• Wind and Structures
• /
• v.32 no.2
• /
• pp.89-104
• /
• 2021

Non-synoptic winds generated by tornadoes, downbursts or gust fronts exhibit significant non-stationarity and can cause significant wind load effect on flexible structures such as long-span bridges. However, conventional assumptions on stationarity used to evaluate the structural wind-induced vibration are inadequate. In this paper, an efficient frequency domain scheme based on fast CQC method, which can predict non-stationary buffeting random responses of long-span bridges, is presented, and then this approach is applied to evaluate the buffeting response of a long-span suspension bridge located in a complex mountainous wind environment as an example. In this study, the data-driven method based on one available measured wind speed sample is firstly presented to establish non-stationary wind models, including time-varying mean wind speed, time-varying intensity envelope function and uniformly modulated fluctuating spectrum. Then, a linear time-variant (LTV) system based on the proposed scheme can be generally applied to calculate the non-stationary buffeting responses. The effectiveness and accuracy of the proposed scheme are verified through Monte Carlo time domain simulation implemented in ANSYS platform. Also, the transient effect nature of the bridge responses is further illustrated by comparison of the non-stationary, quasistationary and steady-state cases. Finally, buffeting response analysis with traditional stationary treatment (10 min constant mean plus stationary wind fluctuation) is performed to illustrate the importance of the non-stationary characteristics embedded in original wind speed samples.

• The Bulletin of The Korean Astronomical Society
• /
• v.44 no.1
• /
• pp.50.3-50.3
• /
• 2019

The EUV wave is a disturbance that has been believed to be the fast-mode (shock) wave, which can propagate passing through magnetic field lines. After the passage of EUV waves, coronal streamers start to show kink-mode oscillations, and the footpoints, i.e., magnetic separatrices, of the oscillating streamers are observed as the so-called stationary front. We compare the stationary front observed by EUV imagers and coronal streamers observed in coronagraphic images. We analyze the successive events occurred in September 2011. We find that the stationary fronts are consistent with the coronal streamer boundaries, and they are located along the boundaries of coronal holes and active regions. Our results confirm that EUV waves are in fact fast-mode waves and demonstrate that the stationary front is a promising tool to probe into the source of slow solar wind that is the boundary of coronal streamers on the solar surface.

• Asian Journal of Atmospheric Environment
• /
• v.11 no.2
• /
• pp.71-78
• /
• 2017

Monthly characteristics of rainwater chemistry at a coastal site in southwestern Japan were examined based on an eight year record. In the period November-May when rain was mainly caused by cyclones, the monthly mean concentrations of $nss-{SO_4}^{2-}$, ${NO_3}^-$, ${NH_4}^+$, $nss-Ca^{2+}$, $Na^+$ and $Cl^-$ over the eight years were 25.1-57.8, 9.9-25.0, 11.3-31.4, 5.5-18.7, 24.2-154.9 and $30.0-178.5{\mu}eq\;L^{-1}$, respectively. In June and July when rain was mainly caused by stationary fronts, i.e. Meiyu fronts, the concentrations were 14.4-20.7, 7.2-9.5, 7.7-12.9, 4.1-6.8, 21.7-33.6 and $26.4-40.5{\mu}eq\;L^{-1}$, respectively. In August and September when typhoons contributed substantial rainfall, the respective concentrations of $Na^+$ and $Cl^-$ were as high as 97.7-105.3 and $116.8-122.9{\mu}eq\;L^{-1}$, while the concentrations of other ions were low. These results indicate a large variation of monthly rainwater chemistry, which is basically dependent on the synoptic weather patterns causing rain. From later autumn to early spring, rain contains ions in high concentration and large variation ranges. In the Meiyu season, rain contains less ions which vary in a range much smaller than that in later autumnearly spring. In summer and autumn, the concentrations are low, except $Na^+$ and $Cl^-$ which can be large due to typhoons' contribution.

• Wind and Structures
• /
• v.13 no.1
• /
• pp.37-55
• /
• 2010

In this paper South Africa is divided into strong wind climate zones, which indicate the main sources of annual maximum wind gusts. By the analysis of wind gust data of 94 weather stations, which had continuous climate time series of 10 years or longer, six sources, or strong-wind producing mechanisms, could be identified and zoned accordingly. The two primary causes of strong wind gusts are thunderstorm activity and extratropical low pressure systems, which are associated with the passage of cold fronts over the southern African subcontinent. Over the eastern and central interior of South Africa annual maximum wind gusts are usually caused by thunderstorm gust fronts during summer, while in the western and southern interior extratropical cyclones play the most dominant role. Along the coast and adjacent interior annual extreme gusts are usually caused by extratropical cyclones. Four secondary sources of strong winds are the ridging of the quasi-stationary Atlantic and Indian Ocean high pressure systems over the subcontinent, surface troughs to the west in the interior with strong ridging from the east, convergence from the interior towards isolated low pressure systems or deep coastal low pressure systems, and deep surface troughs on the West Coast.

• 한국연소학회:학술대회논문집
• /
• 2005.10a
• /
• pp.198-203
• /
• 2005

The LES-based level-set flamelet model has been applied to analyze the turbulent propane/air premixed bluff-body flame with a highly wrinkled flame fronts. The present study has been motivated to investigate the interaction between the flame front and turbulent eddies. Special emphasis is given to study the effect of G equation filtering treatment on the precise structure of turbulent premixed flames as well as the effect of sub-grid scale (SGS) eddies on the wrinkling of the flame surface. The level-set/flamelet model has been adopted to account for the effect of turbulence-flame interaction as well as to properly capture the flame front. Numerical results indicate that the present LES-based level-set flamelet approach has a capability to realistically simulate the highly non-stationary turbulent premixed flame.

