Study On Mechanisms Of Cavitating And Oscillating Jet Induced By Axis-symmetric Cavity

The self-sustained oscillations induced by flow past various cavities had beenobserved widely in nature and fluid engineering, which also is one essential researchtopic in fluid mechanics. Taking advantage of such principle, the cavitating andoscillating jet generated by water jet past axis-symmetric cavity had been applied inwater jet engineering to improve working efficiency of water jet. However, the flowcharacteristics and mechanisms of this phenomenon have not been understoodcompletely. The experimental study for flow patterns of water jet in axis-symmetriccavity is the fundamental method to solve this problem. In this paper, both experimentaland numerical methods were used to study the flow mechanisms of cavitating andoscillating jet induced by axis-symmetric cavity.Based on experimental observations and measurements of cavitating andoscillating jet flow in axis-symmetric cavity, it is observed that there are two states inaxis-symmetric cavity, i.e. non-submerged jet and submerged jet; and the low frequencyoscillation occurs at non-submerged jet state. The geometric condition, Reynoldsnumber (Re) range, and non-dimensional frequency range for occurences of lowfrequency oscillation are obtained by summaring the experimental data. The mechanismof the dynamic balance between jet entrainment and impinging shear layer feedback offluid mass in cavity is presented to explain low frequency oscillation of jet. Forsubmerged jet state, the variation of flow patterns with Reynolds numbers inaxis-symmetric cavity show four different stages, namely cavitation inception anddevelopment in jet shear layer, cavitation mutation, relative steady cavitation state andunsteady cavitation state. The critical Re of cavitation mutation occurrence in Reincreasing process are larger than ones in Re decreasing process, which exhibithysteresis effects between them. And the critical Re varies linearly with cavity length;the slope of Re of increasing process is larger than that of decreasing process.Consistent with boundary conditions in experimental measurement, large eddysimulation (LES) and Schnerr-Sauer (S-S) cavitation model are employed to simulateinception cavitation flow at condition Re=2×105and σ=0.59. The computational resultsshow good agreement with experimental results. The cavitating bubble is slender, just like “rice grain shape”, and the collapse of bubble induces the instantaneous andsignificantly pressure fluctuation in cavity. The shear layer structure in cavity exhibitsdeflection and non-linear diffusion due to round imping shear layer action. Thestrongest mixing process zone locates in near half width of5≤x/d1≤7domain.In order to further insight into self-sustained oscillation flow, large eddy simulationof impinging shear layer flow of rectangular cavity is performed. The presentpredictions correctively capture the primary frequency variation of pressure fluctuationsin shear layer. Three mechanisms inuce the self-sustained oscillation of impingingshear layer flow: the first one is vortex-impinging edge intreraction; the second one isthe centrifugal instability of large vortex structure in cavity; the last one is the existenceof standing wave in cavity. The transition from linear region to nonlinear region ofshear layer occurs at the same position x/H=0.7for both conditions. And the slopes ofthe linear region vary with cavity length solely, showing decreasing slope with increaseof the cavity length.