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Lock-in is of great importance in many engineering applications due to its practical implications for structural safety. The influence of composite bend-twist coupling on the wake dynamics and vortex-induced vibration around a carbon fiber composite hydrofoil is investigated and compared to a similar stainless-steel hydrofoil. Experiments are conducted by varying linearly and slowly the upstream velocity back and forth between 5 and 15 m/s, which allows reaching lock-in conditions for both hydrofoils. Due to the blunt truncation of the trailing edge, both hydrofoils produce strong and alternate vortices in their wake, whose effect is visible on vibration spectrograms. The steel hydrofoil produces a classical lock-in onto its first torsion mode, while the composite hydrofoil exhibits two lock-in phenomena onto both torsion and second bending modes. Interestingly, for the second bending mode, the vibration spectrogram reveals the existence of two frequencies: (i) the resonance frequency, which remains almost constant during the lock-in phase, and (ii) the Strouhal frequency, which increases linearly with the upstream velocity. Using flow visualization, we found that this peculiar behavior is the result of the twist-bending coupling, which leads to the co-existence of two different vortex-shedding mechanisms. Close to the hydrofoil tip, the large vibration amplitude dictates the shedding frequency while the shedding follows the Strouhal law elsewhere. This partial lock-in gradually fades away as the velocity is increased. This result provides guidance for the safe design of composite structures. [ABSTRACT FROM AUTHOR]

In the present paper, a new control method for tip-leakage vortex (TLV) cavitation is proposed. This method, which combines the effects of grooves and winglets, is made of overhanging grooves (OHGs) fitted at the tip of a hydrofoil. Experimental and numerical investigations are conducted to evaluate the performances of OHGs in terms of TLV cavitation suppression. The results are systematically compared with the baseline, conventional grooves (CGs) and anti-cavitation lip (ACL) and a significant improvement of TLV cavitation suppression is obtained with the OHGs. We also carried out a primary optimization design of the OHGs and a more effective suppression on TLV cavitation is obtained for small gaps and TLV cavitation is almost suppressed for middle and large gaps. The underlying reasons for TLV cavitation mitigation are discussed in detail with the help of numerical simulations. It indicates that for small gap sizes, OHGs can effectively weaken the strength of both TLV and tip-separation vortex (TSV) and mitigate TLV cavitation. For middle and large gap sizes, it is found that OHGs will induce an increase in the TLV core size, which further increases local minimum pressure. The influence of OHGs on the performance of hydrofoil is also examined, which indicates that the fluctuation of TLV cavitation can be effectively suppressed by OHGs and no significant alteration of time-averaged drag and lift is induced by OHGs. Our work shows that the OHGs can effectively suppress TLV cavitation with limited influence on the performances of hydrofoil in a large range of the gap sizes, which is a promising method for the control of TLV cavitation in hydraulic machinery. [ABSTRACT FROM AUTHOR]

We investigate the cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of highspeed cavitation tunnel. Hydrofoils with smooth and rough leading edge were tested for different upstream velocities and incidence angles. Our observations clearly revealed that cavitation incipience is enhanced by roughness at incidence angle below 2°. This is in line with the former reports, according to which roughness element decreases the wettability and traps a larger amount of gas. As a result, surface nucleation is enhanced with an increased risk of cavitation. Surprisingly, for higher incidence angles (>3°), we have found that cavitation incipience is significantly delayed by roughness while developed cavitation is almost the same for both smooth and rough hydrofoils. We argue that this unexpected incipience delay is related to the change in the boundary layer structure due to roughness. We also report a significant influence of roughness on the dynamic of developed cavitation and shedding of transient cavities. [ABSTRACT FROM AUTHOR]

An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-stream velocity, and the adverse pressure gradient. They were controlled through the incidence angle of the elliptical hydrofoil, the free-stream velocity and the sphere diameter. A single helical breakdown of the vortex was systematically observed over a wide range of experimental parameters. The helical breakdown coiled around the sphere in the direction opposite to the vortex but rotated along the vortex direction. We have observed that the location of vortex breakdown moved upstream as the swirl number or the sphere diameter was increased. LDV measurements were corrected using a reconstruction procedure taking into account the so-called vortex wandering and the size of the LDV measurement volume. This allows us to investigate the spatio-temporal linear stability properties of the flow and demonstrate that the flow transition from columnar to single helical shape is due to a transition from convective to absolute instability. [ABSTRACT FROM AUTHOR]

