Research On The Mechanism And Passive Control Of Attached Cavitation On Hydrofoil

Attached cavitation is a common and severe phenomenon on hydrofoils and rotating blades in fluid machinery.When the cavity length of an attached cavitation extends,cavitation becomes unstable with cloud cavitation periodically shedding.The cloud cavitation shedding induces large loading and pressure fluctuation on blades or hydrofoils,and the collapse of the cloud cavity at downstream causes severe cavitation erosion,noise,and vibration.Investigating the mechanism of attached cavitation and developing methods to control it are necessary.In this study,we studied the mechanism of attached cavitation on a 2-D NACA0015hydrofoil experimentally.The dynamic behaviors of re-entrant jet and cavity shedding were investigated using synchronous high-speed video observation and surface pressure measurement.We applied a span-wise obstable on suction surface of the hydrofoil to control the attached cavitation.The effect of obstacle position on cavitation control was investigated experimentally through the Response Surface Methodology?RSM?.In addition,we applied a row of micro counter-rotating vortex generators?VGs?at the leading edge of the hydrofoil to control the boundary layer separation and control the attached cavitation.The effect of micro VGs on attached cavitation dynamics was investigated experimentally and numerically.?1?On the smooth hydrofoil,in the partial cavity oscillation?PCO?,the sheet cavitation grew along the chord with good spanwise uniformity,and the middle-entrant jet played a dominant role in cavity shedding.Meanwhile,in the transitional cavity oscillation?TCO?,the previous shedding cavity exhibited a prohibitive effect on the growth of sheet cavitation on the hydrofoil,resulting in concave cavity closure line.Moreover,two symmetrical side-entrant jets originated at the near-wall ends and induced the two-stage shedding phenomenon.The aft and fore parts of the sheet cavitation shed separated as different forms and eventually merged into the large-scale cloud cavity.?2?A span-wise obstacle on the suction surface of the hydrofoil is an effective passive control method for cloud cavitation.The position of the obstacle significantly influences the performance of cavitation control.We derived regression equations and built response surfaces using RSM to illustrate the influence mechanism quantitatively between individual factors?obstacle position,cavitation number,and angle of attack?and cavitation dynamics response parameters?cavity length,acoustic intensity,and energy flux?.Sheet cavitation was effectively suppressed because the obstacle increased the pressure at the near-wall region.However,the obstacle induced a shear cavitation at times when its position was too close to the leading edge of the hydrofoil.Under PCO conditions,the obstacle was consistently performed well in cloud cavitation control.Three cavitation dynamics response parameters all decreased considerably.However,under TCO conditions,the obstacle could not suppress the cavitation at its location because the TCO was mainly caused by system instability.?3?Under non-cavitating conditions,the proposed micro VGs effectively suppress laminar separation.However,under cavitating conditions,even very small micro VGs within the boundary layer promote the formation of counter-rotating cavitating vortices.In comparison to the smooth hydrofoil surface?without micro VGs?,cavitation onset is shifted towards the leading edge.Additionally,classical“fingering structures”and Tollmien–Schlichting waves are no longer present.Since the onset of the cavity does no longer appear at?or close to?the laminar separation line,a novel onset mechanism is observed experimentally.The mechanism consists of stable vortex cavitation,followed by vortex break-down into bubbly structures that are finally accumulated into an attached cavity region.By reduction of the height of the micro VGs a delayed vortex break-down is found,leading to an increase in length of the cavitating vortex pattern.This allows for enhanced control on the cavity dynamics,especially with respect to the penetration depth of the re-entrant jet.?4?The effect of the height of micro VGs on the cavity length7)₍?8???8 was quantitatively evaluated by regression equations through RSM.The micro VGs increased cavity length under the experimental conditions.The counter-rotating streamwise vortices induced by micro VGs had a rectifying effect on the near-wall flow and withstood the flow disturbance in the spanwise direction.Under PCO condition,the sheet cavitation grew with a trimmed closure line,and the cloud cavity shedding had a strict periodicity with a smaller Strouhal number?St?than the smooth hydrofoil.The shedding cloud cavity showed an integral and consistent structure in the spanwise direction.The transition from PCO to TCO occurred at?2=4.5,corresponding to7?₍?8??(8 of 0.8.Under TCO condition,the concave cavity closure line of sheet cavitation on hydrofoil showed perfect symmetry and the St was nearly constant.