Development of Scan-Type Laser-Doppler Velocimeter and Some Examples of Measurement

LDV（激光多普勒测速仪）有一个很好的特点，即它可以测量速度，而不需要在流场中插入证明，从而不干扰流场。然而，它不能摆脱欧拉式测速仪的缺陷，欧拉式测速仪被设置在流场的固定点上；也就是说，这些测速仪无法一目了然地测量出流动的瞬时空间特征。而近年来发展起来的各种流场可视化技术，虽然可以让我们对流场的空间特征一目了然，但由于没有昂贵的专用设备和繁琐的工时，无法给出流场的详细定量信息。在这个项目中，我们试图开发一种新型的仪器；即SLV（扫描型激光多普勒测速仪），它可以定量地、几乎连续地测量空间（一维，如果引入冻结流场的泰勒假设，则为二维）流场。用快速旋转的多边形反射镜扫描聚焦在测量点并产生干涉条纹的分裂激光束。电子处理单元一方面接收驱动单元发出的运动测点位置的信号，同时接收通过干涉条纹的示踪粒子的散射激光束携带的速度信号。最大扫描周期为0.2s，最大扫描宽度为10cm。为了测试SLV的性能，测量了三个典型流场，即(1)卡门涡街、(2)层流振荡边界层和(3)涡环。结果令人满意。该仪器可用于测量非稳态或不能重复实现的流场，如有组织湍流的瞬时结构。

LDV (Laser Doppler Velocimeter) has an excellent feature that it can measure velocity without inserting a prove into a flow field and thus without disturbing it. Nevertheless, it cannot be free from the defect common with the Eulerian type of velocimeters which are set at a fixed point in a flow field; i.e., these velocimeters cannot measure the instantaneous spatial feature of flow at a glance.On the contrary, the recently advanced various techniques of flow visualization can afford us a spatial feature of a flow field at a glance but they cannot give a detailed quantitative information of the flow, without expensive special equipments and tedious time consuming labor.We attempted in this project ot develop a novel type of instrumentation; i.e., SLV (Scan-type Laser Doppler Velocimeter) Hich can measure quantitatively and nearly continuously the spatial (one-dimensional and if the Taylor hypothesis of a frozen flow field is introduced, two-dimensional) flow field.Splitted laser beams which focus at a measuring point and make the interference fringes are scanned by a rapidly rotating polygon-mirror. An electronic processing unit receives, on the one hand, signals on the position of the moving measuring point from the driving unit and, at the same time, signals of velocity carried on the scattering laser beam from tracer particles passing through interference fringes. The maximum scanning period is 0.2s, and the maximum scanning width is 10cm.In order to test the performance of the SLV, three typical flow fields are measured, i.e., (1) Karman vortex street, (2) laminar oscillatory boundary layer and (3) vortex ring. The results were satisfactory.The instrument may be a powerful tool for measurement of flow fields which are non-steady or cannot be realized repeatedly such as an instantaneous structure of organized turbulence.