Tomographic PIV measurement of a re-entrant jet in a cavitating venturi

Partial cavitation occurs when low-pressure regions caused by separated shear layers are filled with vapours. Partial cavitation is inherently unsteady and leads to periodic cloud shedding. The periodically generated re-entrant jet travelling beneath the vapour cavity is considered as one of the mechanisms responsible for the periodic cloud shedding (Callenaere et al. (2001)). However, the exact physical mechanism that drives the shedding remains unclear. The re-entrant flow exists as a thin liquid film wedged between the wall and the vapour cavity. The flow in this thin film is generally assumed to move with the same order of magnitude as the bulk flow, yet in the opposite direction. There have been several attempts to measure the velocity of the re-entrant flow to get insight into the physics of re-entrant flow and its contribution to cloud shedding. However, the flow topology of the re-entrant jet poses a major challenge to experimentally study it. The unsteady nature of the flow and the opacity of the cavitation cloud adds to the further complexity. In this work, we show that tomographic PIV (Elsinga et al. (2006)) can be extended to exploit the flow topology to accurately measure the velocity and thickness of the re-entrant flow. This in turn provides better insight into the role of re-entrant flow in periodic cloud shedding.

Influence of Rectifier Nozzles on the Flow Distribution Characteristics of Parallel Pipes

The inhomogeneous distribution of parallel pipe flow leads to difficulty in the efficient and reliable operation of fluid power equipment. In view of this, a new type of rectifier nozzle has been proposed in parallel pipelines. Numerical simulation and experimental studies were used to reveal the influence of the rectification nozzle on the flow distribution characteristics. The hydraulic characteristics of the parallel pipelines with and without rectifier nozzles were compared and analyzed. The effects of the temperature and inlet flow on the flow uniformity were studied. The results showed that the initial temperature had little effect on the flow distribution of parallel pipelines, and the flow rates of the branches were not much different. The inlet flow had great influence on the distribution characteristics of the parallel pipelines, but the rectifier nozzles changed the local resistance structure and pressure distribution at the shunt, thereby improving the non-uniformity of the flow distribution of the parallel pipelines, and the maximum difference between the two pipes was optimized from 28.89 t/h (20.3%) to 2.2 t/h (1.5%). The rectifying nozzle could distort the flow field of each branch during the split, making the distribution of flow rate and flow state more uniform and stable. At high inlet fluid temperatures, cavitation could occur under the pressure drop of the nozzle, and partial cavitation had little effect on the flow distribution.

Estimation of Ventilation Impact on Fluid–Structure Interaction for Partially Cavitating Hydrofoils

Fluid–structure interaction is analyzed for natural and ventilated partial cavitation of conventional hydrofoils. Quasi-linear two-dimensional (2D) analysis of ideal fluid incompressible flow outside the cavity is coupled with one-dimensional analysis of compressible flows within the cavity and with analysis of hydrofoil bending vibration under impact of periodical oscillations of hydrodynamic forces. The old experimental data for hydrofoils Clark Y-11.7% and NACA 0015 are used for validation of this coupling. Estimations based on obtained computational results show that the force oscillations can be significantly mitigated by ventilation, whereas the ventilation effect on the cavity volume oscillation is less significant. The presented estimations also show that ventilation can suppress generation of shock waves in the cavity tail and affect their propagation.