Comparative Accuracy Study Of LDV Profile-Sensor Acquisition Modes And Particle Diameters

Abstract

The laser Doppler velocity profile sensor (LDV-PS) system offers the possibility to measure flow velocities with high spatial resolution and is therefore a promising method for the investigation of flows with high velocity gradients. The present experimental study revolves around the interplay between a broad range of chosen LDV-PS acquisition parameters, the dynamic velocity range within the observed measurement volume, and the accuracy of the velocity and position estimation from an application perspective. In the chosen set-up, thin wires of different diameters rotate at varying velocity levels and are captured instead of tracer particles by the applied system. It is observed that constant sample rate and number for the Fast Fourier Transform (FFT) evaluation of the detected burst signal allow straightforward measurements of unknown velocity profiles, but imply a velocity-dependent spatial measurement uncertainty. In contrast, velocity-adjusted acquisition settings allow to acquire particle bursts with equal signal resolution and length effecting a constant measurement accuracy at all velocity levels when adjusted appropriately. The position standard deviation is furthermore observed to increase with the wire diameter.Hence, the size of the scattering object should be chosen appropriately small during calibration and measurement. The gained knowledge offers the possibility to adapt the evaluation parameters more specifically to a given measurement problem.Consequently, measurements can be conducted in flows with small velocities with a high spacial resolution, when the FFT parameters and processing routines are adjusted accordingly, small particles are chosen and vibrations in the set-up are avoided.