• Journal of Environmental Science International
• /
• v.1 no.1
• /
• pp.29-39
• /
• 1992

A study on acidity in precipitation was carried out during May 1990 - April 1991 at two sites in Chongwon, Choongbook. We observed variations of pH from 4.0 to 7.0. Annual mean value of pH was 5.21 in the area. In particular, strong acidity of rain fall, pH 4.0 were observed during winter to early spring. Neutral values were observed during June to July and were due to wet deposition of atmospheric pollutants by stationary fronts in the rainy season. Interestingly, acidity of snow observed in winter was neutral and it was weaker than the acidity of rain in winter by a value of 2.0. Discussion is made on meteorological and chemical analyses and seasonal variations of acidity of precipitation.

• Korean Journal of Agricultural and Forest Meteorology
• /
• v.4 no.3
• /
• pp.169-174
• /
• 2002

When the long - lasting stationary fronts were pushed northward by the Pacific Highs in late July, mostly clear skies with intermittent showers were a typical weather of August in Korea. However, torrential rains and flash floods are now a seasonal event of August in recent years. Some meteorologists suspect this unusual phenomenon might be connected with the global change and are concerned about the possibility of change in summer climatic pattern in Korea. August of year 2002 must be remembered to be one of the record breaking months with respect to the rainfall events. In this paper, we analyzed the weather and crop data nationwide for August in 2002, and suggest a few countermeasures necessary to overcome the wet and cool summer impacts on agricultural sector.

• Journal of the Korean Geographical Society
• /
• v.42 no.1 s.118
• /
• pp.1-14
• /
• 2007

This study aimed to evaluate weather forecast accuracies of minimum temperature, maximum temperature, precipitation and sky cover by the localized land forecast areas over South Korea Average forecast accuracy score of precipitation was the lowest while that of sky cover was the highest during the study period Overall forecast accuracy scores for Gangwon-do was the lowest while those for Gyeongsangnam-do and Gyeongsangbuk-do were higher than other areas. The frequencies of perfect forecast(eight points) by seasons, were the highest during winter and the lowest during summer. pressure pattern analyses for days when forecast accuracy scores were poor, showed that precipitation forecast accuracy scores were lower due to the movement of the stationary fronts during summers. When continental polar air masses expanded, forecast accuracy of temperature became greatly lower during autumns and winters As the migratory anticyclone pattern rapidly moved, forecast accuracy became lower during springs and autumns. Forecast accuracies were compared by wind directions at 850hPa for the Young-dong region where forecast accuracy was the lowest. Forecast accuracy scores on minimum and maximum temperatures were low when winds were westerlies and forecast accuracy scores of precipitation were low when winds were easterlies.

• The Journal of the Acoustical Society of Korea
• /
• v.20 no.2E
• /
• pp.30-45
• /
• 2001

This study considers the environmental factors affecting propagation loss and sonar performance in the continental regions of the East Coast Sea of Korea. Water mass distributions appear to change dramatically in a few weeks. Simple calculation with the case when the NKCW (North Korean Cold Water) develops shows that the difference in propagation loss may reach in the worst up to 10dB over range 5km. Another factor, an eddy, has typical dimensions of 100-200km in diameter and 150-200m in thickness. Employing a typical eddy and assuming frequency to be 100Hz, its effects on propagation loss appear to make lower the normal formation of convergence zones with which sonars are possible to detect long-range targets. The change of convergence zones may result in 10dB difference in received signals in a given depth. Thermal fronts also appear to be critical restrictions to operating sonars in shallow waters. Assuming frequency to be 200Hz, thermal fronts can make 10dB difference in propagation loss between with and without them over range 20km. An observation made in one site in the East Coast Sea of Korea reveals that internal waves may appear in near-inertial period and their spectra may exist in periods 2-17min. A simulation employing simple internal wave packets gives that they break convergence zones on the bottom, causing the performance degradation of FOM as much as 4dB in frequency 1kHz. An acoustic experiment, using fixed source and receiver at the same site, shows that the received signals fluctuate tremendously with time reaching up to 6.5dB in frequencies 1kHz or less. Ambient noises give negative effects directly on sonar performance. Measurements at some sites in the East Coast Sea of Korea suggest that the noise levels greatly fluctuate with time, for example noon and early morning, mainly due to ship traffics. The average difference in a day may reach 10dB in frequency 200Hz. Another experiment using an array of hydrophones gives that the spectrum levels of ambient noises are highly directional, their difference being as large as 10dB with vertical or horizontal angles. This fact strongly implies that we should obtain in-situ information of noise levels to estimate reasonable sonar performance. As one of non-stationary noise sources, an eel may give serious problems to sonar operation on or under the sea bottoms. Observed eel noises in a pier of water depth 14m appear to have duration time of about 0.4 seconds and frequency ranges of 0.2-2.8kHz. The 'song'of an eel increases ambient noise levels to average 2.16dB in the frequencies concerned, being large enough to degrade detection performance of the sonars on or below sediments. An experiment using hydrophones in water and sediment gives that sensitivity drops of 3-4dB are expected for the hydrophones laid in sediment at frequencies of 0.5-1.5kHz. The SNR difference between in water and in sediment, however, shows large fluctuations rather than stable patterns with the source-receiver ranges.