SPHERE (which stands for Spectro-Polarimetric High-contrast Exoplanet REsearch) is a second-generation Very Large Telescope (VLT) instrument dedicated to high-contrast direct imaging of exoplanets whose first-light is scheduled for 2011. Within this complex instrument one of the central components is the apodized Lyot coronagraph (ALC). The principal aim of this paper is to report the first laboratory experiment of the ALC designed for the SPHERE instrument. The performance and sensitivity of the optical configuration was first numerically studied with an end-to-end approach (see the results in paper I subtitled ??Detailed numerical study??). Made confident by the results, we then tested a prototype on an infrared coronagraphic bench. We measured the transmission profiles of the apodizer prototype and the coronagraphic performance of the apodized Lyot coronagraph in Y, J, and H bands. The coronagraph sensitivity to lateral and longitudinal misalignments of its three main components (apodizer, coronagraphic mask and Lyot stop) was finally studied in H band. We can conclude that the prototype meets the SPHERE technical requirements for coronagraphy. 相似文献
When a high-speed body with cavity passes through water-air free surface and exits water, its mechanical environment and dynamic characteristics change significantly due to the great difference in density and viscosity between water and air. With focusing on this problem, the Computational Fluid Dynamics (CFD) method is applied to perform numerical calculation on the process of this vapor-liquid-gas flow during the water exit of a high-speed cylinder, with the Volume of Fraction (VOF) multiphase flow interface-capturing techniques and the overset grid technology. After the verification and validation of the CFD model through mesh convergence study and a water-entry experiment, cavity evolution and flow characteristics including pressure and velocity distribution during the water exit are analyzed. The effects of different initial velocities on the pressure distribution and drag characteristics of the cylinder are investigated. Calculated results show that the cavity collapse during water exit causes strong pressure fluctuation on the cylinder; when the cylinder exits water enveloped in a supercavity, the pressure distribution on its wall surface and surrounding water region is relatively uniform, and the drag changes gently, and thus the cylinder has good motion stability.
In this paper, the transient fluid resonance phenomenon inside a narrow gap between two adjacent boxes excited by the incident focused waves with various spectral peak periods and focused wave amplitudes is simulated by utilizing the open-sourced computational fluid dynamics software, OpenFOAM. The weather-side box is allowed to heave freely under the action of waves, and the lee-side box keeps fixed. This paper mainly focuses on how both the spectral peak period and the focused wave amplitude affect the free-surface amplification inside the gap, the motion of the weather-side box, and the wave loads (including the vertical and the horizontal wave forces) acting on both boxes. For comparison, another two-box system with both boxes fixed is also considered as a control group. It is found that the motion of the weather-side box significantly changes the characteristics of the transient gap resonance, and it would cause that the fluid resonant period becomes 1.4–1.6 times that of the two-box system with both boxes fixed. All the concerned physical quantities (i.e., the free-surface amplification in the gap, the motion of the weather-side box, the wave loads) are found to closely depend on both the spectral peak period and the focused wave amplitude.
Timely identification of disaster-prone neighborhoods and examination of disparity in disaster exposure are critical for policymakers to plan efficient disaster management strategies. Many studies have investigated racial, ethnic, and geographic disparities and populations most vulnerable to disasters. However, little attention has been paid to the development of easily accessible and reusable tools to enable: (1) the prompt identification of vulnerable neighborhoods; and (2) the examination of social disparity in disaster impact. In this research, we have developed a visual analytics tool that allows users to: (1) delineate neighborhoods based on their selection of variables; and (2) explore which neighborhoods are susceptible to the impacts of disasters based on specific socioeconomic and demographic characteristics. Through an exploration of COVID-19 data in the case study, we revealed that the tool can provide new insights into the identification of vulnerable neighborhoods that need immediate attention for disaster control, management, and relief. 相似文献
The multifractal measure enables an examination of the characteristics of a quantity distributed over a domain. This study examined the multifractal properties of turbulent intensities obtained from jet discharge experiments, where three types of nozzle geometries were examined in terms of the velocity fields and turbulent characteristics using particle image velocimetry. Depending on the nozzle geometry, the experimental results showed that the distribution of turbulent intensities and resulting dilution exhibited different behaviors. The experiment also showed that the transversal velocity profiles are similar to each other regardless of the outfall nozzle shapes and demonstrates the traditional similarity assumption at the same time. The multifractal exponents of the turbulent intensities were obtained with Box Count Method in a two-dimensional space. The results showed that the turbulent intensities obtained in two-dimensional space have a common multifractal spectrum, which was not the case for the velocity or shear stress observed in the same space. Although the transversal velocity profiles are similar, the multifractal exponent clearly shows a difference depending on the outfall geometries. In particular, the minimum value of the Lipschitz–Hölder exponent (αmin) and the entropy dimension (α1) tends to increase as turbulent intensity and dilution increase. These results suggest that the multifractal properties can be utilized potentially to categorize and evaluate the discharge outfall capabilities in terms of the resulting dilution. 相似文献
