12:15–1:15 pm
KPTC 206 5740 S. Ellis Avenue
Fluid flows subject to time-dependent external forces or boundary conditions are ubiquitous in aeronautical applications. Whether one considers pitching wings, dynamic stall or the gust response of wind turbines, the flow is unsteady or non-autonomous. We investigate the influence of unsteadiness on the non-linear flow evolution, as well as on the linear response to small disturbances that determines their stability and the subsequent transition to turbulence. The simulations are performed with a high-order spectral-element method (SEM) with the domain discretized by up to several billion grid points. The capabilities of our SEM solvers are presented and two flow cases are studied in more detail. First, a small amplitude pitching wing where the laminar-turbulent interface drastically changes its cordwise location, and subsequently the dynamic stall of an airfoil undergoing a large pitchup motion.
We assess the potential of the optimally time-dependent (OTD) framework for transient linear stability analysis of flows with arbitrary time-dependence using a localized linear/non-linear implementation. This framework is first tried on oscillating plane Poiseuille flow to show the potential of the method and subsequently used to track the linear stability of laminar separation bubbles on unsteady wings. For the pitching case the global mode corresponding to an absolute local instability is identified at the rear of the separation bubble, causing its breakdown to turbulence.
The influence of low-amplitude free-stream disturbances on the onset of dynamic stall is also investigated and the onset of intermittent vortex shedding during the bubble bursting is documented. Here the Proper Orthogonal Decomposition framework is extended to include time dependence. This allows for the objective extraction of transient structures from data. Large structures shedding the bubble are identified as precursors of the detachment of the dynamic stall vortex.