Experimental study on the gap entrance profile affecting rudder gap cavitation     

Bu-Geun Paik  Kyung-Youl Kim  Jong-Woo Ahn  Yong-Soo Kim  Sung-Pyo Kim  Je-Jun Park 《Ocean Engineering》2008,35(1):139-149

The unsteady cavity patterns around the gap of the conventional and newly developed semi-spade rudders for marine ships are visualized qualitatively using a high-speed CCD camera. Time-resolved PIV analysis is also performed with the bubble tracers to study the flow behavior over the rudder surface. In addition, pressure measurements are conducted on the rudder surface and inside the gap to find out the flow characteristics around the gap entrance of the rudder. Both the rudders are tested without a propeller wake at the various cavitation numbers and at the rudder deflection angle of −8°θ10°. The strong cavitation patterns around the conventional rudder gap are significantly reduced by adopting a newly developed entrance profile, and a time-resolved velocity field is found to be very effective in catching the vortical cavity flow around the rudder gap. The stagnation point near the gap entrance of the conventional rudder can cause unsteady cavity flow. However, the developed rudder has very stable pressure distribution along the horn surface and decreases the pressure inside the gap because of the modification of the gap entrance. The pressure distribution around the gap of the suction side is closely related to the variation of the rudder deflection angle. The cavitation inception speed is delayed by about 4 knots in the angle range of −5°θ5° by employing the developed profile of the gap entrance.  相似文献   

Time‐varying,nonlinear suspended sediment rating curves to characterize trends in water quality: An application to the Upper Hudson and Mohawk Rivers,New York     

Kuk‐Hyun Ahn  Scott Steinschneider 《水文研究》2019,33(13):1865-1882

This work proposes two modelling frameworks for diagnosing temporal variations in nonlinear rating curves that describe suspended sediment–discharge relationships. A variant of the weighted regression on time, discharge, and season model is proposed and is compared against dynamic nonlinear modelling, a newly developed nonlinear time series filter based on sequential Monte Carlo sampling. Both approaches estimate a time series of rating curve parameters, with uncertainty, that can be used to diagnose variability in the sediment–discharge relationship over time. We evaluate the models with a variety of synthetic scenarios to highlight their ability to estimate signals of known rating curve change. Results reveal important bias‐variance trade‐offs unique to each approach, and in general, suggest that dynamic nonlinear modelling is better suited for rapid rating curve changes, whereas the weighted regression on time, discharge, and season variant more precisely estimates slow change. The techniques are then applied in two case studies in the Upper Hudson and Mohawk Rivers in New York. We conclude with a discussion of the implications of dynamic rating curves for the management of water quality in riverine and estuary systems.  相似文献   

Scientific instrumentation for the 1.6 m New Solar Telescope in Big Bear     

W. Cao  N. Gorceix  R. Coulter  K. Ahn  T. R. Rimmele  P. R. Goode 《Astronomische Nachrichten》2010,331(6):636-639

The NST (New Solar Telescope), a 1.6 m clear aperture, off‐axis telescope, is in its commissioning phase at Big Bear Solar Observatory (BBSO). It will be the most capable, largest aperture solar telescope in the US until the 4 m ATST (Advanced Technology Solar Telescope) comes on‐line late in the next decade. The NST will be outfitted with state‐of‐the‐art scientific instruments at the Nasmyth focus on the telescope floor and in the Coudé Lab beneath the telescope. At the Nasmyth focus, several filtergraphs already in routine operation have offered high spatial resolution photometry in TiO 706 nm, Hα 656 nm, G‐band 430 nm and the near infrared (NIR), with the aid of a correlation tracker and image reconstruction system. Also, a Cryogenic Infrared Spectrograph (CYRA) is being developed to supply high signal‐to‐noise‐ratio spectrometry and polarimetry spanning 1.0 to 5.0 μm. The Coudé Lab instrumentation will include Adaptive Optics (AO), InfraRed Imaging Magnetograph (IRIM), Visible Imaging Magnetograph (VIM), and Fast Imaging Solar Spectrograph (FISS). A 308 sub‐aperture (349‐actuator deformable mirror) AO system will enable nearly diffraction limited observations over the NST's principal operating wavelengths from 0.4 μm through 1.7 μm. IRIM and VIM are Fabry‐Pérot based narrow‐band tunable filters, which provide high resolution two‐dimensional spectroscopic and polarimetric imaging in the NIR and visible respectively. FISS is a collaboration between BBSO and Seoul National University focussing on chromosphere dynamics. This paper reports the up‐to‐date progress on these instruments including an overview of each instrument and details of the current state of design, integration, calibration and setup/testing on the NST (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Fuzzy modeling of holding capacity of offshore suction caisson anchors          下载免费PDF全文

Yeesock Kim  Jaehun Ahn  Jongwon Jung 《国际地质力学数值与分析法杂志》2017,41(7):1038-1054

A fuzzy algorithm, the Takagi–Sugeno model, is implemented to develop a fuzzy inference system for predicting the holding capacity of suction caisson foundations for offshore platforms. The premise parameters of the fuzzy model are optimized by using a subtractive clustering algorithm. The consequent parameters are optimally determined via a weighted least square estimation. The input variables used for training the fuzzy model include the aspect ratio of the caisson, the undrained shear strength of the clay, and the angle that the chain force forms with the horizontal. The output of the proposed fuzzy model is the capacity of the suction caisson anchor. To demonstrate the effectiveness of the fuzzy modeling framework, the results of extensive finite element analyses are investigated. Comparisons of the trained fuzzy model with the data demonstrate that the proposed modeling framework is an effective method to estimate the holding capacity of offshore suction caisson systems. Moreover, the performance of the fuzzy model is robust against higher levels of input data uncertainties. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Inter-comparison of the infrared channels of the meteorological imager onboard COMS and hyperspectral IASI data     

