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An adaptive integration surface for predicting transonic rotor noise in hovering and forward flights

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Chinese Journal of Aeronautics 2019年第9期32卷 2047-2058页

作者： Siyu CHEN Qijun ZHAO Yiyang MA national key laboratory of rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

In this paper, a new permeable adaptive integration surface is developed in order to evaluate transonic rotor noise in accordance with FW-H_pds equations(Ffowcs Williams-Hawkings equations with penetrable data surface). Firstly, a nonlinear near-field solution is computed on the basis of Navier-Stokes equations, which is developed on moving-embedded grid *** solution calculated through the present CFD method is used as the input for acoustic calculations by FW-H_pds equations. Then, two criteria for constructing integration surfaces are established based on the analysis of the quadrupole source strength and the nonlinear characteristic.A new surface is determined adaptively by the pressure gradient or density in a given flowfield,eschewing the uncertainties associated with determining cylinder-shaped integration surfaces. For varying hover cases, transonic noises are simulated with new integration surfaces for a UH-1 model rotor. Furthermore, numerical results of the new integration surface derived from the density perturbation value conform better to experimental data than results derived from the pressure ***, the integration surface given by jrqj being 0.1, which is an applicable criterion obtained from hover cases, is used to predict transonic rotor noise in forward flight. The computational accuracy of the new integration surface method has been validated in predicting transonic rotor noise of an AH-1 model rotor at different advance ratios.

关键词： FW-H_pds equations Integration surface Navier–Stokes equations Rotor Transonic rotor noise

Active control for performance enhancement of electrically controlled rotor

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Chinese Journal of Aeronautics 2015年第5期28卷 1494-1502页

作者： Lu Yang Wang Chao national key laboratory of rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

Electrically controlled rotor （ECR） system has the potential to enhance the rotor perfor- mance by applying higher harmonic flap inputs. In order to explore the feasibility and effectiveness for ECR performance enhancement using closed-loop control method, firstly, an ECR rotor performance analysis model based on helicopter flight dynamic model is established, which can reflect the performance characteristics of ECR helicopter at high advance ratio. Based on the simulation platform, an active control method named adaptive T-matrix algorithm is adopted to explore the feasibility and effectiveness for ECR performance enhancement. The simulation results verify the effectiveness of this closed-loop control method. For the sample ECR helicopter, about 3% rotor power reduction is obtained with the optimum 2/rev flap inputs at the advance ratio of 0.34. And through analyzing the distributions of attack of angle and drag in rotor disk, the underlying physical essence of ECR power reduction is cleared. Furthermore, the influence of the key control parameters, including convergence factor and weighting matrix, on the effectiveness of closed-loop control for ECR performance enhancement is explored. Some useful results are summarized, which can be used to direct the future active control law design of ECR performance enhancement.

关键词： Electrically controlled rotor(ECR) Helicopter Higher harmonic control Performance enhancement Swashplateless

Performance analysis of variable speed tail rotors with Gurney flaps

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Chinese Journal of Aeronautics 2018年第11期31卷 2104-2110页

作者： Chen DONG Dong HAN Lei YU national key laboratory of rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

Gurney Flaps(GFs) are used for improving the performance of variable speed tail rotors. A validated analytical helicopter model able to predict the main and tail rotor power is utilized. The fixed height GF has substantially small influence on the tail rotor power in hover and low to medium speed forward flight, and can obtain significant power reduction in high speed *** ability can be enhanced by decreasing the tail rotor speed. With the deployment of GF, the collective pitch of the tail rotor decreases, and the maximum tail rotor thrust increases. The GF can compensate the reduction of the maximum thrust by the decrease in the tail rotor speed. The GF with a height of 5% of the chord length can almost remedy 50% of the thrust reduction introduced by decreasing 10% of the tail rotor speed. With the increase of GF height, the maximum thrust generated by the tail rotor increases. The GF with larger height can cause the increase in the tail rotor power in hover and low to medium speed flight. The retractable GF can obtain more power savings than the fixed height GF. However, the benefit is substantially small even in high speed flight. Considering the side effects introduced by the active GF, the fixed height GF may be more preferable. The mechanism for the retractable GF to generate more tail rotor thrust is to increase the lift in advancing side due to the higher dynamic pressure.

关键词： Tail rotor Variable speed Performance Gurney flap Tail rotor thrust Flap height High speed flight

Efficient prediction of ground noise from helicopters and parametric studies based on acoustic mapping

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Chinese Journal of Aeronautics 2018年第2期31卷 273-284页

作者： Fei WANG Guohua XU Yongjie SHI national key laboratory of rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

