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Sound pressure levels and flow characteristics of a synthetic jet actuator (SJA) are investigated experimentally using the following orifice configurations: a) a slender rectangular slot orifice and b) an array of circular orifices with two different orifice diameters. All configurations have similar total orifice neck area, orifice height, and cavity volume, resulting in a similar Helmholtz frequency. Experiments are conducted for orifices mounted on a flat plate under a quiescent condition. The mean jet velocity exhibits resonant peaks at several excitation frequencies, which also gave rise to the sound pressure levels. The resonant frequencies and peak jet velocities were found to depend on the excitation amplitude and orifice configuration. Investigation of the jet momentum penetration into the quiescent air above the different orifice configurations shows that the momentum issuing from the rectangular slot decays much quicker than that of the circular orifices, irrespective of whether the SJA is excited at resonance or not. The most favorable performance (i.e., the highest momentum in the jet core) was obtained with the array of circular orifices having a smaller orifice diameter, even at off-resonance excitations. The work herein shows the possibility of reducing the SJA noise by 8-10 dB by operating the SJA at a frequency away from the Helmholtz frequency while still achieving comparable levels of jet momentum penetration into the crossflow. [ABSTRACT FROM AUTHOR]

Termoakustyczna maszyna cieplna (TAHE) przekształca ciepło w energie akustyczna bez uzycia czesci ruchomych. W porównaniu z maszynami tradycyjnymi ma ona szereg zalet, takich jak prosta konstrukcja, stabilnosc działania oraz wykorzystanie gazów przyjaznych dla srodowiska. W celu dalszej poprawy osiagów maszyny TAHE nalezy zoptymalizowac parametry stosu (zespołu waskich kanałów miedzy płytami wymiennika ciepła). Trzema głównymi parametrami analizowanymi w przedstawionej pracy sa pozycja stosu, długosc i odstep miedzy płytami. Pozycja stosu decyduje zarówno o sprawnosci jak maksymalnej mocy akustycznej wytwarzanej przez maszyne cieplna. Umieszczenie stosu blizej strzałki fali stojacej moze zapewnic wieksza sprawnosc kosztem maksymalnej wytwarzanej mocy akustycznej. Stwierdzono, ze dalsza poprawa sprawnosci maszyny TAHE moze byc osiagnieta gdy zastosuje sie odstep płyt stosu równy 2,4 cieplnej głebokosci wnikania, k . Zmiany długosci stosu nie maja wielkiego wpływu na sprawnosc tak długo, jak stosunek krytycznego gradientu cieplnego jest wiekszy od 1. Na podstawie interpretacji efektów tych zmian podjeto wysiłki dla uzyskania najlepszych osiagów maszyny.

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]