Boundary layer velocity measurements by a laser Doppler profile sensor with micrometre spatial resolution

We have developed a differential laser Doppler profile sensor for distributed one-component velocity measurements with high spatial resolution. Two Doppler frequencies are measured simultaneously in order to determine the position as well as the velocity of individual tracer particles passing through the measurement volume. In the centre of the measurement volume the obtained uncertainty of the position is about 1.6 mum. The profile measurement has the advantage that no mechanical scanning is needed to obtain flow velocity profiles over a length of 5 mm. The profile sensor thus provides a tool for highly resolved instantaneous measurements of shear flows, which have strong velocity gradients.