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

Cavitation in Kármán Vortex Shedding from 2D Hydrofoil: Wall Roughness effects

The present study deals with the shedding process of the Kármán vortices in the wake of a NACA0009 hydrofoil at high Reynolds number. This research addresses the effects of the foil leading edge roughness on the wake dynamic with a special focus on the vortex shedding frequency, vortex-induced vibration and three-dimensionality of vortex shedding. For smooth leading edge, the shedding frequency of Kármán vortices occurs at constant Strouhal number, St=0.24. The wake exhibits 3D instabilities and the vortex induced vibration signals strong modulation with intermittent weak shedding cycles. Direct relation between vibration amplitude and vortex spanwise organization is shown. In the case of rough leading edge, the Kármán shedding frequency is notably decreased compared to the smooth one, St=0.18. Moreover, the vortex induced vibration level is significantly increased and the vibration spectra sharply peaked. The occurrence of vortex dislocations is shown to be less frequent with the roughness. The shedding of the vortices is considered on the whole as in phase along the hydrofoil span. Obviously, the shedding process of the Kármán vortices is highly related to the state of the boundary layer over the entire hydrofoil. It is believed that in the case of smooth leading edge, slight spanwise non-uniformities in the boundary-layer flow lead to slight instantaneous variation in vortex shedding frequency along the span which is enough to trigger vortex dislocations. On the contrary, for the rough leading edge, the location of transition to turbulence is uniformly forced which leads to the reduction of the spanwise boundary-layer non-uniformities and therefore to the enhancement of the coherence length of the Kármán vortices

Hydrofoil Roughness Effects on von Kármán Vortex Shedding

The shedding process of the von Kármán vortices is shown to be highly related to the state of the boundary layer over the entire hydrofoil: The selected hydrofoil has a laminar-turbulent boundary layer transition at mid-chord for an incidence angle of 0° and tested Reynolds number range. With the help of a distributed roughness, the transition to turbulence is triggered at leading edge. The vortex shedding frequency and the vortex-induced vibration are compared For the two roughness configurations. Cavitation is used as a mean of visualization of the wake flow: Since vortex-induced vibration and high speed visualization are synchronized, the hydrofoil vibration level is considered versus the vortex span-wise organization. The occurrence of the 3D structures of shed vortices and the modulation of the vortex-induced vibration signals are investigated

Cavitation in Kármán Vortices and Flow Induced Vibrations

The shedding process of the Kármán vortices at the trailing edge of a 2D hydrofoil at high Reynolds numbers is investigated. The focus is put on the effect of the cavitation on the vortex street morphology. A direct insight is also provided on the role of the structural vibration on the cavitation inception. The shedding frequency, derived from the measurement of flow induced vibration, is found to follow the Strouhal law as far as none of resonance frequencies of the hydrofoil is excited. For lock-off condition, PIV measurements in cavitation free regime and high speed visualizations for developed cavitation reveal strong spanwise 3D instabilities. The comparison of instantaneous velocity fields in cavitation free regime and images of cavitating vortices does not show notable influence of the cavitation on the vortex street morphology. It is also observed that the cavitation inception index increases linearly with the square root of the Reynolds number. For Reynolds numbers ranging from 35’000 to 40’000, the torsion mode of the hydrofoil is excited with a substantial increase of the vibration level. In this case, the spatial coherence of the Kármán vortices is enhanced with a quasi 2D shape and the shedding frequency is locked onto the vibration frequency. The cavitation inception index is found to be significantly increased compared to lock-off conditions. It is thought that the vortex roll-up is amplified by the phase locked vibration of the trailing edge. Former model, successful in describing the cavitation inception for fixed bluff bodies, is extended for taking into account the hydrofoil trailing edge vibration velocity. In addition, we have found that the vortices strength increases for lock-in condition and is directly related to the vibration velocity of the trailing edge normalized by the upstream velocity

Vortex shedding from blunt and oblique trailing edge hydrofoils

The phenomenon of vortex shedding behind a hydrofoil is an important issue from both scientific and technical point of view. The resulting fluctuating forces may lead to excessive vibrations and premature cracks in the hydraulic machines. According to previous studies, it is well known that an oblique trailing edge, also called "Donaldson cut", reduces the vibration in comparison with a blunt trailing edge; however the physics of the problem is still poorly understood. The purpose of the present work is the experimental investigation of vortex shedding dynamics in the wake of oblique and blunt trailing edge NACA0009 hydrofoils. Experiments are performed at zero incidence angle and high Reynolds numbers, ReL =5 10 5 - 2.9 10 6. The wake velocity profile is measured by two-component Laser Doppler Velocimetry. Cavitation occurrence in the core of the vortices is used as a mean of wake visualization with the help of a high speed camera. We have found that vortex induced vibration is significantly reduced for oblique trailing edge hydrofoil in comparison with the truncated one, which is in agreement with former reports. A disorganization of the Karman vortex street in the near wake is believed to be the reason of this vibration reduction. The high speed movies clearly show that the alternate shedding of the vortices turns into almost simultaneous shedding at the hydrofoil trailing edge. As a result, the upper and lower vortices pair with a significant thickening of the lower vortex core and a reduction of its strength. Consequently, the fluctuating lift, which is the cause of the structural vibration, is also reduced by the oblique truncation. We believe that this result stands for a basis to better optimize the trailing edge of turbine blades in order to further decrease the flow induced vibration

Experimental investigation of the vortex shedding dynamic in the wake of oblique and blunt trailing edge hydrofoils using PIV-POD method

