For over 100 years, researchers have attempted to predict transition to turbulence in fluid flows by analyzing the spectrum of the linearized Navier-Stokes equations. However, for many simple flows, this approach has failed to match experimental results. Recently, new scenarios for transition have been proposed that are based on the non-normality of the linearized operator. These new "mostly linear" theories have increased our understanding of the transition process, but the role of nonlinearity has not been explored. The main goal of this work is to begin to study the role of nonlinearity in transition. We use model problems to illustrate that small unmodeled disturbances can cause transition through movement or bifurcation of equilibria. We also demonstrate that small wall roughness can lead to transition by causing the linearized system to become unstable. Sensitivity methods are used to obtain important information about the disturbed problem and to illustrate that it is possible to have a precursor to predict transition. Finally, we apply linear feedback control to the model problems to illustrate the power of feedback to delay transition and even relaminarize fully developed chaotic flows. / Ph. D.

While research studies have investigated postsecondary planning for high-need student populations, few studies have explored the intersectionality of students experiencing emotional disturbances and students living in urban environments and the postsecondary planning perspectives of school counselors with this demographic. The purpose of this study was to explore the current perspectives, practices, and experiences of 10 high schools counselors working within a large school district of the northeastern U.S. on postsecondary planning with students experiencing emotional disturbances in urban environments. An ecological perspective framed the discussion to provide a holistic picture of the postsecondary needs of adolescents experiencing emotional disturbances in urban environments. Using a constructivist grounded theory qualitative approach; themes emerged and provided a voice to urban high school counselors on the concepts related to the postsecondary planning process. Two primary themes were attributed to the postsecondary planning perspectives of urban high school counselors with each theme including three sub-themes. Elements hindering postsecondary planning included (1) school resources, (2) poor parent engagement, and (3) higher-level needs; while elements supporting postsecondary planning incorporated (1) school personnel, (2) high parent engagement, and (3) computer-based planning resources. Three themes were linked to the postsecondary planning practices of urban high school counselors: (1) college campus connection, (2) information dissemination, and (3) student-counselor connection. Three themes were also associated with the postsecondary planning experiences of urban high school counselors: (1) learning from mistakes, (2) planning multiple options, and (3) similar planning for all students. These eight primary themes and six sub-themes provide evidence that offers a greater understanding of the postsecondary planning process for students experiencing emotional disturbances in urban environments. Implications for school counselors, counselor educators, and higher education support staff are presented. Study limitations are discussed and recommendations for future research ideas are suggested. By conducting research on this underserved student population, the emerging themes intend to create more equitable postsecondary planning procedures for students experiencing emotional disturbances in urban environments / Ed. D.

Large-scale atmospheric disturbances play important roles in determining the general circulation of the atmosphere during the North Pacific boreal winter. A number of scientific questions have been raised due to these disturbances’ spatial and temporal complexity as well as the hydrological implication associated with them. In this dissertation, the principal goal is to further improve our understanding of the atmospheric high frequency (HF) and intermediate frequency (IF) disturbances active over the North Pacific. The study focuses on their energetics, intraseasonal and interannual variability, and the resulting hydrological impact over the eastern North Pacific and Western U.S. including extreme events. To delineate the characteristics of HF and IF disturbances in the troposphere, we first derive a new set of equations governing the local eddy kinetic energy (EKE), and assess the critical processes maintaining local budgets of the HF and IF EKE. The diagnosis assesses the 3-D patterns of energy flux convergence (EFC), barotropic conversion (BT), baroclinic conversion (BC), and cross-frequency eddy-eddy interaction (CFEI). The local EKE budget analysis is followed by an investigation of the modulation of HF and IF eddy activity by different modes of low frequency climate variability. On interannual timescales, the response of various local energetic processes to El Niño-Southern Oscillation (ENSO) determines the HF and IF EKE anomalies and the role of CFEI process is important in producing these anomalies. Also on interannual timescales, winter precipitation deficits associated with suppressed cyclonic activity, i.e., negative HF EKE anomalies, are linked to severe droughts over the U.S. Southern Great Plain (SGP) region. The suppressed cyclonic activity is, in turn, tied to phase changes in the West Pacific (WP) teleconnection pattern. On intraseasonal timescales, variations in HF disturbances (a.k.a. storm tracks) over the North Pacific are closely coupled with tropical convection anomalies induced by the Madden-Julian Oscillation (MJO), and partly drive larger scale intraseasonal flow anomalies in this region through eddy-eddy interactions. Anomalous HF eddy activity induces subseasonal transitions between “wet” and “dry” regimes over the west coast of North America. Also on intraseasonal timescales, the East Asian cold surge (EACS) is found to provide a remote forcing of the winter precipitation anomalies in the western U.S. This modulation is achieved through “atmospheric rivers” (ARs), which are narrow channels of concentrated moisture transport in the atmosphere and are responsible for over 70% of the extreme precipitation events in the western U.S.. EACS effectively modulates the IF disturbance activity over the North Pacific, and the anomalous IF disturbances lead to the formation of an AR over the eastern North Pacific that ultimately induces precipitation anomalies in the western U.S. Analyses of the simulations from the NCAR Community Climate System Model version 4 (CCSM4) demonstrate that the connections among the EACS, AR and western U.S. precipitation are better captured by a model with higher spatial resolutions. The improved simulation of these connections is achieved mainly through a better representation of the IF disturbances, and the associated scale-interaction processes in the higher resolution model.

