Soogab Lee

Dynamic stall control for advanced rotorcraft application

Advanced concepts designed to improve the lift, drag, and pitching moment characteristics of rotor blades have been investigated for the purpose of enhancing rotor maneuver capability. The advantages and disadvantages of these concepts have been evaluated using both computational and experimental means. The concepts that were considered in this study included a leading-edge slat, a deformable leading-edge, and upper-surface blowing. The results show the potential of these concepts for substantially improving the performance of a rotor. HE next generation of rotorcraft will be required to operate at much higher performance levels than in the past, particularly in the areas of nap-of-the-ea rth (NOE), deep-penetration operations, and air-to-air combat. These new requirements will require highly maneuverable, agile, and survivable rotorcraft, far exceeding the capabilities of those in the current inventory. The objectives of this project include an increase in the maneuverability/agility capability of the helicopter and a reduction in the acoustic detection range. The single most important element of the rotorcraft for meeting these requirements is the rotor itself, since it is the primary source of lift, control, and speed. At the same time, the rotor is also a major source of acoustically detectable radiation. Among the many factors affecting rotorcraft performance, the aerodynamic characteristics of the rotor system are the most important and are the main subject of this paper. The maneuvering capability of a rotorcraft can be improved by re- ducing or suppressing the vibratory loads on the rotor blades caused by aerodynamic separation and stall. This would have the effect of expanding the stall-limiting boundary of the rotor and thereby increase the available load factor in all flight regimes. The con- ventional way to obtain higher lift is to increase the blade area, however, this usually results in a heavier rotor that is also less ef- ficient. With regard to compressibili ty effects and acoustic radia- tion, improvements have been obtained by sweeping, tapering, and thinning the tip region of the rotor blade. As a result, numerous families of airfoils and planform shapes have evolved that offer bet- ter advancing-blade characteristics. However, improvements on the retreating-blade side have not been as impressive. One reason for this imbalance may be that design codes are available for treating blades at low angles of attack and high Mach number (characteris- tic of the advancing side), whereas the design strategy has had to depend heavily on costly empirical studies for blades at high angles of attack and having some amount of separation (characteristic of the retreating side). Increasing the tip speed of the rotor to achieve a maneuvering ad- vantage may produce a dangerous condition with regard to acoustic detection. Rapid advancements in passive acoustic sensor arrays and advanced signal processing technologies pose a serious threat to the mission effectiveness of Army helicopters. Since the rotor blade generates acoustic radiations that can be easily detected and identified, airfoil and planform shapes must be carefully optimized to reduce the detection range of the rotorcraft. The requirements for improved maneuverability and reduced sus- ceptibility will clearly demand a substantial growth in the technolo- gies for addressing rotor aerodynamics. New control techniques must be considered, both passive and active, and these must be ac- companied by a more thorough physical understanding of these flow phenomena along with substantially improved prediction capabili- ties. To meet these requirements, computational and experimental efforts have been initiated to evaluate the effectiveness of various concepts. At present these concepts include airfoils with slats and slots, airfoils that deform, and airfoils with flow energizers. Description of Experiment and Computational Fluid Dynamics Code

Annoyance caused by amplitude modulation of wind turbine noise

A listening test has been performed to investigate the relationship between human annoyance and the amplitude modulation of wind turbine noise. To obtain sound samples for the listening test, sound from a 1.5 MW wind turbine in Korea was recorded. The strength of the amplitude modulation of the sound samples was defined in terms of the modulation depth spectrum, which was approximated by assuming that the sound samples are sinusoidally amplitude-modulated. The stimuli for the listening tests were created by reducing the modulation depth spectrum of the sound samples. A total of 30 participants were involved in the listening tests. The results of the listening tests indicate that the equivalent sound level and the amplitude modulation of wind turbine noise both significantly contribute to noise annoyance.

High frequency formulation for the acoustic power spectrum due to cascade - turbulence interaction

This paper investigates the noise radiated by a cascade of flat-plate airfoils interacting with homogeneous, isotropic turbulence. An analytic formulation for the spectrum of acoustic power of a two-dimensional flat-plate is derived. The main finding of this paper is that the acoustic power spectrum from the cascade of flat airfoils may be split into two distinct frequency regions of low frequency and high frequency, separated by a critical frequency. Below this frequency, cascade effects due to the interaction between neighboring airfoils are shown to be important. At frequencies above the critical frequency, cascade effects are shown to be relatively weak. In this frequency range, acoustic power is shown to be approximately proportional to the number of blades. Based on this finding at high frequencies, an approximate expression is derived for the power spectrum that is valid above the critical frequency and which is in excellent agreement with the exact expression for the broadband power spectrum. The formulation is used to perform a parametric study on the effects on the power spectrum of the blade number, stagger angle, gap-chord ratio, and Mach number. The theory is also shown to provide a close fit to the measured spectrum of rotor-stator interaction.

Head-on parallel blade-vortex interaction

An experimental and computational study was carried out to investigate the parallel head-on blade-vortex interaction (BVI) and its noise generation mechanism. A shock tube, with an enlarged test section, was used to generate a compressible starting vortex which interacted with a target airfoil. The dual-pulsed holographic interferometry (DPHI) technique and airfoil surface pressure measurements were employed to obtain quantitative flowfield data during the BVI. A thin-layer Navier-Stokes code (BVI2D), with a high-order upwind-biased scheme and a multizonal grid, was also used to simulate numerically the phenomena occurring in the head-on BVI. The detailed structure of a convecting vortex was studied through independent measurements of density and pressure distributions across the vortex center. Results indicate that, in a strong head-on BVI, the opposite pressure peaks are generated on both sides of the leading edge as the vortex approaches. Then, as soon as the vortex passes by the leading edge, the high-pressure peak suddenly moves toward the low-pressure peak—reducing in magnitude as it moves—simultaneously giving rise to the initial sound wave. In both experiment and computation, it is shown that the viscous effect plays a significant role in head-on BVIs.

