• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.2 no.4
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• pp.268-278
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• 1990

Hydrodynamic stability equations are formulated for natural convection flows adjacent to a heated or cooled, inclined, isothermal surface in pure water at $4^{\circ}C$, where the density variation with temperature becomes nonlinear. The resulting stability equations, when reduced to ordinary differential equations by a similarity transformation, constitute a two-point boundary-value problem, which was solved numerically. It is found from the obtained stability results that the neutral stability curves are systematically shifted to have lower critical Grashof numbers, as the inclination angle of upward-facing plate increases. Also, the nose of the neutral stability curve becomes blunter as the angle increases. It implies that the greater the inclination of the upward-facing plate, the more susceptible of the flow to instability for the wide range of disturbance wave number and frequency.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.12
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• pp.1784-1795
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• 1998

Natural convection of low Prandtl number fluids with $Pr{\leq}0.2$ in a narrow horizontal annulus is numerically investigated. For $Pr{\leq}0.2$, hydrodynamic instability induces oscillatory multicellular flows consisting of multiple like-rotating cells. For a fluid with $Pr{\approx}0$, the region in which instability of conduction regime first forms is near the vertical section of annulus, and the multiple cells are distributed uniformly in the lower and upper regions of annulus. As Pr increases, however, the cells are shifted upwards. The like-rotating cells drift downward, as time goes on, and the speed of travel increases with increase of Pr. For a fluid with Pr=0.1, a flow with period-4 solution is observed between chaotic states.

• Journal of the Korean Society of Combustion
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• v.17 no.4
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• pp.44-50
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• 2012

The present study was conducted to investigate the flame instability(evaluated by Markstein length and cellular instability) and laminar burning velocity in a constant volume combustion chamber at room temperature and elevated pressure up to 0.3 MPa to suggest the possibility of utilizing mixtures of syngas added DME-air premixed flames in internal combustion engines. The experimentally measured laminar burning velocities were compared to predictions calculated the PREMIX code with Zhao reaction mechanism. Discussions were made on effects of syngas addition into DME-Air premixed flames through evaluating laminar burning velocity, Markstein length, and cellular instability. Particular concerns are focused on cellular instability caused by hydrodynamic instability and diffusive-thermal instability.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2001.04a
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• pp.635-638
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• 2001

This paper presents the dynamic characteristics of r rotor-bearing system supported by an actively controlled hydrodynamic journal bearing. The proportional, derivative and integral controls are adopted for the control algorithm to control the hydrodynamic journal bearing with an axially groove. Also, the cavitation algorithm implementing the Jakobsson-Floberg-olsson boundery condition is adopted to predict cavitation regions in the fluid film more accurately than conventional analysis, which uses the Reynolds condition. The speed at onset of instability of a rotor-bearing system is increased by both proportional and derivative control of the bearing. The integral control has no effect on stability characteristics of hydrodynamic journal bearing. The PD-control is more effective than proportional or derivative control. Results show the active control of bearing can be adopted for the stability improvement of a rotor-bearing system.

• 한국연소학회:학술대회논문집
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• 2012.11a
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• pp.207-209
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• 2012

The present experiment was conducted to measure the unstretched laminar burning velocity and cellular instability of DME-air and syngas (in steps of 25 %) added DME-air premixed flames using propagating spherical flame. The experimental results were discussed in two focuses which are effects of syngas fraction and initial pressure on Markstein length, unstretched laminar burning velocities, and cellular instability. The flame instability was evaluated by the Markstein length and cellularity which is caused by diffusional-thermal instability and hydrodynamic instability.

