Recently leading-edge protuberances have attracted great attention as a passive method for separation control. In this paper, the effect of multiple leading-edge protuberances on the performance of a two-dimensional airfoil is investigated through experimental measurement of aerodynamic forces, surface tuft visualization, and numerical simulation. In contrast to the sharp stall of the baseline airfoil with large hysteresis effect during AOA (angle of attack) increasing and decreasing, the stall process of the modified airfoil with leading-edge protuberances is gentle and stable. Flow visualization revealed that the flow past each protuberance is periodic and symmetric at small AOAs. Streamwise vortices are generated on the shoulders of the protuberance, leading to a larger separation around the valley sections and a longer attachment along the peak sections. When some critical AOA is exceeded, aperiodic and asymmetric flow patterns occur on the protuberances at different spanwise positions, with leading-edge separation on some of the valley sections and non-stalled condition elsewhere. A combined mechanism, involving both the compartmentalization effect of the slender momentum-enhanced attached flows on the protuberance peaks and the downwash effect of the local stalled region with low circulation, is proposed to explain the generation of the aperiodic flow patterns. The influence of the number of protuberances is also investigated, which shows similar aperiodic flow patterns. The distance between the neighboring local stalled valley sections is found to be in the range of 4–7 times the protuberance wavelength. According to the proposed mechanism, it is speculated that the distance between the neighboring local stalled valley sections is inclined to increase with a smaller protuberance amplitude or at a larger AOA.

The removal of surface-attached particles with cavitation bubbles is usually attributed to the jetting or shear stresses when bubbles collapse. In this Letter, we report an unexpected phenomenon that millimeter-sized spherical particles made of heavy metals (e.g., stainless steel), when initially resting on a fixed rigid substrate, are suddenly accelerated like projectiles through the production of nearby laser-induced cavitation bubbles of similar sizes. We show experimentally and theoretically that the motion of a particle with radius $R_p$ is determined by the maximum bubble radius $R_{b,\max }$, the initial distance from the laser focus to the center of the particle $L_0$, and the initial azimuth angle ${\phi }_0$. We identify two dominant regimes for the particle’s sudden acceleration, namely, the unsteady liquid inertia dominated regime and the bubble contact dominated regime, determined by $R_{b,\max }{R}_p/L_0^2$. We find the nondimensional maximum vertical displacement of the particle follows the fourth power and the square power scaling laws for respective regimes, which is consistent with the experimental results. Our findings can be applied to nonintrusive particle manipulation from solid substrates in a liquid.

Owing to the complexity of the load rejection process, the accompanied complex low frequency pressure fluctuations and their sources have not been determined. Herein, the load rejection transient process of a pump-turbine was simulated with a three-dimensional (3-D) large eddy simulation method and a dynamic mesh technology. The simulation results were validated against the experimental data. Through the joint time-frequency analysis of simulated pressure, the complex low frequency pressure fluctuation components were captured, which are generally lower than the rated rotational frequency of the runner. A comprehensive formation mechanism of these complex low frequency pressure fluctuation components was attributed to three transient hydraulic phenomena and their interactions: the water hammer in the pump-turbine, local reverse flow near the runner inlet, and 3-D blocking water ring in the vaneless space. These findings facilitate the elucidation and elimination of complex low frequency high amplitude pressure fluctuations during the load rejection transient process.

