Abstract
The flow field structure of a rotorcraft is complex; specifically, the rotor tip vortex structure has a great influence on the rotor performance. Therefore, in this paper, the evolution characteristics of rotor tip vortices and the dynamic mode decomposition (DMD) of rotor tip vortices in a rotor hovering state are studied. Through a time-resolved particle image velocimetry experiment, a comparative study of the blade tip vortex flow field at a fixed rotation speed (1500 rpm) and a collective pitch of 6° and 9° was performed. The method of DMD is used for the reduced-order analysis of the vorticity field of the blade tip vortex in the hovering state. By this method, these important vortex structures are extracted and discussed; meanwhile, the future flow field is also reconstructed. The results of flow visualization indicate that the trajectory of the blade tip vortex is moving down the axis, while moving toward the hub in the radial direction in the hovering state. The results of DMD analysis show that during the evolution of the blade tip vortex, different modes have different contributions to the rotor as a whole. In addition, the larger the collective pitch, the larger the modal coefficient amplitude and the slower the stabilization speed.
Highlights
The aerodynamic performance of a rotorcraft is more complicated than a fixed-wing aircraft, and its complexity comes from the unique flow field structure around the rotor
In addition to the vortex sheet generated by the blade movement, strong blade tip vortices will appear in the wake of each blade tip. These vortices are always close to the rotor, creating a complex three-dimensional induced flow field, which affects the aerodynamic load of the rotor, aerodynamic performance, vibration level of the rotor, aeroelasticity, and acoustic performance
In view of the above problems, this paper uses the TR-particle image velocimetry (PIV) experimental facilities to carry out proficiency vortex field measurements at the blade tip; the dynamic mode decomposition (DMD) method is used to carry out modal reduction and reconstruction of the flow field, which can further analyze the kinematic development process of the blade tip vortex in the complex wake of the rotor, and further understand the unsteady and nonlinear complex aerodynamic mechanism of the blade tip vortex, providing a theoretical basis for the modeling and control of the wake vortex
Summary
The aerodynamic performance of a rotorcraft is more complicated than a fixed-wing aircraft, and its complexity comes from the unique flow field structure around the rotor. In order to further study the complex flow of the rotor and its aerodynamic characteristics, it is urgent to develop corresponding rapid analysis techniques for the rotorcraft flow field and apply the dynamic modal dimension reduction technology to accurately obtain the complex reduced-order model of the vortex structure. In view of the above problems, this paper uses the TR-PIV experimental facilities to carry out proficiency vortex field measurements at the blade tip; the DMD method is used to carry out modal reduction and reconstruction of the flow field, which can further analyze the kinematic development process of the blade tip vortex in the complex wake of the rotor, and further understand the unsteady and nonlinear complex aerodynamic mechanism of the blade tip vortex, providing a theoretical basis for the modeling and control of the wake vortex.
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