Direct Numerical Simulation of pulsed jets-in-crossflow

Direct Numerical Simulations of a jet with a passive scalar injected vertically into a crossflow (velocity ratio = 6) is performed at a jet Reynolds number of 5000. The role of sinusoidal forcing of the jet on the dynamics of the flow structures, and on the jet trajectory and jet spreading is examined. Sinusoidal excitations selected are at non-dimensional frequencies of 0.2, 0.4, and 0.6. For the unforced jet, shear-layer vortices on the leading edge of the jet have a preferred mode frequency of around 0.35. With forcing, the dominant frequency in the near field of the jet is the forcing frequency, but further downstream, vortex interactions/mergings lead to the growth of the subharmonic modes. For a forcing frequency of 0.2, the jet bifurcates in the vertical plane; at a forcing frequency of 0.4, the jet trifurcates into three jet-streams in the vertical plane; and, at a forcing frequency of 0.6, the jet bifurcates in the horizontal plane. The largest vertical penetration is at a forcing frequency of 0.4, while the largest horizontal spreading occurs at a non-dimensional forcing frequency of 0.6. Wake vortices, with a U-loop structure, are seen for all cases except at a forcing frequency of 0.6, where they are completely suppressed. The U-loop structure is asymmetric for the unforced and 0.2 forcing frequency case, and is consistent with the earlier experimental observations for unpulsed jets-in-crossflow. Through particle visualizations, a mechanism for the development of the wake vortices is presented in the paper. Mean statistics on isosurface contours are also presented, and asymmetry in the mean counter-rotating kidney pair vortex structures are also seen for the unforced and 0.2 forcing cases. The results of this study indicate the potential of using external forcing as a potential control strategy for controlling the jet penetration and mixing with the crossflow in either the vertical plane or the horizontal plane.