Laser-Doppler measurement of vibrating surfaces: A portable instrument

Blood flow in the vessel of microcirculation (that is, red blood cells) usually has very slow velocity and a broadening of the frequency spectrum, so that the Doppler frequency signal component overlaps with the pedestal. To solve this problem, a new method is described for the frequency shifting in LDV, that is, a wedged glass is moved across one of the dual incident light beams. To demonstrate that this method is useful for blood flow measurement, the velocity (0.01–0.1 mm/sec) in the small vessels of a caudal fin of a goldfish was measured in vivo.

A frequency shifting of LDV for blood velocity measurement by a moving wedged glass.

A design using simple lenses and frequency shift by scattering disk has been developed, which allows easy construction, and alignment of one‐, two‐, and three‐orthogonal‐component, frequency shifting laser Doppler velocimeters. A series of instruments designed on this basis have proved to be inexpensive to construct, precise and accurate to use, adaptable to a wide range of applications, and have proven particularly advantageous in studies of turbulent phenomena and complex flows. In this paper the design concept is explained, construction and alignment procedures are outlined for the simplest assembly of the one and two component versions, and performance and application examples are presented. The extension of the system to measure the third orthogonal component is discussed.

Multicomponent frequency shifting self-aligning laser velocimeters.

The conventional laser Doppler velocimeter (LDV) can only furnish the instantaneous velocity at a single point in a fluid. To obtain the complete flow pattern, the measurement has to be repeated for different regions of the flow field. In the authors' laboratory new optical systems for the simultaneous measurement of flow velocity distribution have been developed (Nakatani et al. 1979). The laser beams were arranged to intersect in a line or plane in the flow region, and optical fibres connected the interfering beams at measured points to the photodetectors. By using unsteady flow in a branch tube, it was demonstrated that the velocimeter could not only be applied to obtain the flow pattern but could also be used as a flowmeter by integrating the velocities. In these systems, however, it was required to use the same number of detectors as measured points.

LDV optical system with multifrequency shifting for simultaneous measurement of flow velocities at several points

This paper describes two simple laser instruments which the automotive engineer can use in vibrational studies of diesel engines. The first is a "normal surface velocity" instrument which the engineer can hand hold and simply "point" at the target surface for measurement. Field tests have shown it to be quick and easy to use with excellent accuracy and reliability. It is a particular time-saver in test cell use where the necessity for many accelerometer fittings is removed. The second is a laser torsional vibrometer which can measure the torsional oscillation of rotating machinery parts. This has shown itself to be superior to the more standard techniques of slotted discs and slip rings, etc. and provides advantages of higher frequency response, greater accuracy and non-contact measurement. Conclusions drawn herald a step forward in measurement techniques and demonstrate the great potential of laser technology in heavy engineering applications.

Diesel Engine Health: Laser Diagnostics

Two heterodyne laser anemometers incorporating frequency pre-shifting are described. A thin scattering disc provides scattered light from a focused laser beam to form a reference beam whose frequency can be shifted by rotating the disc. The method is both simple and inexpensive. It extends the range of laser velocity measurements, removes directional ambiguity, and facilitates estimations of instrumental broadening. The instruments are analysed, compared with conventional anemometers, and their range and capability demonstrated.

A. Ballantyne

Papers

Frequency shifting for laser anemometers by scattering