Turbulent vortex shedding from a blunt trailing edge hydrofoil

Placed in a fluid stream, solid bodies can exhibit a separated flow that extends to their wake. The detachment of the boundary layer on both upper and lower surfaces forms two shear layers which generate above a critical value of Reynolds number a periodic array of discrete vortices termed von Kármán street. The body experiences a fluctuating lift force transverse to the flow caused by the asymmetric formation of vortices. The structural vibration amplitude is significantly amplified when the vortex shedding frequency lies close to a resonance frequency of the combined fluid-structure system. For resonance condition, fatigue cracks are likely to occur and lead to the premature failure of the mechanical system. Despite numerous and extensive studies on the topic, the periodic vortex shedding is considered to be a primary damage mechanism. The wake produced by a streamlined body, such as a hydrofoil, is an important issue for a variety of applications, including hydropower generation and marine vessel propulsion. However, the current state of the laboratory art focuses mainly in the wakes produced by hydraulically smooth bluff bodies at low Reynolds numbers. The present work considers 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. Experiments are performed in the test section of the EPFL-LMH high speed cavitation tunnel. With the help of various measurement devices including laser Doppler vibrometer, particle image velocimetry, laser Doppler velocimetry and high speed digital camera, the effects of cavitation on the generation mechanism of the vortex street are investigated. Furthermore, the effects of a tripped turbulent boundary layer on the wake characteristics are analyzed and compared with the condition of a natural turbulent transition. In cavitation free regime and according to the Strouhal law, the vortex shedding frequency is found to vary quasi-linearly with the free-stream velocity provided that no hydrofoil resonance frequency is excited, the so-called lock-off condition. For such regime, the shed vortices exhibit strong span-wise instabilities and dislocations. A direct relation between vortex span-wise organization and vortex-induced vibration amplitude is found. In the case of resonance, the coherence of the vortex shedding process is significantly enhanced. The eigen modes are identified so that the lock-in of the vortex shedding frequency on a free-stream velocity range occurs for the first torsional mode. In the case of liquid flows, when the pressure falls below the vapor pressure, cavitation occurs in the vortex core. For lock-off condition, the cavitation inception index is linearly dependent on the square root of the Reynolds number which is in accordance with former models. For lock-in, it is significantly increased and makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index and considering the trailing edge displacement velocity, a new correlation relationship that encompasses the lock-off and the lock-in conditions is proposed and validated. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. Therefore, the displacement velocity of the hydrofoil trailing edge increases the fluctuating forces on the body and this effect is additional to any increase of vortex span-wise organization, as observed for the lock-in condition. Cavitation developing in the vortex street cannot be considered as a passive agent for the visualization of the turbulent wake flow. The cavitation reacts on the wake as soon as it appears. At early stage of cavitation development, the vortex-induced vibration and flow velocity fluctuations are significantly increased. For fully developed cavitation, the vortex shedding frequency increases up to 15%, which is accompanied by the increase of the vortex advection velocity and reduction of the stream-wise and cross-stream inter-vortex spacings. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation. Besides, it is shown that the cavitation does not obviously modify the vortex span-wise organization. Moreover, hydro-elastic couplings are found to be enabled/disabled by permitting a sufficient vortex cavitation development. The effects on the wake characteristics of a tripped turbulent boundary layer, as opposed to the natural turbulent transition, are investigated. 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 re-establishment 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 induced hydrofoil vibration, the wake velocity fluctuations, the wake energies and the vortex strength are revealed. The vortex shedding process intermittency is decreased and the coherence is increased. 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. This result leads to an effective estimation of the vortex shedding frequency.

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