| High-end aerospace and deep-sea equipment are the frontier technology of my country’s strategic emerging industries.As the key power equipment of aerospace and deep-sea propellers,its performance seriously restricts the development of system equipment.As a new type of propulsion systems,due to the advantages of its high efficiency,low noise and strong maneuverability,cycloidal propellers are attracting more attention in helicopter,micro air vehicles and underwater propulsion devices.It shows that the internal flow structure of a cycloid propeller consisting of several blades,is extremely complicated,including blade-wake and wake-wake interactions,the transition over the blade surface,and turbulent wakes.However,the current research only focuses on the prediction of its overall performance,and ignores how the internal flow structure and near-wall flow induce the performance change of the rotating system and single blade under different working conditions and with different geometrical parameters.In addition,under low Reynolds number condition,the transition from the laminar to turbulent flow can’t be ignored,and the two-equation model used to resolve the fully turbulent flow can’t capture the transition event.This work was completed in cooperation between Research centrer of fluid machinery engineering and technology in Jiangsu University,école nationale supérieure des arts et métiers de Lille(ENSAM)and Virginia Tech(VT),with focus on the application of the transition model on a single airfoil,forward and reversed oscillating airfoils and cycloid propellers.Deeply analyzing the effects of different operating conditions and geometrical parameters on the performance of the rotating system and single blade,and revealing how the near-wall flow influences the performance change,to provide a research basis for further understanding of the complex internal flow of the cycloid propeller and how to better optimize its performance.The main work and innovations are listed as follows:(1)Evaluating the capability of the transition model on a single stationary airfoil.The work verifies the influence of the grid distribution in normal and streamwise directions,inlet turbulence intensity and eddy viscosity ratio,the correlation functions and parameters in the transition model and the turbulence model.Based on the computations and experimental measurements,the pressure coefficient,wall skin friction coefficient,velocity distribution near the wall,and the positions of the separation,transition and reattachment points are displayed,and the transition characteristics at different incidences and Reynolds numbers are clarified.(2)With the attention to the dynamic characteristic of the rotating blade of the cycloidal propeller,with the aid of the sliding mesh technology,the internal flow state,transition characteristic and vortex trajectory over forward and reverse airfoils under different pitching amplitudes,pitching frequencies and Reynolds numbers are studied.The results preliminarily indicate that the flow around the reversed pitching airfoil is more complicated,mainly induced by the earlier flow separation due to the sharp leading edge.In addition,the transition over the reversed pitching airfoil is also completely different from that on the forward airfoil,which is characterized by the primary transition in the separated shear layer caused by the leading-edge vortex and the second transition on the trailing edge of the blade induced by the existence of the laminar separation bubble.Increasing the pitching frequency will not only cause the delay in performance and flow field,but also delay the transition induced by the leading-edge vortex.(3)By directly applying the optimized transition model and other turbulence models to the 2-bladed cycloid,it can be seen that the results obtained by the transition model are closer to the experiments.By the comparison with the experimentlal measurements,it can be observed that the computations can capture the process of the wake-wake and blade-wake interactions inside the cycloidal propellers.Combined with the performance of the single blade,the force acting on the blade,pressure distribution and flow structure near the wall,the performance difference of the rotating system and the transition over the blade under different advance coefficients are clarified.The results also show that the transition on the dynamic airfoil was firstly caused by the instability of the laminar boundary layer induced by the discrete vortices,then the appearance of the laminar separation bubble,finally the laminar boundary layer shortly after the attachment point,and the separation of the turbulent boundary layer,which can shed light on the mechanism of the transition over the reversed airfoil.(4)To optimize the performance of the propellers,several important parameters including the pitching kinematics,chord-to-radius ratio,pitch-pivot-point and blade profile,were considered.The results show that the asymmetrical pitching kinematic with a small positive value of 5~°can achieve a better performance.Then,a cycloidal rotor with a chord-to-radius ratio of 0.45 can produce the highest thrust-based efficiency.In addition,the pitch-pivot-point at the quarter-chord length can greatly reduce power consumption,resulting in the best efficiency.Finally,the symmetrical airfoils involving NACA0012 and NACA0015 can maintain the high efficiency in a wide range of the Reynolds number,and at relatively high Reynolds numbers,the inversed NACA2415is also a good choice.According to the velocity triangle of a single blade,it is observed that changing the advance coefficient and Reynolds number would affect the relative velocity,mainly presented by the location of the stagnation point,which further affects the pressure distribution and lift and drag acting on the blade,and finally impacts the performance of the single blade and the rotating system.The results also show that the Reynolds number effect is smaller than the advance coefficient,which is mainly due to the change of the rotating speed and inlet velocity at the same time.The research results provide basic theoretical support for the aerodynamic optimization of the cycloid propeller. |
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