The present study experimentally investigates the onset of ventilation of surface-piercing hydrofoils. Under steady-state conditions, the depth-based Froude number Fr and the angle of attack α define regions in which distinct flow regimes are either locally or globally stable. To map the boundary between these stability regions, the parameter space (α,Fr) was systematically surveyed by increasing α until the onset of ventilation while maintaining a constant Fr. Two simplified model hydrofoils were examined: a semi-ogive with a blunt trailing edge and a modified NACA 0010-34. Tests were conducted in a towing tank under quasi-steady-state conditions for aspect ratios of 1.0 and 1.5, and for Fr ranging from 0.5 to 2.5. Ventilation occurred spontaneously for all test conditions as α increased. Three distinct trigger mechanisms were identified: nose, tail and base ventilation. Nose ventilation is prevalent at Fr<1.0 and Fr<1.25 for aspect ratios of 1.0 and 1.5, respectively, and is associated with an increase in the inception angle of attack. Tail ventilation becomes prevalent at higher Fr, and the inception angle of attack exhibits a negative trend. Base ventilation was only observed for the semi-ogive profile, but it did not lead to the development of a stable ventilated cavity. Notably, the measurements indicate that the boundary between bistable and globally stable regions is not uniform and extends to significantly higher α than previously estimated. A revised stability map is proposed to reconcile previously published and current data, demonstrating how two alternative paths to a steady-state condition can lead to different flow regimes.

Background: Rowing is a sport that places significant stress on the lower back, often leading to low back pain (LBP) injuries among athletes. Laboratory studies have shown that rowing with an oar blade under an angle is more efficient compared to a traditional blade. The effect of blade angle on the lower back is unknown. Therefore, the aim of this study is to investigate the effect of different oar blade angles on the muscle activation of the lower back muscles during on-water rowing. Methods: Seven collegiate (five males, two females) athletes row 500 m on water twice, once with a traditional (0-degrees blade) and once with an oar blade under a 5-degrees angle. Surface electromyography of the longissimus muscle of the erector spinae was measured bilaterally at the thoracic and lumbar level with a sample frequency of 2,000 Hz. In total 1,443 strokes were analyzed. Statistical Parametric Mapping was used to investigate the differences in muscle activity between the 0-degrees and 5-degrees oar blade. Results: No significant differences in muscle activity were found between the 0- and 5-degrees oar blade. Conclusion: Rowing with an oar blade under 5-degrees did not alter the muscle activity during on-water rowing. This indicates that rowing with an oar blade under 5-degrees may not increase the muscle activation. These results are important as it seems that a change in oar blade angle does not increase the injury risk, longitudinal studies should investigate the effect of oar blade angles on LBP injuries.

The principal aim of the work presented here is to investigate and demonstrate that a forward tilted rowing blade would result in a more efficient and effective motion of the blade through the water that would result in a higher boat speed when an equal input power is provided. A 1:5 scaled rowing boat is used to determine the performance of rowing blades with different sizes and blade angles. This is used to validate the results of a previous study where the optimal blade angle of 15 (Formula presented.) with respect to the oar shaft was determined (1). The input power and speed of the rowing boat can be compared between original and modified oar blades. Measurements in a towing tank demonstrate that a modified rowing blade result in faster rowing by 0.4% at the same input power. Maintaining the same stroke rate, the improvement of the blade efficiency is compensated by using a 4–6% increased blade area to yield the same input power.

The circular Taylor-Couette flow is one of the archetypical model systems for the study of flow transitions and dynamic pattern formation in experimental fluid dynamics. The emergence of the internal vortical flow structures are commonly visualized through a rheoscopic flow visualization, while their spatio-temporal dynamics can be extracted by the construction of a space-time diagram using a single camera. Although the latter is an effective method to map the various flow regimes for different inner and outer cylinder rotations, it suffers from limitations in the frame rate while the full extent of the azimuthal vortex structure along the circumference, together with its dynamic evolution through space and time, remains unclear. In this work, we perform the full 360-degree field of view panorama imaging for the rheoscopic flow visualization of the azimuthal vortex structure that wraps around the circumference. We use a set of 12 GoPro cameras that are commercially available and can be triggered remotely. We calibrate and position our cameras using methods from computer vision while we synchronize their audio channels at an inter-frame precision much greater than the frame rate. We unwrap the physical coordinates along the circumference of the outer cylinder through texture mapping its surface using a spatially weighted image interpolation and present a single representation of the azimuthal vortex structure from the rheoscopic flow visualization. We validate our methods within a submillimeter precision and showcase the application to study the steady-state and transient dynamics of a single- phase wavy vortex flow. Furthermore, we discuss the current limitations as we add neutrally buoyant PMMA particles at increasing volume fractions up to 30 %. Our methods allow us to fully decouple space and time, and study the dynamic pattern formation at bullet time accuracy.

