• Title/Summary/Keyword: Unsteady Fluid Flow

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Numerical simulation of unsteady galloping of two-dimensional iced transmission line with comparison to conventional quasi-steady analysis

  • Yang, Xiongjun;Lei, Ying;Zhang, Jianguo
    • Structural Engineering and Mechanics
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    • v.75 no.4
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    • pp.487-496
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    • 2020
  • Most of the previous works on numerical analysis of galloping of transmission lines are generally based on the quasisteady theory. However, some wind tunnel tests of the rectangular section or hangers of suspension bridges have shown that the galloping phenomenon has a strong unsteady characteristic and the test results are quite different from the quasi-steady calculation results. Therefore, it is necessary to check the applicability of the quasi-static theory in galloping analysis of the ice-covered transmission line. Although some limited unsteady simulation researches have been conducted on the variation of parameters such as aerodynamic damping, aerodynamic coefficients with wind speed or wind attack angle, there is a need to investigate the numerical simulation of unsteady galloping of two-dimensional iced transmission line with comparison to wind tunnel test results. In this paper, it is proposed to conduct a two dimensional (2-D) unsteady numerical analysis of ice-covered transmission line galloping. First, wind tunnel tests of a typical crescent-shapes iced conductor are conducted firstly to check the subsequent quasisteady and unsteady numerical analysis results. Then, a numerical simulation model consistent with the aeroelastic model in the wind tunnel test is established. The weak coupling methodology is used to consider the fluid-structure interaction in investigating a two-dimension numerical simulation of unsteady galloping of the iced conductor. First, the flow field is simulated to obtain the pressure and velocity distribution of the flow field. The fluid action on the iced conduct at the coupling interface is treated as an external load to the conductor. Then, the movement of the conduct is analyzed separately. The software ANSYS FLUENT is employed and redeveloped to numerically analyze the model responses based on fluid-structure interaction theory. The numerical simulation results of unsteady galloping of the iced conduct are compared with the measured responses of wind tunnel tests and the numerical results by the conventional quasi-steady theory, respectively.

Unsteady Flow Characteristics of an Axial Flow Fan Installed in the Outdoor Unit of Air Conditioner (에어콘 실외기용 축류송풍기의 비정상 유동장 특성 연구)

  • Jang, Choon-Man
    • 유체기계공업학회:학술대회논문집
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    • 2005.12a
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    • pp.223-230
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    • 2005
  • The unsteady nature of vortex structures has been investigated by a large eddy simulation (LES) in an axial flow fan with a shroud covering only the rear region of its rotor tip. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the fan. The movements of the vortex structures induce high-pressure fluctuations on the rotor blade and in the blade passage. Frequency characteristics of the fluctuating pressure on the rotor blade are analyzed using wavelet transform. The dominant frequency of the real-time pressure selected at the high pressure fluctuation region corresponds well to that of the fluctuating rotor torque and the experimental result of fan noise. It is mainly generated due to the unsteady behavior of the vortical flow, such as the tip vortex and the leading edge separation vortex.

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An Analysis of the Unsteady Flow-Field and Aerodynamic Sound of a Turbo Fan used in a Vacuum Cleaner (청소기용 터보홴의 비정상 유동장 및 공력소음 해석)

  • Jeon, Wan-Ho;Kim, Chang-Joon;Rew, Ho-Seon
    • The KSFM Journal of Fluid Machinery
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    • v.5 no.2 s.15
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    • pp.36-42
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    • 2002
  • A new method to calculate the aeroacoustic pressure of a centrifugal fan that is used in a vacuum cleaner has been developed. The centrifugal fan consists of the impeller, the diffuser, and the circular casing. Due to the high rotating speed of the impeller and the small gap distance between the impeller and diffuser, the centrifugal fan makes very high noise levels at BPF and its harmonic frequencies. In order to calculate the sound pressure of a centrifugal fan, the unsteady flow field data is needed. This unsteady flow field is calculated by the vortex method. The sound pressure is then calculated by acoustic analogy. In this paper, only dipole term is considered in the equation. The noise generated by moving impeller and stationary diffuser is calculated separately. The predicted acoustic pressures agree very well with the measured data. The difference between the two is less than 4dB

