Study On Tip Leakage Vortex Cavitation Characteristics Of 3D Hydrofoil And The Suppressing Effect Of Double-Curved Hole Structure

Tip Leakage Vortex(TLV)is a kind of flow phenomenon ubiquitous in the area near the tip of axial flow rotating machinery.Tip Leakage Vortex will reduce the performance and efficiency of the fluid machinery,and also induce the tip of the blade.Instability of regional flow cavitation.In this paper,an in-depth study on the leakage vortex and control of the hydrofoil tip is carried out to provide theoretical guidance for the design of axial flow hydraulic machinery.(1)Considering the effect of the vortex viscosity model with rotation and curvature correction on the suppression of eddy viscosity dissipation,analyze the applicability of the existing rotation and curvature correction vortex viscosity model(SST-CC model)to the prediction of gap leakage vortex and its cavitation.The results show that the velocity distribution and vortex core position of the numerical simulation are almost the same as those in the experiment.Moreover,the velocity gradient of the numerical results is also consistent with the experiment.The SST-CC turbulence model predicts the rotation effect of the leakage flow more detailed and accurate than the SST turbulence model.It further verifies the accuracy and validity of the numerical results.(2)Numerical study on the mechanism of three different tip clearances and tip leakage flow and tip leakage vortex cavitation.The results show that the initial point of the tip leakage vortex under the 2mm gap is about 0.1C,and the vortex strength is relatively large,occupying about 50%of the tip front edge area.In the downstream development,affected by the mainstream,the vortex intensity shows a downward trend.As the gap increases,the initial point of the leakage vortex will move back.V_x around the leakage vortex is symmetrically distributed in y,and V_y is symmetrically distributed in the x direction,showing the trajectory and regular shape of the leakage vortex.With the action of the mainstream,the downstream vortex intensity decreases.When the gap is 2mm,the vortex core pressure at the leading edge of the blade tip is the lowest.When the gap of 3mm and4mm,the minimum point of the vortex core pressure will move back.From the distribution of the vortex core pressure,it can be concluded that the tip leakage vortex has a strong vortex at the leading edge.When the gap is small,the pressure drop position of the vortex core is closer to the leading edge,and when the gap is large,the pressure drop position of the vortex core shifts backward,because the initial position of the leakage vortex is closer to the leading edge when the gap is small.The three schemes all have obvious vortex cavitation patterns.The position of cavitation is consistent with that of the leakage vortex,and the cavitation initiation point of the leakage vortex is roughly the same as the initiation point of the leakage vortex.When the gap increases,the form of cavitation and the volume of cavitation also gradually increase.(3)According to the development mechanism of hydrofoil tip leakage vortex,a new type of tip structure—double-curved-hole(DC hole)is proposed.Due to the use of the double-curved hole structure,driven by a large pressure difference,the jet passing through the double-curved hole will directly impact the leakage flow of the blade tip,thereby destroying the structure of the leakage vortex.Under this condition,the length of the tip leakage vortex is reduced by about 50%,and the maximum reduction of the vortex equivalent surface area of the three schemes is 50.65%.The position of the hole is one of the important factors of the suppression effect.The hyperbolic hole located at the initial point of the leakage vortex has the best suppression effect on the leakage vortex.Therefore,under different gaps,the outlet position of the hyperbolic hole should be designed at the best position of the tip leakage vortex initiation point.Under the interaction of the jet and the main flow,the leakage vortex will be destroyed and separated into weakened leakage vortex(WTLV),The new secondary tip leakage vortex(NSTLV)and induced vortex,hole separation vortex(HSV).From the perspective of vortex dynamics,the vorticity first drops sharply along the tip leakage vortex trajectory,and then slowly drops to a lower level.The pressure evolution process along the trajectory of the leakage vortex shows an almost opposite law to the vorticity.The vortex core pressures of the three suppression schemes are generally higher than that of the original scheme.(4)The three DC hole schemes can significantly improve the distribution and vortex cavitation of the TLV low pressure area.The pressure equivalent surface area of the M1hyperbolic pore solution is reduced by 53.2%.As the position of the hyperbolic hole continues to move to the trailing edge,the improvement effect of the M2 and M3 vortex low pressure area gradually weakens.The cavitation performance of the three hyperbolic hole schemes is better than that of the original scheme,and has excellent suppression effects on leakage vortex cavitation under different gaps.The maximum cavity surface area under the three gaps was reduced by 49.6%,72.8%,and 77.7%,respectively.The maximum drop in lift coefficient of different hyperbolic hole schemes under 2mm and 4mm gaps is 0.56%and0.1%,respectively.The hyperbolic hole structure has little effect on the lift of the airfoil,and arranging the hyperbolic hole structure on the top of the blade will not change the lift performance of the hydrofoil.However,from the perspective of the effect of suppressing leakage vortex and cavitation,the hyperbolic hole scheme is an excellent solution.