Study On The Mechanism Of Active Jet Suppressing Cavitation On The Hydrofoil

Cavitation is a common phenomenon of hydrodynamic machinery,which involves turbulence,compressible characteristic,multiphase flow,phase transition of mass transfer and other complex fluid phenomenon.Cavitation will cause the dynamic performance degradation,vibration,noise and fatigue damage of machinery,which makes people have to pay more attention to this complex hydrodynamics process and study the suppresssiom method.In this paper,based on the active control strategy,the suction surface of the hydrofoil?NACA66?is perforated and the jet is carried out.The cavitation mechanism of the flow field and the suppression mechanism of this scheme are studied by the method of experiment and numerical simulation.Therefore,the results of this paper have certain guidance and reference significance for the study of mixing effect of the flow field around and the design of pump blades.The main conclusions are as follows:The specific cavity morphology and evolution process of sheet cavitation and cloud cavitation are studied by experiments.The cavity has the characteristics of unsteady evolution.With the growth and retraction of the attached cavity,the shedding cavity has the dynamic behavior of falling,rolling and collapsing over the hydrofoil.The gray difference between the two shows that the vapor volume fraction is quite different from each other.However,the unsteady development of cloud cavity is suppressed after the jet.The area and the falling off scale of the shedding cavity are decreased.In order to study the suppression mechanism,the particle image velocimetry technology is used to study the velocity distribution of the flow field.It is found that the turbulence fluctuation intensity of the flow field after the jet is reduced near the jet hole,which improves the stability of the whole flow field.Due to the limited of the parameters that can be extracted from the experiment,based on the experimental results,the two-dimensional and three-dimensional numerical simulation of the flow field is further carried out.The characteristics of cavity are simulated when the angle of attack changes based on the same dimensionless criterion number?/2?firstly.It is found that there is a large-scaled flow separation in the case of big angle of attack,and the influence of re-entrant jet on cavity regime is very strong.Therefore,the separation characteristics of the boundary layer under the condition of high angle of attack are studied secondly.It is found that the shear effect caused by the flow around exists at the leading edge of the hydrofoil,while the tip vortex exists at the trailing edge of the hydrofoil.Because of the pressure difference between the suction surface and the pressure surface,the fluid forms a small range of separated flow.The re-entrant jet impinges the attached cavity on the leading edge of the hydrofoil,resulting in the sudden rise of boundary layer contour.After the jet is adopted,the jet shears and blocks the re-entrant jet,which makes the contour of the boundary layer and the whole flow field tend to be stable.The velocity separation of three-dimensional cavitating flow field has two regimes:re-entrant jet and lateral jet.The effect of jet on flow separation is not only that it weakens the strength of the re-entrant jet,blocks the advance of the re-entrant jet,but also that it eliminates the lateral jet.Therefore,the cavity has spanwise symmetry.Vortex dominates the dynamic behavior of the cavity's unsteady evolution.The vortex structure of the flow field is studied by the vortex identification method of Q criterion and?₂ criterion.It is found that the jet can change the location and scale of the shedding cavity.The jet provides energy for the boundary layer by dissipating the small eddies in the low-pressure region of the flow field directly,and thus avoids the large-scaled vortices collapsing at the trailing edge.