| Turbulent shear flow past a slotted plate bounded by a cavity is investigated in absence of acoustic resonant and elastic effects. High-image-density particle image velocimetry is employed to determine simultaneously the flow patterns on either side of the slotted plate, as well as within the slots. This imaging is complemented with pressure measurements at the trailing-edge of the plate. Using these approaches, the primary features of the self-excited oscillations were determined for variations in length of the slotted plate, as well as parameters of the individual slots.; It is demonstrated that initially turbulent shear flow along the slotted plate can give rise to a highly coherent, purely hydrodynamic instability. Irrespective of the plate geometry, the generic features of the instability are: a large-scale swirl (vortical structure), which propagates in the streamwise direction; small-scale swirls, which occur in individual slots; a transversely-oriented jet at the trailing-edge of the plate, which alternates direction during an oscillation cycle; and a time-averaged counterflow along the backside of the plate. All of these unsteady and averaged features are associated with well-defined patterns of vorticity. The relationships between the unsteady features are quantitatively interpreted via patterns of phase, and thereby phase shifts, of fluctuations within and along either side of the plate.; The generic type of oscillation described in the foregoing can exist over a range of parameters of the slotted plate; however, when these parameters approach limiting values, the oscillation may be attenuated. At an optimal value of plate thickness, the oscillation attains its largest amplitude. When the thickness becomes relatively large or small, however, substantial attenuation occurs. Moreover, if the gap width of each slot of the plate exceeds a critical value, attenuation sets in. Further techniques of attenuation of the oscillation involve non-homogeneity of the plate, i.e. a non-spatially repetitive plate geometry. Of these approaches, the most effective is blocking a single slot near the trailing-edge of the plate. A different concept involves alteration of the inflow characteristics via generation of a separated shear layer at the leading-edge of the slotted plate; this approach can also lead to effective attenuation, provided the displacement of the layer above the plate exceeds a critical value. For all of the foregoing approaches to attenuation, wholefield imaging is employed to define the underlying physics. Calculation of both the pressure field and an energy integral, along with measurement of pressure, allow further interpretation of selected attenuation techniques. |
