• Journal of Mechanical Science and Technology
• /
• v.17 no.11
• /
• pp.1820-1831
• /
• 2003

Characteristics of self-excited combustion oscillation are experimentally studied using confined premixed flames stabilized by a rearward-facing step. A new idea to suppress combustion oscillation was applied to the flames. The characteristics of unsteady combustion were examined, which is driven by forced pulsating mixture supply that can modulate its amplitude and frequency. The self-excited combustion oscillation having weaker flow velocity fluctuation intensity than that of the forced pulsating supply can be suppressed by the method. The effects of the forced pulsation amplitude and frequency on controlling self-excited combustion oscillations were also investigated comparing with the steady mixture supply. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillations, and it also exhibits desirable performances, from a practical point of view, such as high combustion load and reduced pollutant emissions of nitric oxide.

• Journal of Welding and Joining
• /
• v.21 no.3
• /
• pp.85-91
• /
• 2003

Narrow gap welding has many advantages over conventional V-grooved butt welding such as high productivity, small deformation and improved mechanical property of joints. With narrower groove gap, less arc heat input is expected will all the other advantages of narrow gap welding. The main defects of narrow gap welding include the lack of root fusion, convex bead surface and irregular surface, all of which have negative effects on the next welding pass. This paper suggests an up-and-down torch oscillation for ultra narrow gap welding with gap size of 5mm and investigates the proper welding conditions to fulfill the reliable and high welding quality. First, GMA welding model was suggested for ultra narrow gap welding system with Halmoy's model referenced for wire melting modeling. And the arc length in ultra narrow gap was defined. Secondly, based on the experimental results of up-and-down torch oscillation welding, phase shift of current and wire extension length were simulated for varying oscillation frequency to show that weld the bead shape in ultra narrow gap welding can be predicted. As the result, it was confirmed that reliable weld quality in ultra narrow gap welding can be achieved with up-and-down torch oscillation above 15Hz due to its ability to provide uniform heat input along the sidewall of gap.

• Nuclear Engineering and Technology
• /
• v.55 no.2
• /
• pp.780-791
• /
• 2023

To study oscillation characteristic of steam and non-condensable gas direct contact condensation through multi-hole sparger at low mass flux, a series of experiments of pure steam and mixture gas condensation have been carried out under the conditions of steam mass flux of 20-120kg/m²s, water temperature of 20-95 ℃ and mass fraction of non-condensable gas of 0-5%. The regime map of pure steam condensation through multi-hole sparger is divided into steam chugging, separated bubble, aggregated bubble and escaping aggregated bubble. The bubbles behavior of synchronization in the same hole columns and desynchronized excitation between different hole columns can be found. The coalescence effect of mixture bubbles increases with water temperature and non-condensable gas content increasing. Pressure oscillation intensity of pure steam condensation first increases and then decreases with water temperature increasing, and increases with steam mass flux increasing. Pressure oscillation intensity of mixture gas condensation decreases with water temperature and non-condensable gas content increasing, which is significantly weaker than that of pure steam condensation. The oscillation dominant frequency decreases with the rise of water temperature and non-condensable gas content. The correlations for oscillation intensity and dominant frequency respectively are developed in pure steam and mixture gas condensation at low mass flux.

• Journal of The Korean Astronomical Society
• /
• v.36 no.2
• /
• pp.67-72
• /
• 2003

The basic objective of helioseismology is to determine the structure and the dynamics of the Sun by analysing the frequency spectrum of the solar oscillations. Accurate frequency measurements provide information that enables us to probe the solar interior structure and the dynamics. Therefore the frequency of the solar oscillation is the most fundamental and important information to be extracted from the solar oscillation observation. This is why many efforts have been put into the development of accurate data analysis techniques, as well as observational efforts. To test one's data analysis method, a realistic artificial data set is essential because the newly suggested method is calibrated with a set of artificial data with predetermined parameters. Therefore, unless test data sets reflect the real solar oscillation data correctly, such a calibration is likely incomplete and a unwanted systematic bias may result in. Unfortunately, however, commonly used artificial data generation algorithms insufficiently accommodate physical properties of the stochastic excitation mechanism. One of reason for this is that it is computaionally very expensive to solve the governing equation directly. In this paper we discuss the nature of solar oscillation excitation and suggest an efficient algorithm to generate the artificial solar oscillation data. We also briefly discuss how the results of this work can be applied in the future studies.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.30 no.6 s.249
• /
• pp.523-530
• /
• 2006

