High-Precision Numerical Simulation On The Tip Clearance Flow And Its Cavitation In A Tur-bomachinery

In axial-flow turbomachinery,such as compressors,ship propellers,and marine propellers,there exists an inevitable clearance between the blade tip and the outer shell.Inside the clearance,the tip clearance flow of a blade is driven by the pressure difference between the pressure and the suction side,and complicated vortical structures are generated in the tip clearance region due to the interaction between the tip clearnce flow and incoming flow,leading to detrimental effects on the turbomachinery,such as the degradation of efficiency,rotational instability and so on.In hydraulic machinery,when the pressure in the tip clearance region is lower than the saturated vapor pressure of water,tip clearance cavitation occurs.Tip clearance cavitation is not only implicated in the noise generation,but also the damage to the blade due to the cavitation erosion.For the axial-flow hydraulic machinery,there exist complex multi-physics and multi-scale processes of interactions among the endwall turbulence,tip clearance flow,blade rotation and tip clearance cavitation in the tip clearance region,where the inner physics remains far from fully understood.Therefore,it is of great theoretical value and engineering guiding significance to carry out highprecision numerical simulations on the complicated physical problems existing in tip clearance flow and its cavitation.In axial-flow turbomachinery,the clearance between the blade and the casing is narrow generally,and a real turbomachinery often operates in an environment with strong turbulent fluctuations,which makes the complex interaction between tip clearance flow and casing wall turbulence near the blade tip of the impeller.Considering the complexity of the geometric structure of a real turbomachinery,the tip clearance flow between the straight NACA0012 hydrofoil and the horizontal endwall has been studied for the sake of simplying the geometry.And then the research has been conducted for the tip clearance flow under different tip clearance sizes by combining the real turbulent inlet and the direct numerical simulation method.The simulation results show that the present direct numerical simulation can accurately capture the tip leakage vortex,tip separation vortex and induced vortex in the tip clearance region,and can also capture the recirculation vortex when the tip clearance size is small.The influence of tip clearance flow structures on the highspeed streaky structures is mainly in the buffer layer and logarithmic layer.The increase of the tip clearance size significantly reduces the turbulent kinetic energy and flow blockage in the tip clearance region,and the flow blockage satisfies a simple power distribution along the streamwise direction when the tip clearance width is large.In addition,it is also found that there exists a specific tip clearance size to maximize the tip leakage loss.As mentioned above,the clearance between the rotor blade and the casing in a turbomachinery is narrow generally,and the relative motion between the blade tip and the casing becomes crucial for the development and evolution of tip clearance flow.In this paper,the moving endwall is applied to mimic the rotation of the blade,and the tip clearance flow for different endwall speeds are studied by using direct numerical simulation.The simulation results show that the endwall motion significantly inhibits the induced vortex,and an increase in the relative endwall speed causes the breakdown of the tip leakage vortex in advance.The endwall motion reduces the flow blockage introduced by the tip leakage vortex,increases the tip leakage loss and the mean mass flow rate.The boundary layer statistics show that the endwall motion decreases and increases the skin friction coefficient at the leading edge of hydrofoil and near the endwall in the wake,respectively.Turbulence statistics demonstrates that there exists a specific endwall speed for which the tip leakage vortex strength and the peak of negative pressure within the vortex core is maximum.The outer shear layer of tip separation vortex,the shear layer in the tip leakage jet and tip leakage vortex are the main source of energy loss in the tip clearance flow.The energy loss in the tip clearance region is significantly enhanced by endwall motion.The analysis of the mean vorticity transport equation shows that the mean convective term,the vortex stretching term,the viscous diffusion tern in the spanwise direction and the turbulent diffusion term in the wall normal direction dominates.The endwall motion significantly enhances the vortex stretching term in the spanwise direction and the turbulent diffusion in the wall-normal direction within the shear layer of tip clearance jet.The analysis of turbulent kinetic energy transport equation demonstrates that an increase in the endwall speed gradually transported the turbulence in the shear layer of tip clearance jet into the tip leakage vortex core.In order to the negative effects caused by the tip clearance flow and its vortices on turmbachinery,a J groove on tip surface is proposed to realize passive flow control in this paper,and the influence of J groove on tip clearance flow for the stationary and moving endwall has been studied by using direct numerical simulations.The simulation results show that the flow blockage can be reduced before the end of the J groove to some extent,but significantly enhanced after that.A J groove also significantly reduces the tip leakage loss and increases the tip clearance mass flow rate.Although the J groove migrated the risk of tip leakage vortex cavitation to the trailing edge of hydrofoil,the J groove significantly increased the probability of cavitation events within the J groove with endwall motion.From the perspective of energy loss and stable operation,it is important for the optimization design of the geometry at the beginning and end of the J groove.In addition,the transport of turbulence statistics are mostly unchanged by the moving endwall.Therefore,the J groove designed for the stator can be directly applied to the rotor in a real turbomachinery when considering the passive flow control strategy.Previously,the inner mechanisms within the tip clearance region have been explored from the perspective of small-scale flows.In this paper,high-precision large eddy simulation method is adopted to study the tip clearance cavitating flow from the perspective of large-scale flows in the laboratory.The simulation results show that cavitation does not significantly alter the tip clearance flow structures and tip leakage vortex wandering,but significantly enhances the turbulence fluctuation and pressure fluctuation in the streamwise domain of tip leakage vortex cavity,and also enhances the unsteady wall-normal motion of tip leakage vortex.In addition,cavitation increases the mean pressure coefficient at the guide edge of the suction side of the hydrofoil and the loading near the leading edge of the middle of the hydrofoil,but reduces the loading at the tip of the hydrofoil as well as the lift and drag of the hydrofoil.