• Title/Summary/Keyword: Onset of Instability

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Analysis for Onset of Changma Using Ieodo Ocean Research Station Data (이어도 기상 관측 자료를 활용한 장마 시작일 분석)

  • Oh, Hyoeun;Ha, Kyung-Ja;Shim, Jae-Seol
    • Atmosphere
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    • v.24 no.2
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    • pp.189-196
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    • 2014
  • The definition of onset date of Changma is revisited in this study using a quality controlled Ieodo ocean research station data. The Ieodo station has great importance in terms of its southwest location from Korean Peninsula and, hence, makes it possible to predict Changma period in advance with less impact of continents. The onset date of Changma using the Ieodo station data is defined by the time that meridional wind direction changes and maintains from northerly to southerly, and then the zonal wind changes from easterly to westerly after first June. This definition comes from a recognition that the establishment and movement of the western North Pacific subtropical high (WNPSH) cause Changma through southwesterly flow. The onset data of Changma has been determined by large-scale dynamic-thermodynamic characteristics or various meteorological station data. However, even the definition based on circulation data at the Ieodo station has a potential for the improved prediction skill of the onset date of Changma. The differences between before and after Changma, defined as Ieodo station data, are also found in synoptic chart. The convective instability and conspicuous circulations, corresponding low-level southwesterly flow related to WNPSH and strong upper-level zonal wind, are represented during Changma.

Thermoelastic Instability of the Layer Sliding between Two Non-conducting Half-planes (비전도 반평판 사이에서 미끄럼 운동하는 평판 층의 열탄성 불안정성)

  • 하태원;조용구;김흥섭;이정윤;오재응
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 2003.05a
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    • pp.483-488
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    • 2003
  • Frictional heating in brakes causes thermoelastic distortion of the contacting bodies and hence affects the contact pressure distribution. The resulting thermo-mechanical coupling can cause thermoelastic instability (TEI) if the sliding speed is sufficiently high, leading to non-uniform heating called hot spots and low frequency vibration known as hot judder. The vibration of brakes to the known phenomenon of frictionally-excited thermoelastic instability is estimated studying the interface temperature and pressure evolution with time. A simple model has been considered where a layer with half-thickness ${\alpha}$ slides with speed V between two half-planes which are rigid and non-conducting. The advantage of this properly simple model permits us to deduce analytically the critical conditions for the onset of instability, which is the relation between the critical speed and the growth rate of the interface temperature and pressure. Symmetrical component of pressure and temperature distribution at the layer interfaces can be more unstable than antisymmetrical component. As the thickness ${\alpha}$ reduces, the system becomes more apt to thermoelastic instability. Moreover, the evolution of the system beyond the critical conditions has shown that even if low frequency perturbations are associated with low critical speed, it might be less critical than high frequency perturbations if the working sliding speed is much larger than the actual critical speed of the system.

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Thermoelastic Instability of the Layer Sliding between Two Rigid Non-conducting Half-planes (단단한 비전도 반평판 사이에서 미끄럼 운동하는 평판층의 열탄성 불안정성)

  • 오재응;하태원;조용구;김흥섭;이정윤
    • Transactions of the Korean Society of Automotive Engineers
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    • v.12 no.1
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    • pp.114-121
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    • 2004
  • Frictional heating in brakes causes thermoelastic distortion of the contacting bodies and hence affects the contact pressure distribution. The resulting thermo-mechanical coupling can cause thermoelastic instability (TEI) if the sliding speed is sufficiently high, leading to non-uniform heating called hot spots and low frequency vibration known as hot judder. The vibration of brakes to the known phenomenon of frictionally-excited thermoelastic instability is estimated studying the interface temperature and pressure evolution with time. A simple model has been considered where a layer with half-thickness$\alpha$slides with speed V between two half-planes which are rigid and non-conducting. The advantage of this properlysimple model permits us to deduce analytically the critical conditions for the onset of instability, which is the relation between the critical speed and the growth rate of the interface temperature and pressure. Symmetrical component of pressure and temperature distribution at the layer interfaces can be more unstable than antisymmetrical component. As the thickness $\alpha$ reduces, the system becomes more apt to thermoelastic instability. For perturbations with wave number smaller than the critical$m_{cr}$ the temperature increases with m vice versa for perturbations with wave number larges than $m_{cr}$ , the temperature decreases with m.