One of the fascinating prospects is the possibility of new hydrodynamics technology on micro-scale system since oscillations of micro-droplets are of practical and scientific importance. It has been widely conceived that the lowest oscillation mode of a pendant droplet is the longitudinal vibration, i.e. periodic elongation and contraction along the longitudinal direction. Nonlinear and forced oscillations of supported viscous droplet were focused in the present study. The droplet has a free contact line with solid plate and inviscid fluid. Natural frequencies of a pendant droplet have been investigated experimentally by imposing the acoustic wave while the frequency is being increased at a fixed amplitude. It is found that a pendant droplet shows the resonant behaviors at each mode similar to the theoretical analysis. The rotation of the droplet about the longitudinal axis is the oscillation mode of the lowest resonance frequency. This rotational mode can be invoked by periodic acoustic forcing and is analogous to the pendulum rotation. It is also found that the natural frequency of a pendant droplet is independent of the drop density and surface tension but inversely proportional to the square root of the droplet size.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.4 no.3
• /
• pp.102-111
• /
• 1996

For the air-fuel ratio control in a fuel injection SI engine, the Jump-Ramp control algorithm has been widely adopted by using the on/off type oxygen sensor. But the Jump-Ramp control method has limitation on treating the frequency and amplitude of the air-fuel ratio oscillation. This study suggests another feedback control logic named modulated fuel feedback control, which has a concept of pre-tuned air-fuel ratio oscillation. In the modulation method, the oxygen sensor output is not treated as on/off signal but as analog signal for feedback. By using the modulation method, the frequency and the amplitude of air-fuel ratio oscillation can be adjustable to some extent for improving the conversion efficiency of the Three-Way Catalyst. The result shows that the performance of the modulation method is better than that Jump-Ramp control method in reducing the amplitude of the air-fuel ratio oscillation as well as in increasing the frequency of the air-fuel ratio oscillation.

• The Transactions of the Korean Institute of Electrical Engineers C
• /
• v.52 no.11
• /
• pp.521-528
• /
• 2003

A varactor diode was utilized in order to expand variable range of the natural oscillation frequency of an active phased-array antenna. We have conformed experimentally that the variable range of the natural oscillation frequency was expanded about three times in the oscillator controlled by the varactor diode. When frequency difference was given to the oscillators in the two elements antenna system, phase difference was appeared between the oscillators. The 2-, 3-, 5-elements patch antenna array was composed for the beam scanning experiments. All the above patch antennas showed good phased array characteristics. The experimental results are as follows that the scanning angle of the 2-elements array antenna is 28.6$^{\circ}$, the 3-elements array antenna is 29.4$^{\circ}$, and the 5-elements array antenna is 26.2$^{\circ}$.

• Journal of the Society of Naval Architects of Korea
• /
• v.50 no.5
• /
• pp.343-348
• /
• 2013

When a natural frequency of the trailing edge of a wing is close to a vortex shedding frequency, an amplitude of the edge oscillation becomes maximal; it makes intensive noise called singing. Motion of the trailing edge may also feedback to the vortex shedding so that self-sustained oscillation appears, and a resonant frequency is locked in some interval of the speed of the incident flow. In this study, we first evaluate main features of oscillating characteristics of the wing. Second we simulate fluid-structure interaction of the wing with a flap using a commercial code, ANSYS-CFX, and investigate lift characteristics in a frequency domain.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.58 no.7
• /
• pp.1255-1262
• /
• 2009

The present study explained the phenomenon that low frequency oscillation is synchronized with discrete data obtained from a wide area system, and a sync search method. When a disturbance occurs in an power system, various controllers operate in order to maintain synchronization. If the system's damping is poor, low frequency oscillations continue for a long time and the oscillations are synchronized with one another at specific frequency. The present study estimated dominant modes, magnitude and phase of signals by applying parameter estimation methods to discrete signals obtained from an power system, and performed sync search among wide area signals by comparing the estimated data. Sync search was performed by selecting those with the same frequency and damping constant from dominant oscillation modes included in a large number of signals, and comparing their magnitude and phase. In addition, we defined sync indexes in order to indicate the degree of sync between areas in a wide area system. Furthermore, we proposed a wide area sync search method by normalizing mode magnitude in discrete data obtained from critical generator of the wide area. By applying the sync search method and sync indexes proposed in this study to two area systems, we demonstrated that sync scanning can be performed for discrete signals obtained from power systems.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.4
• /
• pp.997-1008
• /
• 1994

Heat transfer by laminar oscillating flow in a circular pipe has been studied analytically. The general solution with respect to the arbitrary wall boundary condition is obtained by superposing the fluid temperatures with the sinusoidal wall temperature distributions. The fulid temperature distributions are two dimensional, but uniform flow assumption is used to simplify the velocity distribution. The heat transfer characteristics in the thermally developing regions are analyzed by applying the general solution to the two cases of thermal boundary conditions in which the wall temperature and wall heat flux distributions have a square-wave form, respectively. The results show that the length of the thermally developing region becomes larger in proportion to the oscillation frequency at slow oscillation and eventually approaches to the value comparable to the swept distance as the frequency increases. The time and cross-section averaged Nusselt number in the developing region is inversely proportional to the square root of the distance from the position where the wall boundary condition is changed suddenly. In the developed region, Nusselt number is only determined by the oscillation frequency.