Three-Dimensional Noise Analysis of an Axial-Flow Fan using Computational Aero-Acoustics (공력음향학을 이용한 축류홴의 삼차원 소음 해석)

  • Kim, Joo-Hyung;Kim, Jin-Hyuk;Shin, Seungyeol;Kim, Kwang-Yong;Lee, Seungbae
    • The KSFM Journal of Fluid Machinery
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    • v.15 no.5
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    • pp.48-53
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    • 2012
  • This paper presents a systematic procedure for three-dimensional noise analysis of an axial-flow fan by using computational aero-acoustics based on Ffowcs Williams-Hawkings equation. Flow-fields of a basic fan model are simulated by solving three-dimensional, unsteady, Reynolds-averaged Navier-Stokes equations using the commercial code ANSYS CFX 11.0. Starting with steady flow results, unsteady flow analysis is performed to extract the fluctuating pressures in the time domain at specified local points on the blade surface of the axial flow fan. The perturbed density wave by rotating blades reaches at the observer position, which is simulated by an in-house noise prediction software based on Ffowcs Williams-Hawkings equation. The detailed far-field noise signatures from the axial-flow fan are analyzed in terms of source types, field characteristics, and interpolation schemes.

CFD Analysis of Cavitation Phenomena in Mixed-Flow Pump

  • Sedlar, Milan;Sputa, Oldrich;Komarek, Martin
    • International Journal of Fluid Machinery and Systems
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    • v.5 no.1
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    • pp.18-29
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    • 2012
  • This paper deals with the CFD analysis of cavitating flow in the mixed-flow pump with the specific speed of 1.64 which suffers from a high level of noise and vibrations close to the optimal flow coefficient. The ANSYS CFX package has been used to solve URANS equations together with the Rayleigh-Plesset model and the SST-SAS turbulence model has been employed to capture highly unsteady phenomena inside the pump. The CFD analysis has provided a good picture of the cavitation structures inside the pump and their dynamics for a wide range of flow coefficients and NPSH values. Cavitation instabilities were detected at 70% of the optimal flow coefficient close to the NPSH3 value (NPSH3 is the net positive suction head required for the 3% drop of the total head of the pump).

A Numerical Study of Unsteady Wake Flow Characteristics in a Torque Converter (토크 컨버터 내부의 비정상 후류 유동특성에 대한 수치해석 연구)

  • Won, Chan-Shik;Hur, Nahm-Keon
    • The KSFM Journal of Fluid Machinery
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    • v.9 no.5 s.38
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    • pp.36-41
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    • 2006
  • In the present study, a transient incompressible viscous turbulent flow is simulated for the automotive torque converter with moving mesh technique. For the analysis, entire torque converter flow passages are modeled. Computed torque ratio, capacity factor and efficiency show a good agreement with the experiment data. The flow instabilities characterized by back-flow and wake etc. appeared in some cascade passages are shown to be propagating along tangential direction. These flow patterns are mainly influenced by the pump and turbine blade passing and can't be predicted through conventional steady simulation with a mixing plane approach. The understanding of the unsteady flow characteristics in a torque converter achieved in the present study may lead to the optimal design of a torque converter.

Characteristics of Transonic Flow-Induced Vibration for a Missile Wing Considering Structural Nonlinearity and Shock Inference Effects (구조 비전형성 및 충격파 간섭효과를 고려한 미사일 날개의 천음속 유체유발 진동특성)

  • Kim, Dong-Hyun;Lee, In;Kim, Seung-Ho;Kim, Tae-Hyoun;Lee, James S.
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 2002.11b
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    • pp.914-920
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    • 2002
  • Nonlinear flow-induced vibration characteristics of a generic missile wing (or control surface) are investigated in this study. The wing model has freeplay structural nonlinearity at its pitch axis. Nonlinear aerodynamic flows with unsteady shock waves are considered in the transonic flow region. To practically consider the effects of freeplay structural nonlinearity, the fictitious mass method (FMM) is applied to structural vibration analysis based on a finite element method (FEM). A computational fluid dynamics (CFD) technique is used for computing the nonlinear unsteady aerodynamics of all-movable wings. The aerodynamic analysis is based on the efficient transonic small-disturbance aerodynamic equations of motion using the potential-flow theory. To solve the nonlinear aeroelastic governing equations including the freeplay effect, a modal-based computational structural dynamic (CSD) analysis technique based on fictitious mass method (FMM) is used in time-domain. In addition, CSD and unsteady CFD techniques are simultaneously coupled to give accurate computational results. Various aeroelastic computations have been performed for a generic missile wing model. Linear and nonlinear aeroelastic computations have been conducted and the characteristics of flow-induced vibration are introduced.

