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The effect of the excitation frequency of synthetic jet actuators on the mean jet velocity issuing from an array of circular orifices is investigated experimentally, focusing on the acoustic excitation characteristics of the actuator's cavity. Two cavity configurations are considered. In the first configuration, synthetic jets are generated by exciting a single, large cavity having an array of sixteen orifices via sixteen piezoelectric elements. In the second configuration, the cavity volume of the first configuration is divided into eight isolated compartments, each with two orifices and two piezoelectric elements. Several distinct resonant peaks were observed in the frequency response of the synthetic jet actuator built with a single large-aspect-ratio cavity, whereas the case of compartmentalised cavities exhibited a single resonant peak. Acoustic simulations of the large-aspect-ratio-cavity volume showed that the multiple peaks in its frequency response correspond to the acoustic standing-wave mode shapes of the cavity. Due to its large aspect ratio, several acoustic mode shapes coexist in the excitation frequency range aside from the Helmholtz resonance frequency. When the actuator's cavity volume is compartmentalised, only the Helmholtz resonance frequency is observed within the excitation frequency range. [ABSTRACT FROM AUTHOR]

The flow-excited acoustic resonance phenomenon is created when the flow instability oscillations are coupled with one of the acoustic modes of a confined duct, which in turn generates acute noise problems and/or excessive vibrations. In this study, the effect of the upstream edge geometry on attenuating these undesirable effects is investigated experimentally for flows over shallow rectangular cavities with two different aspect ratios of L/D=1 and 1.67, where L is the cavity length and D is the cavity depth, for Mach number up to 0.45. The acoustic resonance modes of the cavity are self-excited due to the development of free shear layers over the cavity mouth. Twenty four different upstream cavity edges are investigated in this study, including round edges, chamfered edges, vortex generators, and spoilers with different sizes and configurations. The results for each upstream cavity edge are compared with the base case where sharp edge is used. Most of the spoiler edges are found to be effective in suppressing the pressure amplitude of the excited acoustic resonance. Hot-wire measurements that were taken along the lateral direction downstream of the spoilers reveal the existence of secondary vortices generated by the spoilers, orthogonal to the cavity shear layer, which results in suppressing the resonance. The performance of each spoiler depends on its specific geometry (i.e. thickness, height, and angle) and the size and strength of the orthogonal vortices that can be generated. A summary of the results is presented in this paper. [ABSTRACT FROM AUTHOR]

The flow-excited acoustic resonance of isolated cylinders in cross-flow is investigated experimentally where the effect of the cylinder(s) proximity to the acoustic particle velocity nodes of the cross-modes is presented in this paper. For the case of a single cylinder, the cylinder's location does not significantly affect the vortex shedding process; however, it affects the excitation level of each acoustic cross-mode. When the cylinder is moved away from the acoustic particle velocity antinode of a specific acoustic cross-mode, a combination of the cross-modes is excited with intensities that seem to be proportional to the ratio of the acoustic particle velocities of these modes at the cylinder's location. For the cases of two and three hydrodynamically uncoupled cylinders positioned simultaneously side-by-side in the duct, it is observed that the first three acoustic cross-modes are excited. When one cylinder is positioned at the acoustic particle velocity antinode of a specific cross-mode and another cylinder is positioned at its acoustic particle velocity node, i.e., a cylinder that should excite the resonance and another one that should not excite it, respectively; the excitation always takes over and the resonance occurs at a further elevated levels. It is also observed that the acoustic pressure levels in the cases of multiple cylinders are not resulting from a linear superposition of the excited level obtained from each individual cylinder which indicates that the removal of cylinders at certain locations may not be a viable technique to eliminate the acoustic resonance in the case of tube bundles. [ABSTRACT FROM AUTHOR]