Coherent structure in the turbulent planar jet. Part 2. Structural topology via POD eigenmode projection

The topology of the large-scale structure in the similarity region of a turbulent planar jet is investigated experimentally. The large-scale structure is reconstructed in physical space by projection of measured proper orthogonal decomposition eigenmodes onto instantaneous flow-field realizations. The instantaneous flow-field realizations are obtained by a spanwise aligned triple X-wire rake arrangement which is used in conjunction with the linear stochastic estimation technique. Instantaneous realizations are also acquired via a second triple rake arrangement which provides an assessment of the effect of spatial aliasing on the resulting structural topology. Results indicate that the self-similar large-scale structure in the planar jet consists of a dominant planar component consisting of two lines of large-scale spanwise vortices arranged approximately asymmetrically with respect to the jet centreline. This planar component of the structure resembles the classic Karmam vortex street. There is a strong interaction between structures on opposite sides of the jet in the form of nearly two-dimensional lateral streaming motions that extend well across the flow. In addition, results indicate that the effect of the nonplanar spanwise modes is to both tilt and bend the primary spanwise vortex tubes and thereby redistribute large-scale vorticity. The bending occurs primarily in the streamwise direction. The degree to which the spanwise vortices are distorted varies greatly; in some cases they are nearly streamwise oriented and in others only slight distortion of a spanwise vortex is noted. Based upon the experimental results, prospects for low-order modelling of the jet large-scale structure are discussed.