This thesis describes the development of a Mach-Zehnder interferometric filter based
planar Doppler velocimetry (MZI-PDV) flow measurement technique. The technique
uses an entirely new optical system, an unbalanced MZI incorporating glass blocks for
wavefront-matching, to replace the iodine cell currently used in conventional PDV. The
free spectral range of the interferometric filter can be selected by adjusting the optical
path difference of the MZI. This allows the velocity measurement range, sensitivity and
resolution to be varied. This system offers no restricts to the choice of laser wavelength
of operation which is not the case with most techniques. Two techniques to process the
interference fringe images are presented. The first uses the shift of the fringe pattern to
determine the Doppler frequency shift along profiles. The second provides a full-field
measurement by normalising the received light intensity at each pixel in the image.
With the single camera MZI-PDV scheme, exact alignment of the two output images on
the active area of the camera is automatic. This eliminates the pixel-matching problem
in conventional two camera PDV systems. The technique allows the measurement of up
to three components of the flow velocity across a plane defined by a laser light sheet.
The construction of a single velocity component MZI-PDV system that incorporates a
phase-locking system designed to stabilise the filter is described. Measurements are
made on the velocity field of a rotating disc with maximum velocities of ~±70ms-1 and
an axis-symmetric air jet (with a nozzle diameter of 20mm) with an exit velocity of
~85ms-1. Standard deviations in the measured velocities were found to be about 2.9 and
2ms-1 for the two processing methods respectively. The system was then modified to
make 3-component velocity measurements using imaging fibre bundles to port multiple
views to a single detector head, and the standard deviation of the velocity error is
around ±3ms-1 for a maximum velocity of ~±30ms-1 in the field of view.
The factors that will affect the quality of the interference fringe image are investigated
including polarisation sensitivity of the two beam splitters and flatness of the optical
components. The inclination angle and the optical path deviation have little effect on the
contrast of the interference fringes since collimated light beams, rather than divergent
ones, are used in the interferometer.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/7151 |
| Date | January 2008 |
| Creators | Lu, Zenghai |
| Contributors | Tatam, Ralph P. |
| Publisher | Cranfield University |
| Source Sets | CRANFIELD1 |
| Language | English |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
| Rights | © Cranfield University, 2008. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder. |
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