Ⅰ型边缘单裂缝的计算机模拟     

许英姿  张连秀  郝晓峰 《农机化研究》2005,(5):97-98

对素混凝土构件以及具有钢筋强化的混凝土构件在线弹性范围内Ⅰ型边缘单一裂缝的开口进行了模拟。通过模拟发现,在混凝土材料的基体内配置钢筋,使得裂缝的开口位移明显减小。为此,分析了在素混凝土材料的基体内垂直于裂缝的不同位置配置钢筋对构件裂缝开口位移的影响。  相似文献   

Ecological consequences of variation in standard metabolism and dominance among salmon parr     

John D. Armstrong  Karen J. Millidine  Neil B. Metcalfe 《Ecology of Freshwater Fish》2011,20(3):371-376

Abstract – Standard metabolic rate (SMR, closely related to basal and resting metabolism) varies by up to threefold among juvenile Atlantic salmon, Salmo salar L., of similar size at common temperature. Here, consequences of this variation are predicted by combining empirically derived relationships between SMR, specific dynamic action, energy budgets, water velocity, food level in the environment and food availability to fish. The range of velocities across which growth is predicted to occur is inversely related to a fish’s SMR. Growth is positively related to SMR at high but negatively related to it at low‐food levels. The relationship between food level and the range of velocities over which lower SMR fish can grow but higher SMR fish cannot is asymmetrically bi‐phasic and peaked. It is predicted that maternal manipulation of offspring SMR would generate fitness benefits through bet‐hedging against unpredictability in food level and increases in the overall range of velocities that the family of offspring can occupy and thrive in.  相似文献   

直剪法用于糙米内摩擦角的测定与研究     

刘志云  陆琳琳  石翠霞  程绪铎 《粮食储藏》2011,40(5):21-23,28

利用土工合成材料综合测定仪（直剪仪）,以水分含量14.57%的糙米为样品,测定了在不同的法向压应力、不同的剪切速率下糙米的剪切应力并计算出内摩擦角。实验结果表明：糙米的剪切应力随着法向压应力（25kPa～100kPa）的增大而增大;剪切速率（1.33mm/min～2.67mm/min）较低时糙米的内摩擦角随着法向压应力的增大而减小;随着剪切速率（1.33mm/min～5.20mm/min）的增大,糙米的内摩擦角减小。  相似文献   

Flow alteration and thermal pollution depress modelled growth rates of an iconic riverine fish,the Murray cod Maccullochella peelii          下载免费PDF全文

Nick S. Whiterod  Shaun N. Meredith  Paul Humphries  Bradford S. Sherman  John D. Koehn  Robyn J. Watts  Brett A. Ingram  Tom Ryan 《Ecology of Freshwater Fish》2018,27(3):686-698

The serial discontinuity concept (SDC) proposes that hypolimnetic‐releasing impoundments cause major disruptions to the naturally occurring physical, chemical and biological gradients of rivers but that this impact diminishes with distance downstream. Such a gradient in discharge, flow velocity and temperature regime occurs below a large hypolimnetic‐releasing impoundment, the Hume Dam, on the River Murray in south‐eastern Australia. To examine the effects of this disturbance gradient on a warm‐water large‐bodied freshwater fish, the Murray cod (Maccullochella peelii), a bioenergetics model was developed and calibrated to explore energy expended under differing water velocities and temperature regimes. Model simulations predicted negative growth of juveniles directly downstream of the impoundment, due largely to the energetic costs associated with active and, to a lesser extent, standard metabolism outweighing the achievable energetic gains through food consumption. As flow velocity and temperature regimes became more favourable downstream, so did the simulated growth of the species. It was not until +239 km downstream of the impoundment that the model predicted that flow velocity and temperature regimes were suitable for greater weight gains. The modelled growth responses of juvenile Murray cod are consistent with the predictions of the SDC, emphasising that changes in the bioenergetics of individuals are likely to be reflected in reduced growth rates under the changed flow velocity and temperature regimes imposed by disturbance gradients. This research represents a valuable step in the biological understanding of Murray cod within variable riverine environments and emphasises the urgency required to mitigate impacts associated with hypolimnetic impoundments.  相似文献   

Influence of salinity and moisture on the threshold shear velocity of saline sand in the Qarhan Desert,Qaidam Basin of China: A wind tunnel experiment     

Chao LI  Zhibao DONG  Shuyan YIN 《干旱区科学》2019,11(5):674-684

Determination of the threshold shear velocity is essential for predicting sand transport, dust release and desertification. In this study, a wind tunnel experiment was conducted to evaluate the influence of salinity and moisture on the threshold shear velocity of saline sand. Saline sand samples (mean particle size of 164.50-186.08 μm and the total silt, clay and salt content of 0.80%-8.25%) were collected from three saline sand dunes (one barchan dune and two linear dunes) in the Qarhan Desert, Qaidam Basin of China. Original saline sand samples were placed in two experimental trays for wet and dry processing to simulate deliquescence and desiccation, respectively. Surface moisture content ranging from 0.30% to 1.90% was generated by the steam method so that the saline sand can absorb water in a saturated water vapor environment. The motion of sand particles was determined by the observers with a solid laser. The laser sheet (0.80 cm thick), which was emitted by the solid laser, horizontally covered the sand surface and was bound to the sand. Results show that the cohesion of saline sand results from a combination of salt and water. The threshold shear velocity increases exponentially with the increase in crust thickness for the linear sand dunes. There is a positive linear correlation between the original moisture content and relative threshold shear velocity. The threshold shear velocity of dewatered sand is greater than that of wet sand with the same original moisture content. Our results will provide valuable information about the sand transport of highly saline soil in the desert.  相似文献   

