• The Korean Journal of Physiology and Pharmacology
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• v.15 no.6
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• pp.383-388
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• 2011

Regulators of G-protein signaling (RGS) proteins are regulators of $Ca^{2+}$ signaling that accelerate the GTPase activity of the G-protein ${\alpha}$ -subunit. RGS1, RGS2, RGS4, and RGS16 are expressed in the pancreas, and RGS2 regulates G-protein coupled receptor (GPCR)-induced $Ca^{2+}$ oscillations. However, the role of RGS4 in $Ca^{2+}$ signaling in pancreatic acinar cells is unknown. In this study, we investigated the mechanism of GPCR-induced $Ca^{2+}$ signaling in pancreatic acinar cells derived from $RGS4^{-/-}$ mice. $RGS4^{-/-}$ acinar cells showed an enhanced stimulus intensity response to a muscarinic receptor agonist in pancreatic acinar cells. Moreover, deletion of RGS4 increased the frequency of $Ca^{2+}$ oscillations. $RGS4^{-/-}$ cells also showed increased expression of sarco/endoplasmic reticulum $Ca^{2+}$ ATPase type 2. However, there were no significant alterations, such as $Ca^{2+}$ signaling in treated high dose of agonist and its related amylase secretion activity, in acinar cells from $RGS4^{-/-}$ mice. These results indicate that RGS4 protein regulates $Ca^{2+}$ signaling in mouse pancreatic acinar cells.

• Nuclear Engineering and Technology
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• v.31 no.6
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• pp.572-585
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• 1999

The characteristics of two-phase flow instability in a vertical boiling channel connecting with an unheated riser are investigated through the linear stability analysis model. Various two-phase flow models, including thermal non-equilibrium effects, are taken into account for establishing a physical model in the time domain. A classical approach to the frequency response method is adopted for the stability analysis by employing the D-partition method. The adequacy of the linear model is verified by evaluating experimental data at high quality conditions. It reveals that the flow-pattern-dependent drift velocity model enhances the prediction accuracy while the homogeneous equilibrium model shows the most conservative predictions. The characteristics of density wave oscillations under low-power and low-quality conditions are investigated by devising a simple model which accounts for the gravitational and frictional pressure losses along the channel. The necessary conditions for the occurrences of type-I instability and flow excursion are deduced from the one-dimensional D-partition analysis. The parametric effects of some design variables on low quality oscillations are also investigated.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.10a
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• pp.157-163
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• 2003

A systematic numerical experiment has been conducted to study droplet gasification in high pressure environments with pressure oscillations. The general frame of previous rigorous model[1] is retained but tailored for flash equilibrium calculation of vapor-liquid interfacial thermodynamics. Time-dependent conservation equations of mass, momentum, energy, and species concentrations are formulated in axisymmetric coordinate system for both the droplet interior and ambient gases. In addition, a unified property evaluation scheme based on the fundamental equation of state and empirical methods are used to find fluid thermophysical properties over the entire thermodynamic domain of interest. The governing equations with appropriate physical boundary conditions are numerically time integrated using an implicit finite-difference method with a dual time-stepping technique. A series of calculation have been carried out to investigate the gasification of an isolated n-pentane droplet in a nitrogen gas environment over a wide range of ambient pressures and frequencies. Results show that the mean pressures and frequencies of the ambient gas have strong influences on the characteristics of the droplet gasification. The amplitude of the response increases with increasing pressure, and the magnitude of the vaporization response increases with the frequency.

• Computational Structural Engineering
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• v.8 no.4
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• pp.189-198
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• 1995

The conventional finite element formulations for incompressible materials show pressure oscillations or pressure modes in four-node quadrilateral elements of commonly used displacement and pressure interpolations. The criterion for the stability in the pressure solution is the so-called Babugka-Brezzi stability condition, and the above elements do not satisfy this condition. In this study, a pressure continuity residual based on the pressure discontinuity at element interfaces is used to study the stabilization of pressure solutions in bilinear displacement-constant pressure four-node quadrilateral elements. This pressure residual is implemented in Q1P0 element derived from the conventional incompressible elasticity. The pressure solutions can be stable with the pressure residual though they exhibit sensitivity to the stabilization parameters. Parametric study for the solution stabilization is also discussed.

• International Journal of Ocean System Engineering
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• v.4 no.1
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• pp.19-28
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• 2014

The tension leg platform (TLP) is a vertically moored structure with excess buoyancy. The TLP is regarded as moored structure in horizontal plan, while inherit stiffness of fixed platform in vertical plane. In this paper, a numerical study using modified Morison equation was carried out in the time domain to investigate the influence of nonlinearities due to hydrodynamic forces and the coupling effect between surge, sway, heave, roll, pitch and yaw degrees of freedom on the dynamic behavior of TLP's. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables and the nonlinear equations of motion were solved utilizing Newmark's beta integration scheme. The effect of tethers length and wave characteristics such as wave period and wave height on the response of TLP's was evaluated. Only uni-directional waves in the surge direction was considered in the analysis. It was found that for short wave periods (i.e. 10 sec.), the surge response consisted of small amplitude oscillations about a displaced position that is significantly dependent on tether length, wave height; whereas for longer wave periods, the surge response showed high amplitude oscillations about that is significantly dependent on tether length.

• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.294-298
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• 2003

The complex, unsteady, self-sustained pressure oscillations induced by supersonic flow past a rectangular cavity is investigated using numerical simulations. The present numerical study is performed using a parallel, multiblock solver for the two-dimensional, compressible Navier­Stokes equations. Open cavities with length-to-depth (L / D) ratio in the range 0.5 - 3.3 are considered. This paper sheds light on the cavity physics, cavity oscillatory mechanism, and the organisation of vortical structures inside the cavity. The vortex shedding phenomenon, the shear layer impingement event at the aft wall and the movement of the acoustic/compression wave within the cavity are well predicted. The vortical structures· and the source of the acoustic disturbances are found to be located near the aft wall of the cavity. With the increase in the cavity length, strong recompression of the flow near the aft wall leading to a sudden jump in the cavity form drag is observed. The estimated cavity tones are in good agreement with the available semi­empirical relation. Multiple peaks are noticed in deep and long cavities. For the present free­stream Mach number 1.71, it is observed that around L/D=2.0, the cavity oscillatory mechanism changes from the transverse to longitudinal oscillatory mode. The effects of this transition on various fluid dynamics and acoustic properties are also discussed.

• International Journal of Railway
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• v.5 no.1
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• pp.38-47
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• 2012

At present among the most important problems in the field of traction systems of railway vehicles are the following: 1) the minimization of the mass-and-overall dimensions of the drive systems; 2) the increase of the drive systems reliability and obtaining their higher fault-tolerance abilities; 3) the minimization of the motion speed pulsations and its oscillations, etc. The results of the researches received by the authors of this paper show that the use of the multiphase (i.e. having the number of phases more than four) inverter-fed induction motors in these traction systems is the most effective way of solving the above mentioned problems. In this case the motion speed oscillations can be decreased only by the increase of the drive phase number without any change in the inverter control algorithm. In addition, the application of some non-traditional control methods in the multiphase asynchronous traction drive system of a railway vehicle allows to decrease the mass-and-overall dimensions of the system and to improve its reliability and some other technical-and economic characteristics.

• Journal of Ocean Engineering and Technology
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• v.18 no.5
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• pp.15-21
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• 2004

An edge tone is the discrete tone or narrow-band sound produced by an oscillating free shear layer, impinging on a rigid surface. In this paper, we present a 2-D edge tone to predict the frequency characteristics of the discrete oscillations of a jet-edge feedback cycle, using the finite difference lattice Boltzmann method (FDLBM). We use a modified version of the lattice BGK compressible fluid model, adding an additional term and allowing for longer time increments, compared to a conventional FDLBM, and also use a boundary fitted coordinates system. The jet is chosen long enough in order to guarantee the parabolic velocity profile of the jet at the outlet, and the edge consists of a wedge with an angle of ${\alpha}$ = 23. At a stand-off distance, the edge is inserted along the centerline of the jet, and a sinuous instability wave, with real frequency, is assumed to be created in the vicinity of the nozzle and propagates towards the downstream. We have succeeded in capturing very small pressure fluctuations, resulting from periodical oscillations of a jet around the edge. The pressure fluctuations propagate with the speed of sound. Its interaction with the wedge produces an non-rotational feedback field, which, near the nozzle exit, is a periodic transverse flow, producing the singularities at the nozzle lips.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.311-317
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• 2011

The present paper deals with the continuous works of extending the multi-dimensional limiting process (MLP) for compressible flows, which has been quite successful in finite volume methods, into discontinuous Galerkin (DG) methods. From the series of the previous, it was observed that the MLP shows several superior characteristics, such as an efficient controlling of multi-dimensional oscillations and accurate capturing of both discontinuous and continuous flow features. Mathematically, fundamental mechanism of oscillation-control in multiple dimensions has been established by satisfaction of the maximum principle. The MLP limiting strategy is extended into DG framework, which takes advantage of higher-order reconstruction within compact stencil, to capture detailed flow structures very accurately. At the present, it is observed that the proposed approach yields outstanding performances in resolving non-compressive as well as compressive flaw features. In the presentation, further numerical analyses and results are going to be presented to validate that the newly developed DG-MLP methods provide quite desirable performances in controlling numerical oscillations as well as capturing key flow features.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.12
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• pp.3980-3990
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• 1996

When a moist air is rapidly expanded in a supersonic nozzle, nonequilibrium condensation occurs at a supersaturation state. Condensation shock wave appears in the nozzle flow if the releasing latent heat due to condensation goes beyond a critical value. It has been known that self-excited oscillations of the condensation shock wave generate in an air or a steam nozzle flow with a large humidity. In the present study, the passive control technique using porous wall with a cavity underneath was applied to the condensation shock wave. The effects of the passive control on the steady and self-excited condensation shock waves were experimentally investigated by Schlieren visualization and static pressure measurements. The result shows that the present passive control is a useful technique to suppress the self-excited oscillations of condensation shock wave.