An Experimental Study of Unsteady Partial Cavitation

Abstract

Unsteady partial cavitation can cause damage to hydraulic machinery and understanding it requires knowledge of the basic physics involved. This paper presents the main results of a research program based on wall-pressure measurements aimed at studying unsteadiness in partial cavitation. Several features have been pointed out. For cavity lengths that did not exceed half the foil chord the cavity was stated to be stable. At the cavity closure a peak of pressure fluctuations was recorded originating from local cavity unsteadiness in the closure region at a frequency depending on the cavity length. Conversely, cavities larger than half the foil chord were stated to be unstable. They were characterized by a cavity growth/destabilization cycle settled at a frequency lower than the previous ones. During cavity growth, the closure region fluctuated more and pressure fluctuations traveling in the cavity wake were detected. When the cavity was half the foil chord, cavity growth was slowed down and counterbalanced by large vapor cloud shedding. When the cavity length was maximum (l/c∼0.7–0.8), it was strongly destabilized. The reason for such destabilization is discussed at the end of the paper. It is widely believed that the cavity instability originates from a process involving the shedding of vapor clouds during cavity growth, a re-entrant jet, and a shock wave phenomenon due to the collapse of a large cloud cavitation.

Numerical Investigation for Steady and Unsteady Cavitating Flows

“…These excellent results obtained for 2-D cavitating flow confirm the correct assumptions of the proposed numerical model in terms of vaporization and condensation processes, and to verify it's performance with 3-D considerations, we present in the next some numerical results compared to experimental measurement [16] for cavitating flow around hydrofoil NACA66 (Fig 10). In this section, a numerical simulation was performed to further study the performance of the proposed model and unsteady three-dimensional, the choice is justified because of the availability of experimental measurements.…”

“…This different ways of cavities shedding have an important influence on the pressure field at the hydrofoil wall, and there are characterized by the temporal evolution of the lift coefficient Cl compared to experimental result [16] …”

Investigation of Cavitation Models for Steady and Unsteady Cavitating Flow Simulation

“…The transducers locations were aligned along the chord on the suction side of the hydrofoil at mid-span, starting from the foil leading edge at a reduced coordinate of x/c = 0.1 to the trailing edge at x/c = 0.9, with increments of 0.1 c. Lift and drag were measured using a resistive gauge hydrodynamic balance with a range up to 1500 N in lift and 150 N in drag. Readers should refer to [3,44] for additional details about the rigid hydrofoil experimental setup and results. The experimental results presented in this paper are taken from [3] for cases with steady sheet cavitation and from [44] for cases with unsteady sheet/cloud cavitation.…”

“…Readers should refer to [3,44] for additional details about the rigid hydrofoil experimental setup and results. The experimental results presented in this paper are taken from [3] for cases with steady sheet cavitation and from [44] for cases with unsteady sheet/cloud cavitation.…”

“…Numerous experimental tests have been carried out for two popular cases: hydrofoils [3][4][5] and Venturi sections [6,7] to understand the physics of the cavitating flows. The characteristics of cavitation have been reported in several books and articles [8][9][10], describing various forms of cavitation where the unsteady sheet/cloud is one of the most complicated and destructive states for hydraulic machines.…”