Inspired by the importance of Redfield-type C:N:P ratios in global soils,we looked for analogous patterns in peatlands and aimed at deciphering the potential affecting factors.By analyzing a suite of peatlands soil data(n = 1031),mean soil organic carbon(SOC),total nitrogen(TN) and total phosphorous(TP) contents were 50.51%,1.45% and 0.13%,respectively,while average C:N,C:P and N:P ratios were 26.72,1186.00 and 46.58,respectively.C:N ratios showed smaller variations across different vegetation coverage and had less spatial heterogeneity than C:P and N:P ratios.No consistent C:N:P ratio,though with a general value of 1245:47:1,was found for entire peatland soils in China.The Northeast China,Tibet,Zoigê Plateau and parts of Xinjiang had high soil SOC,TN,TP,and C:P ratio.Qinghai,parts of the lower reaches of the Yangtze River,and the coast zones have low TP and N:P ratio.Significant differences for SOC,TN,TP,C:N,C:P and N:P ratios were observed across groups categorized by predominant vegetation.Moisture,temperature and precipitation all closely related to SOC,TN,TP and their pairwise ratios.The hydrothermal coefficient(RH),defined as annual average precipitation divided by temperature,positively and significantly related to C:N,C:P and N:P ratios,implying that ongoing climate change may prejudice peatlands as carbon sinks during the past 50 years in China. 相似文献
Oceanic red beds are widely distributed in the global oceans and across the entire Phanerozoic period, which mostly appeared after oceanic anoxic events. They represent typical oxygen-rich sedimentary environment and play a significant role on ocean science research. Numerous studies have been carried out since the oceanic red beds were discovered. However, previous studies mainly focused on the Cretaceous oceanic red beds, and the understanding of the characteristics and scientific significance of oceanic red beds are not comprehensive. Therefore, we here summarized the global distribution characteristics and compared mineral and element compositions of various lithological oceanic red beds, including marly, clayey and cherty oceanic red beds. The main mineral and element components of oceanic red beds have no direct relationship with the color of the sediments, and mainly are affected by the regional environment and provenances. Therefore, the mineralogical and geochemical characteristics of oceanic red beds should be analyzed in combination with the regional background. The red coloration of oceanic red beds is controlled mainly by hematite, goethite and manganese-bearing calcite, which have two main mechanisms: ① Colored minerals formed in oxic conditions; ② Colored minerals formed due to low deposition rates. These two mechanisms are not completely independent, but complement one another with either dominance in most oceanic red beds. Lithological characteristics of oceanic red beds are controlled by three factors, including water depth, productivity and nutrients. Therefore, the formation of oceanic red beds should be considered with global changes and regional events. The unique origin mechanism and global distribution characteristics of long time-scale oceanic red beds can be used to indicate sedimentary paleoenvironment, paleo-oceanic current, and paleoclimate change. In addition, hydrothermal or magmatic activities on the ocean floor could also produce red-color deposits that are strongly different from sedimentary oceanic red beds. Based on the existing research, we also put forward the future in-depth studies on the oceanic red beds from multidisciplinary perspectives. 相似文献
This paper discusses the geological and hydrogeological features of Quaternary deposits in Tianjin as well as the geohazards related to groundwater hydrology in this region. The soft soil deposits, comprising silt, sand, silty clay and clay, are composed of four aquifer groups. In the first aquifer group, one phreatic aquifer and two confined aquifers have relationships with underground construction in the urban area. These three aquifers are separated by two aquitards and collectively form a multi-aquifer system. During geotechnical construction, potential geohazards present are related to the groundwater, which include water-in-rushing, quicksand and piping hazards. To prevent the aforementioned geohazards, dewatering is conducted; however, groundwater pumping may result in large settlements of the surrounding ground. To reduce pumping-induced settlement, the dewatering–waterproofing system has been adopted. According to the characteristics of the subsoil, excavation depth and the surrounding environment, the dewatering system can be divided into five patterns. In the first four patterns, when pumping is conducted in the excavation pit, the groundwater head in the adjacent aquifers outside the pit decreases due to the leakage effect of the aquitards located between the aquifers. In the fifth pattern, waterproof curtain has cut off the aquifers completely and dewatering in the pit cannot result in settlement around excavation pit. To avoid geohazards related to groundwater hydrology, countermeasures recommended include construction of an effective waterproof curtain, selection of a reasonable excavation dewatering pattern and withdrawal of required groundwater. 相似文献