Uncertainty Analysis of 3D LDA System

Abstract

The uncertainty of three component laser Doppler anemometry (LDA), applied to investigation of the turbulent swirl flow in jet behind the axial fan is considered. The main sources of uncertainties are identified and estimated. They can be classified into two groups: one associated to single component velocity measurement, and the other, generated by LDA probes settings. Expressions for axial, radial and circumferential velocity component uncertainties are derived. They show that for velocity components that are not measured directly, but are calculated using transformation matrix (here axial and circumferential component), uncertainties depend on their ratio. For specific setting of LDA probes, uncertainty dependences on component value ratios are calculated. The results show that, for small velocity component ratio, impact of beam separation uncertainty and the uncertainty due to fringe distortion on measurement result is comparable to the impact of the uncertainty of angular positions of probes. The latter is remarkably dominant if the velocity component if the velocity component ratio is high, which is the case in some area of the here considered swirl flow. That indicates the importance of precise angular positioning in 3D LDA systems.