Photomicroscopic Sublayer Velocity Measurement

The measurement of velocity in the boundary layer region can be a difficult problem using probes. The use of a microscope to observe the motion of small tracer particles can yield quantitative velocity data without disturbing the flow. The application of this technique to turbulent open channel flow is described. Micron particles which act as indicators of the instantaneous velocity vector were photographed through a microscope as they passed through the focal field which was oriented parallel to the channel bottom. By varying the location of the field, a profile normal to the bottom was obtained. The high speed motion picture film was analyzed to obtain particle velocity data which was in turn used to compute a mean velocity profile, turbulence intensity and skewness of the particle velocity distribution. It is pointed out that due to uncertainty of particle location within the focal field, errors in the computation of mean velocity and turbulence intensity can arise. These errors are analyzed quantitatively.