Attenuation of boundary layer disturbances by means of streamwise vortices

Abstract: The stability of the boundary layer developed on a flat plate is studied in the presence of streamwise streaks where disturbances of different types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at flow control by means of passively damping the energy growth of the disturbances in the streaky boundary layer and furthermore delay the onset of the transition in the streamwise direction.The streamwise streaks are introduced to the two-dimensional Falkner-Skan boundary layer by means of a spanwise-periodic array of miniature vortex generators (MVGs), mounted on the flat plate, that generate pairs of counterrotating vortices elongated in the streamwise direction. The spanwise modulation of the boundary layer into regions of high and low speed streaks is hence obtained passively since the MVG blades utilize the existing momentum in the shear flow to set up the streamwise vortices with no energy input to the boundary layer. The disturbances are introduced as planar Tollmien-Schlichting (TS) waves as well as three dimensional oblique and pair of oblique waves. In an attempt to obtain a more realistic configuration, the disturbance slot is moved upstream of the MVG array as opposed to the earlier studies. It is shown that the passive control method is capable of stabilizing the disturbance waves in the linear regime for a wide range of frequencies albeit an initial receptivity of the disturbance amplitudes is observed immediately downstream of the MVG array. The control method is proven to be extended in the non-linear regime of the disturbances where transition to turbulence delay is obtained. This results in significant drag reduction when comparing the skin friction drag of a laminar boundary layer to a turbulent one. In addition, a parameter study on a wide range of MVG configurations is performed in order to investigate the transient growth of the streaks as well as the damping effects. A universal scaling of the streak amplitudes is found based on empiricism where an integral amplitude definition is proposed for the streaks. The damping effect is found to be optimized for an integral streak amplitude of 30% of the free-stream velocity which takes into account the periodic wavelength of the streaky base flow. Moreover, it is shown that the passive control method can be extended in the streamwise direction by placing a second array of the MVGs in an already streaky boundary layer which results in further delay of the transition to turbulence location.