实验流体力学-流体力学测量仪器(简化)讲义.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ADCP’s can be deployed from a boat or mounted on the streambed. This particular graphic shows an upward looking ADCP mounted on the stream bed. We mentioned the the ADCP measured a complete velocity profile. It is comparable to having a whole string of velocity meters deployed. It measures the velocity in discrete water layers by range gating the reflected acoustic energy. In other words it processes small parts of the entire echo individually thus producing multiple velocity measurements. The water layer is frequently referred to as a depth cell or bin. A measurement consisting of a single ping (burst of acoustic energy) can be made in less than a second. Typically a single profile measurement consist of several pings averaged together. Still the typical time for a profile measurement is 3-5 seconds. * 图是超声多普勒流量计示意图。超声换能器安装在管外。从发射晶体T发射的超声波束遇到流体中运动着颗粒或气泡，再反射回来由接收晶体R1接收。发射信号与接收信号的多普勒频率偏移与流体流速成正比。如忽略管壁影响，并假设流体没有速度梯度，以及粒子是均匀分布的，可得方程 ?涡街流量计?? ?涡街流量计??? ? ?电压式涡街流量计??? ?插入式涡街流量计 ?分体式涡街流量计? ?潜水型涡街流量计 精度：±0.1% 适用流体：液体、蒸汽（饱和、过热） 流体振动流量计是60年代末期发展起来的一种较新的流量测量技术。它具有如下一些特点：可得到与流量成正比的频率输出信号；被测流体本身就是振动体，无机械可动部件，几乎不受流体组成、密度、粘度、压力等因素的影响；所以，该测量方法越来越受到人们的重视， 涡街流量计（Vortex flow meter) 是利用流体流过阻碍物时产生稳定的漩涡，通过测量其漩涡产生频率而实现流量计量的。涡街流量计由涡街流量传感器和流量显示仪表两部分构成， 振动式流量计 涡街流量计实现流量测量的理论基础是流体力学中著名的“卡门涡街”原理。在流动的流体中放置一根其轴线与流向垂直的非流线性柱形体(加三角柱、圆柱等)，称之为漩涡发生体，见图。当流体沿漩涡发生体绕流时，会在漩涡发生体下游产生如图所示不对称但有规律的交替漩涡列，这就是所谓的卡门涡街。 When a fluid flow encounters a body, the layers of fluid close to the surfaces of the body are slowed down. With a streamlines body, these boundary layers follow the contours of the body until virtually meeting at the rear of the object. This results in very little wake being produced. With a non-streamlined body, a so-called bluff-body, the boundary layers detach from the body much earlier and a large wake is produced.When the boundary layer leaves the b