Experimental studies on laminar boundary layers: instability, transition and control
Time: Thu 2024-12-19 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Language: English
Subject area: Engineering Mechanics
Doctoral student: André Weingärtner , Strömningsmekanik
Opponent: Prof. Edward White, The University of Texas at Dallas
Supervisor: Prof. Jens H. M. Fransson, Strömningsmekanik; Prof. Ramis Örlü, Strömningsmekanik, Department of Mechanical, Electrical and Chemical Engineering, OsloMet – Oslo Metropolitan University
QC241204
Abstract
The laminar-turbulent transition in boundary layers is a pivotal process in fluid dynamics, with significant implications for engineering applications. Typical examples are aviation and energy systems, where this process has a tremendous impact on large-scale effects like friction drag and heat transfer. As such, boundary layers and their transition to turbulence have been an active topic of research for more than 100 years, making considerable progress in understanding the underlying physics. For example, we know today that the transition process is not unique. However, even though many phenomena are now understood, significant challenges remain. The present thesis aims to shed light on some aspects of boundary layer transition and control thereof.
We present a series of experimental studies focusing on the instability, transition, and control of laminar boundary layers. Flow instabilities are generally the precursor of turbulence and are therefore studied first, where three phenomena are investigated separately. Natural transition of a boundary layer often occurs due to two-dimensional Tollmien-Schlichting waves. In higher background disturbance flows, the dominating instability in the flow are streamwise streaky structures that govern the transition to turbulence. The third type of investigated flow instability are the structures that develop in the boundary layer behind an isolated roughness element.
If the instabilities in the flow grow beyond a critical amplitude, they will break down to turbulence. A considerable section of the thesis is dedicated to boundary-layer transition under free-stream turbulence. Even though this topic is of great interest for applications such as turbomachinery, it is still not possible to accurately predict the location where transition happens for given flow conditions, even for strongly simplified cases like flat plates. Amongst other topics, we investigate effects that might influence this transition process and have been overlooked previously. Furthermore, phenomena that happen during the transition process, such as the emergence and development of turbulent spots, are studied under various free-stream turbulence conditions.
Finally, we explore passive control strategies to delay the transition to turbulence. This includes the assessment of the feasibility of established techniques to use in realistic engineering applications. Here, flow control devices are designed with the goal to delay transition on the fuselage of an aircraft. On the other hand, new approaches to accomplish transition delay are investigated, where the unfavorable direct disturbance of the boundary layer is minimized, leading to potentially improved transition delay.