The response of an axisymmetric jet placed at various positions in a standing wave

The hydrodynamic response of an axisymmetric jet placed at various positions in a standing wave oriented normally to the jet is investigated. At the velocity and pressure nodes the axisymmetric (m = 0) and first azimuthal (m = +/- 1) modes are excited, respectively, through manipulation of the jet exit boundary conditions. At positions between the nodes, both the m = 0 and m = +/- 1 modes are simultaneously excited resulting in asymmetric forcing due to the phase difference between the transverse and longitudinal acoustic fluctuations. This leads to the asymmetric formation of vortices in the near field and bifurcation into two or more momentum streams further downstream. The dominant momentum stream is deflected in the direction of the velocity node. It is shown that the asymmetric response can be well approximated by a superposition of the boundary conditions at the pressure and velocity nodes where the contributions from each mode are proportional to the acoustic pressure and velocity. A method is proposed to characterize the bifurcation behaviour statistically via moments of the probability density functions constructed from profiles of streamwise momentum. The jet symmetry and momentum spreading are shown to be proportional to the magnitude of the transverse acoustic velocity. Finally, the streamwise velocity is reconstructed as a superposition of Gaussian profiles providing a robust method to characterize the number of individual momentum streams which also shows that each of the streams behave self-similarly.

Automated summary

The study investigates the hydrodynamic response of an axisymmetric jet placed at various positions in a standing wave oriented normally to the jet. Different modes of fluctuations are excited at velocity and pressure nodes, causing asymmetric forcing and bifurcation of momentum streams between the nodes. The asymmetrical response can be approximated by superposition of boundary conditions at pressure and velocity nodes, with the jet symmetry and momentum spreading proportional to the transverse acoustic velocity magnitude.