Transient Development of a Laminar Separation Bubble Over a Low Reynolds Number Airfoil

Abstract

Airfoils operating in low Reynolds number (Re) conditions frequently have a laminar separation bubble (LSB) form as a part of the natural boundary layer (BL) transition. A transient analysis of the Kelvin-Helmholtz (K-H) rolls in the LSB uncovered a new pressure feedback process that alters the development of the BL transition. The SD 7037 airfoil at a modest Re of 41,000 is studied using Large Eddy Simulation (LES) where a free-shear flow filter length criterion is applied to capture the K-H roll development. Multiple detailed experimental data sets are used to validate the chordwise positioning of the time averaged LSB and the dominant K-H roll frequency. The K-H rolls are shown to pinch-off from the reverse flow region of the LSB and the lift-off and subsequent touchdown of the K-H rolls are the source of the pressure feedback. The pressure feedback occurs consistently at 1 deg angle of attack (AOA) resulting in a dominance of the K-H roll frequency throughout the transitional BL. The dominant frequency is relevant to the aeroacoustic performance of low Re airfoils, where K-H rolls at a consistent frequency can generate tonal noise. The intermittent feedback at 5 deg AOA provides a clear distinction between the transition structures during a natural BL transition and feedback-initiated transition. The analysis of the K-H rolls and the discovery of a novel feedback mechanism provides invaluable information for the aerodynamic and aeroacoustic performance of low Re airfoils.