Ziel der Untersuchung war die Ermittlung der Prävalenz schlafbezogener Atmungsstörungen (SBAS) in einer Gruppe von Herzschrittmacherpatienten. Durch Gegenüberstellung mit der in der Literatur angegebenen Prävalenz SBAS in vergleichbaren Patientengruppen ohne Herzschrittmacher sollte eine Aussage über einen möglichen Zusammenhang zwischen SBAS und schrittmacherbedürftigen Herzrhythmusstörungen getroffen werden. Dieser Zusammenhang könnte indirekt über die bekannte Koinzidenz zwischen SBAS und arterieller Hypertonie bzw. KHK bestehen. Es wurden 192 Patienten (100 Männer, 92 Frauen) der Rhythmussprechstunde des Universitätsklinikums Charité in Berlin mit einem Durchschnittsalter von 62,2 +/- 12,2 Jahren ambulant für eine Nacht untersucht. Zum Einsatz kam das tragbare Schlafapnoe-Monitoring-Gerät MESAM IV. Schlafbezogene Atmungsstörungen lagen dann vor, wenn der respiratorische Gesamtindex (RDI = respiratory disturbance index) als Summe der Apnoen und Hypopnoen pro Stunde Schlaf größer oder gleich 10 war. Anhand des RDI erfolgte eine Einteilung der Patienten in Diagnosegruppen: Gruppe A (RDI < 5, kein Vorliegen SBAS), Gruppe B (RDI >= 5 und < 10, Grenzbefund) und Gruppe C (RDI >= 10, Vorliegen SBAS). Ergebnisse (Gesamtkollektiv / Männer / Frauen): Gruppe A: 100 / 44 / 56 Patienten (52,1 / 44,0 / 60,9 %) Gruppe B: 30 / 16 / 14 Patienten (15,6 / 16,0 / 15,2 %) Gruppe C: 62 / 40 / 22 Patienten (32,3 / 40,0 / 23,9 %). Signifikant häufiger wurden SBAS (Gruppe C) bei Männern als bei Frauen nachgewiesen. Ebenso liegen das Durchschnittsalter (Gruppe A: 58,5 +/- 14,5 Jahre, Gruppe C 67,3 +/- 6,8 Jahre) sowie der durchschnittliche Body-Mass-Index (Gruppe A: 24,8 +/- 3,4 kg/m2, Gruppe C 27,1 +/- 3,3 kg/m2) bei Patienten mit SBAS signifikant höher als bei Patienten ohne SBAS. Patienten mit unterschiedlichen, der Schrittmacherimplantation zugrundeliegenden, Herzerkrankungen (z. B. Sick-Sinus-Syndrom, AV-Block, Bradyarrhythmia absoluta) wiesen keine signifikanten Differenzen in der Häufigkeit SBAS auf. Gleichermaßen konnte das Vorliegen zusätzlicher kardiovaskulärer Erkrankungen und Risikofaktoren (z. B. KHK, arterielle Hypertonie, Rauchen) nicht als Einflußfaktor für das gehäufte Auftreten SBAS nachgewiesen werden. Angaben zur Prävalenz SBAS in der Literatur schwanken für die entsprechende Altersgruppe von Patienten ohne Herzschrittmacher zwischen 24 % und 73 %. Die für die Herzschrittmacherpatienten ermittelte Prävalenz der SBAS mit 32,3 % liegt innerhalb dieser großen Spannbreite. Die Untersuchungsergebnisse ergaben daher keinen Zusammenhang zwischen SBAS und schrittmacherbedürftigen Herzrhythmusstörungen. Auch konnte kein Unterschied im Auftreten SBAS in Abhängigkeit von der Art der Herzrhythmusstörung nachgewiesen werden. Weiterführende Untersuchungen (z. B. in Form einer Fall-Kontroll-Studie) erscheinen notwendig. / The study was aimed to determine prevalence of sleep-related respiratory disturbances (SRRD) in a group of pacemaker patients. It was investigated if SRRD are more prevalent in pacemaker patients compared to corresponding patients without pacemaker. This question was raised in background of the wellknown coincidence of SRRD and arterial hypertension respectively coronary heart disease. References about the prevalence of SRRD in corresponding elderly people served as standard of comparison. 