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he presence of a cavity in aerodynamic surfaces can generate unsteady disturbances in the surrounding flow field. Such unsteady cavity flows are known to cause acoustic, vibration and aerodynamic problems. For example, the deep wheel well in the undercarriage of an aircraft can |
develop high-intensity acoustic tones, which in turn can induce vibrations in the surrounding structure leading to structural fatigue. In addition, for a bomb or missile bay (a shallow cavity), an adverse static pressure gradient in the cavity can cause the separating store to experience a large nose-intothe-cavity pitching moment, thus preventing safe store separation.
This research studies flow-past-cavity at transitional and supersonic flow speeds. Experimental investigation of such phenomena are often rather involved due to the difficulty in seeding the flow suitably at supersonic speed and the unavailability of high speed cameras.
The methodology used included flow visualization, cavity surface pressure measurement and PIV measurement. It also involved the setting up of a small supersonic wind tunnel to enable the work to be carried out. The supersonic wind tunnel designed and built is a blow-down type wind tunnel, which is shown in Figure 1. The compressed air flows from the supply through a pipe and the various components of the wind tunnel in the following sequence: compressed air supply  pressure regulator settling chamber contraction 50 mm x 50 mm test chamber (in which the cavity model is located) diffuser silencer.
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Figure 1: In-house designed supersonic wind tunnel. |
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For flow visualization, both Schlieren and shadowgraphic techniques were used. A comparison of the flow images obtained with the respective techniques, at flow Mach number (M) of 1.78 and cavity’s length-to-depth (L/D) ratio of 4, are shown in Figure 2. This figure shows that both techniques reveal the same flow features, which include a shock wave at the leading edge of the cavity, an expansion wave at the trailing edge of the cavity and a shear layer that separates from the leading edge. It further shows that the Schlieren technique results in a picture with better contrast than the shadowgraphic technique.
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Figure 2: Flow visualization by (a) Schlieren technique and (b) shadowgraphic technique, both taken at M = 1.78 and L/D = 4. Flow in both pictures is from left to right. |
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Figure 3 shows the measured pressure distribution over the surfaces of a long cavity (with L/D = 10 and M = 1.78). Comparison was made with the data of Zheng et al. (2003) for a cavity of the same L/D value but with M = 2.0. Good agreement is noted between both sets of measurements.
Figure 4 shows the results of the PIV measurements (vorticity contours at L/D = 4, U = 280 m/s). It is clear that at L/D = 4, the resultant flow structure is of the “open cavity” type. That is, the shear layer that separates from the cavity leading edge only deflects downwards and eventually reattaches onto the cavity trailing edge without touching the cavity floor. For PIV measurements, the highest flow Mach number investigated in the present work is 0.85, which is limited by the frame rate of the high-speed camera used. A new (and faster) camera has been ordered, which will enable us to study flow at supersonic speeds.
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Figure 3: Cavity surface pressure distribution for L/D = 10 at
M = 1.78. Open circles are data from Zhang et al. (2002) for
L/D = 10 and M = 2.0. |
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Dr Luo Siao Chung is an Associate Professor in
the Department of Mechanical Engineering. He
obtained his B.Eng, M.Eng and PhD/DIC from
University of Melbourne, National University
of Singapore, and Imperial College (University
of London), respectively. His research interests
include Aerodynamics, Bluff Body Aerodynamics,
Drag Reduction Methods, Flow-Induced Vibration
of Structures, and Experimental Methods in Fluid
Mechanics.
Email: mpeluosc@nus.edu.sg
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