A study on the generation and development of high amplitude steady streamwise streaks in a flat plate boundary layer is presented. High amplitude streamwise streaks are naturally present in many by-pass transition scenarios, where they play a fundamental role in the break-down to turbulence process. On the other hand, recent experiments and numerical simulations have shown that stable laminar streamwise streaks of alternating low and high speed are also capable to stabilize the growth of Tollmien-Schlichting waves as well as localized disturbances and to delay transition. The larger the streak amplitude is, for a prescribed spanwise periodicity of the streaks, the stronger is the stabilizing mechanism. Previous experiments have shown that streaks of amplitudes up to 12% of the free-stream velocity can be generated by means of cylindrical roughness elements. Here we explore the possibility to generate streaks of much larger amplitude by using a row of miniature vortex generators (MVGs) similar to the ones used in the past to delay or even prevent boundary layer separation. In particular, we present a boundary layer experiment where streak amplitudes exceeding 30% have been produced without having any secondary instability acting on them. Furthermore, the associated drag with the streaky base flow is quantified and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one. In this way it is possible to make the control more persistent in the downstream direction. It must be pointed out that the use of MVGs opens also the possibility to set-up a control method which acts twofold in the sense that both transition and separation are delayed or even prevented.
On the generation of steady streamwise streaks in flat-plate boundary layers
TALAMELLI, ALESSANDRO
2012
Abstract
A study on the generation and development of high amplitude steady streamwise streaks in a flat plate boundary layer is presented. High amplitude streamwise streaks are naturally present in many by-pass transition scenarios, where they play a fundamental role in the break-down to turbulence process. On the other hand, recent experiments and numerical simulations have shown that stable laminar streamwise streaks of alternating low and high speed are also capable to stabilize the growth of Tollmien-Schlichting waves as well as localized disturbances and to delay transition. The larger the streak amplitude is, for a prescribed spanwise periodicity of the streaks, the stronger is the stabilizing mechanism. Previous experiments have shown that streaks of amplitudes up to 12% of the free-stream velocity can be generated by means of cylindrical roughness elements. Here we explore the possibility to generate streaks of much larger amplitude by using a row of miniature vortex generators (MVGs) similar to the ones used in the past to delay or even prevent boundary layer separation. In particular, we present a boundary layer experiment where streak amplitudes exceeding 30% have been produced without having any secondary instability acting on them. Furthermore, the associated drag with the streaky base flow is quantified and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one. In this way it is possible to make the control more persistent in the downstream direction. It must be pointed out that the use of MVGs opens also the possibility to set-up a control method which acts twofold in the sense that both transition and separation are delayed or even prevented.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
