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Unsteady boundary layer transition on axial compressor blades

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Solomon, W. J.(William J.) (1996) Unsteady boundary layer transition on axial compressor blades. PhD thesis, University of Tasmania.

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Abstract

The unsteady laminar to turbulent transition of boundary layers on the blading of axial
turbomachinery has been investigated experimentally. Measurements were made on and
around the outlet stator row of a 1.5-stage axial compressor using thermal anemometry.
Pitch-wise hot-wire probe traverses were made upstream and downstream of the blade
row to define the freestream disturbance field; these clearly show the interaction of the
inlet guide vane and rotor wakes. Boundary layer traverses were taken with a single
hot-wire on the stator suction surface.
A blade instrumented with 61 surface hot-film sensors distributed around the surface
at mid-blade height was used to obtain high frequency response skin friction measurements.
High speed data acquisition triggered by a rotor angle position reference signal
allowed ensembles of data to be acquired corresponding to a specific set of rotor wakes.
This data was processed in a variety of ways to reveal the nature of the unsteady transition
process. This work corroborates and complements that of Halstead et al. (1995)
and provides a useful resource for validation of unsteady flow calculations.
A new intermittency detection algorithm has been developed to aid in the interpretation
of the surface hot-film data. Intermittency is a more reliable measure of the
transitional boundary layer state (and is easier to interpret) than other statistics such
as skew, especially as the boundary layer nears separation. The intermittency detection
algorithm has also been adapted to detect the region of relaxing flow commonly
observed following turbulent spots where the skin friction relaxes from a turbulent level
to the laminar level. This region is more stable and more resistant to separation than
the undisturbed laminar layer and has a lower skin friction than the turbulent layer (on
average). It may be possible to exploit the properties of this relaxing flow to improve
blade profile designs.
The experimental data show the transition process to be periodically unsteady
and dominated by the presence of turbulent spots. Not only do the transition onset
and completion points oscillate markedly in stream-wise position with the passage of
wakes from upstream, but small regions of boundary layer separation may appear in
the transition region during certain parts of the cycle. In flows with large separations
through the whole cycle, the re-attachment point has also been observed to oscillate in
stream-wise position.
Using experimental data recently compiled by Gostelow et al. (1995) a new method
of predicting the transition length in rapidly changing pressure gradients has been
proposed and tested in typical turbomachine pressure distributions. The new method,
which extends the commonly used Chen-Thyson technique, allows the spot propagation
and growth parameters to respond to changes in the local pressure gradient. This
can be especially important in turbine blade boundary layers where transition may commence in a region of favourable pressure gradient and then move into a region of
strong adverse pressure gradient. Only the new method is able to predict the shortening
of the transition zone due to this change in pressure gradient.

Item Type: Thesis (PhD)
Copyright Holders: The Author
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Copyright 1996 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Additional Information:

Investigates experimentally the unsteady laminar to turbulent transition of boundary layers on the blading of axial turbomachinery. A new intermittency detection algorithm is developed to aid in the interpretation of the surface hot-film data. Library has additional copy on microfiche. Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references. Investigates experimentally the unsteady laminar to turbulent transition of boundary layers on the blading of axial turbomachinery. A new intermittency detection algorithm is developed to aid in the interpretation of the surface hot-film data

Date Deposited: 03 Feb 2015 03:23
Last Modified: 11 Mar 2016 05:55
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