The interaction between a turbulent boundary layer flow and compliant surfaces is investigated experimentally. Three viscoelastic coatings with different material stiffnesses are used to identify the general surface response to the turbulent flow conditions. For the softest coating, the global force measurements show two obvious regimes of interaction with an indicated transition at Ub/Ct∼3.5, where Ub is the bulk flow velocity and Ct is the coating shear velocity. The one-way coupled regime shows friction values comparable to those of the rigid wall, while the two-way coupled regime indicate a significant increase in fluid friction. Within the one-way coupled regime for Ub/Ct>1.2, the flow measurements show a low level of two-way coupling represented by the change of the velocity profile as well as the increase in the Reynolds stresses in the near-wall region. This is supported by the surface deformation measurements. Initially, the turbulent flow structures induce only an imprint on the coating surface, while a change in surface response occurs when the surface wave propagation velocity cw equals the shear wave velocity of the coating Ct (i.e. cw/Ct∼1). Above Ub/Ct>1.2, a growth in wavelength is observed with increasing flow velocity, most probably due to the surface wave formation generated downstream the pressure features of the flow. The surface response is stable and correlates with the high-intensity turbulent pressure fluctuations in the turbulent boundary layer, with a wave propagation velocity cw∼0.7–0.8Ub. Within the two-way coupled regime, additional fluid motions and a downward shift in the logarithmic region of the velocity profile are observed due to substantial surface deformation and confirm the frictional drag increase. Another type of surface response is initiated by phase-lag instability in combination with surface undulations that start to protrude the viscous sublayer, where the propagation velocity of surface wave trains is cw∼0.17–0.18Ub.

