Expanding the Boundaries of Technology

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Crossed beams showing interference fringes

Laser Doppler Velocimetry is a powerful technique used for highly accurate measurements of fluid velocity in liquid or gaseous flows. A pair of coherent beams of laser radition are focused down to create interference fringes at the optical probe volume formed by the intersection of the two beams. The fringe spacing is a function of the optical wavelength and the angle between the beams. Scattered light from particles passing through the probe volume contains a Doppler frequency proportional to the particle velocity.

Doppler pulse obtained from light scattered by particle

A Doppler burst signal represents scattered light intensity from a particle passing through the probe volume. Signal amplitude modulates at the Doppler frequency as the particle alternates between bright and dark fringes. The velocity component orthogonal to the fringes is determined from the frequency of transit time between the known spacing of adjacent fringes.

Because the fluid velocity is inferred from these naturally occuring or introduced particles, the particles must be small enough to follow the flow field accelerations, yet large enough to scatter sufficient light intensity. Optimum particle size is generally between 0.5 and 2 micrometers. Aerometrics LDV systems can be supplied with a particle size discriminator circuit which insures that only particles of a desired size are included during data acquisition.

Very small probe volumes can provide highly resolved velocity profiles and allow measurements close to a wall or object in the flow. Simultaneous dynamic measurements can be made in two or three dimensions for full turbulence analysis. Successful measurements have been made in harsh environments including under water, in combustion processes, and in explosions.

Frequency shifting applied to the input beams acts to offset the Doppler frequency, providing improved dynamic range and allowing distinctions between forward and reverse flow directions. The transmitter implements frequency shifting via a Bragg cell, giving a fixed frequency shift of 40 or 80 MHz, for high velocity or high turbulence measurements.

With two or three component systems, several wavelengths are used to form independent overlapping probes for orthogonal flow directions. This greatly reduces the time required for mapping multi-dimensional flows.

Aerometrics optical systems provide superior alignment stability and are designed to virtually eliminate the need for internal adjustments. The standard optical transmitter is rugged and easy to set up. Selectable beam spacings, beam expansion, and interchangeable focusing lenses provide a combination of fringe spacings and probe volume sizes to accommodate a wide range of flow conditions.

Aerometrics' Real-time Signal Analyzer LDV / PDPA analysis system and software

The Phase Doppler Method is based upon the principles of light scattering interferometry. Measurements are made using the same optical probe volume as for LDV . Scattered light from a particle moving through the probe volume is picked up by a receiving lens strategically located at an off-axis collection angle. This light is split and a portion is projected onto several detectors. Each detector produces a Doppler burst signal with a frequency proportional to the particle velocity. The phase shift between the Doppler burst signals from the different detectors is proportional to the size of the spherical particles.

Aerometrics has developed a unique method to directly measure the sample volume simultaneously with respect to particle size and velocity. This enables an accurate determination of the particle number density and volume flux. The Phase Doppler method requires no calibration because the measured particle size and velocity are dependent only on the laser wavelength and optical configuration. PDPA measurements are not based on light intensity, and consequently are not subject to errors from beam attenuationor deflection which occur in dense particle and combustion environments.

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