Based on the acoustic mapping, a prediction model for the ground noise radiated from an in-flight helicopter is established. For the enhancement of calculation efficiency, a high-efficiency second-level acoustic radiation model capable of taking the influence of atmosphere absorption on noise into account is first developed by the combination of the point-source idea and the rotor noise radiation characteristics. The comparison between the present model and the direct computation method of noise is done and the high efficiency of the model is validated. Rotor free-wake analysis method and Ffowcs Williams-Hawkings（FW-H） equation are applied to the aerodynamics and noise prediction in the present model. Secondly, a database of noise spheres with the characteristic parameters of advance ratio and tip-path-plane angle is established by the helicopter trim model together with a parametric modeling approach. Furthermore, based on acoustic mapping, a method of rapid simulation for the ground noise radiated from an in-flight helicopter is developed. The noise footprint for AH-1 rotor is then calculated and the influence of some parameters including advance ratio and flight path angle on ground noise is deeply analyzed using the developed *** results suggest that with the increase of advance ratio and flight path angle, the peak noise levels on the ground first increase and then decrease, in the meantime, the maximum Sound Exposure Level（SEL） noise on the ground shifts toward the advancing side of rotor. Besides, through the analysis of the effects of longitudinal forces on miss-distance and rotor Blade-Vortex Interaction（BVI） noise in descent flight, some meaningful results for reducing the BVI noise on the ground are obtained.

关键词： Acoustic mapping Helicopter Noise footprint Rotor noise Second-level acoustic radiation model

Robust Navier-Stokes method for predicting unsteady flowfield and aerodynamic characteristics of helicopter rotor

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Chinese Journal of Aeronautics 2018年第2期31卷 214-224页

作者： Qijun ZHAO Guoqing ZHAO Bo WANG Qing WANG Yongjie SHI Guohua XU national key laboratory of rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

A robust unsteady rotor flowfield solver CLORNS code is established to predict the complex unsteady aerodynamic characteristics of rotor flowfield. In order to handle the difficult problem about grid generation around rotor with complex aerodynamic shape in this CFD code,a parameterized grid generated method is established, and the moving-embedded grids are constructed by several proposed universal methods. In this work, the unsteady Reynolds-Averaged Navier-Stokes（RANS） equations with Spalart-Allmaras are selected as the governing equations to predict the unsteady flowfield of helicopter rotor. The discretization of convective fluxes is accomplished by employing the second-order central difference scheme, third-order MUSCL-Roe scheme, and fifth-order WENO-Roe scheme. Aimed at simulating the unsteady aerodynamic characteristics of helicopter rotor, the dual-time scheme with implicit LU-SGS scheme is employed to accomplish the temporal discretization. In order to improve the computational efficiency of holecells and donor elements searching of the moving-embedded grid technology, the ‘‘disturbance diffraction method＂ and ‘‘minimum distance scheme of donor elements method＂ are established in this work. To improve the computational efficiency, Message Passing Interface（MPI） parallel method based on subdivision of grid, local preconditioning method and Full Approximation Storage（FAS） multi-grid method are combined in this code. By comparison of the numerical results simulated by CLORNS code with test data, it is illustrated that the present code could simulate the aerodynamic loads and aerodynamic noise characteristics of helicopter rotor accurately.

关键词： Aerodynamic characteristicsHelicopter rotor Moving-embedded grid Navier-Stokes equations Upwind schemes

Experiments on unsteady vortex flowfield of typical rotor airfoils under dynamic stall conditions

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Chinese Journal of Aeronautics 2016年第2期29卷 358-374页

作者： Wang Qing Zhao Qijun national key laboratory of Science and Technology on rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

A new experiment for airfoil dynamic stall is conducted by employing the advanced par- ticle image velocimetry （PIV） technology in an open-return wind tunnel. The aim of this experimen- tal investigation is to demonstrate the influences of different motion parameters on the convection velocity, position and strength of leading edge vortex （LEV） of airfoil under different dynamic stall conditions. Two different typical rotor airfoils, OA209 and SC1095, are measured at different free stream velocities, oscillation frequencies, and angles of attack. It is demonstrated by the measured data that the airfoil with larger leading edge radius could notably decrease the strength of LEV. The angle of attack （AoA） of airfoil can obviously influence the dynamic stall characteristics of airfoil, and the LEV would be effectively inhibited by decreasing the mean pitch angle. In addition, the con- vection velocity of LEV is estimated in this measurement, and the results demonstrate that the influ- ence of airfoil shape on convection velocity of LEV is limited, but the convection velocity of LEV would be increased by enlarging the oscillation frequency. Meanwhile, the convection velocity of LEV is a time variant value, and this value would increase as the LEV convects to the trailing edge of airfoil.

关键词： Convection velocity Dynamic stall Leading edge vortex PIV Rotor airfoil

Augmented flight dynamics model for pilot workload evaluation in tilt-rotor aircraft optimal landing procedure after one engine failure

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Chinese Journal of Aeronautics 2019年第1期32卷 92-103页

作者： Xufei YAN Renliang CHEN national key laboratory of Science and Technology on rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

