• Korean Chemical Engineering Research
• /
• v.53 no.5
• /
• pp.609-613
• /
• 2015

The onset of instability induced by impulsive spin-down of the rigid-body flow placed in the gap between two coaxial cylinders is analyzed by using the energy method. In the present stability analysis the growth rate of the kinetic energy of the base state and also that of disturbances are taken into consideration. In the present system the primary flow is a transient, laminar one. But for the Reynolds number equal or larger than a certain one, i.e. $Re{\geq}Re_G$ secondary motion sets in, starting at a certain time. For $Re{\geq}Re_G$ the dimensionless critical time to mark the onset of vortex instabilities, ${\tau}_c$, is here presented as a function of the Reynolds number Re and the radius ratio ${\eta}$. For the wide gap case of small ${\eta}$, the transient instability is possible in the range of $Re_G{\leq}Re{\leq}Re_S$. It is found that the predicted ${\tau}_c$-value is much smaller than experimental detection time of first observable secondary motion. It seems evident that small disturbances initiated at ${\tau}_c$ require some growth period until they are detected experimentally.

• The Bulletin of The Korean Astronomical Society
• /
• v.35 no.2
• /
• pp.45.2-45.2
• /
• 2010

We analyzed onset times of the largest six solar proton events during 1997-2006 of solar cycle 23, as observed at 1AU by two satellites of GOES/SEM (Geostationary Operational Environmental Satellites/the Space Environment Monitor) and SOHO/ERNE (Solar and Heliospheric Observatory/the Energetic and Relativistic Nuclei and Electron). We adopted the time shifted method suggested by Leon Kocharov and determined the path length by Sam Krucker's fitting method. We found some problems of those methods and tried to improve those. In this presentation, we will give details of the energy spectra of the 6 SPE events from the ERNE/HED, and onset time comparison among the SPE, flare, type II burst, and CME.

• Journal of Physiology & Pathology in Korean Medicine
• /
• v.16 no.5
• /
• pp.1075-1078
• /
• 2002

Objective: The management for the stroke should ,given as soon as possible to be effect. But Patients with stroke symptoms commonly delay many hours before seeking medical attention. We evaluated the factors which are related to the time of hospital arrival after acute stroke. Method: Data were obtained from 317 patients admitted to our hospital within 72 hours of stroke onset. We assessed demographic variables, stoke subtype. referral routes. history of previous stroke, level of consciousness, distance from the place where stroke occurred to hospital, and the time interval between onset of stroke and arrival at the hospital. Results: Mean patient age was 65.99±9.57 years. The mean time interval between onset of stroke and hospital arrival was 17.26±18.69 hours and 128 (40.38%) patients arrived within 6 hours. The patients whoes stoke subtype was infarction, who arrived our hospital by way of other hospital, who had no suffered from previous stroke and who showed no impairement of consciousness was arrived at the hospital late(p<0.05). Conclusion: The majority of patients arrive at the hospital after prolonged delays for multiple reasons, and patients with milder symptoms, for whom treatment might be more effective, were less likely to arrive in time for therapy. Our study suggest that effective education about stroke to the patients and public would be highly necessary.

• Journal of the Korean earth science society
• /
• v.39 no.6
• /
• pp.593-599
• /
• 2018

The onset times of P- and S-waves are important information to have reliable earthquake locations, 1D or 3D subsurface velocity structures, and other related studies in seismology. As the number of seismic stations increases significantly in recent years, it becomes a formidable task for network operators to pick phase arrivals manually. This study used a simple method to estimate additional P- and S-wave arrival times for local earthquakes when a priori information (event location and time) is available using the Akaike Information Criterion (AIC). We applied the AIC program to the earthquake data recorded at the seismic station located in Gyeongsan (DAG2). The comparisons of automatically estimated phase arrival times with manually picked onset times showed that 95.1% and 93.7% of P-wave and S-wave arrival time estimations, respectively, are less than 0.1 second difference. The higher percentage of agreement presented the method which can be successfully applied to large data sets recorded by high-density seismic arrays.

• Korean Chemical Engineering Research
• /
• v.57 no.5
• /
• pp.723-729
• /
• 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.

