| Shear flow past a deep cavity can generate self-sustained oscillations, including locked-on flow tones, due to coupling between the inherent instability of the separated shear layer and an acoustic mode of the cavity resonator. This investigation focuses on the pressure amplitude response within a deep cavity, as a function of both the streamwise length of the cavity opening and the cavity depth; for each geometrical configuration, the inflow velocity is varied over a wide range. In addition, the flow structure along the mouth of the cavity is characterized via a technique of high-image-density particle image velocimetry.; Criteria for locked-on flow tones include: a strength of lock-on SoL criterion and the quality factor Q. All self-excited oscillations are assessed using both of these criteria, in relation to different dimensionless forms of the fluctuation pressure amplitude.; The pressure response characteristics within the cavity, due to variations of streamwise length L of the cavity opening, involve several successive regimes, which are defined in relation to L/theta, in which theta is the momentum thickness of the inflow boundary layer. Pressure response characteristics due to variations of cavity depth LC also exhibit successive, identifiable regimes. All of the foregoing regimes are defined in terms of the possible hydrodynamic modes (stages) of the unsteady shear layer and the acoustic modes of the deep cavity. A common feature of all flow tones is generation of sharply defined higher harmonics of the fundamental flow tone. Furthermore, it is demonstrated that hysteresis of the pressure amplitude response characteristics does not occur. On the other hand, intermittency of the flow-acoustic coupling of a given combination of a hydrodynamic mode and an acoustic mode does occur for closely-spaced acoustic modes.; With the aim of characterizing locked-on flow tones, the following characteristics have been derived: (i) universal representations of the strength of lock-on SoL; (ii) the critical (onset) velocity Ucr for generation of a flow tone; (iii) the minimum cavity length Lmin for onset of a tone; and (iv) Strouhal number Sr correlations involving all identifiable non-locked-on and locked-on peaks. |