This thesis describes the investigation of the dynamics of turbulent shear flows over non-smooth surfaces. The research was conducted in two parts, related to the experimental facility used in combination with the applied functional surface. The first part describes the experiments of a turbulent Taylor-Couette flow over a riblet surface. The Taylor-Couette facility proves to be an accurate measurement device to determine the frictional drag of surfaces under turbulent flow conditions. Sawtooth riblets are applied on the inner cylinder surface and have the ability to reduce the total measured drag by 5.3% at Res=4.7x104.  Under these conditions, a small shift is observed in the azimuthal velocity profile that indicates the change in the net system rotation, which on its turn affects the quantity of drag change, the so-called rotation effect. A model based on the angular momentum balance is proposed and quantifies the drag change due to the rotation effect. Using the total measured drag change, the model accurately predicts the velocity shift in the azimuthal direction. In addition to the steady operational conditions, periodically driven Taylor-Couette flows were investigated by modulating the velocity between the two cylinders as a sinusoidal function, while maintaining RΩ = 0. The main scaling parameters are the shear Reynolds number Res, the oscillation Reynolds number Reosc and the Womersley number Wo, such that the required power to overcome the frictional drag becomes equal to = f(Res,Reosc, Wo). Large velocity amplitudes A = Reosc/Res > 0.10 induce the growth of frictional drag due to the additional turbulent fluctuations. The required power to overcome the frictional drag is given by = (f(A)+ K*Wo4A2). The first term represents the analytical quasi-steady state solution with the accompanying velocity modulation, while the second term involves the magnitude of the boundary acceleration with the associated velocity fluctuation, where K* is the conditional scaling-factor between the additional drag and the dimensionless acceleration. Riblets are still able to reduce the frictional drag under small accelerations of the periodically driven boundaries, but the effect declines drastically or even enhances the frictional drag when the boundary acceleration becomes more significant.   The second part of this thesis describes the assessment of the applied water tunnel and the interactional behavior between a compliant coating and a turbulent boundary layer flow in the tunnel. In the assessment of the water tunnel, the Clauser chart method showed to be a suitable procedure to quantify the local wall shear stress τw. The interaction between a compliant wall and the near-wall turbulent flow was examined by applying in-house produced visco-elastic coatings with three different stiffnesses.  Two typical flow-surface interaction regimes were identified; the one-way coupled regime and the two-way coupled regime. The one-way coupled regime is valid when the turbulent flow initiates moderate coating surface deformation, while the fluid flow remains undisturbed. All of the three coatings exhibited the one-way coupled interactional behavior, where the surface modulations ζ were smaller than the viscous sublayer thickness δv and scale with the turbulent pressure fluctuations over the coating shear modulus, i.e. ζrms ~ prms/|G*|. In this regime, the surface waves have the propagation velocity in the order of cw = 0.70-0.80 Ub, indicating a strong correlation with the high-intensity pressure fluctuations in the turbulent boundary layer away from the wall. The two-way coupled regime has only been observed for the coating with the lowest shear modulus when Ub > 4.5 m/s, indicating significant surface deformation accompanied by additional fluid motions (u',v') and an increase in the local Reynolds stresses. The velocity profile shifts downwards Δu+ in the log region, which verifies the drag increase due to the significant surface undulations. The visualizations of the surface deformation showed the formation of wave-trains with high amplitudes originating from the initial surface undulations caused by the pressure fluctuations in the turbulent boundary layer (i.e. one-way coupling). When these early surface undulations start to protrude the viscous sublayer, the turbulent flow is capable of transfering more energy towards the coating and initiates the wave-train with high amplitudes. The wave-trains dominate the coating surface incrementally with increasing bulk velocity and propagate with a wave velocity of cw = 0.17-0.18 Ub. The 1-way/2-way regime transition is estimated to occur around ζrms > δv/2. The turbulent flow along the slow-moving wave-trains resembles the classical phenomenon of a turbulent flow over a rigid wavy surface, with a local acceleration and deceleration of the fluid. When the wave-trains start to dominate the coating surface, a linear correlation determines the abovementioned downward shift Δu+, based on the wall-normal velocity component dζ/dt. No frictional drag reduction under turbulent flow conditions was found in this study with this type of visco-elastic compliant coatings.  

We investigate the deformation of a linear viscoelastic compliant coating in a turbulent flow for a wide range of coating parameters. A one-way coupling model is proposed in which the turbulent surface stresses are expressed as a sum of streamwise-travelling waves with amplitudes determined from the stress spectra of the corresponding flow over a rigid wall. The analytically calculated coating deformation is analysed in terms of the root-mean-square (r.m.s.) surface displacement and the corresponding point frequency spectra. The present study systematically investigates the influence of five coating properties namely density, stiffness, thickness, viscoelasticity and compressibility. The surface displacements increase linearly with the fluid/solid density ratio. They are linearly proportional to the coating thickness for thin coatings, while they become independent of the thickness for thick coatings. Very soft coatings show resonant behaviour, but the displacement for stiffer coatings is proportional to the inverse of the shear modulus. The viscoelastic loss angle has only a significant influence when resonances occur in the coating response, while Poisson's ratio has a minor effect for most cases. The modelled surface displacement is qualitatively compared with recent measurements on the deformation of three different coatings in a turbulent boundary-layer flow. The model predicts the order of magnitude of the surface displacement, and it captures the increase of the coating displacement with the Reynolds number and the coating softness. Finally, we propose a scaling that collapses all the experimental data for the r.m.s. of the vertical surface displacement onto a single curve.

