• The Journal of the Acoustical Society of Korea
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• v.37 no.2
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• pp.83-91
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• 2018

The noise sources of a tip-jet driven rotor can be separated by rotor blade noise and jet noise. The rotor blade noise consists of thickness noise, loading noise, nonlinear quadrupole noise, and jet noise is divided into nozzle momentum noise and jet radiation noise. The flow analysis for the prediction of rotor blade noise is performed by CFD (Computational Fluid Dynamics) analysis, and the noise source of the rotor blade noise is identified by simultaneously applying the permeable and impermeable surface based FW-H (Ffowcs Williams-Hawkings) acoustic analogy. The nozzle momentum noise is obtained by permeable surface FW-H, and jet radiation noise is predicted by using empirical method for the fixed-wing jet. Both of jet noises use nozzle exit condition for noise analysis. The accuracy of the technique is verified based on the noise measurements of the tip-jet driven rotor, and the unique noise characteristics of the tip-jet driven rotor is confirmed by spectrum analysis.

• Aerospace Engineering and Technology
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• v.2 no.1
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• pp.35-43
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• 2003

In this study, a rotor planform shape with high performance and low noise has been designed and its aerodynamic and aeroacoustic characteristics are analysed. First of all, rotor blade planform with low noise characteristics, has been designed based on the paddle-shape blade by applying vane-tip concept. Finally, noise characteristics of the designed next-generation rotor blade have been investigated and the results are compared with those of BERP blade.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.11
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• pp.1133-1143
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• 2009

Vibro-acoustic characteristics of a simplified disk-brake rotor containing a narrow radial slot are studied using a semi-analytical procedure. First, modal sound radiations for flexural and radial modes of a generic annular disk having identical key dimension and slot(with free boundaries) are defined using pre-developed analytical solutions based on the modal vibrations from finite element model. The analytical solutions are validated by fully computational methods. Second, sound radiation from a simplified brake rotor simulated using sound radiation solution of the generic disk based on the rotor eigensolutions computed using a finite element code. Predictions by the semi-analytical method matched well numerical calculations using finite element and boundary element method. Finally, sound radiation and vibration characteristics for the example rotor due to a harmonic excitation fixed to the rotor or rotating around the rotor are also obtained.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1991.04a
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• pp.111-116
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• 1991

주파수 영역에서 로터의 thickness noise는 블레이드의 여러가지 기하학적인 조건, 즉 블레이드 수, 블레이드의 사잇각, 에어포일의 모양 그리고 반경방향의 블레이드 끝모양에 따라서 해석되어진다. 본 논문은 비 등간격 로터의 기하학적 조건에 따른 음신호의 위상간섭 영향을 고찰하기 위해서 등간격을 이루는 로터에 대해서 해석된 Hanson의 방법을 확장하였다. thickness noise를 6개의 블레이드를 가진 로터에 대하여 15.deg., 30.deg., 45.deg., 비 등간격을 이루는 경우와 각각에 45 tip angle로 sweepback되었을 때 먼 거리의 음향학적 음신호와 그 스펙트럼을 계산하였다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.3
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• pp.10-16
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• 2004

In this paper the aeroacoustic characteristics of a helicopter main rotor system is measured by using a pair of scaled rotor blades. A low noise rotor test jig is developed for noise measurement and the rotational speed, thrust and torque are measured simultaneously in order to match the aerodynamic conditions with the full scale rotor. The accuracy of the force measurement device was checked through a calibration procedure. The measured thurst and torque with a 1.2m rotor are compared to the results of analytical prediction and showed that the thrust data at various rotational speed followed the prediction relatively well, but the torque data considered less accurate. It is also found that the background noise level of the test rig is sufficiently low, and the measured noise level from the rotor can be scaled with rotor tip speed. However, the Mach number dependancy and the directivity changes depend on the noise source characteristics.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.8
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• pp.699-708
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• 2021

In this paper, the aerodynamic and aeroacoustic characteristics that vary depending on the rotation axial distance between the upper and lower rotor, which is one of the design parameters of the coaxial rotor, is analyzed in the hovering condition using the computational fluid dynamics. Aerodynamic analysis using the Reynolds Averaged Navier Stokes equation and the aeroacoustic analysis using the Ffowcs Williams ans Hawkings equation is performed and the results were compared. The upper and lower rotor of the coaxial rotor have different phase angle which changes periodically by rotation and have unsteady characteristics. As the distance between the upper and lower rotors increased, the aerodynamic efficiency of the thrust and the torque was increased as the flow interaction decreased. In the aeroacoustic viewpoint, the noise characteristics radiated in the direction of the rotational plane showed little effect by axis spacing. In the vertical downward direction of the axis increased, the SPL maintains its size as the frequency increases, which affects the increase in the OASPL. As the axial distance of the coaxial rotor increased, the noise characteristics of a coaxial rotor were similar with the single rotor and the SPL decreased significantly.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.7
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• pp.469-478
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• 2019

