Kinematic aspects of flow separation in external aerodynamics are investigated in the Lagrangian frame. Specifically, the initial motion of upwelling fluid material from the wall is related to the long-term attracting manifolds in the flow field. While the short-time kinematics are governed by the formation of a material spike upstream of the zero-skin-friction point and ejection of particles in direction of the asymptotic separation line, the trajectories of the fluid tracers are guided by attracting ridges in the finite-time Lyapunov exponents once they leave the vicinity of the wall. The wall signature of this initial fluid upwelling event, the so-called spiking point [Serra, M., Vetel, J., Haller, G., "Exact theory of material spike formation in flow separation", J. Fluid Mech., Vol. 845, 2018], is computed from the curvature of advected material lines and, for the first time, from high-order numerical derivatives of the wall-normal velocity obtained from direct numerical simulations of a circular cylinder and a cambered NACA 65(1)-412 airfoil. As the spline-based boundary parametrization of the airfoil profile induces oscillations, the principle spiking point can be recovered robustly through appropriate filtering. The short-term kinematics correlate strongly with the scaling lengths in the boundary layer. Keywords flow separation, Lagrangian, backbone of separation 1 IntroductionAs practical small-scale flying devices proliferate and as interest in turbomachinery at various scales develops, it becomes increasingly important to understand and characterize flows at moderate Reynolds number. In this flow regime, laminar boundary layer separation, reattachment, and transition results in significant changes of the lift and drag, affecting the performance of the airfoil. The control and prevention of flow separation can therefore yield a substantial extension of the operating range of the device. The dynamics of flow separation are highly non-linear, however, making the design of an effective passive and active flow control system challenging. arXiv:1909.04129v1 [physics.flu-dyn]

An improved non-stationary two-layer model is presented for the simulation ofwind speed maxima in the nocturnal boundary layer. The model is based on the idea of Blackadar (1957), who proposed as forcing mechanism an inertial oscillation of the ageostrophic component of the wind vector in the levels above the top of the nocturnal radiation inversion. First, the time-dependent variation of the nocturnal boundary-layer height is studied by means of prognostic equations; there is a good agreement between observed and calculated height data for three days of the Wangara experiment. Furthermore, a diurnal variation of the drag coefficient is considered in the lower layer by decreasing the coefficient by a factor of lo-20 due to stabilization of this layer during the night, The marked temporal decrease (increase) of the dra coefficient in the first hours after sunset (sunrise) is described by a function C Doj = Co, -A de sm(x/2T) t. The incorporation of these two effects into the model gives results which are in good agreement with observed wind data for Wangara days 13/14, 30/31, and 33/34.

Many active flow control strategies have been developed to mitigate flow separation and improve aerodynamic performance. Most studies have focused on optimizing the control action for a given actuator configuration; however, actuator placement is intimately tied to achievable performance. In this paper, we formulate a systematic approach for determining the optimal actuator location for separation control from numerical and experimental fluids data for both stable and unstable systems. High-fidelity numerical fluids simulations have been performed to compute the lift and separation-angle responses to a pulse of localized body-force actuation applied at six distinct locations on the upper surface of the airfoil. These pulse response data are then used to determine the actuator location among the set that can drive the system output to an arbitrary value with the minimum amount of input energy. The results of this study indicate that the optimal actuator locations for controlling lift and separation angle are not identical.

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We present a systematic approach for determining the optimal actuator location for separation control from input-output response data, gathered from numerical simulations or physical experiments. The Eigensystem Realization Algorithm is used to extract state-space descriptions from the response data associated with a candidate set of actuator locations. These system realizations are then used to determine the actuator location among the set that can drive the system output to an arbitrary value with minimal control effort. The solution of the corresponding minimum energy optimal control problem is evaluated by computing the generalized output controllability Gramian.We use the method to analyze high-fidelity numerical simulation data of the

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