The influence of land-use practices and economics on plant species richness in meadows

Thirty-one meadows were investigated within five sites representing various farming styles found in Austrian cultural landscapes. The meadows were analysed regarding (a) biodiversity (vascular plant and bryophyte species richness), (b) land-use practices (fertiliser input, mowing intensity, the use of silage), and (c) economic aspects (variable costs, profit margin and subsidies per ha). There were significant negative correlations between plant species richness and mowing intensity and intensity of fertiliser application. Bryophytes were good indicators of low nutrient regimes, having high species richness at low fertiliser input. Vascular plants showed highest species richness at an intermediate nitrogen supply. The total plant species richness decreased with increasing nitrogen supply. Intensive silage production was also negatively correlated with plant diversity. Species with a very narrow ecological niche of soil moisture and nutrients declined, whereas species adapted to wider ecological conditions increased. Profit margin and variable costs correlated negatively with plant species richness, with meadows that offered low or no profit margins showing highest species richness. There was no significant relationship between species richness and the amount of subsidies invested at the study meadows. Estimated costs of maintaining a species are shown. It is concluded that if plant species richness are to be maintained in these meadows, farmers have to receive increased financial incentives through agro-environmental subsidies for appropriate meadow management, and these have to be linked to clearly defined measures.     

风电齿轮箱传动误差分析及振动噪声预估

针对一级行星两级平行轴风电齿轮传动系统,综合考虑齿轮时变啮合刚度、啮合阻尼、传递误差等因素,建立31个自由度的弯扭轴耦合集中参数动力学模型,采用变步长Runge-Kutta法对系统动力学微分方程进行求解,得出齿轮传动系统各级传动误差;借助软件建立风电齿轮箱刚柔耦合动力学模型,并导入传动误差,采用模态叠加法求得齿轮箱轴承支反力,并将其作为声振耦合模型的边界条件,采用声学有限元法对风电齿轮箱进行振动噪声预估,并与试验结果对比分析,两者吻合良好。     

轴流泵流动噪声数值模拟

为了研究轴流泵内部压力脉动和流动噪声在不同工况下的变化规律及其关系,采用数值模拟方法,应用计算流体动力学软件Fluent和声学软件LMS Virtual Lab分别模拟轴流泵流场和声场分布,并进行时域和频域分析.取叶片非定常脉动力作为声源,运用边界元法对比分析了有泵壳振动影响和无泵壳振动影响下泵壳体边界声场分布的不同.结果表明：叶轮叶片、导叶叶片和动静交界面处监测点的静压均表现出明显的离散频谱特性,叶片通过频率（BPF）是压力脉动和流动噪声的主频,这是由叶轮和导叶之间的动静干涉引起的;而流动噪声在2倍谐频（133.4 Hz）和3倍谐频（200.1 Hz）处也有明显峰值,这是由叶轮叶片和泵壳壳体振动引起的.忽略泵壳振动影响的情况下,噪声水平偏大,考虑声振耦合的噪声情况更接近于实际,所以结构振动是噪声辐射分析的重要因素.噪声指向性分布图表明了叶片噪声辐射具有明显的偶极子特性.     

Simple and High Performance Timing Synchronization in OFDM System

One ML timing offset estimation method for orthogonal frequency-division multiplexing(OFDM) systems is presented.The performance of our timing offset estimator are compared to that of H.Minn's estimator in terms of estimator variance error in both AWGN channel and multipath fading channel. Because no performance floor exists under high SNR condition,the new estimator can get more accurate estimation result with low computational complexity even under multipath fading environment.     

小缸径涡流室柴油机非稳态工况噪声研究

以N485型小缸径涡流室柴油机为样机,探讨了柴油机在非稳态工况中暖机过程和部分负荷中间转速工况加速噪声的变化规律。其中整机噪声在暖机过程中随着水温、机油温度的降低而增加;对于加速工况,噪声在加速初期增加迅速,中间阶段变化较平缓,而后期噪声有所下降。     

防霜风机圆弧板叶型降噪结构设计与试验

夜间作业时茶果园防霜风机产生的噪声严重影响周边居民,为降低圆弧板叶型防霜风机的气动噪声,进行了锯齿降噪结构设计。以锯齿长度、宽度、个数和位置为试验因素,以叶轮轴线正前方1 m处声压级和出口面平均风速为评价指标,采用数值模拟和响应面法结合的方式优化锯齿结构参数。优化结果表明：锯齿参数对声压级影响由大到小为锯齿宽度、个数、长度、位置;对风速影响由大到小为锯齿位置、宽度、个数、长度。最优参数组合为锯齿长度26.0 mm、宽度11.6 mm、个数16、位置480.0 mm。对锯齿叶型进行了性能测试,试验结果表明,锯齿叶型正前方、正后方和径向右侧各监测点的声压级比对照叶型平均降低7.71%,距离锯齿叶型风机30 m处的声压级低于国标限值45.0 dB。两款叶型风速分布趋势相近,在12 m处测得,对照叶型和锯齿叶型的最大风速分别为3.19、3.17 m/s。两款叶型均能够产生较好的温升效果,锯齿叶型在10、15 m处平均温升分别为2.8、3.2℃;对照叶型在10、15 m处平均温升分别为3.0、3.1℃,两者无显著差异。结果表明,在保证防霜性能的前提下,锯齿叶型可有效降低风机噪声。     

柴油发电机组噪声分析和降噪方案设计

通过对柴油发电机组噪声源的分析,采取隔音、吸音、消音、减振等措施对云内YN25型柴油发电机组进行优化设计;根据发电机组噪声源分布特点设计了静音箱、进风罩、消音器、进气管、减振器等结构并进行试验验证,试验结果表明:发电机组噪声从90dB降低到30dB,验证了计算和设计的合理性;在有限的空间内实现了较大的降噪量,取得良好的效果。为柴油发电机组降噪减振研究提供了理论参考。     

基于车内噪声控制技术的研究

随着汽车保有量的不断增加,车辆噪声污染已经成为人们越来越关注的环境问题。本文分折了车内噪声声源机理,并针对汽车车内噪声的来源提出相应的控制技术,以达到降低车内噪声的目的。     

某发动机缸盖异响原因浅析

发动机缸盖内部异响是一种普遍发生在各个厂家生产的发动机上的一种故障类型,也是影响发动机总成一次下线合格率的主要原因之一。本文以分析影响某公司4GA系列发动机缸盖异响的关键因素作为主线,依据发动机组成结构、工作原理作为论据,通过考察现场工艺标准、外购件质量、现场质量控制等手段,利用大量的实例实验数据作为论证支持,有条不紊的对影响发动机缸盖异响的关键因素一一进行阐述。     

汽车加速过程噪声分析与控制

提出了基于变时基短时傅里叶变换来分析和处理时变信号的技术。运用该技术对某汽车的加速噪声进行测量和分析,发现发动机进气噪声是该车主要的加速噪声源。通过对该车进气系统的改进设计,使该车车外加速噪声达到了限值要求。