叶片数对离心泵振动噪声性能的影响

叶片数是离心泵的主要几何参数之一。为研究叶片数对离心泵振动噪声性能的影响,以比转速为97的离心泵为例,对比了不同叶片数下的水力和振动噪声性能,并采用FEM\BEM声振耦合计算方法对流动激励下的振动及其声辐射噪声进行了数值模拟,同时与试验数据进行对比分析。结果表明:提出的数值模拟方法可用于预测泵的流动诱导振动和声辐射性能,且在模拟中考虑口环泄漏的影响能够提高计算精度,有口环方案预测得到的振幅较无口环方案的预测精度提高了13.5%。随着叶片数的增加,扬程和轴功率均逐渐增大,最大增幅分别为15.9%和14.1%;效率随叶片数的增加呈先增大后减小再增大的趋势。离心泵蜗壳的压力脉动幅值随叶片数的减小而增大。由于叶轮蜗壳动静干涉的作用,蜗壳隔舌处、第1到第2断面间和扩压管壁面等3个区域的压力脉动幅值相对较高。随着叶片数的减少,蜗壳壁面的振动位移有所增大,最大位移主要发生蜗壳第8断面处。振动速度随着叶片数的增大后减小,与振动位移的规律有一定的差异,振动高速区主要集中在隔舌、蜗壳的第4与第6断面之间和靠近扩压管的第8断面处。设计工况下,泵在叶频对应的声压级和声强随着叶片数的增加先增大后减小,高声压级区域主要出现在泵出口附近的高振动速度引起的垂直方向。综合考虑水力和振动噪声性能,确定该模型泵的最佳叶片数为6。     

基于点格局分析的林地表层土壤优先路径水平分布特征

将景观生态学中的空间点格局分析方法应用于土壤优先路径的水平空间分布研究,采用Ripley’s K(x)函数单种点格局分析及多元点格局分析方法,对重庆四面山针叶林地、阔叶林地及针阔混交林地10-20cm土壤深度上不同影响半径范围优先路径的空间位置及其与土壤生物活动的关系进行分析,经Monte-Carlo方法检验,发现优先路径在0-15cm土壤水平剖面半径尺度上呈聚集分布,>15cm尺度向均匀分布发展;在不同尺度上与生物活动呈显著正相关关系;较小影响半径范围优先路径的空间分布数量随较大影响半径范围优先路径的空间分布数量的增加而减小。结果表明,空间点格局分析能较好地反映优先路径在土壤水平空间分布的状态,并能从数学空间形态分析角度解释土壤优先路径水平空间分布与生物活动的关系,以及不同影响半径范围优先路径间的分布关系,适用于量化分析土壤优先路径水平空间分布特征。     

我国低碳农业发展技术路径及支撑体系研究

从我国低碳农业发展中存在的主要问题来看,推广应用"节省"型农业技术、循环再利用技术、资源化技术以及农作制度创新技术等,能够有效降低农业生产系统中的碳排放量,是现阶段我国发展低碳农业的关键所在。文章主要从降低碳排放的视角出发,在深入探讨低碳农业科学内涵和我国低碳农业发展中存在主要问题的基础上,提出我国发展低碳农业的技术路径,并构建包含科技硬支撑及政策制度软支撑在内的低碳农业发展的支撑体系。     

食葵联合收获割脱一体式割台设计与试验

针对食葵机械化收获过程割台损失大、葵盘输送过程籽粒表皮易划伤、脱粒过程籽粒破损严重等问题,根据食葵生物力学特性、种植模式及机械化收获要求,在传统割台的基础上增设脱粒装置,设计了集分禾、扶禾、拨禾、切割、输送及脱粒等功能于一体的食葵联合收获割台装置,葵盘在割台上实现脱粒,有效缩短了葵盘输送路径,为后续提高清选质量奠定基础。为降低割台损失,依据适收期食葵植株姿态,设计了一种不对行拨杆式拨禾轮,并设计了侧边倾角30°的分禾器,同时在相邻分禾器之间增加软毛刷收集碰撞飞溅籽粒;为减少脱粒过程籽粒破损,设计一种轴流螺旋滚筒式脱粒装置;基于物料抛送过程动力学和运动学分析,得出螺旋输送器拨板安装倾角为18°时葵盘较顺畅进入脱粒装置。为验证割台结构设计的可行性,开展了田间试验,结果表明,留茬高度为700 mm时,联合收获机在1.21～2.11 m/s范围内5组不同速度条件下进行田间作业,割台损失率不大于3%、未脱净率不大于2%、破损率不大于3%,均能够满足食葵收获要求。     