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Study of the Unsteady Gas Flow in a Critical Nozzle (임계노즐에서 발생하는 비정상유동에 관한 연구)

  • Kim, Jae-Hyung;Kim, Heuy-Dong;Park, Kyung-Am
    • 유체기계공업학회:학술대회논문집
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    • 2002.12a
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    • pp.337-345
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    • 2002
  • The present study addresses a computational result of unsteady gas flow through a critical nozzle. The axisymmetric, unsteady, compressible, Wavier-Stokes equations are solved using a finite volume method that makes use of the second order upwind scheme for spatial derivatives and the multi-stage Runge-Kutta integral scheme for time derivatives. The steady solutions of the governing equation system are validated with the previous experimental data to ensure that the present computational method is valid to predict the critical nozzle flows. In order to simulate the effects of back pressure fluctuations on the critical nozzle flows, an excited pressure oscillation with an amplitude and frequency is assumed downstream of the exit of the critical nozzle. The results obtained show that for low Reynolds numbers, the unsteady effects of the pressure fluctuations can propagate upstream of the throat of critical nozzle, and thus giving rise to the applicable fluctuations in mass flow rate through the critical nozzle, while for high Reynolds numbers, the pressure signals occurring at the exit of the critical nozzle do not propagate upstream beyond the nozzle throat. For very low Reynolds number, it is found that the sonic line near the throat of the critical nozzle remarkably fluctuateswith time, providing an important mechanism for pressure signals to propagate upstream of the nozzle throat, even in choked flow conditions. The present study is the first investigation to clarify the unsteady effects on the critical nozzle flows.

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Flow Characteristics of Centrifugal Impeller Exit under Rotating Stall (선회실속하의 원심 임펠러 출구 유동 특성)

  • Shin, You-Hwan;Kim, Kwang-Ho
    • The KSFM Journal of Fluid Machinery
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    • v.2 no.2 s.3
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    • pp.5-12
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    • 1999
  • This study presents the measured unsteady flctuation of impeller discharge flow for a centrifugal compressor in an unstable operating region. The characteristics of the blade-to-blade flow at rotating stall onset were investigated by measuring unsteady velocity fluctuations at several different diffuser axial distances using a hot wire anemometer. The flow characteristics in terms of the radial and tangential velocity components and the flow angle distribution at the impeller exit were analyzed using phase-locked ensemble averaging techniques. As a result, increase or decrease of the radial velocity component during the rotating stall is dominated by that of the suction side. The radial velocity distributions show the opposite trends in the regions where the radial velocity during rotating stall onset increases and decreases.

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A Study on Unsteady Flow and Movement around a Check Valve in a Scroll Compressor (스크롤 압축기 밸브주변의 비정상유동과 밸브거동에 관한 연구)

  • Lee, Jin-Kab;Rew, Ho-Seon
    • The KSFM Journal of Fluid Machinery
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    • v.2 no.1 s.2
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    • pp.108-113
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    • 1999
  • In a scroll compressor it is generally accepted that a check valve is necessary to prevent reverse rotation of the scrolls. The check valve is subjected to discharge pulsations and their resultant forces. The flow phenomena around the check valve may affect the efficiency and the noise level significantly. The motivation of this study is to understand the flow phenomena and the unstable motion of the check valve on operating conditions in order to identify reasons raising noise and improve the performance of the check valve. In this study, unsteady flow simulation was performed using CFD and the pressure distribution around the check valve was obtained. This paper also shows that unstable motion of the check valve on standard operating conditions through theoretical analysis and flow visualization.

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