The role of the large-scale structure in the development of turbulent wall jets

The development of the large-scale turbulent structures was investigated in impinging jets exiting long pipes and three-dimensional wall jets exiting long channels. An azimuthal decomposition of the fluctuating wall pressure in the impinging jet revealed that the pressure fluctuations in the stagnation region were caused by three-dimensional azimuthal modes, whereas in the wall jet region a significant portion of energy was contained in both azimuthal mode 0 and 1, indicating that the large-scale structures are more dominant in this region of the jet. In the wall jet region, azimuthal mode 1 was due to the asymmetric evolution of the ring structures over the plate. The contribution of the large-scale structures in the radial wall jet region decreased when the nozzle to plate distance of the jet was increased from H/D = 2 to 4 indicating that the structures in the jet shear-layer were breaking down more before interacting with the wall. A similar comparison of the effect of Reynolds number on the development of the large-scale structures in the radial wall jet region indicated that the large-scale structures are more prominent in the impinging jet with Reynolds number of Re_D = 50,000, than 23,300.

The development of three-dimensional turbulent wall jets exiting rectangular channels with aspect-ratios from 1 to 8 was investigated using profiles and contours of the turbulent velocity in the region x/h = 3 to 60. The development was found to be delayed by increasing the channel aspect-ratio, but the development of the length-scales in the different jets could be collapsed by scaling the streamwise coordinate by the square root of the channel cross-sectional area and normalizing by the size of the channel. The development of the large-scale structures in the jet formed using the rectangular channel with aspect-ratio of 4 was investigated using measurements of the fluctuating wall pressure and the turbulent velocity field in the near and intermediate regions. Contours of the turbulence intensity and the mean streamwise vorticity, indicated that there are two regions of counter-rotating mean vorticity on either side of the jet centreline associated with the lateral shear-layers. The Proper Orthogonal Decomposition (POD) was applied to the fluctuating pressure measurements and the majority of energy was found to be approximately equally divided between a symmetric and antisymmetric mode. Reconstructions of the contributions from the first three modes indicated that the passage of the structures in the jet were asymmetric. These results were compared to similar measurements in a three-dimensional wall jet exiting a square channel at the same normalized locations. The results indicated that the structures in the jet issuing from the channel with A_r = 1 were more antisymmetric and pass closer to the wall than in the jet exiting the channel with A_r = 4.