Set 2), and runs with ventilation (Set 3). During the experiment, the loads are measured with force transducers on a frame that measures forces in 6 degrees of freedom. The calibration of the force transducers is performed prior to the experiment. The calibration of the pressure sensors is done first using a 9 meter water column before the experiment and during the experiment in the towing tank, extra re-calibration (stepped runs) data is collected. The collected force measurement data is processed to attain the lift and drag coefficients. The pressure measurements are translated to the pressure coefficient and placed in a matrix on the right location of the lifting surface.

Results - Based on repeated experimental runs the percentage differences are obtained on sensor and array levels. 30% of the sensors have a percentage difference of more than 10%, while the maximum difference for an entire array is 2.5%. A cross-check between the upper and lower arrays showed a percentage difference above 15% for 28% of the sensors, with a maximum difference of 1.2% at the array level. The pressure distributions at 0∘ roll angle, from Set 1, match the general pressure distribution characteristics. Comparing the 3D lift coefficient computed from the pressure measurements to that of the force data or an empirical method yields ambiguous results. The 3D lift coefficient based on the pressure measurement is within a 4% difference to Xfoil. The result of the high roll angle runs, Set 2, show the effect free-surface proximity has on the chord-wise pressure distribution. For the shallowest pressure reconstructions, close to the free-surface, the pressure distribution shows a minor peak near the leading edge after which the curve drops to near zero quickly. The operational conditions do not affect the quality of measurements. During two runs ventilation occurred, this is Set 3. The time-traces of the three forwardmost sensors show a dip prior to ventilation, with one sensor displaying an oscillating response. It is hypothesised that this is the position where ventilation is induced, but based on the results this can not be proven. All sensor response follows the same pattern when the ventilation bursts over the surface. The set-up is able to capture quick pressure changes.

Conclusion -The percentage difference is too high for too many sensors. On the array level, however, the difference is considered small enough. The results of set 1 are in good agreement with known data. The results obtained at 60∘ roll angle are concluded to be of the same quality as for 0∘ roll. The method is capable of providing results in a wide range of operational conditions. Based on the ventilation runs, no correlation to pressure reading has been found. The time-traces of the sensors near the leading edge of these runs do show fluctuations prior to ventilation. With more ventilation runs available, it is highly likely that a correlation can be established. Due to the slim data set, it is possible that the conclusions drawn will be refuted in the future when a larger data set is available. Currently, the goal has not been achieved, and it cannot be stated with certainty that the method used here to attain a full surface pressure reconstruction can be used to correlate pressure to ventilation.

Over the last few decades, the bypass ratio and nacelle diameter of modern turbofans have been increasing to achieve higher efficiencies. This trend has forced the underwing-mounted engines to be coupled more closely to the wing surface and the flap system, which significantly enhances the low-frequency noise, known as jet installation noise (JIN).

In this project, lobed nozzles are proposed and tested as a passive flow control technique for the reduction of the JIN. Measurements were performed using hot-wire anemometry and a linear array of microphones to investigate the effects of lobed nozzles on the near-field structure of a jet. The lobed nozzles were found to alter the convection velocities of the pressure fluctuations and the transition to turbulence of the jet. Moreover, far-field pressure measurements were performed inside the anechoic wind tunnel to examine the effects of the near-field structure on the perceived far-field noise. These findings can be used to assess the effectiveness of lobed nozzles for the reduction of the JIN in specific jet installation configurations.