192 hospital outpatients (100 men and 92 women) from rhythmological ambulance of university hospital "Charité" in Berlin were randomly selected and examined for one night by means of MESAM IV device, a Non-laboratory-monitoring-system". The mean age of these patients was 62,2 +/- 12,2 years. A respiratory disturbance index (RDI; the sum of apneas and hypopneas per hour of sleep) equal or greater than 10 was laid down for the existence of SRRD. The RDI built the basis for dividing the patients into three groups: Group A (RDI < 5, no SRRD), Group B (RDI >= 5 and < 10, borderline SRRD) an Group C (RDI >= 10, SRRD). Results (all patients / men / women): Group A: 100 / 44 / 56 patients (52,1 / 44,0 / 60,9 %) Group B: 30 / 16 / 14 patients (15,6 / 16,0 / 15,2 %) Group C: 62 / 40 / 22 patients (32,3 / 40,0 / 23,9 %). Significant differences occured in the frequency of SRRD (Group C) between men and women. Apart from that the mean age (Group A: 58,5 +/- 14,5 years, Group C: 67,3 +/- 6,8 years) as well as the Body-Mass-Index (Group A: 24,8 +/- 3,4 kg/m2, Group C: 27,1 +/. 3,3 kg/m2) were significantly higher in patients with SRRD than in patients without SRRD. No significant differences in the frequency of SRRD could be demonstrated in dependence on underlying rhythm disturbances (e. g. Sick-Sinus-Syndrom, atrioventricular block, Bradyarrhythmia absoluta). In the same manner no evidences regarding significant influences of additionally existing cardiovascular diseases or risk factors (e. g. coronary heart disease, arterial hypertension, smoking) on the appearance of SRRD were shown. References about the prevalence of SRRD in corresponding elderly people without pacemaker vary from 24 % to 73 %. The prevalence of SRRD in pacemaker patients (32,3 %) fits right in the frame of this great variety. A more frequent appearance of SRRD in patients with pacemaker than in patients without could not be proved. Equally, the underlying rhythm disturbance has apparently no correlation to SRRD. Extended studies are necessary.

香港的山火主要是由於氣候及人類活動所引發，嚴重威脅著一些陸地上生物多樣性豐富的生境（如灌木林和林地）。在華南地區，有關火燒地的動物群落恢復生態學硏究的資料非常少，所以本硏究旨在調查山火後動物群落的復原，以及比較天然復原的林區及人工植林的生境恢復之差異。此外，枯枝落葉、物理因素及山火後的微生物對動物群落的影響亦會在此硏究中詳细分析。 / 在亞熱帶地區，地棲無脊椎動物群落的復原速度相對為慢，例如螞蟻群落需要12年的時間才能復原，但人工種植於火燒地能成功地加快其復原速度。 / 無脊椎動物群落與植被結構、山火之後的時間及生境發展都有一定的關係。山火發生兩年後火燒地的鳥類和無脊椎動物的數量、種或科的豐富度、單一性、多樣性及所有無脊椎動物的生物量都比參考樣地低，這意味著山火對動物群落有著長遠的影響。Pheidole spp. 及Tapinoma sp. 1 屬的螞蟻、隠翅甲科及小蠹科的甲蟲、管巢蛛科及狼蛛科的蜘蛛，牠們的數量，以及數量與生物量之比 (ABC plot) 都能用作反映火燒地的生態復原及演替進度的指標。 / 人工種植能夠使生態系統得到長遠的持續建立，對於動物群落的恢復非常重要。是次研究結果顯示人工種植能促進本地無脊椎動物的建群，其數量和群落亦會隨時間增加而增加。這很可能是由於山火後人工種植能為無脊椎動物創造較好的生存條件。其中，相對於原生樹種，外來樹種更有利於無脊椎動物的建群。在外來品種的人工種植區，微生物的數量較低，枯枝落葉的被分解速度較慢，加上生長速度快的外來品種能製造大量的枯枝落葉，這些都顯著增加了枯葉層的深度。較深的枯葉層為無脊椎動物提供了適宜的微生境，有利於其建群。 / 總的來說，在火燒地進行人工植樹能加快動物在火燒地的復原速度，這種方法是有效且成功的。人工種植時採用適當的品種能夠促進及預測早期的植被演替。根據此次硏究結果，我們建議在火燒地上種植生長快速的外來樹種，以加快無脊椎動物的建群。植被蓋度對動物的建群很重要，但植物和結構的多樣性及枯枝落葉層對無脊椎動物的建群更為重要。 / Hill fire in Hong Kong is mainly caused by a combination of weather and human activities. Fire is a major threat to terrestrial animal communities especially in shrublands and woodlands which have a rich faunal diversity. Since research done on hill fire ecology in southern China, especially on faunal colonization after fire, is very scarce, this research attempted to investigate the recovery of fauna after fire disturbance, and the difference in recovery between natural regeneration and artificial planting. Besides, the effects of litter and physical parameters on fauna community as well as the effect of fire on soil microbial community structure were studied. / The recolonization of ground-dwelling invertebrates was relatively slow in the study area, while proactive planting has been successful because they have considerably accelerated