Large eddy simulations of bifurcating jets

We perform large-eddy simulations (LES) of bifurcating jets to investigate the conditions that are necessary for their onset. A bifurcating jet refers to the splitting of an initially axisymmetric jet into two distinct branches, resulting in a Y-shaped mean flow pattern. Experiments by Reynolds et al. (2002) demonstrated that bifurcation occurs when the axisymmetric mode ($m=0$) is forced at the fundamental frequency and the helical modes ($m=\pm1$) are simultaneously forced at the subharmonic frequency, for Reynolds numbers up to $Re = 100,000$. Extending this further using LES, a comprehensive parameter study is conducted by varying the state of the boundary layer, Reynolds number, forcing frequencies, forcing amplitudes, and azimuthal wavenumber combinations. Bifurcation is found to be highly sensitive to the nozzle-exit boundary layer state: it occurs when the boundary layer is initially laminar but is suppressed when the boundary layer is tripped and initially turbulent. For an initially laminar boundary layer, bifurcation is further suppressed as the Reynolds number increases from 50,000 to 500,000. The increase in small-scale structures leads to the breakdown of vortex rings, thereby inhibiting bifurcation. Owing to azimuthal symmetries, only forcing with $m=0$ and $m=\pm1$ results in bifurcation. Other azimuthal combinations produce multi-lobed structures in the mean flow but do not lead to bifurcation. Finally, bifurcation is also influenced by the forcing frequency of the axisymmetric mode $m=0$ and the corresponding subharmonic frequencies in the $m=\pm1$ modes. It is observed when the forcing frequency excites convectively unstable modes in the initial shear layer. Forcing the most amplified frequency results in bifurcation at upstream locations, while forcing lower amplified frequencies shifts the onset of bifurcation farther downstream.