Flowfield Imaging Laboratory: High-Speed Cavity Flow

planar laser imaging of high-speed cavity flow dynamics

Ö.H. Ünalmis, Pablo C. Bueno, N.T. Clemens, and D.S. Dolling

An important issue facing military aircraft design is the problem of opening weapons bay doors at high speeds. The strong instabilities within the bay (or cavity) induce intense pressure fluctuations which consist of both broadband small amplitude fluctuations typical of turbulent shear layers, as well as discrete resonances whose frequency, amplitude and harmonic properties depend on the cavity geometry and external flow conditions. These intense pressure fluctuations, driven by self-sustaining vortex oscillations, can excite vibrations of the local bay structure or the ordnance within the cavity and cause material failure. The oscillatory flow field can also adversely affect the stable release of weapons.

The objective of this project is to understand the underlying physics of the high-speed cavity flow dynamics, using non-intrusive laser diagnostic techniques combined with fast response pressure measurements in a Mach 5 wind tunnel. Two outstanding issues that we seek to address are (1) the cause of the deflection of the shear layer around the cavity rear wall region, and (2) the effects of the incoming boundary layer and/or the individual boundary layer structure in this process.

The two imaging techniques to be implemented in this study are planar laser scattering (PLS) and particle image velocimetry (PIV). A schematic of the optical arrangement is given above. The method of approach will be to obtain quasi-real time movies which will be used to understand the cavity oscillation cycle. In this approach, a sequence of images is obtained, where each image is delayed by a different amount from a well-defined event. In the case of the cavity flow, the impingement of the shock on the rear wall is an ideal event for this purpose since this event produces a clear spike in the pressure, as recorded by a transducer mounted on the rear wall. When the pressure exceeds a threshold value, the triggering circuit simultaneously fires the laser and signals the frame grabber to store the next video frame.

The figure above gives a sample PLS image from preliminary application of the imaging system, where the flow direction and cavity position and dimensions are as indicated. Uncorrelated samples of images illustrating different phases of the cavity oscillation cycle can be seen as an animated series of images (323 kB).

This work has direct applications to new aircraft under development for the U.S. Air Force, including the F-22 supersonic fighter. The data generated in this study, particularly the details of the shear layer impingement and mass inflow/outflow cycle, should also be useful in the validation of large eddy simulations.

Publications:

Experimental Study of Shear-Layer/Acoustics Coupling in Mach 5 Cavity Flow, Ünalmis, Ö.H., Clemens, N.T., and Dolling, D.S., AIAA Journal, Vol. 39, No. 2, Feb. 2001, pp.242-252.
Planar Laser Imaging of a Supersonic Side-Facing Cavity, Ünalmis, Ö.H., Clemens, N.T., and Dolling, D.S., AIAA 99-0297, 37th Aerospace Sciences Meeting & Exhibit, Jan. 1999, Reno, NV, U.S.A.
Planar Laser Imaging of High-Speed Cavity Flow Dynamics, Ünalmis, Ö.H., Clemens, N.T., and Dolling, D.S., AIAA 98-0776, 36th Aerospace Sciences Meeting & Exhibit, Jan. 1998, Reno, NV, U.S.A.

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