Determination of the measuring position in laser-Doppler anemometry

Methods for the determination of the measuring position of an LDA relative to a wall are discussed. The known location of a 5-micron hot-wire is used to find the location of the center of a LDA-measuring control volume. The LDA optical system is traversed perpendicular to the wire axis, moving the measuring volume across the wire, and the light scattered from the wire at each location of the beam is detected by the light-receiving optics. A second method is described which has been used for velocity measurements in test sections with refractive index matched fluids. Under these conditions, the entrance of the LDA-measuring volume into the wall is marked by a distinct change in the slope of the measured velocity profile.