Effects of a trapped vortex cell on thick wing profile

Experimental investigation on the effects originated from a trapped vortex cell on the NACA0024 airfoi

Bluff Body Flow Control Through Piezoelectric Actuators

Delay separation from bluff bodies leads to drag reduction. An active flow control technique based on piezoelectric actuators has been arranged on a 2D cilynder in subsonic flow. Experimental investigation in precritical and postcritical conditions of the flow has been conducted varying the configurations of the actuators. Drag reduction up to 10% without a complete optimization of the flow control parameters have been obtained

Linearised Reynolds-Averaged predictions of secondary currents in turbulent channels with topographic heterogeneity

A rapid predictive tool based on the linearised Reynolds-averaged Navier-Stokes equations is proposed in this work to investigate secondary currents generated by streamwise-independent surface topography modulations in turbulent channel flow. The tool is derived by coupling the Reynolds-averaged momentum equation to the Spalart-Allmaras transport equation for the turbulent eddy viscosity, using a nonlinear constitutive relation for the Reynolds stresses to capture correctly secondary motions. Linearised equations, describing the steady flow response to arbitrary surface modulations, are derived by assuming that surface modulations are shallow. Since the equations are linear, the superposition principle holds and the flow response induced by an arbitrary modulation can be obtained by combining appropriately the elementary responses obtained over sinusoidal modulations at multiple spanwise length scales. The tool permits a rapid exploration of large parameter spaces characterising structured surface topographies previously examined in the literature. Here, channels with sinusoidal walls and with longitudinal rectangular ridges are considered. For sinusoidal walls, a large response is observed at two spanwise wavelengths scaling in inner and outer units respectively, mirroring the amplification mechanisms in turbulent shear flows observed from transient growth analysis. For longitudinal rectangular ridges, the model suggests that the analysis of the response and the interpretation of the topology of secondary structures is facilitated when the ridge width and the gap between ridges are used instead of other combinations proposed in the literature

Periodic Shadowing Sensitivity Analysis of Chaotic Systems

The sensitivity of long-time averages of a hyperbolic chaotic system to parameter perturbations can be determined using the shadowing direction, the uniformly-bounded-in-time solution of the sensitivity equations. Although its existence is formally guaranteed for certain systems, methods to determine it are hardly available. One practical approach is the Least-Squares Shadowing (LSS) algorithm (Q Wang, SIAM J Numer Anal 52, 156, 2014), whereby the shadowing direction is approximated by the solution of the sensitivity equations with the least square average norm. Here, we present an alternative, potentially simpler shadowing-based algorithm, termed periodic shadowing. The key idea is to obtain a bounded solution of the sensitivity equations by complementing it with periodic boundary conditions in time. We show that this is not only justifiable when the reference trajectory is itself periodic, but also possible and effective for chaotic trajectories. Our error analysis shows that periodic shadowing has the same convergence rates as LSS when the time span $T$ is increased: the sensitivity error first decays as $1/T$ and then, asymptotically as $1/\sqrt{T}$. We demonstrate the approach on the Lorenz equations, and also show that, as $T$ tends to infinity, periodic shadowing sensitivities converge to the same value obtained from long unstable periodic orbits (D Lasagna, SIAM J Appl Dyn Syst 17, 1, 2018) for which there is no shadowing error. Finally, finite-difference approximations of the sensitivity are also examined, and we show that subtle non-hyperbolicity features of the Lorenz system introduce a small, yet systematic, bias

Transitional regime control in a fully developed channel flow

Friction drag reduction is one of the main topic of investigation because of the great beneficial fallout in different engineering field. Laminar transition can be efficiently controlled using suitable flow control techniques [1]. Recent studies focused on the possibility of controlling transitional regimes in wall bounded flow have shown that the generation of streaks with appropriate intensity are able to control the transition in boundary layer. The concept of such control was shown by A1 et al. [2] using small cylinder mounted in the spanwise direction of a flat plate a zero incidence. These devices were able to generate streamwise vortices that in turn gave rise to streaks of well specific amplitude able to attenuate the transition. The paper will present results of an experimental investigation related to the transition control in a fully developed channel flow. For the streaks generation couples of convergent jets are positioned in the spanwise direction. In figure 1 the channel with the injection system are shown

Controlling fluid flows with positive polynomials

A novel nonlinear feedback control design methodology for incompressible fluid flows aiming at the optimisation oflong-time averages of key flow quantities is presented. The key idea, first outlined in Ref. [1], is that the difficulties of treatingand optimising long-time averages are relaxed by shifting the analysis to upper/lower bounds for minimisation/maximisationproblems, respectively. In this setting, control design reduces to finding the polynomial-type state-feedback controller thatoptimises the bound, subject to a polynomial inequality constraint involving the cost function, the nonlinear system, the controlleritself and a tunable polynomial function. A numerically tractable approach, based on Sum-of-Squares of polynomials techniquesand semidefinite programming, is proposed. As a prototypical example of control of separated flows, the mitigation of thefluctuation kinetic energy in the unsteady two-dimensional wake past a circular cylinder at a Reynolds number equal to 100,via controlled angular motions of the surface, is investigated. A compact control-oriented reduced-order model, resolving thelong-term behaviour of the fluid flow and the effects of actuation, is first derived using Proper Orthogonal Decomposition andGalerkin projection. In a full-information setting, linear state-feedback controllers are then designed to reduce the long-timeaverage of the resolved kinetic energy associated to the limit cycle of the system. Controller performance is then assessed indirect numerical simulations