High night-time temperature during flowering and pod filling affects flower opening,yield and seed fatty acid composition in canola     

Meghnath Pokharel  Anuj Chiluwal  Michael Stamm  Doohong Min  Davina Rhodes  S.V. Krishna Jagadish 《Journal of Agronomy and Crop Science》2020,206(5):579-596

Winter canola (Brassica napus L.) is highly sensitive to increasing temperatures during the reproductive and pod-filling stages. Although the impact of high day-time temperature stress on yield and quality has been documented in canola, similar information under high night-time temperature (HNT) stress is not available. Using six hybrids and four open-pollinated cultivars, we observed a marked shift in peak flowering towards earlier, cooler hours of the morning under HNT. Averaged across two independent experiments, the photochemical efficiency of photosystem II was significantly decreased (3%), with a significant increase in thylakoid membrane damage (13%) in the leaves of susceptible cultivars under HNT stress. Similarly, the susceptible cultivars also recorded significant reduction in biomass (34%), pod number (22%), pod weight (37%) and total seed weight (40%) per plant while the same set of agronomic traits were not affected among the tolerant cultivars. Quantitative impact of heat stress was confirmed with increased sensitivity to HNT exposure from gametogenesis until maturity resulting in a significantly higher yield loss compared to stress exposure from post-flowering till maturity. HNT significantly decreased oil concentration, but increased protein concentration and saturated fatty acid levels in seeds of the susceptible cultivars. However, HNT had no impact on the unsaturated fatty acids in both hybrids and the open-pollinated cultivars. Breeding targets based on fatty acid composition for enhancing canola seed quality may not be easily amenable due to the inconsistency documented with the compositional changes under heat stress. In summary, our findings conclude that canola hybrids are better suited to regions experiencing heat stress, compared to open-pollinated cultivars, indicating the possibility of a complete shift to hybrid canola cultivation under predicted hotter climates in the future.  相似文献   

基于定向定量堆放的马铃薯收获机设计   总被引：1，自引：0，他引：1  

潘志国  尚书旗  杨然兵  于镇伟  马庆驰 《农机化研究》2015,(8):88-91

在研究小型马铃薯收获机的基础上,设计了一款能根据收集薯块质量往特定方向堆放的马铃薯收获机,主要由挖掘装置、输送装置及定向定量堆放装置等组成。在马铃薯收获机的后部设计一个四分区临时集薯器,能根据所收获马铃薯的质量来打开集薯器,将收集的马铃薯向中间或者一侧堆放,从而实现了马铃薯的连续挖掘及收集薯块的定向定量间隔堆放,可在较大程度上降低人工捡拾的工作量,同时避免拖拉机碾压伤薯。  相似文献   

边界滑移对螺旋槽上游泵送机械密封性能的影响     

陈汇龙  徐成  胡吉  刘志斌  赵斌娟  李述林 《排灌机械》2015,(1)

为了研究边界滑移对上游泵送机械密封性能的影响,建立液膜三维几何模型和计算模型,基于 Navier 线性滑移模型对液膜壁面边界条件进行修正,采用商用软件 Fluent 的 SIMPLEC 算法及层流模型求解三维 Navier －Stokes 方程,并分析相对滑移量对液膜静压分布、开启力、摩擦扭矩、泄漏量的影响规律。结果表明：相对于边界滑移发生的位置,滑移速度的大小对密封性能的影响更大;当相对滑移量较小时,存在边界滑移与无滑移的模拟结果无明显区别,能很好地解释宏观无滑移边界假设的应用,当相对滑移量较大时液膜动压效应随滑移的增加而减弱,开启力、摩擦扭矩、泄漏量都随边界滑移的增加而减小;相对于开启力的降低,边界滑移的减阻和降低泄漏的效果更为明显;当开启力较小,应避免边界滑移发生;当开启力足够大时,加工成超疏水表面形成边界滑移,可极大地减少摩擦扭矩,降低能耗。  相似文献   

海河二道闸和防潮闸优化调度研究   总被引：1，自引：0，他引：1  

高学平  张晨  于丽丽  果有娜 《中国农村水利水电》2005,(5):30-32

汛期为腾空海河河道并保证二道闸下咸水不致上溯，二道闸闸下水位应控制在1.0m左右。利用一维非恒定流模型对海河自西河闸至入海口河段进行数值模拟，研究了西河闸分流量为80、100、120m3/s3种情况下，海河防潮闸和二道闸优化调度结果。  相似文献   

Nonlinearity of Electronic Throttle and Its Control     

DU Kai-ming  Qin Da-tong  LIU Zhen-jun  JIANG Feng-xiang  DING Rui 《保鲜与加工》2005,(4):14-18

This paper introduces the development of the electronic throttle control system (ETCS) , the structure and the principle of the system are also involved. After analyzing the nonlinearity problem existing in the mechanical structure of the system, a mathematical model of the ETCS is built up. Then three controllers (PID controller, Fuzzy controller, sliding mode controller) were designed and simulated in the matlab simulink. The result indicates that the PID controller has an overshoot,but the Fuzzy controller and the sliding mode controller can settle the nonlinearity problem better.  相似文献