Dohyeong Kim  Myoung-Hwan Ahn  Minjin Choi 《大气科学进展》2015,32(7):0-990

The successful launch and commissioning of the first geostationary meteorological satellite of Korea has the potential to enhance earth observation capability over the Asia Pacific region. Although the specifications of the payload, the meteorological imager(MI), have been verified during both ground and in-orbit tests, there is the possibility of variation and/or degradation of data quality due to many different reasons, such as the accumulation of contaminants, the aging of instrument components, and unexpected external disturbance. Thus, for better utilization of MI data, it is imperative to continuously monitor and maintain the data quality. As a part of such activity, this study presents an inter-calibration, based on the Global Space-based Inter-Calibration System(GSICS), between the MI data and the high quality hyperspectral data from the Infrared Atmospheric Sounding Interferometer(IASI) of the Metop-A satellite. Both sets of data, acquired for three years from April 2011 to March 2014, are processed to prepare the matchup dataset, which is spatially collocated, temporally concurrent,angularly coincident, and spectrally comparable. The results show that the MI data are stable within the specifications and show no significant degradation during the study period. However, the water vapor channel shows a rather large bias value of-0.77 K, with a root-mean-square difference(RMSD) of around 1.1 K, which is thought to be due to the shift in the spectral response function. The shortwave channel shows a maximum RMSD of around 1.39 K, mainly due to the coarse digitization at the lower temperature. The inter-comparison results are re-checked through a sensitivity analysis with different sets of threshold values used for the matchup dataset. Based on this, we confirm that the overall quality of the MI data meets the user requirements and maintains the expected performance, although the water vapor channel requires further investigation.  相似文献   

Does radiative feedback by the first stars promote or prevent second generation star formation?     

Kyungjin Ahn  Paul R. Shapiro 《Monthly notices of the Royal Astronomical Society》2007,375(3):881-908

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Wave propagation in nonlinear one‐dimensional soil model     

J. Ahn  G. Biscontin  J. M. Roesset 《国际地质力学数值与分析法杂志》2009,33(4):487-509

The objective of the research conducted by the authors is to explore the feasibility of determining reliable in situ values of shear modulus as a function of strain. In this paper the meaning of the material stiffness obtained from impact and harmonic excitation tests on a surface slab is discussed. A one‐dimensional discrete model with the nonlinear material stiffness is used for this purpose. When a static load is applied followed by an impact excitation, if the amplitude of the impact is very small, the measured wave velocity using the cross‐correlation indicates the wave velocity calculated from the tangent modulus corresponding to the state of stress caused by the applied static load. The duration of the impact affects the magnitude of the displacement and the particle velocity but has very little effect on the estimation of the wave velocity for the magnitudes considered herein. When a harmonic excitation is applied, the cross‐correlation of the time histories at different depths estimates a wave velocity close to the one calculated from the secant modulus in the stress–strain loop under steady‐state condition. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

A review of the National Groundwater Monitoring Network in Korea     

Jin‐Yong Lee  Myeong‐Jae Yi  Young‐Kwon Yoo  Kyung‐Hwan Ahn  Gyoo‐Bum Kim  Jong‐Ho Won 《水文研究》2007,21(7):907-919

The drastic expansion of cities and the rapid economic growth in Korea have caused dramatic increases to demand from groundwater supplies for drinking, domestic, agricultural and industrial water usage. The Ministry of Construction and Transportation and the Korea Water Resources Corporation have constructed and operated the National Groundwater Monitoring Network (NGMN) throughout the country since 1995. The NGMN, an official project establishing a total of 320 groundwater monitoring stations, was completed in 2005. Each national groundwater monitoring station serves as a baseline and primary station to monitor long‐term general trends in water‐level fluctuations and in groundwater quality. The present NGMN and its monitoring capabilities were evaluated to enhance the efficiency of groundwater monitoring and to meet the new societal conditions. Based on reviews and evaluations, some suggestions and recommendations are made with regard to improvements of the national network, including the installation of rainfall gauges in groundwater monitoring stations, gathering groundwater data every hour instead of every 6 h as at present, involving major cations and anions in the regular and periodic chemical analyses, regular periodic analyses of collected groundwater data, and construction of 199 additional groundwater monitoring stations to supplement the existing groundwater monitoring network. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Formation and evolution of self-interacting dark matter haloes     

Kyungjin Ahn  Paul R. Shapiro 《Monthly notices of the Royal Astronomical Society》2005,363(4):1092-1110

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Magnetospheric convection at a low level power ϵ     

S.-I. Akasofu  B.-H. Ahn 《Planetary and Space Science》1982,30(10):1057-1059

It is demonstrated that the magnetospheric convection becomes evident in terms of the AE index only when the power ? of the solar wind-magnetosphere dynamo becomes greater than ～ 10¹⁸ erg s^?1 or a slightly lower value. An enhanced conductivity is a crucial factor for the magnetospheric convection to manifest even in a low-level increase of the AE index of ～ 50–100 γ.  相似文献