Mixing of A Non-Circular Jet into A Counterflow

An elliptic jet and a square jet flowing into a counterflow with different jet-to-current velocity ratios are investigated by using realizable k–ε model.Some computed mean velocity and scalar features agree reasonably well with experimental measurements,and more features are obtained by analyzing the computed results.After fluid issues from a nozzle,it entrains ambient fluid,and its velocity and concentration on the centerline decay with the distance downstream from the potential core(l0).The decay ratio increases with the decreasing jet-to-current velocity ratio α.For an elliptic jet,the evolution of the excess velocity half-width b and the concentration half-width bc merely remains constant near the jet exit on major-axis plane while they increase linearly on the minor-axis plane.However,the half-widths on the major-axis and minor-axis plane become proportional to the axial distance downstream after equaling each other.For a square jet,b and bc increase linearly with the distance downstream from the jet exit,but the spread ratio is larger on the middle plane than that on the diagonal plane before they equal each other.The radial extent of the dividing streamline rs or the mixing boundary rsc increases linearly downstream,and decreases exponentially after reaching a peak at xb.The ratio on the minor-axis plane is larger than that on the major-axis plane for an elliptic jet.The characteristics are the same for the square jet.b,bc,rs,and rsc on two corresponding planes become equal to each other more rapidly for the square jet than for the elliptic jet,because the sharp corner of the square nozzle induces secondary structures that are more intense.The distributions of the excess axial velocity and scalar concentration exhibit self-similarity for either the elliptic jet or square jet in the region of l0 < x < xb.On the cross section,four counter-rotating pairs of vortices,which enhance the entrainment between the jet and counterflow,form at the four corners of the square jet or at the two ends of the major-axis plane of the elliptic jet.The recirculation pattern formed by these axial vortices is more complex for the square jet than that for the elliptic jet.The turbulent kinetic energy k have large value in the region near the jet exit and stagnation point.The maximum value of k for the square jet is larger than that of the elliptic jet near the jet exit.This results in the square jet mixing more strongly than the elliptic jet.