Inhibition Of Cloud Cavitation By Placing An Obstacle On The Flat Hydrofoil

Cloud cavitation causes a lot of negative effects:vibration noise,load impact,and cavitation erosion.Therefore,it is of great significance to carry out research on how to effectively inhib-it cloud cavitation.In order to reveal the mechanism of inhibition of cloud cavitation,the flat hydrofoil is selected in the experiments of cloud cavitation.A large number of two-dimensional numerical simulations are performed for obstacles with different locations and heights to analyze the effects of inhibition.Meanwhile,three-dimensional numerical verifications of inhibition of cloud cavitation are performed by the analysis of flow field with specific cavitation number.Experiments of the cavitation on flat hydrofoil are carried out at different cavitation numbers.The high-speed camera captures the features of cavitation from top and side views,and the image processing methods are applied to evaluate the effects of obstacle.The experimental results show three typical processes:the growth of the cavity,the formation of reflow towards to the leading edge,the shedding and collapsing of cloud cavitation in the downstream.Due to wall effects,U-type cloud cavity are detected as approaching to the trailing edge of hydrofoil.The large-scale shedding which occurs in the case without obstacles can hardly be spotted.Instead,the small-scale shedding can be observed at the rear part of the cavity.The majority of the vapor mass stays in front of the obstacle.To make a quantitative evaluation on the effects of the obstacle,the cavity length and cavity volume are presented by image processing methods.The results show a significant decrease of cavity length,and the cavity volume reduces more than 50%in the case with obstacle.On the basis of experiments,numerical simulations of cavitation are conducted on the flat hydrofoil with different locations and different heights.It shows that the evolution of cavitation matches well between experiments and numerical simulations for both with and without obstacle cases.Adding obstacle makes the cavity length and cavity thickness both decrease apparently,while different features of cavity development occur in different cases.The evolution of cavity is changed by placement of an obstacle.Indeed,the small-scale shedding of cavity is dominant in the case with obstacle,which replaces the large-scale shedding in the case without obstacle.The relationship between thickness of cavity and re-entrant jet is influenced by placing the obstacle gradually closes to the leading edge.the small-scale shedding occurs when the thickness of cavity and re-entrant jet are close to each other.The height of the obstacle plays significant role on the effects of inhibition of cavity.Oversize of obstacle promotes the production of cavity,thus it is no good for inhibition of cavitation.While the obstacle height with small size is not enough to obstruct the re-entrant jet.From the experimental and numerical results,it shows that the cavitation is inhibited by plac-ing an obstacle on the flat hydrofoil,and the mechanisms of the inhibition of cavitation are revealed.High pressure is detected at the tail of the cavity in the case without obstacle,then pressure gradi-ent leads to the formation of the re-entrant jet.The re-entrant jet moves to the leading edge,and large-scale shedding occurs.The obstacles change the pressure distributions on the surface of the hydrofoil,decrease the pressure gradient in the rear part of the cavity,weaken the strength of the re-entrant jet,and even obstruct the re-entrant jet towards to upstream.