Numerical Investigation of Unsteady Laminar Separation Bubble Dynamics over a Pitching Airfoil

This study presents a numerical investigation of laminar separation bubble dynamics over a modified NACA 64₃–618 airfoil under both static and sinusoidal pitching conditions. Using the Transition SST model within an unsteady Reynolds-averaged Navier–Stokes framework, the effects of pitching motion and reduced frequency on the transition process, bubble length, and lift force are examined at a chord-based Reynolds number of 2×10⁵. Comparison between the static and pitching cases (k = 0.15) shows that pitching motion primarily modifies the reattachment point, producing a clear hysteresis in laminar separation bubble length during the upstroke and downstroke phases. Intermittency and turbulent kinetic energy fields further support this behavior and reveal enhanced shear-layer instability growth under dynamic motion. When the reduced frequency is increased to k = 0.75, the transition shifts upstream, the separation bubble becomes strongly suppressed, and a pronounced lift hysteresis loop develops. These findings demonstrate that laminar separation bubble dynamics are highly sensitive to reduced frequency and highlight the significant role of unsteady excitation in governing separation, transition, and overall aerodynamic performance.