ON A MECHANISM OF DELAYING LAMINAR-TURBULENT TRANSITION

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In this paper we investigate the boundary layer flows over a flat plate on which smooth localized imperfections are located. The localized imperfections have the width scale (d) comparable with the wavelength of the Tollmien-Schlichting (T-S) waves and the height scale (h) less than the boundary layer thickness . The existence of the localized imperfection gives rise to the change of the instability property of the boundary layer. The investigations are focused on the interaction between the T-S waves and the base flows distorted by smooth forward-facing steps and aim to forge links between the localized imperfections and the mechanisms of stabilizing the T-S waves. Numerical investigations show that isolated smooth forward-facing steps can perform as a robust strategy of delaying laminar-turbulent transition. Finally, direct numerical simulations are implemented to validate the strategy.