Effect of background noise levels on community annoyance from aircraft noise

A study of community annoyance caused by exposures to civil aircraft noise was carried out in 20 sites around Gimpo and Gimhae international airports to investigate the effect of background noise in terms of dose-effect relationships between aircraft noise levels and annoyance responses under real conditions. Aircraft noise levels were mainly measured using airport noise monitoring systems, B&K type 3597. Social surveys were administered to people living within 100 m of noise measurement sites. The question relating to the annoyance of aircraft noise was answered on an 11-point numerical scale. The randomly selected respondents, who were aged between 18 and 70 years, completed the questionnaire independently. In total, 753 respondents participated in social surveys. The result shows that annoyance responses in low background noise regions are much higher than those in high background noise regions, even though aircraft noise levels are the same. It can be concluded that the background noise level is one of the important factors on the estimation of community annoyance from aircraft noise exposure.

Reduction of blade-vortex interaction noise through porous leading edge

The effect of the porous leading edge of an airfoil on the blade-vortex interaction noise, which dominates the far-lield acoustic spectrum of the helicopter, is investigated. The thin-layer Navier-Stokes equations are solved with a high-order upwind-biased scheme and a multizonal grid system. The Baldwin-Lomax turbulence model is modified for considering transpiration on the surface. The amplitudes of the propagating acoustic wave in the near field are calculated directly from the computation. The porosity effect on the surface is modeled in two ways: 1) imposition of prescribed transpiration velocity distribution and 2) calculation of transpiration velocity distribution by Darcys law

Prediction method for tire air-pumping noise using a hybrid technique

Air-pumping noise from a car tire is investigated with a hybrid technique composed of three stages: (1) small-scale air-pumping noise generation process is modeled as a piston-like movement of the base-side of the tire groove and then numerically simulated; (2) the flow properties in the tire groove are used as air-pumping sources and noise propagation is simulated with emphasis placed on scattering process with full tire/road geometry; (3) the far-field acoustic pressure is predicted from a Kirchhoff integral method by using unsteady flow data in space and time which is provided by the computational fluid dynamics (CFD) calculation of full tire-road domain. The comparison of predicted results shows that the nonlinearity of the air-pumping noise generation mechanism affects not only noise characteristics in frequency domain but also in the directivity pattern. It seems that this approach can overcome the weakness of the acoustic monopole theory which stems from the usual assumption of a small amplitude ac...

The relationship between railway noise and community annoyance in Korea.

A study of community annoyance caused by exposures to railway noise was carried out in 18 areas along railway lines to accumulate social survey data and assess the relationship between railway noise levels and annoyance responses in Korea. Railway noise levels were measured with portable sound-level meters. Social surveys were administered to people living within 50 m of noise measurement sites. A questionnaire contained demographic factors, degree of noise annoyance, interference with daily activities, and health-related symptoms. The question relating to noise annoyance was answered on an 11-point numerical scale. The randomly selected respondents, who were aged between 18 to 70 years of age, completed the questionnaire independently. In total, 726 respondents participated in social surveys. Taking into consideration the urban structure and layout of the residential areas of Korea, Japan, and Europe, one can assume that the annoyance responses caused by the railway noise in this study will be similar to those found in Japan, which are considerably more severe than those found in European countries. This study showed that one of the most important factors contributing to the difference in the annoyance responses between Korea and Europe is the distance between railways and houses.

The effects of long-term exposure to railway and road traffic noise on subjective sleep disturbance

The exposure-response relationships between subjective annoyance with sleep disturbance from railway trains and road traffic noise were established from an extensive social survey by CENVR (Center for Environmental Noise and Vibration Research) in Korea. The objectives of this research are to determine the long-term effects of noise on sleep and to compare the exposure-response relationships from different noise sources with those from other studies and to elucidate the effects of some modifying factors on subjective responses to noise. From an investigation of the percentage of a highly sleep-disturbed population (%HSD) in response to railway and road traffic noise, it was found that sleep is affected more by railway noise than by road traffic noise. The effects of non-acoustical factors on the responses were examined and sensitivity was shown to be a significant modifying factor, as it pertains to subjective sleep disturbance. A comparison of the response curves from an analysis of pooled data from predominantly European surveys by Miedema and Vos [Behav. Sleep Med. 5, 1-20 (2007)] with the response curves from this survey showed more of a subjective sleep disturbance response in this survey to railway noise, whereas there was no significant difference in terms of a response to road traffic noise.

Noise-induced annoyance from transportation noise: Short-term responses to a single noise source in a laboratory

An experimental study was performed to compare the annoyances from civil-aircraft noise, military-aircraft noise, railway noise, and road-traffic noise. Two-way within-subjects designs were applied in this research. Fifty-two subjects, who were naive listeners, were given various stimuli with varying levels through a headphone in an anechoic chamber. Regardless of the frequency weighting network, even under the same average energy level, civil-aircraft noise was the most annoying, followed by military-aircraft noise, railway noise, and road-traffic noise. In particular, penalties in the time-averaged, A-weighted sound level (TAL) of about 8, 5, and 5 dB, respectively, were found in the civil-aircraft, military-aircraft, and railway noises. The reason could be clarified through the high-frequency component and the variability in the level. When people were exposed to sounds with the same maximum A-weighted level, a railway bonus of about 3 dB was found. However, transportation noise has been evaluated by the time-averaged A-weighted level in most countries. Therefore, in the present situation, the railway bonus is not acceptable for railway vehicles with diesel-electric engines.