Coherent structures in the wake behind a trailing edge cutback

An experimental study was performed to investigate the influence of coolant ejection on the aerodynamic characteristics of the unsteady wake behind a trailing edge cutback. A high-resolution planar particle image velocimetry (PIV) system was emlpoyed to conduct detailed flowfield measurements at a Reynolds number of Reh=2000. The time-averaged velocity fields, the distributions of reverse-flow intermitteny and velocity deficts were comparatively studied, and contours of streamwise Reynolds stress and shear stress were examined to disclose the influence of coolant ejection on wake aerodynamic performance. Proper orthogonal decomposition (POD) ananlysis was applied to the velocity flowfileds to disclose the evolution of coherent structures and the mixing process between the cooling stream and mainstream. Large scale coherent structures shed from both the lip and lower plates were identified in low-order modes, indicating that these are the most energtic structures in the wake. Moreover, a “strip” structure was found in the unpaired third eigemode for lower blowing cases (VR = 0.5 and 0.75). By POD filtering with first six eigenmodes, the flapping motion of the separated shear layer from the upper shear layer of the lower plate was observed. Coherent strucutres were identified from the contours of swirling strength. It is speculated that the “strip” structure and the flapping of the seprarated shear layer is resuled from vortex merging, i.e., the merging of two clockwise-rotating vortical structures shed from the upper shear layers of the lip and lower plates.