Measurement of velocity gradients in boundary layers by a spatially resolving laser Doppler sensor

A novel laser Doppler technique is presented to measure flow fields with micro-scale resolution. The realized laser Doppler sensor provides distributed velocity measurements inside a defined measurement volume. Two laser wavelengths are used to accomplish two Doppler frequency measurements, which determine the position and the velocity of a scattering particle. Repeating of these measurements determines the velocity gradient of the flow. One application area is the evaluation of the velocity variations in boundary layers of flows close to a wall. This contribution presents the principle and experimental results of the spatially resolving laser Doppler velocity sensor. Different concepts on the optical arrangements as well as the signal processing technique are discussed. A scheme for the construction of a miniaturized fiber-coupled sensor is presented, which allows its integration into flow channels.