Effect of an angular bias on LDA measurements in supersonic turbulent flow

Abstract

The problem of a laser Doppler anemometer's statistical response to the velocity direction of particles scattering in space has been studied for many years. This phenomenon introduces a statistical bias distinct from the classical bias related to velocity fluctuations. Though the latter has corrected some controversial application, the only answer found to counter the angular bias remains, for most experimenters, the utilization of Bragg cells. It is not always possible to use such a device, however, and this is particularly true in supersonic flows. The angular bias, inevitable in this case, shows increased effects when the normal to the fringe plane is inclined with respect to the mean velocity direction. Analysis of the phenomenon in the more complex case of a two-component anemometer and a supersonic flow helps to show how measurements of velocity fluctuations in a boundary layer quite close to the wall may include significant errors because of this bias.