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41.
Harald Wolf 《Behavioral ecology and sociobiology》2008,62(3):415-425
In the habitat of desert ants, Cataglyphis fortis, a constant wind is usually blowing during the daytime. When visiting a familiar food source, the ants steer some distance
downwind of the feeder, rather than attempting a direct approach that might miss small food sources, in particular. In the
downwind area, the ants pick up the odor plume emanating from the food and follow it upwind to the prey. This strategy saves
considerable walking distance and time. The additional path necessitated by the downwind strategy is only about 0.75 to 2 m,
depending on nest–feeder distance, while missing the food on the upwind side results in much longer search trajectories. During
the initial three to five visits to a feeding site, downwind distance and length of the approach path are shortened notably,
and the approach trajectory is straightened. Desert ants further exhibit considerable short-term flexibility in their approach.
Experienced individuals are evidently able to decide upon leaving the nest which direction to choose toward the feeder, depending
on current wind direction (that fluctuates slightly during the day). Notable changes in wind direction occur primarily overnight.
For larger nest–feeder distances, the animals adjust their approach en route to the altered wind direction during their first
foraging trip in the morning. 相似文献
42.
Many environmental problems are linked to multiphase flows encompassing ecological issues, chemical processes and mixing or
diffusion, with applications in different engineering fields. The transition from a supercritical flow to a subcritical motion
constitutes a hydraulic jump. This flow regime is characterised by strong interactions between turbulence, free surface and
air–water mixing. Although a hydraulic jump contributes to some dissipation of the flow kinetic energy, it is also associated
with increases of turbulent shear stresses and the development of turbulent eddies with implications in terms of scour, erosion
and sediment transport. Despite a number of experimental, theoretical and numerical studies, there is a lack of knowledge
concerning the physical mechanisms involved in the diffusion and air–water mixing processes within hydraulic jumps, as well
as on the interaction between the free-surface and turbulence. New experimental investigations were undertaken in hydraulic
jumps with Froude numbers up to Fr = 8.3. Two-phase flow measurements were performed with phase-detection conductivity probes.
Basic results related to the distributions of void fraction, bubble frequency and mean bubble chord length are presented.
New developments are discussed for the interfacial bubble velocities and their fluctuations, characterizing the turbulence
level and integral time scales of turbulence representing a “lifetime” of the longitudinal bubbly flow structures. The analyses
show good agreement with previous studies in terms of the vertical profiles of void fraction, bubble frequency and mean bubble
chord length. The dimensionless distributions of interfacial velocities compared favourably with wall-jet equations. Measurements
showed high turbulence levels. Turbulence time scales were found to be dependent on the distance downstream of the toe as
well as on the distance to the bottom showing the importance of the lower (channel bed) and upper (free surface) boundary
conditions on the turbulence structure. 相似文献
43.
瓦斯气体在煤层中的扩散机理及模式 总被引:10,自引:2,他引:8
根据气体在多孔介质中的扩散模式 ,结合煤结构的实际特点 ,分析了瓦斯气体在煤孔隙中的扩散机理 ,得出瓦斯在煤体中几种扩散模式 :菲克型扩散、诺森扩散、过渡型扩散、表面扩散和晶体扩散。从微观上看 ,影响瓦斯气体在煤层中扩散的主要因素是瓦斯气体的平均自由程和煤不同尺寸微孔隙的分布情况。根据不同孔隙分布的煤层 ,探讨了各种扩散模式的适用条件 相似文献
44.
分析了1978~2007年庆阳市环境-经济系统的能值流量与能值效率的演变轨迹.并以环境负荷率(ELR)、能值废弃率(EWR)作为环境压力指标,运用协整检验、Granger因果检验和向量误差修正模型(VECM),通过分析人均GDP与环境压力指标变量之间在时序维度上的因果关系和相互影响程度,进一步阐述经济与环境系统的耦合关系及其动态特征.结果表明,庆阳市经济发展对不可更新资源的依赖度大,环境-经济系统的产出效率总体下降,但环境负荷不断上升.计量分析表明,经济增长与环境压力之间存在单向因果关系,且已形成"恶性循环圈",环境-经济耦合系统表现出不可持续的发展态势. 相似文献
45.