Ultrasound Imaging Velocimetry (UIV) is applied to a Taylor-Couette flow, for the case of pure inner cylinder rotation. By imaging a radial-azimuthal plane, two velocity components are obtained simultaneously in a two-dimensional plane. For the single-phase flow studies, Iriodin flakes (commonly used for visualizing flow structures) are used as “flow tracers” for the backscatter of ultrasound. This allows for a simultaneous mapping of the flow regime, via flow visualization, as well as extracting quantitative velocity information in the radial gap. After validating UIV against the analytically well-defined laminar Circular Couette flow as well as turbulent Taylor-Couette flow, other regimes are probed as well, in particular, the Wavy Vortex flow. Finally, the application of UIV to a particle-laden Taylor-Couette flow (particle volume fraction, f_0:01) is considered, under the conditions of oscillatory pure inner cylinder rotation. The results presented here serve as a proof-of-concept for the application of UIV to the Taylor-Couette flow and will be applied to denser particle-laden flows (f _ 0:05) in the future.

The present study introduces an experimental technique based on a Free Surface-Synthetic Schlieren (FS-SS) method in order to characterize free surfaces subjected to strong deformations. Current synthetic Schlieren methods are based on local image correlation and thus limited to rather weak image deformations, implying that they can only resolve rather large surface wavelengths and limited wave amplitude. The present method is a substantial improvement that allows to measure much stronger image deformations, providing access to shorter surface wavelengths and larger amplitudes (i.e. larger surface curvatures).

The objective of this study was to determine the advantage of the application of speedstrips to rowing oars for a lightweight single sculler. The research method comprehended three steps: (1) the analysis of the rowing oar movement, (2) the determination of the change in drag and (3) the composition of a rowing model to establish the advantage that could be achieved. The parameters needed for the model: boat velocity, oar angle velocity and power delivered by the rower, were recorded on a real single sculler. The change in drag due to speedstrips on cylinders was determined by performing wind tunnel experiments. The rowing model (Matlab) simulates a race by using real stroke data of a world-class rower as input, while calculating the drag with the coefficients determined by the wind tunnel experiments. The output of the model is the final advantage by the application of speedstrips to rowing oars. Speedstrips induce a 0.1% advantage over a 2000 m race under calm wind conditions. The advantage increases up to.4% with a headwind velocity of 5 m s^-1. For bigger boats, the advantage could be even more significant.

The flow motions in the turbulent boundary layer between water and a rowing boat initiate a turbulent skin friction. Reducing this skin friction results in better rowing performances. A Taylor-Couette (TC) facility was used to verify the power losses due to velocity fluctuations P_V′ in relation to the total power, as a function of the velocity amplitude A. It was demonstrated that an increase of the velocity fluctuations results in a tremendous decrease of the velocity efficiency eV. The velocity efficiency eV for a typical rowing velocity amplitude A of 20-25% was about 0.92-0.95%. Suppressing boat velocity fluctuations with 60% will increase boat speed with 1.6%. Riblet surfaces were applied on the inner and outer cylinder wall to indicate the drag reducing ability of such surfaces. The results of the measurements at constant velocity are identical as the results reported earlier, while the experimental configuration was different. This confirms once more the consistency of the TC-system for drag studies. The maximum drag reduction DR was 3.4% at a Reynolds number Res 4.7 × 104, which corresponds to a shear velocity in this TC-system with water of V 4.7 m/s. For typical rowing velocity fluctuations, the riblets maintain to reduce the drag with 2.8% and corresponds to a averaged velocity increase of 0.9%. The drag reducing ability of riblets is partly lost due to velocity fluctuations with high amplitudes (A > 20%). From these results, it is concluded that the friction coefficient Cf will vary within one cycle. Higher acceleration/deceleration leads to a additional level of turbulent kinetic energy.

Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with a riblet surface and indicated a drag reduction for a wide range of shear Reynolds numbers, with a maximum of 5.3% at Res = 4.7 × 104 (s+ = 14). Tomographic PIV verified that the friction coefficients are strongly related to the flow regimes and structures. The bulk fluid rotation was changed by the application of the riblets, as the wall-bounded flow conditions at the inner cylinder wall were changed due to the surface modification and is called the rotation effect. A simple model was used to indicate the averaged bulk velocity shift (1.4%), after which the drag changes due to the rotation effect (-1.9%) and the riblet effect (-3.4%) were determined. The bulk velocity shift of 1.4% was verified by PIV measurements. Compliant surfaces will be further investigated to check their required conditions for drag reduction of wall-bounded flows.