Experiments on vortex shedding from a blunt trailing edge symmetric hydrofoil operating at zero angle of attack in a uniform high speed flow, Reh = 16.1 ⋅ 10³ - 96.6 ⋅ 10³, where the reference length h is the trailing edge thickness, are reported. The effects of a tripped turbulent boundary layer on the wake characteristics are analyzed and compared with the condition of a natural turbulent transition. The foil surface is hydraulically smooth and a fully effective boundary layer tripping at the leading edge is achieved with the help of a distributed roughness. The vortex shedding process is found to be strongly influenced by the boundary layer development: the tripped turbulent transition promotes the reestablishment of organized vortex shedding. In the context of the tripped transition and in comparison with the natural one, significant increases in the vortex span-wise organization, the vortex-induced hydrofoil vibration, the wake velocity fluctuations, and the vortex strength are revealed. Although the vortex shedding frequency is decreased, a modified Strouhal number based on the wake width at the end of the vortex formation region is constant and evidences the similarity of the wakes in terms of spatial distribution for the two considered boundary layer transition processes. [ABSTRACT FROM AUTHOR]

An experimental work has been carried out to investigate the dynamic behavior and the intensity of erosive partial cavitation on a 2-D hydrofoil. Both sheet (stable) and cloud (unstable) cavitation have been tested in a cavitation tunnel for various free stream velocities. Special attention has been given to validate the use of acceleration transducers for studying the physical process. In particular, the modulation in amplitude of the cavitation induced vibrations in a high frequency band has allowed us to determine the shedding frequency and the relative intensity of the collapse process for each testing condition. Regarding the cavity dynamics, a typical Strouhal value based on its length of about 0.28 has been found for cloud cavitation; meanwhile, for sheet cavitation, it presents a value of about 0.16. Furthermore, the level of the vibration modulation in the band from 45 kHz to 50 kHz for cloud cavitation shows a power law dependency on the free stream velocity as well as a good correlation with the pitting rate measured on stainless steel samples mounted on the hydrofoil. [ABSTRACT FROM AUTHOR]

Despite the numerous remedies prescribed so far, tip vortex cavitation (TVC) remains a major issue in design and operation of diverse applications. In this paper, we experimentally investigate the effectiveness of winglets in suppressing TVC. An elliptical hydrofoil is selected as the baseline geometry and various winglets are realized by bending the last 5 or 10% of the span at ± 45° and ± 90° dihedral angles. To better focus on the physics of the problem, we have intentionally avoided any optimization on the geometries and our winglets are only smooth non-planar extensions of the original cross-section. Modifying no more than 3.7% of the lifting surface, lift-and-drag force measurements demonstrate that the hydrodynamic performances of the winglet-equipped hydrofoils are not substantially different from the baseline. Nevertheless, cavitation inception–desinence tests reveal that undeniable advantages are achieved by the winglets in TVC alleviation. It is found that the 10%-bent 90° winglets are more effective than the 45° cases, with − 90° (bent down toward the pressure side) performing superior to + 90°. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestream velocity and 14° incidence angle. In addition, the study on the bending length effect conducted for the 90° configurations shows that the 5%-bent winglets are not as striking as the 10% ones. Employing Stereo-PIV technique, the influence of winglets on non-cavitating flow structures is examined. For the most effective winglet (10%-bent 90°-downward), we observe that the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the vortex core size increases significantly (by almost 70%). These effects are accompanied by a tangible reduction in the axial velocity at the vortex core leading to further mitigation of TVC. [ABSTRACT FROM AUTHOR]

Flow induced vibration on a hydrofoil may be significantly reduced with a slight modification of the trailing edge without alteration of the hydrodynamic performance. Particularly, the so called Donaldson trailing edge shape gave remarkable results and is being used in a variety of industrial applications. Nevertheless, the physics behind vibration reduction is still not understood. In the present study, we have investigated the hydrodynamic damping of a 2D hydrofoil with Donaldson trailing edge shape. The results are compared with the same hydrofoil with blunt trailing edge. The tests are carried out in EPFL high speed cavitation tunnel and two piezoelectric patches are used for the hydrofoil excitation in non-intrusive way. It was found that the hydrodynamic damping is significantly increased with the Donaldson cut. Besides, as the flow velocity is increased, the hydrodynamic damping is found to remain almost constant up to the hydrofoil resonance and then increases linearly, for both tested trailing edge shapes and for both first bending and torsion modes. [ABSTRACT FROM AUTHOR]