A Beam Scanning LDV to Measure Velocity Profile of Unsteady Flow

Abstract

The Laser Doppler Velocimeter (LDV) is known to have excellent spatial resolution and frequency characteristics. However, it is essentially a device for measuring velocity at a single point. It can not be used, without further development, for the measurement of velocity profiles in unsteady flows. In this study, a beam scanning LDV system in which the point of measurement is controlled by rotating mirrors is described. A prototype system has been constructed and tested. The use of a microcomputer to control the angles of the mirrors allows a high scan rate to be achieved. The time to measure a velocity profile is 0.13 seconds and the system can measure the profile of oscillatory flow up to the frequency of 0.12 Hz within the accuracy of 5%. The validity of the system has been assessed by comparing measured and theoretical results for the velocity profile in the damped oscillatory flow in a U-tube.