The stall margin of compressor could be improved effectively by rotor tip injection, and the periodic injection is commonly used in the research. The purpose of this work is to investigate the influence of injection frequency on the rotor stall margin. An unsteady CFD code was employed to simulate the flow field of the rotor with injections of different frequencies. Comparing the stall margin of the rotor with injections of different frequencies, it is shown that there is an optimal injection frequency, around which the rotor stability enhancement is the largest. When the injection frequency is away form the optimal frequency, the improvement in stable flow range decreases correspondingly. For the rotor in this paper, the optimal frequency was 1.5 times the frequency of tip leakage vortex (for short, TLV) fluctuation. Time-averaged loading distribution at 98.5% span indicates that the loading of the rotor near the leading edge is decreased through injection with the optimal frequency, and therefore, the stall could be delayed. periodic injection, axial compressors, unsteady flow, stall margin, tip leakage vortex, loading distribution Citation:Zhou J W, Hou A P, Zhou S. Effects of injection frequency on the rotor stall margin.
A submerged tidal energy device with contra-rotating Diffuser Augmented Tidal Turbines (DATTs) has been investigated in this paper. The device is moored to the seabed with a single mooring line, which limit it to operate at mean water depth, but otherwise allows it to float freely with the tidal current, like a kite in the wind, to harness tidal current energy. This research focuses on the evaluation of stability and power generation of the submerged tidal energy device based on 1:5 th scaled model tests and the full-scaled prototype sea trials.A 1:5 th scaled model had been manufactured and tested in a circulating water channel to observe the power generation performance and working attitude around the designed inflow velocity. A full-scaled prototype was manufactured and tested near the CHU Island in Shandong Province, China. The results show that the device can change its direction automatically to make the DATTs face the tidal inflow, as the tidal current changes direction. The device has a good stability in pitch and roll motions. But the device's stability in yaw motions is worse than the other two, which will significantly affect the power generation performance and introduce more demanding structural requirements.
Taking the rigid NACA0012 airfoil as the object, the key structural parameters of the spring–mass system that govern the dynamics of the double-elastic-constrained flapping hydrofoil are numerically studied in this paper. A two-dimensional numerical model, based on the CFD software FINE/Marine, is established to investigate the influence of the spring stiffness coefficient, frequency ratio, and damping coefficient on the motion and performance of the flapping hydrofoil. This study demonstrates that when the structural parameters are adequately adjusted, the power factor exceeding 1.0 has been achieved, and the corresponding efficiency is up to 37.8%. Moreover, this system can start and work within a wide range of damping coefficients. However, the hydraulic efficiency and power coefficient are sensitive to the change in damping coefficient, so it is very necessary to design an appropriate power output. Lastly, the most obvious parameter affecting the energy acquisition performance is the spring stiffness coefficients. Frequency ratios in the two directions have little influence on the peak value of the power coefficient, but they will cause the change of damping coefficients of the peak point. The key structural parameters studied in this paper provide a useful guideline for an optimized design of this interesting system through searching for the best performance.
The semi-active flapping foil driven by the swing arm is a simple structure to realize the propulsion of the flapping foil. The motion trajectory of this semi-active flapping foil mechanism is a circular arc, and its hydrodynamic characteristics are not clear. This paper systematically investigates the working characteristics and hydrodynamic performance of this semi-active flapping foil with a circular arc track. Compared with the traditional flapping foil structure, the special design parameters of the semi-active flapping foil driven by the swing arm mainly include the length of the swing arm and the stiffness of the torsion spring. In this paper, the three-dimensional fluid-structure coupling method is used by solving the fluid dynamics equation and the structural dynamics equation, and the working characteristics of the structure with different motion and geometric parameters are analyzed. From the results, increasing the swing arm length is beneficial to improving the peak efficiency of the flapping foil, and also to improving the thrust coefficient corresponding to the peak efficiency point. Under a certain swing arm length, reducing the spring stiffness is also conducive to improving the peak efficiency of the propulsion system, but it is adverse to the thrust coefficient. Further analysis shows that the maximum angle of attack is the key factor affecting the efficiency of this flapping foil propulsion. For the flapping foil described in this paper, its peak efficiency is usually concentrated near αmax=0.2 rad. However, for the thrust coefficient of this kind of flapping foil propulsion, the influencing factors are relatively complex, including swinging arm, the spring stiffness, and the advance coefficient. The maximum angle of attack remains the key factor affecting the peak thrust in the range of advance coefficient far from the starting state.
A thick tip foil blade was designed to reduce the tip leakage flow and weaken the tip vortexes in ducted propeller 19A/Ka4-55. The thick tip foil was constructed by normal extrusion of the original tip foil. The maximum thickness of the foil increased from 1.64% chord length to 7.64%. The commercial CFD code CFX was employed to simulate the flow field of the two ducted propellers. From the performance curves, it could be concluded that the tip foil thickening will not affect the thrust and torque of the ducted propeller much. The tip leakage vortex (TLV) in the thick tip foil propeller was obviously weaker than that in the original propeller. The pressure drop in TLV middle chord position decreased by 32.8%. In the tip clearance, both propellers have the tip clearance vortex. The difference is that the tip clearance vortex in the thick tip foil propeller could form an expansionary channel in the tip clearance, which made the tip leakage flow slow down, and hence weaken the TLV.
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