The effects of scaling a cavity with respect to a fixed incoming boundary layer
thickness on its flow dynamics and control was studied experimentally. Three
cavity models with constant length-to-depth ratio of 5 and length-to-width ratio
of 2 and with corresponding linear dimensions in the ratio 0.5 : 1 : 2 were
tested at freestream Mach number 0.71. Additionally, the 0.5 and 1 scale models
were tested at freestream Mach number of 0.85. The experiments involved timeaveraged
pressure measurements, unsteady pressure measurements, and PIV
measurements. Time-averaged pressure measurements made at the floor were
used to study the ’flow-type’ of the cavities. Unsteady pressure measurements
were used to study the acoustic characteristics of the cavity. The cavity length-to-boundary
layer thickness ratios tested were 10, 20 and 40.
The Cp distribution on the clean cavities indicated a change in the cavity flowtype
with change in the cavity scale. Varying the L/δ from 10 to 40 changed the
cavity flow-type from open to transitional. Analysis of the frequency spectra of
the cavity revealed an increase in tonal amplitudes and OASPL with increasing
L/δ . The PIV measurements indicated that this could be caused by an increase in
energy exchange between the freestream and the cavity. The velocity magnitudes
inside the cavities were found to increase with increase in L/δ .
A comparative study of different passive control methods on the largest cavity
showed that leading-edge spoilers were superior in cavity tone suppression. Of
these, the effectiveness of a sawtooth spoiler on the three cavities of different
scales was tested. The results showed that while the spoiler was effective in
eliminating tones and suppression of noise for the smaller cavities, it was unable
to eliminate the tones completely for the largest cavity. To find the correct method
for scaling the spoilers with the cavity dimensions, different spoiler heights were
tested on the three cavities. The results showed that the cavity noise suppression
for a given cavity attains saturation level at a particular spoiler height, called the
critical spoiler height. An increase in spoiler height beyond the critical spoiler
height was found to have no effect on the noise suppression. It is also found that
this critical spoiler height can be scaled with the length of the cavity (for given
L/D, M and spoiler profile) irrespective of the boundary layer thickness.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/8646 |
| Date | 15 August 2014 |
| Creators | Thangamani, V |
| Contributors | Saddington, A J, Knowles, Prof K |
| Source Sets | CRANFIELD1 |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
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