Microscopic open channel boundary layer velocity measurement using a virtual image method

Abstract

A method for velocity measurement is described which employs a microscope and high speed motion picture camera to record the motion of small tracer particles suspended in the flow. The procedure, termed the image method, involves particle illumination such that the particle itself and its virtual image created by reflection from a boundary both appear on the film. The boundary is then located equidistant from the two images. The plane of focus is oriented normal to the boundary giving a picture of the entire velocity profile normal to the boundary within the limits of the field of view. A frame by frame analysis of the particle motion can yield velocity profile and turbulence information. The image method was adapted to a small open channel. Considerable difficulty was encountered in illuminating the particles with sufficient intensity to impress their image on the film. Thus, quantitative data was not obtained. It was concluded that the lens system used to focus the light source was inadequate. However, the method has potential applications in velocity measurement near a boundary and flow visualization ' studies.