Verification and Validation of Large Eddy Simulation for Tip Clearance Vortex Cavitating Flow in a Waterjet Pump

In this paper, large eddy simulation (LES) was adopted to simulate the cavitating flow in a waterjet pump with emphasis on the tip clearance flow. The numerical results agree well with the experimental observations, which indicates that the LES method can make good predictions of the unsteady cavitating flows around a rotor blade. The LES verification and validation (LES V&V) analysis was used to reveal the influence of cavitation on the flow structures. It can be found that the LES errors in cavitating region are larger than those in the non-cavitating area, which is mainly caused by more complicated cavitating and tip clearance flow structures. Further analysis of the interaction between the cavitating and vortex flow by the relative vorticity transport equation shows that the stretching, dilatation and baroclinic torque terms have major effects on the generation and transport of vortex structure. Meanwhile the Coriolis force term and viscosity term also exacerbate the vorticity transport in the cavitating region. In addition, the flow loss characteristics of this pump are also revealed by the entropy production theory. It is indicated that the tip clearance flow and trailing edge wake flow cause the viscous dissipation and turbulent dissipation, and the cavitation can further enhance the instability of the flow field in the tip clearance.

Experimental Study on Different Tip Clearance of Low-Speed Axial Fan

Tip clearance flow is not only the source of undesirable noise but also a potential indicator for critical operating conditions with rotating stall or surge. It can induce blade vibration, which would cause premature blade failure when the vibration is strong enough. The paper presents experimental studies on the effects of tip clearance on the stall inception process in a low-speed high-load single stage fan with different tip clearance. From the point of view of flow range, it has been proved by computations that there is an optimal gap value, and an explanation is given according to different stall mechanisms of large and small tip clearance. However, the experiment of no tip clearance is not easy to achieve. In this experiment, a wearable soft wall casing was used to achieve “zero clearance”, and an explicit conclusion was obtained. The pressure rise and efficiency are improved at small tip clearance. Instantaneous Casing Pressure Field Measurement was carried out: instantaneous casing pressure fields were measured by 9 high response pressure transducers mounted on the casing wall. At the near stall point with large tip clearance, a narrow band increase of the amplitudes in the frequency spectrum at roughly half of the blade passing frequency can be observed according to the spectrum of static pressure at points on the endwall near the leading-edge and above the rotor. This phenomenon was explained from two aspects: tip clearance flow structure and pressure signal spectrum.

Analysis Method of NSV and Influence of Tip Clearance Flow Instabilities on NSV in an Axial Transonic Compressor Rotor

The relationship between tip clearance flow (TCF) and blade vibration in locked-in region is numerically investigated on a transonic rotor. The numerical method is verified by citing references. The phase of TCF changes with the operating condition. A separation method of the unsteady pressure caused by TCF and blade vibration is developed. The unsteady pressure during NSV is separated into the TCF and vibration components under 1B and 8th modes. The unsteady pressure of TCF is similar with that of rigid blade. The unsteady pressure of blade vibration is larger at part span, and its distribution depends on the modal shape and vibrating amplitude. The unsteady pressure of TCF and blade vibration determine the aerodynamic damping in locked-in region. The aerodynamic damping of TCF changes with the TCF phase. TCF provides positive damping at some phases and negative damping at other phases. The aerodynamic work of TCF and blade vibration increases linearly and at the rate of square with the vibrating amplitude, respectively. TCF is dominant in the initial stage of vibration. With the vibrating amplitude increasing, the aerodynamic work of vibration catches up gradually. NSV occurs when TCF provides negative damping and the unsteady pressure of vibration provides positive damping. If the work of vibration is negative, vibration will be enlarged until failure. The maximum amplitude of NSV canbe obtained by calculating the balance of work. For the 8th mode, the limit amplitude under 0ND is 0.0926%C corresponding to vibration stress of 60MPa.

Locked-In Phenomenon Between Tip Clearance Flow Instabilities and Enforced Blade Motion in Axial Transonic Compressor Rotors

In this paper, tip clearance flow (TCF) instabilities and their relationship to blade motion are investigated numerically on a transonic transonic rotor with a large tip clearance. The numerical methods are verified by comparing with the experimental data of NACA0012 and show reliable results. It is found that the TCF instabilities are caused by the radial vortex formed in passage, which is induced by the interaction of tip clearance vortex (TCV) and main flow. When the blade is enforced vibrating with small amplitude, the results show that TCF instabilities are hardly affected by the blade vibration, and almost no phenomenon of locked-in is found. However, when the amplitude of blade vibration is increased, the interaction becomes stronger and the pressure fluctuation is enhanced. A wider locked-in region is observed. In addition, the simulation results show that the locked-in region is affected significantly by modal shapes. For the rotor here, it seems that the bending mode has a greater effect on the TCV instabilities than the torsional mode and causes a wider locked-in region. In locked-in region, the phase differences between TCV and the blade motion change with the flow conditions. In unlocked region, the period of TCF instabilities fluctuates over time, and the process is similar to that in the locked-in region.

Numerical Simulation of the Effect of the Tip Clearance Flow on Rotor-Stator Interaction Tone Noise in Axial-Flow Fan

The present study is focused on the sound generation due to the rotor tip clearance flow interaction with stator in an axial flow fan. A hybrid URANS/Goldstein’s equations method is applied to calculate the unsteady flow and tone noise in a high loaded axial-flow fan with different rotor tip clearance. The numerical simulation results show that the main sound sources of fan tip clearance tone noise are concentrated in the leading edge of downstream stator blades. It is found that when the rotor tip clearance increases from zero to 2.5 mm (0.94% relative blade height), the mass flow of the fan decreases by about 2% and the efficiency of the fan decreases by about 1 percentage, and the sound power level at 1BPF forward tone increases by 1.47dB, and that of backward tone increases by 0.65dB. However, the influence of tip clearance on the tone noise intensity at 2BPF and 3BPF is more complex, and the variation range is less than 1dB. It is found that the wake width and wake strength at the rotor exit increase with the increase of tip clearance. The tip secondary flow caused by rotor clearance seriously affects the circumferential inhomogeneity of stator leading edge inflow conditions.