External disturbances such as free-stream turbulence (FST), and isolated three-dimensional roughness are strong disturbance sources to the laminar boundary layers (BLs), which can lead to a rapid transition to turbulence. The transition process eventuates to increase in skin-friction coefficient and heat transfer rate and hence, both of the aforementioned disturbance sources have practical importance. The current thesis is an experimental work, with investigations carried out in low-turbulence wind-tunnels to study the influence of these disturbance sources on boundary layer transition. Today, in FST transition, it is known that the turbulence intensity and the streamwise integral length scale in the free stream are the two influential characteristics that decide the transition onset, location and the extent. Unsteady, elongated streaks in the streamwise direction dominate this scenario, whose amplitudes and spanwise scales are set by the FST conditions prevalent at the leading edge (LE). In reality, a LE is unavoidable and the influence of the inherent LE pressure gradient region on BL transition was always doubted and not investigated in detail. The first part of the current thesis explores the FST transition scenario for a wide range of FST conditions and pressure gradients providing an input to the future transition prediction models. An important result in this thesis is that the entire energy spectrum needs to be known if an accurate prediction of the transition onset is desired, i.e. the LE condition in terms of characteristic length scale and turbulence intensity is not sufficient. In the second part, isolated roughness-induced transition is investigated thoroughly by changing the roughness height in micrometer precision at various diameters. In the previous experimental studies, the investigations were performed by altering the free-stream velocity at a fixed aspect ratio and hence modifying the base flow. In contrast, here, the aspect ratio of the roughness element is altered in an extensive range and the influence of the aspect ratio on the roughness Reynolds number that causes transition is studied without affecting the base flow. Instabilities that occur prior to the transition onset were examined in detail by performing flow visualization experiments. Moreover, interaction of secondary disturbances like Tollmien-Schlichting waves with the roughness was investigated.

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.

Omslaget från laminär till turbulent strömning i gränsskikt är en central process inom strömningsmekaniken med betydande innebörd i tekniska tillämpningar. Typiska exempel på tillämpningar återfinns framför allt inom flygsektorn och inom olika energisystem där omslaget har en stor inverkan på både friktionsmotstånd och värmeöverföring. Gränsskikt och dess omslag till turbulens har varit ett aktivt forskningsområde i över 100 år och därmed har betydande framsteg gjorts för den fysikaliska förståelsen av denna process. Idag vet vi exempelvis att omslaget kan ta olika vägar till ett turbulent tillstånd, men trots många insikter kvarstår betydande utmaningar. Denna avhandling syftar till att belysa några aspekter på omslaget i gränsskikt och dess kontroll.

Vi presenterar en serie experimentella studier med fokus på instabiliteter, omslag och kontroll av laminära gränsskikt. I allmänhet kan man säga att turbulens föregås av flödesinstabiliteter och därför fokuserar vi inledningsvis på tre kända störningar. Vid låg bakgrundstörning domineras omslaget av två-dimensionella vågor, även kallade Tollmien-Schlichting vågor. Om nivån av bakgrundsstörningen ökar kommer omslaget i stället att domineras av tillväxten av hög- och låghastighetsstråk i strömningsriktningen. Den tredje typen av flödesinstabilitet som undersöks här är de strukturer som utvecklas bakom ett isolerat ytråhetselement i gränsskiktet.

Om instabiliteterna i flödet växer över en kritisk amplitud kommer störningarna att initiera omslaget till turbulens. Avhandlingen handlar till stor del om hur friströmsturbulens (hög bakgrundsstörning) påverkar omslaget i gränsskikt. Trots de många tillämpningar som finns, som exempelvis turbomaskiner, så är det fortfarande svårt att förutspå var omslaget sker på idealiserade geometrier. I detta avhandlingsarbete har vi bland annat undersökt effekter som kan påverka denna typ av omslag och som tidigare har förbisetts. Vidare studeras de fenomen som inträffar under omslagsprocessen, såsom uppkomst och utveckling av turbulenta fläckar.

Slutligen utforskar vi passiva kontrollmetoder för att senarelägga omslaget till turbulens. Detta inkluderar att bedöma genomförbarheten av etablerade tekniker för användning i verkliga tillämpningar. Här har virvelgeneratorer för flödeskontroll designats med målet att senarelägga omslaget till turbulens på en flygplanskropp. Dessutom har nya tillvägagångssätt undersökts för att senarelägga omslaget, där den destabiliserande effekten av virvelgeneratorerna i sig har minimerats. Detta kan potentiellt leda till förbättrad kontroll och fördröjning av omslaget.