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 Kármán 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.
Instantaneous flow field measurements in the interstage section between a fan and the outlet guiding vanes at different axial positions