Towards the understaing of the role of large-scale structures in the plane turbulent wall-jets

The present study examines the large-scale structures in the turbulent plane wall jet flow and its influence in the region below velocity maximum. To capture these structures in the flow, we use two-dimensional particle image velocimetry (PIV) with four cameras placed side-by-side. Large ensembles of PIV data are collected using this facility to look for the turbulent statistics in spatial fields at three different Reynolds numbers. First, energy maps are plotted using single hot wire time series and PIV spatial fluctuation fields. These plots show the presence of outer and inner turbulent intensity peaks where the outer peak is seen to be more energetic. With an increase in Reynolds number, the energies in the inner peak increase, clearly indicating the influence of large-scales in the near-wall region. This argument is further supported by plotting co-spectra and force spectra fields from PIV data giving the scalewise contributions to Reynolds shear stress. Results from these plots apparantly show the presence of bi-modal structures in the region below velocity maximum, where Jet mode and wall mode appear to be interacting. Next, we carry out Fourier mode decomposition as a tool to look for statistical evidence of these large-scale structures. Here, first we binned PIV fields based on dominant streamwise Fourier modes and then plot the two-point correlation. This study shows the underlying periodicity in the outer region of the wall jet, where the first two dominant modes, $5z_{T}$ and $2.5z_{T}$, occur in almost 50 \% of total fields. Evidences of these dominant modes have also been found in the meso-layer overlap where almost 25 \% of total field shows these modes. This shows that the region below velocity maximum could no longer behave like a boundary layer, in fact, a strong influence of the Jet scale structures increases their activity in the near-wall region with an increase in Reynolds number.