Singular properties of flow separation as a real cause of cavitation inception

Abstract

At the very beginning stage of cavitation an infinitesimal cavity suddenly emerges and rapidly grows on the wall close to the point of flow separation. The present paper intends to make clear the mechanism of this peculiar phenomenon previously discovered by the present authors in oil flows, by experimentally examining the characteristics of a separating flow. First, pressure and temperature distributions along a cylindrical smooth wall in an oil flow were measured across the point of separation, which has proved that both tensile stress and heat are really generated near the separation point. Second, a laminar air flow passing over a sharp edge was inspected by particle image velocimetry (PIV). The inspection of the separating flow based on the PIV data has revealed that fluid particle near the separation point is not only drastically stretched but also intensely sheared along the separation streamline. With all the results integrated, it has been concluded that the singular structure of a separating flow is the real cause of both tensile stress and heat generation. When thus generated tension exceeds a threshold, it possibly breaks the adhesion between the liquid and the wall, leaving a microscopic rift on the interface that would grow to a visible cavity.

Numerical and experimental investigation on the development of cavitation in a centrifugal pump

“…For the purpose, various kinds of methods have been adopted to research the development of cavitation in hydraulic machineries, including numerical simulations and experimental investigations. [2][3][4][5][6][7][8] With the development of cavitation, Tsujimoto et al divided the process of cavitation into cavitation surge, rotating cavitation, and asymmetric cavitation, and explained the mechanisms of the phenomenon, as well as the characteristics of those instabilities caused by cavitation, based on two-dimensional stability analyses. [9][10][11] The most common method used to identify the presence of cavitation was based on the observations of the drop in head and 3% head drop from noncavitation was regarded as the critical cavitation.…”

“…For the purpose, various kinds of methods have been adopted to research the development of cavitation in hydraulic machineries, including numerical simulations and experimental investigations. 2–8…”

Visual experimental investigation on the stability of pressure regulating poppet valve

“…A large tensile strain rate near the valve port causes cavitation when the poppet moves to the lowest valve lift. 21 In addition, the cavitation at the valve port appears although the vibration amplitude of poppet is small with a flow rate of 2.4–3.0 L/min in experiment of N-S21-F15; this is because the valve cavity pressure is higher due to the larger flow rate. In the experiment of N-S21-B10, the valve cavity pressure is higher than 3.2 MPa when the flow rate is greater than 3.0 L/min, and due to the higher pressure, the cavitation occurs at the end of the valve port.…”

“…Washio 20–22 used a high-speed camera to observe the flow in the hydraulic valve. It was found that the periodic shedding of the bubble at the valve port caused the same period pressure fluctuation of the hydraulic valve.…”

Attached cavitation in laminar separations within a transition to unsteadiness

“…The inception of cavitation is commonly associated in the literature 25 to a nuclei which fills with vapor when it experiences a pressure drop below the liquid vapor pressure. Washio, Takahashi, and Yoshimori, 26 Washio et al, 27 and Washio, Kikui, and Takahashi 28 observed vaporous cavitation inception in viscous mineral oil separated flows and proposed another inception mechanism which does not depend on the presence of a nuclei. According to their studies, vapor cavities can emerge close to a separation point on the wall where the liquid undergoes high tensions which can, when the threshold is exceeded, induce a microscopic rift between the liquid and the wall leading to the emergence of a cavity.…”