Acoustic response of an isothermal coaxial swirling jet

This experimental study concerns acoustic response of internal recirculation zone (IRZ) in an unconfined isothermal coaxial swirling jet (geometric swirl number: 2-3). Two IRZ modes with characteristic modified Rossby number \textit{Ro}$_{m}$ \textgreater 1 and \textit{Ro}$_{m} \le $ 1 are considered. It is observed that as the amplitude of excitation is increased till a critical magnitude the IRZ topology (time-mean streamline plot obtained from PIV) with \textit{Ro}$_{m} \le $ 1 \textit{fans-out}/widens. At super-critical amplitude the IRZ transits back and finally undergoes a transverse shrinkage. However, \textit{fanning-out} is absent in \textit{Ro}$_{m}$ \textgreater 1 IRZ mode as this flow is dominated by pressure deficit due to entrainment (at the interface between central and co-annular jet) effect when compared to radial pressure gradient due to rotational influence. Thus the central jet with high kinetic pressure exists in \textit{Ro}$_{m}$ \textgreater 1mode (swirl fails to penetrate till the central axis and impart recirculation) but not in \textit{Ro}$_{m} \le $ 1. The imparted acoustics fails to disrupt IRZ in \textit{Ro}$_{m}$ \textgreater 1against the high kinetic pressure of the central stream, failing to impart a \textit{fan-out}.