HELMHOLTZ CAVITY IN INCOMPRESSIBLE FLOW
Installed in the National Center for Physical Acoustics (NCPA) Mach 0.05 Low Speed Wind Tunnel is a Helmholtz cavity apparatus (small Plexiglas box in the center of the photograph) that incorporates a piezoelectric actuator capable of producing vibrations that match the natural cavity resonance frequency. The cavity orifice is flush-mounted to the inside top of the tunnel test section.

The purpose of this experiment is to determine: (1) if the phase of the cavity resonance can be controlled; (2) if the fluctuations induced by the cavity can be sensed (using a hot-wire probe downstream of the cavity orifice) in the incompressible boundary layer; and (3) the power efficiency of the system (i.e., the relationship between electric power input to the piezo device and the resultant acoustic power transferred to the boundary layer). This research is important because it provides insights into potential methodologies that may be used to control naturally evolving instability modes. Such control may be used to reduce unwanted fluctuating pressures caused by these natural flow instabilities.
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