Onset of Buoyancy-Driven Convection in a Fluid-Saturated Porous Layer Bounded by Semi-infinite Coaxial Cylinders

  • Kim, Min Chan
    • Korean Chemical Engineering Research
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    • v.57 no.5
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    • pp.723-729
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    • 2019
  • A theoretical analysis was conducted of convective instability driven by buoyancy forces under transient temperature fields in an annular porous medium bounded by coaxial vertical cylinders. Darcy's law and Boussinesq approximation are used to explain the characteristics of fluid motion and linear stability theory is employed to predict the onset of buoyancy-driven motion. The linear stability equations are derived in a global domain, and then cast into in a self-similar domain. Using a spectral expansion method, the stability equations are reformed as a system of ordinary differential equations and solved analytically and numerically. The critical Darcy-Rayleigh number is founded as a function of the radius ratio. Also, the onset time and corresponding wavelength are obtained for the various cases. The critical time becomes smaller with increasing the Darcy-Rayleigh number and follows the asymptotic relation derived in the infinite horizontal porous layer.

Numerical Simulation on the Onset of Radial Fingering in a Hele-Shaw Cell or a Porous Medium

  • Min Chan Kim
    • Korean Chemical Engineering Research
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    • v.62 no.1
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    • pp.112-117
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    • 2024
  • Numerical simulations on the onset and the growth of viscous fingering during the miscible displacement due to the radial source flow were conducted. With introduction of a new stability criterion, the critical log-viscosity ratio, Rc, was found as a function of the Peclet number, Pe. Similar to the previous linear stability analyses, Pe made the system unstable, i.e., accelerated the onset of instability. For a large Pe system, the present numerical simulation yielded much stable results than the previous theoretical predictions This discrepancy was commonly encountered in the comparison between the theoretical prediction and the experimental finding. Additionally, the difference between the rectilinear system and the present one was also discussed. The present system was found more insensitive to the Peclet number than the rectilinear one.

Energy Stability Analysis on the Onset of Buoyancy-Driven Convection in a Horizontal Fluid Layer Subject to Evaporative Cooling

  • Kim, Min Chan
    • Korean Chemical Engineering Research
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    • v.57 no.1
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    • pp.142-147
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    • 2019
  • The onset of buoyancy-driven convection in an initially isothermal and quiescent horizontal fluid layer was analyzed theoretically. It is well-known that at the critical Rayleigh number $Ra_c=669$ convective motion sets in with a constant-heat-flux cooling through the upper boundary. Here, based on the momentary instability concept, the dimensionless critical time ${\tau}_m$ to mark the onset of convective motion for Ra > 669 was analyzed theoretically. The energy method under the momentary stability concept was used to find the critical conditions as a function of the Rayleigh number Ra and the Prandtl number Pr. The predicted critical conditions were compared with the previous theoretical and experimental results. The momentary stability criterion gives more reasonable wavenumber than the conventional energy method.

Effects of Viscous Dissipation on the Thermal Instability of Plane Couette Flow Heated from Below (밑으로부터 가열되는 평면 Couette 유동에서 점성소산이 열적 불안정성에 미치는 영향)

  • Yoo, Jung Yul;Park, Young Moo
    • The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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    • v.17 no.4
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    • pp.489-498
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    • 1988
  • An analysis has been given for the effect of viscous dissipation on the thermal instability of plane Couette flow between two parallel plates maintained at different constant temperatures. Under the assumption that the principle of the exchange of stabilities holds, stationary disturbance quantities in the form of longitudinal vortices are considered. The magnitudes of disturbance quantities are then represented as fast convergent power series so that the eigenvalue problem for determining the onset conditions of the thermal instability may be reduced to a simplified problem of finding the roots of a $4{\times}4$ determinant. It is shown that as the magnitude of the visucous dissipation increases the flow becomes more susceptible to instabilities, which is in very good agreement with previous results obtained in some related researches.

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A Study on Dynamic Characteristics of Synchronously Controlled Hydrodynamic Journal Bearing (동기 제어되는 동압 베어링의 동특성에 관한 연구)

  • Rho, Byoung-Hoo;Kim, Kyung-Woong
    • Proceedings of the KSME Conference
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    • 2001.06c
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    • pp.311-315
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    • 2001
  • In this paper synchronous whirl of bearing is employed as control algorithm of actively controlled hydrodynamic journal bearing to suppress the whirl instability and unbalance response of a rotor-bearing system. Also, the cavitation algorithm implementing the Jakobsson-Floberg-Olsson boundary condition is adopted to predict cavitation regions in the fluid film more accurately than conventional analysis which uses the Reynolds condition. The stability and unbalance responses of a rotor-bearing system are investigated for various control gain and phase difference between the bearing and journal motion. It is shown that the unbalance response of a rotor-bearing system can be greatly improved by synchronous whirl of the bearing, and there is an optimum phase difference, which gives the minimum unbalance response of the system, at given operating condition. It is also found that the speed at onset of instability can be greatly increased by synchronous whirl of the bearing.

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Investigation of Self-Excited Combustion Instabilities in Two Different Combustion Systems

  • Seo, Seonghyeon
    • Journal of Mechanical Science and Technology
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    • v.18 no.7
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    • pp.1246-1257
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    • 2004
  • The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300㎐ are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifest that a self-excited high frequency instability has strong nonlinear characteristics.