Turbulence Control In Wall Jets PDF Download

The turbulent boundary layer has been the focus of many studies over the last century or so due to its engineering significance. Recent studies have shown that the turbulent boundary layer encapsulates extremely energetic large-scale structures which play a much more significant role in its dynamics than has been previously thought. Further, these large scales amplitude and frequency modulate the finer scales of turbulence. This offers a path to control wall turbulence by controlling the large-scales of the flow. The focus of this thesis is to establish ways to manipulate the energy of the large-scale structures in a model turbulent boundary layer and characterize its effect. The plane wall jet was chosen as the model flow field because it is possible to perturb/excite the large-scales at the onset of turbulence independent of the finer scales of turbulence. Hot-wire anemometry was used to carryout velocity measurements in at plane wall jet at streamwise locations up to three hundred times the jet exit slot height. Three different passive perturbation configurations were used to manipulate the energy of the large-scale structures of the plane wall jet. The different perturbation configurations used were a rod, sandpaper and sandpaper and a still wire. All the perturbations were carried out in the vicinity of the jet exit. The unforced wall jet was used as the baseline to compare the effect of these perturbations. All the perturbation cases were able to alter the energy of the large-scale structures in the flow. However, the large-scale structures were shown to be most energetic and reached closest to the wall in the flow that was perturbed using the rod than in all other flow cases at all streamwise positions considered. Last but not least, tic and reached closest to the wall in the flow that was perturbed using the sandpaper and the sandpaper and string configurations showed less energetic large-scale structures at all streamwise locations than in an unforced wall jet. However, the large-scale structures in the sandpaper and string configuration were more energetic than the large scale structures in the sandpaper configuration at all streamwise locations.