• 한국연소학회:학술대회논문집
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• 2015.12a
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• pp.109-112
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• 2015

This article describes a cellular instability and laminar burning velocity of simulated synthetic natural gas(SNG) including 3% hydrogen. In this study, experimental apparatus is employed using cylindrical bomb combustor, and investigation is carried out with high speed camera and Schlieren system. The cellular instability is caused by the buoyancy, hydrodynamic instability. Unstretched burning velocity can be determined by extrapolated stretch rate of zero point from measured results. These results were also compared with numerical calculation by Chemkin package with GRI 3.0, USC-II, WANG, C3 Fuel mechanism. As an experimental conditions, equivalence ratios was adjusted from 0.8 to 1.3. From results of this work, the one was found that the cellular instability has occurred by effect of thermal expansion rate and flame thickness. As the other results, unstretched laminar burning velocity was best coincided with GRI 3.0 mechanism.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.4
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• pp.247-257
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• 2003

This paper presents an analytical method to Investigate the stability of a hydrodynamic journal bearing with rotating herringbone grooves. The dynamic coefficients of the hydrodynamic Journal bearing are calculated using the FEM and the perturbation method. The linear equations of motion can be represented as a parametrically excited system because the dynamic coefficients have time-varying components due to the rotating grooves, even in the steady state. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving Hill's infinite determinant of these algebraic equations. The validity of this research is proved by the comparison of the stability chart with the time response of the whirl radius obtained from the equations of motion. This research shows that the instability of the hydrodynamic journal bearing with rotating herringbone grooves increases with increasing eccentricity and with decreasing groove number, which play the major roles in increasing the average and variation of stiffness coefficients, respectively. It also shows that a high rotational speed is another source of instability by increasing the stiffness coefficients without changing the damping coefficients.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.05a
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• pp.166-174
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• 2002

This paper presents an analytical method to Investigate the stability of a hydrodynamic journal bearing with rotating herringbone grooves. The dynamic coefficients of the hydrodynamic journal bearing are calculated using the FEM and the perturbation method. The linear equations of motion can be represented as a parametrically excited system because the dynamic coefficients have time-varying components due to the rotating grooves, even in the steady state. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving Hill's infinite determinant of these algebraic equations. The validity of this research is proved by the comparison of the stability chart with the time response of the whirl radius obtained from the equations of motion. This research shows that the instability of the hydrodynamic journal bearing with rotating herringbone grooves increases with increasing eccentricity and with decreasing groove number, which play the major roles in increasing the average and variation of stiffness coefficients, respectively. It also shows that a high rotational speed is another source of instability by increasing the stiffness coefficients without changing the damping coefficients.

• Journal of Advanced Marine Engineering and Technology
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• v.16 no.5
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• pp.67-76
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• 1992

This paper describes the vibration characteristic of second-order nonlinear systems subjected to parametric excitation. Emphasis is put on the examination of the hydrodynamic nonlinear damping effect on limiting the response amplitudes of parametric vibration. Since the parametric vibration is described by the Mathieu equation, the Mathieu stability chart is examined in this paper. In addition, the steady-state solutions of the nonlinear Mathieu equation in the first instability region are obtained by using a perturbation technique and are compared with those by a numerical integration method. It is shown that the response amplitudes of parametric vibration are limited even in unstable conditions by hydrodynamic nonlinear damping force. The largest reponse amplitude of parametric vibration occurs in the first instability region of Mathieu stability chart. The parametric excitation induces the response of a dynamic system to be subharmonic, superharmonic or chaotic according to their dynamic conditions.

• Bulletin of the Society of Naval Architects of Korea
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• v.19 no.4
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• pp.19-29
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• 1982

Galerkin's finite element method is applied to a two-dimensional heat convection-diffusion problem arising in the hydrodynamic lubrication of thrust bearings used in naval vessels. A parabolized thermal energy equation for the lubricant, and thermal diffusion equations for both bearing pad and the collar are treated together, with proper juncture conditions on the interface boundaries. it has been known that a numerical instability arises when the classical Galerkin's method, which is equivalent to a centered difference approximation, is applied to a parabolic-type partial differential equation. Probably the simplest remedy for this instability is to use a one-sided finite difference formula for the first derivative term in the finite difference method. However, in the present coupled heat convection-diffusion problem in which the governing equation is parabolized in a subdomain(Lubricant), uniformly stable numerical solutions for a wide range of the Peclet number are obtained in the numerical test based on Galerkin's classical finite element method. In the present numerical convergence errors in several error norms are presented in the first model problem. Additional numerical results for a more realistic bearing lubrication problem are presented for a second numerical model.