We document experimentally four different interactions of a laser-induced bubble and a free-settling particle, with different combinations of the geometric and physical parameters of the system. Our force balance model shows that four nondimensional factors involving the particle radius $a$, the maximum bubble radius $R_{\max }$, the initial separation distance $l_0$ between the particle center and the bubble center, the fluid viscosity ${\mu }_f$, and the particle and fluid densities ${\rho }_p$ and ${\rho }_f$, respectively, in detail $l_0/R_{\max }$, $a/R_{\max }$, ${\rho }_p/{\rho }_f$, and ${\mu }^{*}={\mu }_f{T}_c/{\rho }_f{R}_{\max }^2$, where $T_c=0.915R_{\max }\sqrt{{\rho }_f/(p_{\infty }-p_v)}$, influence the particle-bubble dynamics, and reasonably predict the maximum particle velocity and the limiting condition when the particle starts to “bounce off” the bubble during bubble growth. In particular, we also discover the high-speed ejection of the particle, and a cavity behind the particle, in cases when initially the particle is in very close proximity to the bubble. These observations offer new insights into the causal mechanism for the enhanced cavitation erosion in silt-laden water.

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.

Leading-edge protuberances on airfoils or hydrofoils have been considered as a viable passive method for flow separation control recently. In this article, the hydrodynamic performance of a NACA 63₄-021 (baseline) foil and two modified foils with leading-edge protuberances was numerically investigated using the Spalart–Allmaras turbulence model. It was found that modified foils performed worse than the baseline foil at pre-stall angles, while the lift coefficients at high angles of attack of the modified foils were increased. Both the deterioration of pre-stall and the improvement of post-stall performance were enhanced with larger amplitude of protuberance. Near-wall flow visualizations showed that the leading-edge protuberances worked in pairs at high angles of attack, producing different forms of streamwise vortices. An attached flow along some valley sections was observed, leading to a higher local lift coefficient at post-stall angles. The leading-edge protuberances were considered as sharing a similar mechanism as delta wings, increasing nonlinear lift at large angles of attack. The specific stall characteristics of this leading-edge modification could provide some guidelines for the design of some special hydrofoils or airfoils.

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.