This paper presents an experimental investigation of the vortex shedding in the wake of blunt and oblique trailing edge hydrofoils at high Reynolds number, Re=5 10 5 - 2.9 10 6. The velocity field in the wake is surveyed with the help of Particle-Image-Velocimetry, PIV, using Proper-Orthogonal-Decomposition, POD. Besides, flow induced vibration measurements and high-speed visualization are performed. The high-speed visualization clearly shows that the oblique trailing edge leads to a spatial phase shift of the upper and lower vortices at their generation stage, resulting their partial cancellation. For the oblique trailing edge geometry and in comparison with the blunt one, the vortex-induced vibrations are significantly reduced. Moreover, PIV data reveals a lower vorticity for the oblique trailing edge. The phase shift between upper and lower vortices, introduced by the oblique truncation of the trailing edge, is found to vanish in the far wake, where alternate shedding is recovered as observed with the blunt trailing edge. The phase shift generated by the oblique trailing edge and the resulting partial cancellation of the vortices is believed to be the main reason of the vibration reductio

2D Oscillating Hydrofoil

This paper presents a validation of numerical simulations in case of a forced oscillating hydrofoil. As a representative case study for vibrating blade in hydraulics machines, a 2D NACA 0009 oscillating hydrofoil is considered. Pressure coefficients from experiments and numerical simulations are presented. The fluid torque is then investigated in the frequency domain. A good agreement between experiments and numerical simulation is observed. This study is a first step for more thorough investigations in the fluid structure interaction field

Cavitation Influence on Kármán Vortex Shedding and Induced Hydrofoil Vibrations

The present study deals with the shedding process of the von Kármán vortices at the trailing edge of a 2D hydrofoil at high Reynolds number. This research focuses mainly on the effects of cavitation and fluid-structure interaction on the mechanism of the vortex generation. The vortex shedding frequency, derived from the flow-induced vibration measurement, is found to follow the Strouhal law provided that no hydrofoil resonance frequencies are excited, i.e., lock-off. For such a regime, the von Kármán vortices exhibit strong spanwise 3D instabilities and the cavitation inception index is linearly dependent on the square root of the Reynolds number. In the case of resonance, the vortex shedding frequency is locked onto the hydrofoil eigenfrequency and the spatial coherence is enhanced with a quasi-2D shape. The measurements of the hydrofoil wall velocity amplitude and phase reveal the first torsion eigenmotion. In this case, the cavitation inception index is found to be significantly increased compared to lock-off conditions. It makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index, a new correlation relationship that encompasses the entire range of Reynolds numbers, including both the lock-off and the lock-in cases, is proposed and validated. In contrast to the earlier models, the new correlation takes into account the trailing edge displacement velocity. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. This effect is important in the context of the fluid-structure interaction, since it implies that the velocity of the hydrofoil trailing edge increases the fluctuating forces on the body. It is also demonstrated that cavitation developing in the vortex street cannot be considered as a passive agent for the turbulent wake flow. In fact, 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 streamwise vortex spacing. In addition, a significant increase of the vortex-induced vibration level is found at cavitation onset. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation

Cavitation effects on fluid-structure interaction in the case of a 2D hydrofoil

In the present study, we have carried out an experimental investigation on the fluid-structure interaction caused by Karman vortices in the wake of a truncated 2D hydrofoil. The instrumentation involves a high frequency accelerometer and high speed visualisation. The mechanical response of the hydrofoil to the hydrodynamic excitation is monitored with the help of a portable digital vibrometer. Moreover, a specific optical device is developed to investigate the dynamic of the cavitating wake. The survey of the generation frequency of the Karman vortices with respect to the flow velocity reveals a Strouhal behaviour and three resonances of the hydrofoil. Out of hydro-elastic coupling conditions, the observation of the vortex structures reveals a strong 3D pattern despite the fact that the hydrofoil is 2D. The maximum fluid-structure interaction occurs for the torsional mode where lock-in is observed for upstream velocities ranging from 11 to 13 m/s. In this case, the vortices exhibit a 2D structure. The cavitation occurrence within the core of Karman vortices leads to a significant increase of their generation frequency. We have observed that hydrofoil resonance may be whether avoided or triggered by cavitation development. The study of the Karman vortices dynamic reveals that their advection velocity increases (4%) with the development of the wake cavitation meanwhile their streamwise spacing decreases

FEDSM2005-77477 CAVITATION EFFECTS ON FLUID STRUCTURE INTERACTION IN THE CASE OF A 2D HYDROFOIL

ABSTRACT In the present study, we have carried out an experimental investigation on the fluid-structure interaction caused by Karman vortices in the wake of a truncated 2D hydrofoil. The instrumentation involves a high frequency accelerometer and high speed visualisation. The mechanical response of the hydrofoil to the hydrodynamic excitation is monitored with the help of a portable digital vibrometer. Moreover, a specific optical device is developed to investigate the dynamic of the cavitating wake. The survey of the generation frequency of the Karman vortices with respect to the flow velocity reveals a Strouhal behaviour and three resonances of the hydrofoil. Out of hydro-elastic coupling conditions, the observation of the vortex structures reveals a strong 3D pattern despite the fact that the hydrofoil is 2D. The maximum fluid-structure interaction occurs for the torsional mode where lock-in is observed for upstream velocities ranging from 11 to 13 m/s. In this case, the vortices exhibit a 2D structure. The cavitation occurrence within the core of Karman vortices leads to a significant increase of their generation frequency. We have observed that hydrofoil resonance may be whether avoided or triggered by cavitation development. The study of the Karman vortices dynamic reveals that their advection velocity increases (4%) with the development of the wake cavitation meanwhile their streamwise spacing decreases