Control of flow in forced jets: a comparison of round and square cross sections

Direct numerical simulation of incompressible, spatially developing round and square jets at a Reynolds number of 1,000 is performed. The effect of two types of inlet perturbation on the flow structures is analyzed. First, dual-mode excitation, which is a combination of axisymmetric perturbation at preferred mode frequency and helical perturbation at sub-harmonic frequency is used, having a disturbance frequency ratio equal to R-f = 2. It is observed that the circular and square jets bifurcate and spread on one of the orthogonal planes forming a Y-shape jet in the downstream while no spreading is visible on the other plane. The second type of perturbation is a flapping excitation at a sub-harmonic frequency, St(F) = 0.2. It leads to a Y-shape bifurcation for both square and circular jets. On the other hand, for flapping excitation at the preferred mode frequency, namely, St(F) = 0.4, a circular jet bifurcates into a W-shape whereas the square jet reveals simple spreading.