• Title, Summary, Keyword: reduced frequency

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Unsteady Aerodynamic Characteristics depending on Reduced Frequency for a Pitching NACA0012 Airfoil at Rec=2.3×104

  • Kim, Dong-Ha;Chang, Jo-Won;Sohn, Myong Hwan
    • International Journal of Aeronautical and Space Sciences
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    • v.18 no.1
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    • pp.8-16
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    • 2017
  • Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number $2.3{\times}10^4$. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

Reduced Frequency Effects on the Near-Wake of an Oscillating Elliptic Airfoil

  • Chang, Jo-Won;Eun, Hee-Bong
    • Journal of Mechanical Science and Technology
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    • v.17 no.8
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    • pp.1234-1245
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    • 2003
  • An experimental study was carried out to investigate the reduced frequency effect on the near-wake of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched around the center of the chord between -5$^{\circ}$and +25$^{\circ}$angles of attack at an airspeed of 3.4 m/s. The chord Reynolds number and reduced frequencies were 3.3 ${\times}$10$^4$, and 0.1, 0.7, respectively Phase-averaged axial velocity and turbulent intensity profiles are presented to show the reduced frequency effects on the near-wake behind the airfoil oscillating In pitch. Axial velocity defects in the near-wake region have a tendency to increase in response to a reduced frequency during pitch up motion, whereas it tends to decrease during pitch down motion at a positive angle of attack. Turbulent intensity at positive angles of attack during the pitch up motion decreased in response to a reduced frequency, whereas turbulent intensity during the pitch down motion varies considerably with downstream stations. Although the true instantaneous angle of attack compensated for a phase-lag is large, the wake thickness of an oscillating airfoil is not always large because of laminar or turbulent separation.

Visualization Study of Dragonfly Type Wing : Reduced Frequency (잠자리 유형 날개의 가시화 연구 : 무차원 진동수)

  • Kim Song Hak;Chang Jo Won
    • 한국가시화정보학회:학술대회논문집
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    • pp.14-17
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    • 2004
  • A purpose of this visual experiment is to investigate the effect of reduced frequency qualitatively by examining wake pattern change for insect flying motion. Insect is composed of two pair wing with forewing and hindwing, flying motion of insect is performed pitching and plunging so it makes a separation over the wings. The separation affects at the wake pattern and changed wake pattern has an influence on lift, drag and propulsion. This experiment is conducted by using a smoke wire technique and a camera is fixed at hindwing to take a photograph of wake. An electronic device is mounted below test section to find exact the mean positional angle of wing. The reduced frequency in experiment is 0.15, 0.3 and 0.45. We obtained the result which that reduced frequency is closely related to wake pattern that determines flight efficiency.

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Visualization Study on a Reduced Frequency of a Dragonfly type wing (잠자리 유헝 날개의 무차원 진동수에 따른 가시화 연구)

  • Kim Song Hak;Chang Jo Won
    • Journal of the Korean Society of Visualization
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    • v.2 no.2
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    • pp.58-65
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    • 2004
  • The purpose of this visualization study is to investigate the effect of reduced frequency qualitatively by examining wake patterns for dragonfly flight motion. Dragonflies have two pairs of wing (a forewing and hindwing) and flight is achieved by a pitching and plunging, so it makes a separation over the wings. The separation affects the wake pattern and changed wake pattern has an influence on lift, drag, and propulsion. This experiment was conducted by using a smoke wire technique and a camera fixed above the test section used to take a photograph of the wake. An electronic device is mounted below the test section to find the exact mean positional angle of the wing. The reduced frequency in the experiment is 0.15, 0.3 and 0.45. Results show that reduced frequency is closely related to the wake pattern that determines flight efficiency.

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The Assessment on the Sound Quality of Reduced Frequency Selectivity of Hearing Impaired People (난청인의 주파수 선택도 둔화현상이 음질에 미치는 영향 평가)

  • An, Hong-Sub;Park, Gyu-Seok;Jeon, Yu-Yong;Song, Young-Rok;Lee, Sang-Min
    • The Transactions of The Korean Institute of Electrical Engineers
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    • v.60 no.6
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    • pp.1196-1203
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    • 2011
  • The reduced frequency selectivity is a typical phenomenon of sensorineural hearing loss. In this paper, we compared two modeling methods for reduced frequency selectivity of hearing impaired people. The two models of reduced frequency selectivity were made using LPC(linear prediction coding) algorithm and bandwidth control algorithm based on ERB(equivalent rectangular bandwidth) of auditory filter, respectively. To compare the effectiveness of two models, we compared the result of PESQ (perceptual evaluation of speech quality) and LLR(log likelihood ratio) using 36 Korean words of two syllables. To verify the effect on noise condition, we mixed white and babble noise with 0dB and -3dB SNR to speech words. As the result, it is confirmed that the PESQ score of bandwidth control algorithm is higher than the score of LPC algorithm, on the other hands, and the LLR score of LPC algorithm is lower than the score of bandwidth control algorithm. It means that both non-linearity and widen auditory filter characteristics caused by reduced frequency selectivity could be more reflected in bandwidth control algorithm than in LPC algorithm.

