Laser-Doppler-Strömungsmessung

Hübner

1993

Essentially the laser two-focus (L2F) and laser Doppler anemometer (LDA) are time-of-flight anemometers. The velocity of particles in the micrometre range is determined within the limits of an optically fixed measurement distance. For this only one optoelectronic receiver for the detection of the scattered light is required [l, 21. With an arrangement of two receiving optics, positioned under an off-axis angle q and an elevation angle f y to the optical axis of the measurement system, a resolution of three-dimensional structures can be achieved and with regard to spherical particles it is possible to determine the dimensions by means of the temporal or phase shift of the signals in the receiving optics. A particle size-dependent distance inside the measurement volume can be fixed, which has to be passed in order to change the signal from one receiving optics to the other. An LDA with an arrangement of two receiving optics for particle sizing is known as a phase Doppler anemometer (PDA); an L2F designed with two receiving optics can be termed a pulse-displacement two-focus anemometer (P2F). The physical analysis of the two methods with respect to a temporal signal displacement in the receivers yielded new results.

Generalized Theory for the Simultaneous Measurement of Particle Size and Velocity using laser doppler and laser two‐focus methods

Albrecht

Hübner

1993

Influence of the Measurement Volume on the Phase Error in Phase Doppler Anemometry. Part 1: Reflective mode operation

Albrecht¹,

Wenzel

et al. 1994

In phase Doppler anemometry (PDA), phase difference errors are caused by the Gaussian intensity distribution of the laser beam and the curvature of the phase fronts outside of the beam waist. This results in erroneous particle sizes. Based on a physical analysis by geometrical optics [1], this phase difference error is evaluated for reflected mode operation (Part I) and for refracted mode operation (Part II). By varying the particle trajectories statistically, the error in determining the particle size and the mass flow can be simulated. By the method described the error in the measured phase difference can easily be estimated.

Influence of the Measurement Volume on the Phase Error in Phase Doppler Anemometry. Part 2: Analysis by extension of geometrical optics to the laser beam; Refractive mode operation

Albrecht¹,

Wenzel²,

1996

The influence of the measurement volume can be investigated by using extended geometrical optics, which is based on geometrical optics by including the amplitude and phase distribution in the laser beam. The dynamics in phase Doppler anemometry can be analysed, in addition to effects of the particle size‐dependent detection volume. Extended geometrical optics has been developed as a powerful tool to investigate these influences for each order of light scattering separately. Phase errors caused by Gaussian‐beam intensity distribution and the curvature of the wave fronts beyond the beam waist can easily be calculated. According to Part 1 (Reflective Mode Operation), the influence of the particle trajectories on measured phase and mass concentration is simulated for refractive mode operation.