| Resumo : |
In this work a modi_ed turbulent boundary layer and shear layer are studied. The base ow is changed by the introduction of spanwise periodic disturbances. An experimental investigation is carried out, where the spanwise periodic disturbances are introduced by an array of cylinders upstream of a backwards-facing step (BFS). The cylinders of height k are positioned with their axes normal to the wall, partially blocking the boundary layer of height _. Recent developments have seen the stabilizing e_ects of streamwise velocity streaks on several di_erent base ows. Here, it is found that the array of roughness elements incite spanwise periodic disturbances with elongated streamwise presence. It is shown that for large height (k > _) and small height (k < _) the streamwise streaks survive the e_ects of a backwards facing step and are present until at least twelve boundary layer thicknesses from their initiation. Previous research has shown how streaks modulate the spanwise uctuation pro_les in a near wall range for a zero pressure gradient turbulent boundary layer. Streaks resulting from the forcing by the roughness elements show the same type of behaviour on a scale that is comparable to the height of the roughness elements. A loudspeaker is used to force a single phase, single frequency disturbance upstream of the roughness elements. A linear stability analysis is carried out on the shear layer at a distance of one step height downstream from the BFS. The eigenmodes that are found in the stability analysis match with the Fourier modes that result from a phase averaging procedure. The amplitude of the Fourier modes for a baseline case are compared to the forced base ow and show that the streaks have a stabilizing e_ect on the shear layer, and attenuate velocity uctuations that are present in the shear layer, while increasing the shear layer thickness. The way in which streaks are currently studied can be greatly simpli_ed by using roughness elements to force streaks to the desired speci_cation. Another application may be found in the acoustic noise generated by jets. As the uctuations in the shear layer are attenuated, the shear layer may produce less noise while the increased thickness may allow uctuations to exist at longer wavelengths and produce a lower noise. This motivates subsequent studies on acoustics and potential engineering applications. |