Multirotor configurations such as VTOL and urban air mobility have been focused on today due to the high maneuverability. Aerodynamic and aeroacoustic characteristics of multirotor have much difference to those of a single rotor. In this study, a numerical analysis based on the free wake vortex lattice method is used for identifying the wake interaction effect. In order to compare the various configurations and operating conditions, the effects of the spacing between the rotors in hovering flight and the effects of the advancing ratio and the formation in forward flight are discussed. In the hovering flight, the unsteady loading of multirotor changes periodically and loading fluctuation increases as decreasing the spacing. It causes the variation in unsteady loading noise and the noise directivity pattern. In the forward flight, the difference in loading fluctuation and noise characteristics are observed according to the diamond and square formation of rotors. By comparing with results of single rotor analysis, multirotor configurations have different directivity pattern and amplitude of loading noise according to the location of each rotor. As a result, wake interaction effect becomes a highly important factor for aerodynamic and aeroacoustic analysis according to multirotor configurations and operating conditions.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.1
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• pp.20-25
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• 2005

The thickness, loading, and broadband noise generated from the trailing edge of the UH-1H main rotor are numerically compared each other. The Kocureck and Tangler's prescribed wake model is adopted to represent the wake geometry during the hovering motion. Three tip Mach numbers of $M_{T}$ = 0.2, 0.4, and 0.8, are selected to analyze the effects of different tip Mach numbers. At $M_{T}$ = 0.8, in considering the A-weighting and audible frequency band, the random noise is smaller than the tonal noises such as the thickness and the loading noise which have the low frequency characteristics. Especially most of the random noise frequency spread on the ultrasound region. On the other hand, below $M_{T}$ = 0.4, the band of random noise moves to the audible frequency region, and the random noise becomes larger than the tonal noise. It turns out that the random noise analysis of the rotor should be necessary at low speed operating condition.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.6
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• pp.516-525
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• 2015

In this paper, the acoustic pressure of a helicopter rotor in hovering and low speed descent flight is predicted and compared with experimental data. Ffowcs Williams-Hawkings equation is used to predict the acoustic pressure. Two different wind tunnel test data are used to validate the predicted results. Boeing 360 model rotor test results are used for the low-frequency noise in hover, and HART II test results are employed for the mid-frequency noise, especially BVI noise, in low speed descent flight. A simple free-wake model as well as the state-of-the-art CFD/CSD coupling method are adopted to perform the analysis. Numerical results show good agreement against the measured data for both low-frequency and mid-frequency harmonic noise signal. The noise carpet results predicted using the FFT(Fast Fourier Transform) shows also reasonable correlation with the measured data.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.10a
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• pp.64-69
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• 1997

정격속도 100,000RPM용 원심분리기(centrifuge) 로터베어링계에 대해 회전체동역학 해석이 수행된다. 시스템은 원심분리기 로터, 유연축, 모터 로터와 축, 그리고 모터축 지지용 두 개의 구름베어링으로 구성된다. 설계목표는 정격속도가 위험속도(critical speed)에 대해 충분한 분리여유를 갖고, 위험속도에서 로터의 양호한 불균형응답특성을 이루어 내는 것이다. 후자의 요구조건은, 시스템이 다수의 위험속도를 통과하며 정격속도 주위에서 충분한 분리 여유를 갖지 않을 수도 있기 때문에 특히 중요하다. 시스템에 초유연축(extra-flexible shaft)을 도입함으로써, 비록 1차 위험속도에서 만족스럽지 못한 큰 불균형응답을 가질지라도 고차 위험속도에서 만족스런 작은 불균형응답을 보인다. 1차 위험속도에서 로터의 큰 변위를 억제하기 위해서 범퍼링(bumper ring) 또는 안내베어링(guide bearing)을 유연축의 적절한 위치에 설치할 필요가 있다. 비록 유연축계라 할지라도 정격속도와 가까운 4차 이상의 고차 위험속도를 정확히 규명하기 위해서는 모터의 동역학을 전체시스템에 결합하여야 함을 볼 수 있다. 해석은 유한요소법(finite element method)에 의해 수행된다.