An augmented flight dynamics model is developed to extend the existing flight dynamics model of tilt-rotor aircraft for optimal landing procedure analysis in the event of one engine *** with the existing flight dynamics model, the augmented model involves with more pilot control information in cockpit and is validated against the flight test data. Based on the augmented flight dynamics model, the optimal landing procedure of XV-15 tilt-rotor aircraft after one engine failure is formulated into a Nonlinear Optimal Control Problem(NOCP), solved by collocation and numerical optimization method. The time histories of pilot controls in cockpit during the optimal landing procedure are obtained for the evaluation of pilot workload. An evaluation method which can synthetically quantify the pilot workload in time and frequency domains is proposed with metrics of aggressiveness and cutoff frequencies of pilot controls. The scale of the pilot workload is compared with those of the shipboard landing procedures, bob-up/bob-down and dash/quickstop maneuvers of UH-60 helicopter. The results show that the aggressiveness of pilot collective and longitudinal controls for the tilt-rotor aircraft optimal landing procedure after one engine failure are higher than those for UH-60 helicopter shipboard landing procedures up to the condition of sea state 4, while the pilot cutoff frequency of collective control is lower than that of the bob-up/bob-down maneuver but the pilot cutoff frequency of longitudinal control is higher than that of the dash/quick-stop maneuver. The evaluated pilot workload level is between Cooper–Harper HQR Level 2 and Level 3.

关键词： Landing procedure One engine failure Optimal control problem Pilot workload Tilt-rotor aircraft

Numerical analysis on noise of rotor with unconventional blade tips based on CFD/Kirchhoff method

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Chinese Journal of Aeronautics 2013年第3期26卷 572-582页

作者： Wang Bo Zhao Qijun Xu Guohua Ye Liang Wang Junyi national key laboratory of Science and Technology on rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

A solver is developed aiming at efficiently predicting rotor noise in hover and forward flight. In this solver, the nonlinear near-field solutions are calculated by a hybrid approach which includes the Navier–Stokes and Euler equations based on a moving-embedded grid system and adaptive grid methodology. A combination of the third-order upwind scheme and flux-difference splitting scheme, instead of the second-order center-difference scheme which may cause larger wake dissipation, has been employed in the present computational fluid dynamics （CFD） method. The sound pressure data in the near field can be calculated directly by solving the Navier–Stokes equations, and the sound propagation can be predicted by the Kirchhoff method. A harmonic expansion approach is presented for rotor far-field noise prediction, which gives an analytical expression for the integral function in the Kirchhoff formula. As a result, the interpolation process is simplified and the efficiency and accuracy of the interpolation are improved. Then, the high-speed impulsive （HIS） noise of a helicopter rotor at different tip Mach numbers and on different observers is calculated and analyzed in hover and forward flight, which shows a highly directional characteristic of the rotor HIS noise with a maximum value in the rotor plane, and the HSI noise weakens rapidly with the increasing of the directivity angle. In order to investigate the effects of the rotor blade-tip shape on its aeroacoustic characteristics, four kinds of blade tips are designed and their noise characteristics have been simulated. At last, a new unconventional CLOR-II blade tip has been designed, and the noise characteristics of the presented CLOR-II model rotor have been simulated and measured compared to the reference rotors with a rectangular or swept-back platform blade tip. The results demonstrate that the unconventional CLOR-II blade tip can significantly reduce the HSI noise of a rotor.

关键词： Aeroacoustics Computational fluid dynam- ics Helicopters Kirchhoff Rotors Unconventional blade tip

CFD calculations on the unsteady aerodynamic characteristics of a tilt-rotor in a conversion mode

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Chinese Journal of Aeronautics 2015年第6期28卷 1593-1605页

作者： Li Peng Zhao Qijun Zhu Qiuxian national key laboratory of Science and Technology on rotorcraft aeromechanics Nanjing University of Aeronautics and Astronautics

In order to calculate the unsteady aerodynamic characteristics of a tilt-rotor in a conver- sion mode, a virtual blade model （VBM） and an real blade model （RBM） are established respec- tively. A new multi-layer moving-embedded grid technique is proposed to reduce the numerical dissipation of the tilt-rotor wake in a conversion mode. In this method, a grid system generated abound the rotor accounts for rigid blade motions, and a new searching scheme named adaptive inverse map （AIM） is established to search corresponding donor elements in the present moving- embedded grid system to translate information among the different computational zones. A dual-time method is employed to fulfill unsteady calculations on the flowfield of the tilt-rotor, and a second-order centered difference scheme considering artificial viscosity is used to calculate the flux. In order to improve the computing efficiency, the single program multiple data （SPMD） model parallel acceleration technology is adopted, according to the characteristic of the current grid system. The lift and drag coefficients of an NACA0012 airfoil, the dynamic pressure distributions below a typical rotor plane, and the sectional pressure distributions on a three-bladed Branum- Tung tilt-rotor in hover flight are calculated respectively, and the present VBM and RBM are val- idated by comparing the calculated results with available experimental data. Then, unsteady aero- dynamic forces and flowfields of an XV-15 tilt-rotor in different modes, such as a fixed conversion mode at different tilt angles （15°, 30°, 60°） and a whole conversion mode which converses from 0° to 90°, are numerically simulated by the VBM and RBM respectively. By analyses and comparisons on the simulated results of unsteady aerodynamic forces of the tilt-rotor in different modes, some meaningful conclusions about distorted blade-tip vortex distribution and unsteady aerodynamic force variation in a conversion mode are obtained, and these investigation

关键词： CFD Conversion mode Multi-layer moving-embedded grid Tilt-rotor Unsteady flowfield