Experimental analysis of an acoustically modulated free swirling jet

The present work reports the experimental analysis of a pulsed isothermal incompressible free swirling jet, characterized by high Reynolds and swirl numbers. The aim of this research is to investigate the influence of an acoustic modulation on the instabilities of this kind of flow, focussing on the effects on the Precessing Vortex Core and on boundary turbulence, which can be important in combustion applications (swirling flames). In fact, it is reported that an acoustic modulation can strongly affect the jet development, especially under the point of view of boundary turbulence generation and increase of entrainment effect. Experimental analysis on the flow field has been performed by laser Doppler anemometry (LDV) and hot wire anemometry (HWA). Data are then analyzed by Fast Fourier Transform algorithm. The acoustic modulation at the natural frequency influences turbulence intensity, reducing it especially in the critical regions near the nozzle exit (central recirculation zone). Frequency analysis shows the influence of modulation with an increase of the energy content associated to instability phenomena, even if there is no evident effect on flow instability characteristic frequencies.