• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.12 s.255
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• pp.1588-1595
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• 2006

The stabilities of a pinned-pinned straight pipe conveying fluid are investigated by complexification-averaging method. The flow is assumed to vary harmonically about a constant mean velocity. Instability conditions of a governing equation are analytically obtained about parametric primary, secondary and combination resonances. The resulted stability conditions show that instabilities exist when the frequency of flow fluctuation is close to one and two times the natural frequency or to the sum of any two natural frequencies. In case that the fluctuated flow frequency is close to the difference of two natural frequencies, instabilities does not exist.

• International Journal of Fluid Machinery and Systems
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• v.10 no.2
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• pp.99-118
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• 2017

ECHO progress was defined to depict the rotordynamic pump theory development. Experience (E) era for pumps lasted nearly one and a half hundred years before the Industrial Revolution due to the low rotation speed of motor and undeveloped manufacture ability. Classic (C) theory referring to quasi-static performance as well as the items those were not able to be steadily resolved under the level were briefly and sophisticated outlined. Since 1962, flow instabilities and the dynamic responses had come into main attention with the development of the modern technologies such as ballistic missile, rocket and space shuttle main engine, and were finally heuristically (H) elucidated by talented scholars and researchers. Recently, new applications for the pumps open (O) to the surrounding fluid and diversity of the medium such as multiphase flow need more studies and some examples were briefly introduced to display the potential problems lastly.

• Journal of The Korean Astronomical Society
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• v.34 no.4
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• pp.247-249
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• 2001

The accretion disks are usually supposed symmetric to reflection on the Z=0 plane. Asymmetries in the flow are be ver-y small in the vicinity of the compact accretor. However their existence can have a important role in the case of subkeplerian accretion flows onto black holes. These flows lead to strong heating and even to the formation of shocks close to the centrifugal barrier. Large asymmetries are due to the development of the KH instability triggered by the small turbulences at the layer separating the incoming flow from the out coming shocked flow. The consequence of this phenomenon is the production of asymmetric outflows of matter and quasi periodic oscillations of the inner disk regions up and down the Z=0 plane.

• 유체기계공업학회:학술대회논문집
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• 1998.02a
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• pp.144-149
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• 1998

• Solar Energy
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• v.8 no.1
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• pp.33-40
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• 1988

The characteristics of thermal instabilities of natural convection in a horizontal fluid layer bounded below by a rigid plate and above by an interface with a passive gas is presented. The critical Grashof number decreases as the surface tension gradient effect (Marangoni effect) at the interface increases and the flow remains unstable for a critical Marangoni number depending on Prandtl numbers. These results are in substantial agreement with those of Smith and Davis.

• Wind and Structures
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• v.37 no.1
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• pp.57-78
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• 2023

In this study, a generalized three-degree-of-freedom (3-DoF) analytical model is formulated to predict linear aerodynamic instabilities of a prism under quasi-steady (QS) conditions. The prism is assumed to possess a generic cross-section exposed to turbulent wind flow. The 3-DoFs encompass two orthogonal horizontal directions and rotation about the prism body axis. Inertial coupling is considered to account for the non-coincidence of the mass center and the rotation center. The aerodynamic force coefficients-drag, lift, and moment-depend on the Reynolds number based on relative flow velocity, angle of attack, and the angle between the wind and the cable. Aerodynamic forces are linearized with respect to the static equilibrium configuration and mean wind velocity. Routh-Hurwitz and Liénard and Chipart criteria are used in the eigenvalue problem, yielding an analytical solution for instabilities in galloping and static divergence types. Additionally, the minimum structural damping and stiffness required to prevent these instabilities are numerically determined. The proposed 3-DoF instability model is subsequently applied to a conductor with ice accretion and a full-scale dry inclined cable. In comparison to existing models, the developed model demonstrates superior prediction accuracy for unstable regions compared with results in wind tunnel tests.

• International Journal of Fluid Machinery and Systems
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• v.9 no.2
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• pp.160-168
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• 2016

In high speed turbopumps, cavitation occurs and often causes the flow instabilities such as cavitation surge and rotating cavitation. The occurrence of these cavitation instabilities is considered to relate to dynamic characteristics of the cavitation, which are modelled using a cavitation compliance and a mass flow gain factor. Various types of cavitation such as a blade surface cavitation, a tip leakage vortex cavitation, and a backflow vortex cavitation occur at the same time in the inducer and the dynamic characteristics of each cavitation have not been clarified yet in experiments. Focusing on the blade surface cavitation as one of fundamental cavitation, we investigated the dynamic characteristics of the blade surface cavitation on a flat plate hydrofoil in experiments in the present study.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.416-421
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• 2000

The performance of gas turbine engines is affected by instabilities, like as rotating stall and/or surge. Rotating Stall is a transient intermediate stage between normal flow and complete flow breakdown leading to engine surge. Rotating Stall is associated with large amplitude nonaxisymmetric flow variations rotating around the compressor annulus. This paper presents the evolutions of stall propagation in a compressor cascade by numerical analysis. The flow phenomena due to stall cells and propagation speed are examined using 2 dimensional Navier - Stokes equations.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.1948-1966
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• 1991

Two-phase cross-flow exists in many shell-tube heat exchangers such as condensers, reboilers and nuclear steam generators. To avoid problems due to excessive vibration, information on vibration excitation in two-phase cross-flow is required. Fluid-elastic instability is discussed in this paper. Four tube bundle configurations were subjected to increasing flow up to the onset of fluid-elastic instability. The tests were done on bundles with one flexible tube surrounded by rigid tubes. The fluid-elastic instability behavior is different for intermittent flows than for bubbly flows. For bubbly flows, the observed instabilities satisfy the relationship V/fd=K(2.pi..zeta. m/rho. $d^{21}$)$^{0.51}$ in which the minimum instability factor K was found to be 2.3 for bundles of p/d=1.22. The lowest critical velocities for fluid-elastic instability were experienced with parallel-triangular tube bundles. For intermittent flow, the observed instabilities did not follow the forgoing relation-ship. Significantly lower flow velocities were required for instability..

• Nuclear Engineering and Technology
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• v.38 no.4
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• pp.343-352
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• 2006

Convenient analytical tools for evaluation of the aperiodic and the fluctuating instabilities of the passive residual heat removal system (PRHRS) of an integral reactor are developed and results are discussed from the viewpoint of the system design. First, a static model for the aperiodic instability using the system hydraulic loss relation and the downcomer feedwater heating equations is developed. The calculated hydraulic relation between the pressure drop and the feedwater flow rate shows that several static states can exist with various numbers of water-mode feedwater module pipes. It is shown that the most probable state can exist by basic physical reasoning, that there is no flow rate through the steam-mode feedwater module pipes. Second, a dynamic model for the fluctuating instability due to steam generation retardation in the steam generator and the dynamic interaction of two compressible volumes, that is, the steam volume of the main steam pipe lines and the gas volume of the compensating tank is formulated and the D-decomposition method is applied after linearization of the governing equations. The results show that the PRHRS becomes stabilized with a smaller volume compensating tank, a larger volume steam space and higher hydraulic resistance of the path $a_{ct}$. Increasing the operating steam pressure has a stabilizing effect. The analytical model and the results obtained from this study will be utilized for PRHRS performance improvement.