Two-Component Dual-Scatter Laser Doppler Velocimeter with Frequency Burst Signal Readout

Abstract

A dual-scatter laser Doppler velocimeter (LDV) system designed for measuring wind tunnel flow velocity is described. The system simultaneously measures two orthogonal velocity components of a flowing fluid at a common point in the flow. Essential single-velocity component dual-scatter concepts are presented to simplify the description of the more sophisticated two-component system. To implement the two-component system three laser beams with a 0 degrees , 45 degrees , and 90 degrees polarization plane relationship are focused to a common point in the flow by the system-transmitting optics. The beams interfere to form two perpendicular sets of interference fringe planes that are orthogonally polarized. The system-receiving optics collect and separate the orthogonally polarized components of laser radiation scattered from micron-size particles moving with the flowing fluid through the ringes. The system requires no artificial seeding, since intrinsic test section aerosols are utilized for radiation scattering. The passage of each scatter particle through the interference fringes simultaneously produces two frequency-burst-type photodetected signals, the frequencies of which are directly proportional to two perpendicular components of particle velocity. The system photodetection, signal-conditioning, and data acquisition instrumentation is specifically designed to process the frequency burst information in the time domain as opposed to spectrum analysis or frequency domain processing. The system was initially evaluated in an AEDC wind tunnel operating over a Mach number range from 0.6 to 1.5. The LDV and calculated wind tunnel mean velocity data agreed to within 1.25%; flow direction deviations of a few milliradians were resolved.

Detection and analysis of photo-acoustic emission in Direct Laser Interference Patterning

“…In order to obtain the characteristics of these signals, the collected data points have been fitted with two Gaussian functions (Fig. 2 a) which, in case of the interference volume, also matches the assumptions of the shape generated by superimposing Gaussian beams 37 . As a result, the FWHM of the interference volume peak (0.65 mm) can be considered as an estimation of the extent of the interference volume.…”

Application of the Laser Velocimeter for Trailing Vortex Measurements

“…Given a signal of high signal-to-noise (S/N) ratio (S/N > 20 db) such as provided by a laboratory oscillator, the DDP will determine the signal period with a combined systematic and random error less than two parts in 10 3 (Ref. 42). However, dependent upon a complexity of factors including available laser power, scatter particle size, scattering angle, background light level, etc., an LV system may produce signals with S/N ratios much less than 20 db.…”

“…The signal suffers both pulse dropouts and pulse additions due to noise mixing with signal information (Ref. 42).…”

“…In the latter experiments two simultaneous velocity components, including directionality, were measured. 42…”

The technology of grating laser Doppler velocimeter for measuring transverse velocity of objects

“…LDV has several advantages such as noncontact sensing, high spatial resolution [1] and the most common type of LDV system is the differential LDV system because its signal-to-noise ratio is superior to that of the reference-beam velocimeter [2] . Up to now this kind of equipment has been commercially available for four decades [3] , however, in the conventional differential LDV systems, the scattered light from objects are usually too weak so we have to utilize powerful lasers or expensive detectors to get the signal.…”