A laser Doppler velocimeter which can measure fluid flow in three dimensions, employing a laser beam projected through a rotating diffraction grating and spaced lenses, with a beam splitter in the major optical path between two lenses ahead of the flow channel through which the fluid passes. Selected pairs of laser-derived beams are directed through the fluid and intersect at a predetermined point in the fluid. The movement of the particles through interference patterns formed by the intersecting laser-derived beams generates further-modulated scattered light, which is focused on a photomultiplier tube, from the output of which a fluid velocity directional component signal is derived for each pair. There are four laser-divided beams, and different pairs of these beams can be selected, to provide measurements from which the components of the fluid velocity along three mutually orthogonal directions can be derived.
Particle velocity is determined optically through the use of crossed interference fringe patterns. The fringe patterns are produced by passing laser beams of distinct wavelength along a common optical fibre and through two crossed diffraction gratings which are bonded to the free end of the fibre. Each diffraction grating is constructed in the manner of an interference filter so as to produce interference fringes at one wavelength but to transmit the other wavelength substantially uniformly across the grating. Other interference elements can be used in place of diffraction gratings and the use of a single diffraction grating is envisaged in certain circumstances. An indication of the sense of particle movement can be achieved by creating a fringe pattern of steadily increasing fringe spacing.
This invention is characterized in that a light beam from a light source is made to enter a diffraction grating and .+-. nth-order (n=1,2, . . . ) diffracted lights from the diffraction grating are made to enter a moving object at an angle substantially equal to the angle of intersection of the angles of diffraction of the two diffracted lights from the diffraction grating. As a result, the two diffracted lights are converged by a lens system provided between the diffraction grating and the light source on the moving object and intersect with each other. Scattered light subjected to Doppler shift by the moving object is detected by photodetector thereby detecting the velocity information of the moving object.
The multiple measuring control volume laser Doppler anemometer includes a laser for providing an optical beam. The beam is transformed by an optical arrangement to produce a number V of measuring control volumes wherein each control volume has two focussed optical beams intersecting at a unique predetermined angle .phi.. A detector collects the scattered light from the measuring control volumes in order to determine the velocity at each of the control volumes.
A laser Doppler velocimeter for near-wall measurements which includes at least one beam-turning device. The beam-turning device receives laser light, reflects and redirects the light at various angles in order to obtain measurements for all three velocity components at grazing incident angles. The beam-turning device includes a mirror or prism at one end which reflects the received light in a particular direction. A collector lens receives the particle scattered light from which the relevant velocity components are determined. The beam-turning device can also be a miniature fiber optic head which outputs laser light and can be turned in any direction.
A method and apparatus for calibrating laser Doppler velocimeters having one or more intersecting beam pairs. These velocimeters measure fluid velocity by observing the light scattered by particles in the fluid stream. Moving fluid particulates are simulated by fine taut wires that are radially mounted on a disk that is rotated at a known velocity. The laser beam intersection locus is first aimed at the very center of the disk and then the disk is translated so that the locus is swept by the rotating wires. The radial distance traversed is precisely measured so that the velocity of the wires (pseudo particles) may be calculated.
