Séminaire de Mécanique d'Orsay

Le Vendredi 25 septembre à 14h00 - Salle de conférences du LIMSI

Transition-delay in 2D boundary layer flows: model-based or adaptive control?

Nicolò Fabbiane
Dpt. of Mechanics, KTH, Sweden

Reducing the energy consumption required for transportation has been one of the main challenges in the past few decades. In this framework, a particular interest has been given to delay the laminar-to-turbulence transition in boundary-layer flows. In a low-turbulence environment, e.g. an airplane in cruise condition, this transition is dominated by local instabilities of the flow - Tollmien-Schlichting (TS) waves - that exponentially grow, eventually breakdown, and lead to turbulence.
Reactive-control techniques are one of the several solutions that have been studied in order to achieve the transition delay. In this scenario, the aim is to remove the disturbances from the flow before they may lead to transition by a system of localised sensors and actuators. Once these devices are fixed, the main effort is to compute the transfer function between those objects, i.e. the control law.
Model-based and adaptive control techniques are two antithetic strategies to approach the control design [1]. The former relies on a model of the flow to guide the actuators' action and it has been shown to be able to effectively delay transition in DNS [2]. This technique may lack of robustness when it comes to experimental investigations or, in general, to cases where an accurate model of the flow is not available. However, adaptive control techniques have been shown to be a possible solution to this problem [3]. In this framework, in fact, the control is not entirely based on a model of the system but also on on-line measurement of the control performance.
This talk will overview advantages and limitations of these two control-design techniques via numerical and experimental results, showing that the adaptive-control approach may be more suitable for real applications where an accurate modelling of the flow is not usually possible.
[1] N. Fabbiane, O. Semeraro, S. Bagheri, and D. S. Henningson (2014) Adaptive and Model-Based Control Theory Applied to Convectively Unstable Flows, Appl. Mech. Rev. 66(6):060801.
[2] O. Semeraro, S. Bagheri, L. Brandt, and D. S. Henningson (2013) Transition delay in a boundary layer flow using active control, J. Fluid Mech. 731 pp 288--311.
[3] N. Fabbiane, B. Simon, F. Fischer, S. Grundmann, S. Bagheri, and D. S. Henningson (2015) On the Role of Adaptivity for Robust Laminar-Flow Control, J. Fluid Mech. 767:R1.