Active Control of Separation Bubbles Exploiting Laminar Baseflow Instabilities

In the present paper laminar separation bubbles are investigated by means of direct numerical simulations with respect to steady and unsteady 2D and 3D Tollmien-Schlichting (TS) like boundary layer disturbances. The specific influence of several disturbance modes on the size and shape of the laminar separation bubble are considered to allow for an effective reduction of the region of separated flow. Based on these results a criterion to detect the extension of the bubble is derived which is a suitable input for a controller. NOMENCLATURE f Disturbance frequency [Hz] h Index denoting higher harmonics of the fundamental disturbance frequency i imaginary unit = √ −1 k Index denoting multiples of the spanwise wavenumber u Streamwise velocity v Wall-normal velocity w Spanwise velocity x Streamwise spatial coordinate y Wall-normal spatial coordinate ∗Research Assistant †Senior Research Scientist ‡Professor, Institute Director z Spanwise spatial coordinate A0 Initial disturbance amplitude C Binary bubble criterion N Accumulated number of points Reδ1 Momentum thickness Reynolds number = Û∞δ̂1 ν̂ L Reference length [m] U∞ Freestream velocity [m/s] αr Streamwise wavenumber β Angular frequency of the disturbance γ Spanwise wavenumber δ1 Momentum thickness [m] λz Spanwise wavelength ν Kinematic viscosity [ m2/s ] ωz Streamwise vorticity ωy Wall-normal vorticity ωz Spanwise vorticity ωz,wall Skin friction Ψ0 Separation stream-surface Ψ0 Time and spanwise averaged separation streamline ∆̃ Modified Laplace Operator ˆ Dimensional variables ′ Non-dimensional disturbance velocities INTRODUCTION Adverse pressure gradient (APG) boundary layers at low 1 Copyright  2002 by ASME disturbance strip integration domain