• Korean Chemical Engineering Research
• /
• v.57 no.1
• /
• pp.142-147
• /
• 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.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2006.11a
• /
• pp.30-33
• /
• 2006

Onset voltage plays a crucial role in the design of a spray microthruster. This paper presents an analytical electrostatic model to predict the behavior of onset voltage in an array of emitters. The basic idea of this method is to superimpose the electric potentials obtained from each individual emitter in an array of emitters. The results show that if one emitter operates and the other neighboring emitters are dry, the potential required for cone-jet spraying generally increases as the emitter spacing decreases (due to electrical shielding). However at very close spacing the potential can decrease. If all emitters operate at the same time, the phenomenon that even at very close spacing the onset voltage required for cone-jet spraying increases merely as the emitter spacing decreases.

• The Bulletin of The Korean Astronomical Society
• /
• v.36 no.2
• /
• pp.87.2-87.2
• /
• 2011

Proper motion of sunspots in several active regions was studied to detect their indicator on flare onset, using data from the Solar Flare Telescope at Mitaka (four flaring active regions), TRACE (e.g. NOAA 0424, M1.7 flare on 5 Aug. 2003) and Hinode (e.g. NOAA 10930, X3.4 flare on 13 Dec. 2006). The proper motion of individual sunspots was derived using a local correlation tracking method. As a result, we found that the sunspots that are located under or close to a part of chromospheric flaring patches showed a change in their moving direction prior to the flare onset. The change in their movements took place a half to two hours before the flare onset. On the other hand, sunspots in non-flaring areas or non-flaring active regions did not show this kind of change. It is likely, therefore, that if a sunspot shows the particular movement, a chromospheric flare is to occur in its nearby region. In the most active regions, the part of flare ribbons was located on an emerging bipolar pair of sunspots. The disturbance in the usual motion of the bipolar sunspots and in other sunspots as well can be interpreted as a sign of magnetic shear development leading to final magnetic energy buildup before its sudden release. We suggest that the change in sunspot motion in a short time scale prior to the flare onset can be regarded as a good indicator in predicting the onset timing and location of chromospheric flares.

• Korean Journal of Community Nutrition
• /
• v.4 no.4
• /
• pp.489-495
• /
• 1999

This study was conducted to investigate the weaning practice of 198 infants in Taejon city in October, 1998. Information on infant feeding and weaning practices were obtained by interviewing mothers in gynecologist and pediatric clinics located in Taejon. The results obtained were as follows : In the survey 37.4% of the infants were breast-feeding, while 31.3% of them were bottle-feeding and 29.3% of them were mixed-feeding. The reason for bottled-feeding was due either to the lack of breast-milk secretion or motner’s job. The educational level of the mother and maternal job affected the feeding methods before weaning. Lower rates of breast-feeding were found among mothers witn a lover level of education. The breast-feeding rate was lower in full-time and part-time job worker groups than in housewife. There was no significant difference in the feeding methods according to family income. Among the subjects, 71.5% of infants began to be weaned within 7 months. The onset of weaning was delayed in the full-time worker group. There was no significant difference in the onset time of weaning according to feeding method, the educational level of the mother, and family income. 60.6% of infants received fruit juice and 29.6% of them received cereals for first-given-supplementary food. ‘For baby’s nutritional status’ was the most common motivation for the onset of weaning. Commercially prepared foods were used more than home-made food for supplementary food.

• Structural Monitoring and Maintenance
• /
• v.4 no.2
• /
• pp.115-131
• /
• 2017

Classical flutter of wind turbine blades indicates a type of aeroelastic instability with fully attached boundary layer where a torsional blade mode couples to a flapwise bending mode, resulting in a mutual rapid growth of the amplitudes. In this paper the monitoring problem of onset of flutter is investigated from a detection point of view. The criterion is stated in terms of the exceeding of a defined envelope process of a specific maximum torsional vibration threshold. At a certain instant of time, a limited part of the previously measured torsional vibration signal at the tip of blade is decomposed through the Empirical Mode Decomposition (EMD) method, and the 1st Intrinsic Mode Function (IMF) is assumed to represent the response in the flutter mode. Next, an envelope time series of the indicated modal response is obtained in terms of a Hilbert transform. Finally, a flutter onset criterion is proposed, based on the indicated envelope process. The proposed online flutter monitoring method provided a practical and direct way to detect onset of flutter during operation. The algorithm has been illustrated by a 907-DOFs aeroelastic model for wind turbines, where the tower and the drive train is modelled by 7 DOFs, and each blade by means of 50 3-D Bernoulli-Euler beam elements.