Longitudinal study of veterinary students and veterinarians: the first 20 years

OBJECTIVE: To describe the career paths of veterinary graduates from the University of Queensland during the first 15 years after they graduated, paying particular attention to the fifteenth year. PROCEDURE: Longitudinal study. Questionnaires were completed by 154 first year veterinary students in 1985 and 1986, then while they were in fifth year, and after 1, 5, 10 and 15 years after they graduated. This paper describes findings from the most recent questionnaire but includes comparisons with previous ones. RESULTS: Completed questionnaires were received from 134 of the 137 graduates. They were spread widely, but 58% were in Queensland and 20% in New South Wales; 17% were overseas. Most (85%) were in private practice, and 56% of them were part or sole owners; 58% were working in cities with > 100,000 people, and 28% in centres with < 10,000 people. Overall 80% of the workload of the group was with dogs and cats, 8% with horses and 11% with farm animals. Most felt that their income was too low. Of those doing less than half-time or no veterinary work, 44% were at home with family, 8% in another business, 11% another profession, and the rest in a wide variety of occupations. Changed interests, need for greater challenges, poor remuneration, loss of interest in veterinary work, and caring for family were, in order, the top five (of 15) reasons for leaving veterinary work. Three-quarters (78%) of all those doing veterinary work, but 52% of those no longer doing veterinary work agreed that their career had lived up to their expectations. CONCLUSION: After 15 years 77% were still doing veterinary work, 85% in private practice, and 80% of their work was with dogs and cats. Almost all were either very glad or generally glad that they had done the veterinary course, though most (78%) felt that their income was too low.     

Nature-based tourism as a form of predation risk affects body condition and health state of Podarcis muralis lizards

Nature-based tourism has experienced a greater increase, and, even although it might have deleterious effects for many wildlife species, its consequences remains little explored. Many lizard species are endangered and tourism has been proposed as a potential factor responsible of the decline of several lizards’ populations, but no study has examined the effect of tourism on lizards’ behaviour, body condition and health state. Many lizards respond to people as if they were predators, by readily escaping to refuges. However, an increase in the frequency of these antipredatory strategies can lead to a loss of body condition, which may have important consequences for short and long term fitness. We analysed the effects of tourism on escape behaviour of common wall lizards, Podarcis muralis, as well as on their body condition and health state (ectoparasites, blood parasites, and cell mediated immune response). Results showed that lizards did not modify their escape behaviour in response to tourism. Lizards had similar approach and flight distances, and escape strategies regardless of the level of tourism pressure. However, lizards inhabiting areas with high tourism levels, where they presumably needed to perform antipredatory behaviours more often, showed higher intensity of infection by ticks and lower body condition at the end of the breeding period. Moreover, lizards with poorer body condition had lower cell mediated immune responses. Therefore, tourism seems to have deleterious effects on body condition and on host-parasite relationships in this lizard species. These effects should be taking into account when designing walking paths in protected areas. Our study reports evidence that regardless lizards showed similar escape behaviour in tourist than in natural areas, their body condition and health state should be also examined to accurately assess the actual effects of tourism on lizards’ populations.     

使用因素对多缸内燃机主轴承润滑的影响

对某多缸内燃机滑动主轴承的热流体动力润滑(THD)计算分析表明，除设计因素外，实际使用因素也对主轴承的润滑状况产生不良影响。通过对主轴承轴心轨迹和最小油膜厚度(MOFT)的计算分析发现，内燃机超负荷运行，主轴颈与轴瓦间隙过大，过热及冷机加载等因素不利于主轴承流体动力润滑膜的形成，并加速主轴承的摩擦和磨损。     

鹤大高速（G11）低路基边坡土壤优先流特征研究

为了探明盐分在低路基公路边坡土壤中的运移方式,以鹤大高速公路(G11)低路基段边坡土壤为研究对象,引入优先流,来探讨盐分随土壤水分运动的过程。采用亮蓝染色示踪和图像解析方法,分析土壤剖面优先路径水平和垂直分布特征。结果显示:染色区和非染色区的土壤性质存在显著差异,染色区土壤存在较多的大孔隙。在水平剖面上,相同影响半径优先路径数量随着土层深度的增加而减少,同一土层中1.0~10.0mm影响半径的优先路径数量要远远多于10.0mm影响半径的优先路径数量。在垂直剖面上,优先流特征指标染色覆盖率(DC)、均匀入渗深度(UF)、优先流分数(PF-fr)、长度指数(LI)和峰值(PI)分别为40.10%,4.10cm,801.96cm2,74.44%,543.6cm和5。低路基边坡土壤这种重塑的土体中存在较为明显的优先流现象,且优先流对氯盐在土壤中的运移起着重要的作用。优先流指标的提出,为今后量化土壤优先流发生程度提供了一个很好的研究方法。     

农业装备企业转型的路径选择及案例研究——基于服务型制造模式下的分析

在剖析我国农业装备企业发展存在的问题和探究服务型制造的价值创造模式的基础上,构建了我国农业装备企业服务型制造转型的5种路径,即服务外包、技术服务化、产品服务化、产品服务一体化和服务产品化;然后,以雷沃福田农业装备服务型制造转型为例,分析了其产品服务系统和服务型制造网络。研究结果对我国农业装备企业转型升级具有一定的借鉴意义。     

路径与方法:农村教师自主发展的思考

农村教师自主发展是解决农村教育质量问题的关键,是农村教师终身学习与自我价值实现的必然选择。目前农村教师自主发展存在着教师自主发展意识薄弱、教师身份认同危机、教师缺乏自主管理等问题,在分析成因的基础上,从教师、学校以及国家三个方面提出农村教师自主发展的具体路径与方法。