The Accuracy of a Three-Component Laser Doppler Velocimeter System Using a Single-Lens Approach

An analysis of the effects of misalignment on the on-axis velocity component measurements made with a three-component LDV system using two colors and five beams issuing from a single lens is discussed. A slight misalignment of the center beam from the optical axis is shown to cause large changes in the measured “on-axis” component of velocity. The effect of misalignment on the ensemble averaged velocity statistics in turbulent flows is also considered. The error analysis of Neti and Clark (1979) is applicable to on-axis component of the present system and it shows that even when the system is perfectly aligned, the relative uncertainty of the on-axis component of velocity is much larger than that of the other components of velocity. Measurements taken in a laminar pipe flow support this conclusion.