Frequency Effects of Upstream Wake and Blade Interaction on the Unsteady Boundary Layer Flow

  • Kang, Dong-Jin;Bae, Sang-Su
    • Journal of Mechanical Science and Technology
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    • v.16 no.10
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    • pp.1303-1313
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    • 2002
  • Effects of the reduced frequency of upstream wake on downstream unsteady boundary layer flow were simulated by using a Wavier-Stokes code. The Wavier-Stokes code is based on an unstructured finite volume method and uses a low Reynolds number turbulence model to close the momentum equations. The geometry used in this paper is the MIT flapping foil experimental set-up and the reduced frequency of the upstream wake is varied in the range of 0.91 to 10.86 to study its effect on the unsteady boundary layer flow. Numerical solutions show that they can be divided into two categories. One is so called the low frequency solution, and behaves quite similar to a Stokes layer. Its characteristics is found to be quite similar to those due to either a temporal or spatial wave. The low frequency solutions are observed clearly when the reduced frequency is smaller than 3.26. The other one is the high frequency solution. It is observed for the reduced frequency larger than 7.24. It shows a sudden shift of the phase angle of the unsteady velocity around the edge of the boundary layer. The shift of phase angle is about 180 degree, and leads to separation of the boundary layer flow from corresponding outer flow. The high frequency solution shows the characteristics of a temporal wave whose wave length is half of the upstream frequency. This characteristics of the high frequency solution is found to be caused by the strong interaction between unsteady vortices. This strong interaction also leads to destroy of the upstream wake strips inside the viscous sublayer as well as the buffer layer.

Flow Visualization for a Dragonfly Type Wing (잠자리 유형 날개에 대한 흐름 가시화)

  • Kim, Song-Hwak;Kim, Hyun-Seok;Chang, Jo-Won;Boo, Joon-Hong
    • Proceedings of the KSME Conference
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    • pp.1586-1591
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    • 2004
  • Flow visualization experiments have been performed to investigate the effects of phase lag, reduced frequency qualitatively by examining wake pattern on a dragonfly type wing. The model was built with a scaled-up, flapping wings, composed of paired wings with fore- and hindwing in tandem, that mimicked the wing form of a dragonfly. The present study was conducted by using the smoke-wire technique, and an electronic device was mounted to find the exact positional angle of wing below the tandem wings, which amplitude is ranged from $-16.5^{\circ}$ to $+22.8^{\circ}$. Phase lag applied on the wings is $0^{\circ}$, $90^{\circ}$, $180^{\circ}$ and $270^{\circ}$. The reduced frequency is 0.15, 0.3 and 0.45 to investigate the effect of reduced frequency. It is inferred through observed wake pattern that the phase lag clearly plays an important role in the wake structures and in the flight efficiency as changing the interaction of wings. The reduced frequency also is closely related to wake pattern and determines flight efficiency.

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Reduced Order H$\infty$ Controller Synthesis

  • Ogawa, Tomohiro;Iida, Michihiko
    • 제어로봇시스템학회:학술대회논문집
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    • pp.161-166
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    • 1998
  • In this paper, an approach to the reduced order H$_{\infty}$ controller synthesis is proposed. This approach employs the frequency weighted model reduction whose frequency weights are deduced from the closed-loop system regarding the controller order reduction errors as uncertainties in a plant, while the resultant reduced order H$_{\infty}$ controller guarantees prescribed H$_{\infty}$ control performances.

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Effect of Reduced Frequency on the Flow Pattern of Pitch Oscillating Elliptic Airfoil (피치 진동하는 타원형 에어포일의 환산주파수가 날개 주위 유동패턴에 미치는 영향)

  • Lee, Ki-Young;Chung, Hyong-Seok;Sohn, Myong-Hwan
    • Journal of the Korea Institute of Military Science and Technology
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    • v.9 no.4
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    • pp.128-136
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    • 2006
  • The purpose of this paper is to examine the dynamic stall characteristics of an elliptic airfoil when subject to constant pitch motions. In this study, which was motivated by the pressing need for a greater understanding of the reduced frequency$({\kappa})$ effects on flow patterns of elliptic airfoil, the various reduced frequencies were considered. The result confirms that the reduced frequency has a profound effects on the flow patterns. The increase of ${\kappa}$ accelerate the separation bubble bursting process up to ${\kappa}=0.10$, then diminish with further increase in ${\kappa}$. Compared with static condition, the dynamic pitching airfoil delays stall angle approximate $4{\circ}{\sim}5{\circ}$ during pitch-up stroke for ${\kappa}=0.10$. Results from this qualitative analysis provided valuable insight Into the control of dynamics stall.

An Eigen Analysis with Out-of-Plane Deformable Ring Element (면외변형 링 요소를 이용한 고유해석)

  • Moon, Won-Joo;Min, Oak-Key;Kim, Yong-Woo
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.17 no.7
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    • pp.1719-1730
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    • 1993
  • This paper presents the theoretical natural frequencies of out-of-plane deformable ring based on the variables such as out-of-plane deflection, torsional rotation and shear rotation. Based on the same variables, a finite element eigen analysis is carried out by using the $C^0$-continuous, isoparametric element which has three nodes per element and three degrees-of-freedom at each node. Numerical experiments are peformed to find the integration scheme which produces accurate natural frequencies, natural modes and correct rigid body motion. The uniformly reduced integration and the selective reduced integration give more accurate numerical frequencies than the uniformly full integration, but the uniformly reduced integration produces incorrect rigid body motion while selective reduced integration does correct one. Therefore, the ring element based on the three variables which employes selective reduced integration is recommended to avoid spurious modes, to alleviate the error due to shear locking and to produce correct rigid body motion, simultaneously.