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Numerical investigations of bypass transition and its control

Time: Fri 2024-06-14 10.00

Location: U1, Brinellvägen 26, Stockholm

Language: English

Subject area: Engineering Mechanics

Doctoral student: José Manuel Faúndez Alarcón , Linné Flow Center, FLOW, Teknisk mekanik

Opponent: Professore Ordinario Flavio Giannetti,

Supervisor: Professor Dan S. Henningson, Linné Flow Center, FLOW; Docent Ardeshir Hanifi, Linné Flow Center, FLOW

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This thesis deals with the laminar-turbulent transition process in boundary layers induced by free-stream turbulence (FST), commonly referred to as bypass transition. The investigation has been carried out using direct numerical simulations (DNS), stability analysis, and control theory. The various aspects of bypass transition considered in this work can be grouped into two categories: open and closed-loop dynamics. 

The open-loop dynamics span from the inception to the breakdown of instabilities, driving the flow from a laminar to a turbulent state. A broader understanding of this process could inspire new and more accurate models for transition prediction, which is of great interest in many engineering applications. In this context, stability theory provides an excellent framework to study the pre-transitional flow. This work has confirmed the relevance of optimal disturbance theory in realistic flow conditions, and how its inexpensive computations can provide valuable information regarding the most 'dangerous' disturbances in terms of their amplification. The key role of streak secondary instabilities in bypass transition has also been studied. They constitute the main cause of transition in a flat plate simulation considering realistic wind tunnel conditions. By comparing the secondary instabilities leading to breakdown in different geometries and FST compositions, it has been found that their hosting streaks feature similar aspect ratios, regardless of their streamwise position. An explanation for this apparent size preference has been provided based on optimal growth and energy propagation due to non-linear interactions.

The closed-loop dynamics address how new inputs can steer the system to a desired state based on operational information extracted from the system. In boundary layers, delaying transition is an attractive idea for energy savings due to the lower drag associated with a laminar state. This work explores this possibility with the use of control theory in reduced-order models constructed solely on input/output data from DNS. The methods are restricted to being equally feasible in experiments. Here, streak attenuation is successfully achieved based only on wall measurements and wall localised actuation. It has been shown that the dissimilar performances regarding transition delay are connected to the controller's capabilities of acting on breaking streaks.