The positive slope on the pump performance curve of pump-turbines suggests potential operational instabilities in pump mode. Previous research has indicated that the increase of the hydraulic loss caused by sudden changes of flow patterns in pump-turbines is responsible for the positive slope, however its detailed flow mechanism is still unclear. A low specific speed model pump-turbine was numerically investigated against experiments in the present study, by applying unsteady RANS (Reynolds-Averaged Navier–Stokes equations) simulations with a v²-f turbulence model. The mechanism of occurrence of the positive slope on the pump performance curve was discussed regarding the energy balance, as this region appears when the value of $\frac{\partial P_u}{\partial Q}$ is larger than the critical value $\frac{P_u}{Q}$ . An unsteady local loss analysis, derived from the energy equation, was conducted to illustrate the contribution of local flow patterns to the loss in corresponding hydraulic components. The variation of the kinetic energy of the mean flow was taken into account for the first time so that this method can be applied to highly time dependent flow patterns, e.g., a rotating stall in the present study. The investigations on the flow patterns revealed that some guide vane channels stalled with a larger discharge coefficient than the positive slope region. Several guide vane channels near the stalled channels were stalling with minor decrease of the discharge coefficient, leading to sudden increases of the input power and the loss. When the discharge coefficient slightly decreased in further, the pump-turbine operated into the positive slope region, and the rotating stall with 3 stall cells appeared, proven by the FFT (Fast Fourier Transform) and cross-phase analysis on the pressure fluctuations.

In this paper, the thermodynamic effect is systematically studied by Venturi cavitation in a blow-down type tunnel for the first time, using water at temperatures up to relatively high levels and at controlled dissolved gas contents in the supply reservoir (measured by dissolved oxygen, DO). The mean attached cavity length $⟨⟨L_{cav}⟩⟩$ is chosen to reveal the thermodynamic effect, and the cavitation characteristics are analyzed from the experiments. With an increase in the thermodynamic parameter Σ^*, a decrease in $⟨⟨L_{cav}⟩⟩$ vs the pressure recovery number κ is observed, which is consistent with suppression of cavitation by the thermodynamic effect, but the decrease is related not only to this effect. Based on the experimental results, a model is presented of the attached cavity cloud that develops from the Venturi throat. It is found that either the length of this cloud oscillates stably around a mean value or the cloud breaks regularly at some upstream position, allowing that a detached cavity cloud is shed, flows downstream, and collapses while the remaining attached cloud regenerates. Applying this model to experimental results obtained first with cold water, then with hot water, we find that when the mean length of the attached cavity cloud oscillates stably, temperature increase causes reduction of the mean cavitation length. This is interpreted to be a consequence of the thermodynamic effect. When detachment of large cavity clouds occurs, the mean length is increased at temperature increase. This is a consequence of cloud configuration changes being superposed on changes due to the thermodynamic effect. These observations explain conflicting results reported for attached cavity clouds in relation to the thermodynamic effect. The gas content in the water is found to be without significance within the range of DO tested.

Geometrical modification of airfoils by leading-edge protuberances has attracted plenty of attention as a passive separation control method. In this research, the effect of a single leading-edge protuberance with different amplitudes on the performance of a two-dimensional airfoil (NACA 63₄-021) is investigated through wind tunnel experiment and theoretical analysis. Force measurement experiment shows that all the modified airfoils demonstrate a special two-step stall performance. When the first step of stall occurs, the lift coefficient of the modified airfoils drops to a medium value between the pre-stall and post-stall lift coefficients of the baseline airfoil and keeps nearly constant within a range of AOA (angle of attack). Tuft visualization reveals that during the first step of stall, the flow pattern on different sides of the protuberance is distinctly different, respectively, showing leading-edge separation and trailing-edge separation, which are compartmentalized by the attached flow originated from the peak of the protuberance. The flow control mechanism is contributed to the combined effect of the early occurrence of the leading-edge separation around the protuberance root, the inherent hysteretic performance, and the flow compartmentalization effect. A theoretical model based on the lifting line theory, which takes the amendment of the hysteresis and compartmentalization effects into consideration, is proposed to analyze the performance of the modified airfoils. The sectional and overall performances by theoretical analysis are highly consistent with the tendency of the experiment results. Based on the theoretical model, the spanwise circulation gradient induced by the protuberance is considered to be responsible for the earlier separation around the protuberance roots at pre-stall AOAs. Further investigation on the effect of the geometrical parameters of the protuberance reveals that the amplitude-to-wavelength ratio plays a dominant role on the performance of the modified airfoils. The special flow phenomenon induced by the single leading-edge protuberance discovered in this research and the proposed mechanism provide an essential point of view for fully understanding the combined mechanism of multiple protuberances.