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

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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.