IntroductionA pedestrian crash occurs due to a series of contributing factors taking effect in an antecedent-consequent order. One specific type of antecedent-consequent order is called a crash causation pattern. Understanding crash causation patterns is important for clarifying the complicated growth of a pedestrian crash, which ultimately helps recommend corresponding countermeasures. However, previous studies lack an in-depth investigation of pedestrian crash cases, and are insufficient to propose a representative picture of causation patterns. Method: In this study, pedestrian crash causation patterns were discerned by using the Driving Reliability and Error Analysis Method (DREAM). One hundred and forty-two pedestrian crashes were investigated, and five pedestrian pre-crash scenarios were extracted. Then, the crash causation patterns in each pre-crash scenario were analyzed; and finally, six distinct patterns were identified. Accordingly, 17 typical situations corresponding to these causation patterns were specified as well. Results: Among these patterns, the pattern related to distracted driving and the pattern related to an unexpected change of pedestrian trajectory contributed to a large portion of the total crashes (i.e., 27% and 24%, respectively). Other patterns also played an important role in inducing a pedestrian crash; these patterns include the pattern related to an obstructed line of sight caused by outside objects (9%), the pattern that involves reduced visibility (13%), and the pattern related to an improper estimation of the gap distance between the vehicle and the pedestrian (10%). The results further demonstrated the inter-heterogeneity of a crash causation pattern, as well as the intra-heterogeneity of pattern features between different pedestrian pre-crash scenarios. Conclusions and practical applications: Essentially, a crash causation pattern might involve different contributing factors by nature or dependent on specific scenarios. Finally, this study proposed suggestions for roadway facility design, roadway safety education and pedestrian crash prevention system development. 相似文献
46.
Seung Min LeeJun Su Ha Poong Hyun Seong 《Journal of Loss Prevention in the Process Industries》2011,24(1):90-97
Communication error has been considered a primary cause of many incidents and accidents in the nuclear industry. In order to prevent these accidents, a method for the analysis of such communication errors is proposed here. This paper presents a qualitative and a quantitative method to analyze communication errors. The qualitative method focuses on finding a root cause of the communication error and predicting the type of communication error which could happen in nuclear power plants. We develop context conditions and antecedent-consequent links of influential factors related to communication errors. The quantitative analysis method focuses on estimating the probability of communication errors. To accomplish the quantification of communication errors, the Cognitive Speaking Process (CSP) is defined and a method to estimate the weighting factors and the probability is suggested. Finally, case studies conducted to validate the applicability of the proposed methods are detailed. From the results, we can foresee the effects of given plant conditions on communication errors and reduce the error occurrences. 相似文献
47.
在线水质自动监测系统质量保证与质量控制 总被引:1,自引:0,他引:1
质量保证与控制是水质自动监测中的非常重要一项工作,在线水质自动分析仪一般情况下都是持续工作,溶液的稳定性、分析仪器的基线漂移、仪器的运行情况都直接影响数据的质量。因此,为取得准确的数据,就必须对自动监测系统进行全程的质量控制。 相似文献
48.
49.
50.
Determination of polycyclic aromatic hydrocarbons in urban street dust: implications for human health 总被引:4,自引:0,他引:4
The determination of sixteen polycyclic aromatic hydrocarbons in urban street dust has been done. Samples were collected from 12 sampling locations in a city centre location (Newcastle upon Tyne, north east England) and extracted using in situ pressurised fluid extraction followed by gas chromatography mass spectrometry. From the results it was possible to identify three groups, with respect to PAH concentration, with PAH contents ranging between 0.6-2.3 mg kg−1, 15.6-22.5 mg kg−1 and 36.1-46.0 mg kg−1. The total PAH content of samples from these sampling sites has been compared to 22 urban locations around the world; comparable levels were found in these samples compared to the other cities around the world.The potential source of PAHs has been investigated by investigating the proportion of pyrogenic and petrogenic material in urban street dust using specific individual PAH ratios. The results indicate that the PAH content of urban street dust from the chosen sites are more likely to be due to pyrogenic sources i.e. vehicle exhaust emissions. The particle size fractions (<63 μm; 63-125 μm; 125-250 μm; 250-500 μm; 500-1000 μm; and 1000-2000 μm) of individual PAHs in three selected sampling sites was investigated. In two of the selected sites the PAH content was independent of particle size whereas in sampling site 10 elevated PAH levels are noted in the <63 μm size fraction. Sampling site 10 is located at the junction of three road tributaries which are used as major access points to the east of the city centre. Finally, the potential health risk for unintentional consumption of PAHs was assessed in terms of a mean daily intake (based on an ingestion rate of 100 mg d−1). It was found that all 4-6 membered ring PAHs had concentrations in excess of the mean daily intake thereby reflecting a potential health risk, particularly in the smallest size particle fractions. 相似文献