  The present paper numerically investigates flow control over NACA0021
(National Advisory Committee for Aeronautics) airfoil by applying a
leading-edge tubercle as a passive flow control device at transitional Reynolds
number (Re) regime. The study includes simulations over the unmodified airfoil
and modified airfoil operating at Re=120000 using kT-kL-{\omega} based Delayed
Detached Eddy Simulation (DDES) model. The calculations include both the
pre-stall and post-stall regions, while the performance of the mean flow
aerodynamic properties like pressure, lift, skin-friction, and drag
coefficients are also investigated in the presence of tubercles. The spanwise
flow over the modified airfoil interacts with the primary flow. The turbulent
separation gets delayed compared to the unmodified airfoil, thereby
significantly improving the stall behavior and flow behavior in the post-stall
region. The modified airfoil provides the gradual stall behavior against the
steep stall behavior for the unmodified airfoil. To optimize tubercle
configuration, the study considers two modified airfoils with different
combinations of amplitude and wavelength. The unsteady analysis, including the
reduced-order modeling, i.e. 3D Proper orthogonal decomposition (POD),
characterizes the dominant vortical structure of the flow to evaluate the
improvement in the aerodynamic performance of the modified airfoil. The model
decomposition for flow over airfoil helps to provide a deeper understanding of
the flow control phenomenon.
