The evolution of a plane mixing layer with spanwise nonuniform forcing

The experimental studies of Nygaard and Glezer (AIAA Paper No. AIAA‐91‐0625, 1991) show that vortical structures shaped like a ‘‘chain‐link fence’’ form in a plane mixing layer subjected to spanwise nonuniform phase excitation. Using direct numerical simulation of temporally evolving mixing layers, initial conditions have been developed that result in flows with vortex structures similar to those observed in the experiments. The simplest initial condition that leads to the experimentally observed structures is composed of a pair of equal‐strength oblique disturbances and this initial condition is a low‐order Fourier approximation to the square‐wave excitation used in the experiments. This simple disturbance suggests an explanation for the short wavelength cutoff observed in the experiments and allows comparison with previous stability analyses. The simulated flow fields show that the chain‐link‐fence vortex structures are different from the usual rib/roller mixing layer structures. Furthermore, detailed examination of the overlap region between adjacent structures shows that ‘‘local pairing’’ does not occur in the chain‐link‐fence flow due to strong self‐induction effects. By increasing the spanwise wavelength, self‐induction effects can be delayed and regions of local pairing are observed. However, the local pairings do not lead to a single amalgamated vortex at late times as is observed in two‐dimensional pairings. Finally, it is shown that the chain‐link‐fence flow produces increased mixing compared to standard rib/roller flows.