Laminar Separation Shear-Layer Instability Influences on Low-Re Airfoil Performance

The aerodynamic efficiency of airfoil geometries operating at transitional Reynolds numbers is intricately linked to the behavior of laminar separation bubbles, which can significantly impact performance coefficients. This study investigates the fluid instability mechanisms within LSBs through a combination of experimental measurements and analytical methods. Particle image velocimetry was employed to acquire detailed shear layer profiles, which were subsequently used in Linear Stability Analysis (LSA) to determine the spatial amplification rates of flow instabilities at various chordwise locations. Two pivotal scenarios were explored: A laminar separation region with off-body transition and reattachment (α = 9°at Re = 50,000 and 100,000), or a laminar separation process without boundary-layer reattachment beyond the airfoil trailing edge (α = 2° at Re = 50,000 and 100,000). The results reveal that the spatial wavenumber estimates from LSA align closely with those calculated using the continuous wavelet transform. Furthermore, it was observed that at the most dominant frequency the spatial growth rate of instabilities is directly proportional to the streamwise spatial wavenumber. Increasing the Reynolds number in both trailing-edge separation cases (with or without reattachment) led to a higher dimensional growth rate of instability, thereby shortening the transition length and enhancing aerodynamic performance.