Mind The Gap:Tip Leakage Vortex Dynamics and Cavitation in Axial Turbines

The tip leakage vortex (TLV), which develops in the clearance between the blade tip and the casing of axial turbomachines, appears in many industrial applications, such as air transportation, space rockets and hydraulic machines. In the latter, cavitation may develop in the core of the TLV, often leading to severe erosion of the runner blades and the casing. Despite the progress achieved in understanding and controlling the dynamics of this particular flow, many associated phenomena are still not sufficiently explored. It remains, for instance, unclear how the clearance size is related to the occurrence of cavitation in the TLV. The present work contributes to this research by assessing the effect of the clearance size on the TLV intensity and dynamics in a simplified case study. The vortex is generated by a two dimensional generic blade in a water tunnel, while the clearance between the blade tip and the wall is varied. The properties of the TLV are established with the help of stereo-PIV and flow visualizations for a wide range of incidence angles, inlet velocities and tip clearances. The measurements clearly reveal the existence of a specific tip clearance for which the vortex intensity is at its maximum and most prone to generate cavitation. By introducing a new dimensionless coefficient $\tau/\Gamma^*_{\infty}$, where $\tau$ is the normalized clearance and $\Gamma^*_{\infty}$ is the normalized circulation in the unconfined case, it is established that the TLV circulation reaches a peak intensity for $\tau/\Gamma^*_{\infty}\approx0.2$, the amplitude of which is in average 45 ($\pm$10) \% higher than in the unconfined case, regardless of the operating conditions. The change in the vortex structure due to cavitation occurrence is also investigated in a different case study by means of PIV using fluorescent seeding particles. A vortex is generated by an elliptical hydrofoil and the velocity field outside the vapor phase is compared with the one in cavitation-free conditions. It is found that the cavitation does not change the vortex circulation, since the tangential velocity distribution of the cavitating vortex is identical to the non-cavitating vortex far from the vapor core. The tangential velocity close to the vapor core is however lower than in cavitation-free conditions. Moreover, the fluid is in solid body rotation in the vicinity of the liquid-gas interface. The alteration of the clearance geometry with shallow grooves to manipulate the gap flow and control the TLV intensity is evaluated in the simplified case study. The cavitation in the TLV and in the clearance region is significantly reduced with grooves located near the foil leading edge, oriented at 45$^{\circ}$ or 90$^{\circ}$ relative to the incoming flow. This result paves the way for further investigations, which may ultimately lead to TLV cavitation mitigation in axial turbines

Surface wave dynamics in orbital shaken cylindrical containers

Be it to aerate a glass of wine before tasting, to accelerate a chemical reaction or to cultivate cells in suspension, the "swirling" (or orbital shaking) of a container ensures good mixing and gas exchange in an efficient and simple way. Despite being used in a large range of applications this intuitive motion is far from being understood and presents a richness of patterns and behaviors which has not yet been reported. The present research charts the evolution of the waves with the operating parameters identifying a large variety of patterns, ranging from single and multiple crested waves to breaking waves. Free surface and velocity fields measurements are compared to a potential sloshing model, highlighting the existence of various flow regimes. Our research assesses the importance of the modal response of the shaken liquids, laying the foundations for a rigorous mixing optimization of the orbital agitation in its applications. Copyright (2014) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Physics of Fluids 26, 052104 (2014) and may be found at http://dx.doi.org/10.1063/1.4874612Comment: 19 pages, 7 figure

Obstacle-induced spiral vortex breakdown

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

Draft tube discharge fluctuation during self-sustained pressure surge: fluorescent particle image velocimetry in two-phase flow

Hydraulic machines play an increasingly important role in providing a secondary energy reserve for the integration of renewable energy sources in the existing power grid. This requires a significant extension of their usual operating range, involving the presence of cavitating flow regimes in the draft tube. At overload conditions, the self-sustained oscillation of a large cavity at the runner outlet, called vortex rope, generates violent periodic pressure pulsations. In an effort to better understand the nature of this unstable behavior and its interaction with the surrounding hydraulic and mechanical system, the flow leaving the runner is investigated by means of Particle Image Velocimetry. The measurements are performed in the draft tube cone of a reduced scale model of a Francis turbine. A cost-effective method for the in-house production of fluorescent seeding material is developed and described, based on off-the-shelf polyamide particles and Rhodamine-B dye. Velocity profiles are obtained at three streamwise positions in the draft tube cone and the corresponding discharge variation in presence of the vortex rope is calculated. The results suggest that 5-10% of the discharge in the draft tube cone is passing inside the vortex rope

Effect of trailing edge shape on hydrodynamic damping for a hydrofoil

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

Obstacle-induced spiral vortex breakdown

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

The effect of cavitation on the natural frequencies of a hydrofoil

Postprint (published version

Mind the gap: a new insight into the tip leakage vortex using stereo-PIV

The tip leakage vortex (TLV), which develops in the clearance between the rotor and the stator of axial hydro turbines, has been studied for decades. Yet, many associated phenomena are still not understood. For instance, it remains unclear how the clearance size is related to the occurrence of cavitation in the vortex, which can lead to severe erosion. Experiments are here carried out on the influence of the clearance size on the tip vortex structure in a simplified case study. A NACA0009 hydrofoil is used as a generic blade in a water tunnel while the clearance between the blade tip and the wall is varied. The 3D velocity fields are measured using Stereo Particle Image Velocimetry (SPIV)in three planes located downstream of the hydrofoil for different values of the upstream velocity, the incidence angle and a large number of tip clearances. The influence of the flow conditions on the structure of the TLV is described through changes in the vortex intensity, core axial flow, vortex center position and wandering motion amplitude. Moreover, high-speed visualizations are used to highlight the vortex core trajectory and clearance flow alteration, turning into a wall jet as the tip clearance is reduced. The measurements clearly reveal the existence of a specific tip clearance for which the vortex strength is maximum and most prone to generating cavitation