the return of fauna at the fire-affected sites. Ground-dwelling invertebrate communities appeared to be less resilient to fire in subtropical environment; the ant composition in the experimental plots needed 12 years to recover. / The composition of the ground-dwelling communities differed among study sites and was correlated with vegetation structure and time since last fire. They showed the trends in the development of invertebrate community composition in association with habitat development. Abundance, species/family richness, evenness, diversity of birds and ground-dwelling invertebrates, and biomass of total invertebrates were lower on the burned site than the reference site even 2 years after fire, implying that fire had longer-term impacts on the faunal communities. Ants such as Pheidole spp., Tapinoma sp. 1, beetles such as Staphylinidae, Scolytidae, and spiders such as Clubionidae, Lycosidae, as well as the abundance biomass comparison (ABC plot) can be chosen to reflect the situation or the recovery process and succession of the fire-affected sites. / The establishment of fauna in proactive planting is vital if ecosystem functions are to be reestablished and restorations to be sustained in the long term. In general the invertebrate data suggest that restoration plantings facilitated the reestablishment of indigenous invertebrate species and invertebrate community changed with time. Proactive plantings created better conditions for ground-dwelling invertebrates after fire, and exotic species with faster growth rate seem to be better than the natives. In exotic plantations, lower decomposition rate of litter provided by the lower microbial population together with higher productivity of exotic species of plants caused a higher litter depth, which provided more microhabitats for ground-dwelling and litter-dwelling invertebrates than the native plantations. / Correct species selection in proactive planting is essential to facilitate and determine the early stages of plant succession. The present study suggests that initial floristic composition is significant in proactive plantings and exotic species with fast growth rate is an effective planting species for invertebrate recolonization. Although plant cover is of importance to developing fauna, the provision of adequate floristic and structural diversity and the presence of logs and litter are important for the full range of native invertebrate recolonization. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Cheung, Kwok Leung. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 192-219). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese; some appendixes also in Chinese. / Abstract --- p.i / Acknowledgements --- p.vi / Table of contents --- p.vii / List of figures --- p.x / List of plates --- p.xvi / List of appendices --- p.xvii / Chapter Chapter 1 --- General Introduction --- p.1 / Chapter 1.1 --- Ecology of Fire --- p.1 / Chapter 1.2 --- Effects of Hill Fire on Soil --- p.3 / Chapter 1.3 --- Effects of Hill Fire on Microbial Communities --- p.7 / Chapter 1.4 --- Effects of Hill Fire on Vegetation --- p.10 / Chapter 1.5 --- Effects of Hill Fire on Fauna Communities --- p.14 / Chapter 1.6 --- The Use of Bioindicators on Restoration Ecology --- p.19 / Chapter 1.7 --- Hill Fire Situation and Research in Hong Kong --- p.23 / Chapter 1.8 --- Study Objectives --- p.28 / Chapter Chapter 2 --- Faunal Colonization after Fire Disturbance --- p.30 / Chapter 2.1 --- Introduction --- p.30 / Chapter 2.2 --- Materials and Methods --- p.32 / Chapter 2.2.1 --- Study sites --- p.32 / Chapter 2.2.2 --- Measurements of soil physicochemical parameters --- p.35 / Chapter 2.2.3 --- Determination of vegetation parameters --- p.36 / Chapter 2.2.4 --- Monitoring of vertebrate communities --- p.36 / Chapter 2.2.5 --- Sampling, sorting and identification of invertebrates --- p.37 / Chapter 2.2.6 --- Data analysis --- p.38 / Chapter 2.3 --- Results --- p.41 / Chapter 2.3.1 --- Soil physicochemical characteristics --- p.41 / Chapter 2.3.2 --- Vegetation parameters --- p.42 / Chapter 2.3.3 --- Vertebrate communities --- p.44 / Chapter 2.3.4 --- Invertebrate communities --- p.47 / Chapter 2.4 --- Discussion --- p.58 / Chapter 2.5 --- Conclusions --- p.66 / Chapter Chapter 3 --- Effect of Different Planting Approaches on Invertebrates --- p.68 / Chapter 3.1 --- Introduction --- p.68 / Chapter 3.2 --- Materials and Methods --- p.70 / Chapter 3.2.1 --- Study sites --- p.70 / Chapter 3.2.2 --- Proactive planting --- p.71 / Chapter 3.2.3 --- Measurement of soil, vegetation, and faunal parameters, and data analysis --- p.73 / Chapter 3.3 --- Results --- p.75 / Chapter 3.3.1 --- Soil physicochemical parameters --- p.75 / Chapter 3.3.2 --- Vegetation parameters --- p.77 / Chapter 3.3.3 --- Invertebrate communities --- p.78 / Chapter 3.4 --- Discussion --- p.92 / Chapter 3.5 --- Conclusions --- p.98 / Chapter Chapter 4 --- Long-term Changes of Invertebrate Community after Hill Fire by a Retrospective Approach --- p.100 / Chapter 4.1 --- Introduction --- p.100 / Chapter 4.2 --- Materials and Methods --- p.102 / Chapter 4.2.1 --- Study sites --- p.102 / Chapter 4.2.2 --- Measurement of soil physicochemical parameters --- p.102 / Chapter 4.2.3 --- Determination of vegetation parameters --- p.103 / Chapter 4.2.4 --- Collection, sorting and identification of invertebrates --- p.104 / Chapter 4.2.5 --- Data analysis --- p.104 / Chapter 4.3 --- Results --- p.105 / Chapter 4.3.1 --- Soil physicochemical characteristics --- p.105 / Chapter 4.3.2 --- Vegetation parameters --- p.106 / Chapter 4.3.3 --- Invertebrate communities --- p.110 / Chapter 4.4 --- Discussion --- p.123 / Chapter 4.5 --- Conclusions --- p.133 / Chapter Chapter 5 --- Relationship of Microbial Community with Recolonization of Invertebrates --- p.135 / Chapter 5.1 --- Introduction --- p.135 / Chapter 5.2 --- Materials and Methods --- p.139 / Chapter 5.2.1 --- Study sites --- p.139 / Chapter 5.2.2 --- Microbial biomass carbon (C[subscript mic]) and nitrogen (N[subscript mic]) --- p.139 / Chapter 5.2.3 --- Fatty acid methyl esters (FAME) extraction --- p.140 / Chapter 5.2.4 --- Data analysis --- p.141 / Chapter 5.3 --- Results --- p.142 / Chapter 5.3.1 --- Fumigation-extraction --- p.142 / Chapter 5.3.2 --- Fatty acid methyl esters --- p.144 / Chapter 5.3.3 --- Relationship of microbial communities with physicochemical, vegetation and invertebrate parameters --- p.145 / Chapter 5.4 --- Discussion --- p.152 / Chapter 5.5 --- Conclusions --- p.159 / Chapter Chapter 6 --- Effects of Critical Factors on Invertebrate Communities --- p.160 / Chapter 6.1 --- Introduction --- p.160 / Chapter 6.2 --- Materials and Methods --- p.162 / Chapter 6.2.1 --- Experimental sites --- p.162 / Chapter 6.2.2 --- Critical factors selection and simulation --- p.163 / Chapter 6.2.3 --- Measurement of microclimatic characteristics --- p.165 / Chapter 6.2.4 --- Collection, sorting and identification of invertebrates --- p.166 / Chapter 6.2.5 --- Data analysis --- p.166 / Chapter 6.3 --- Results --- p.167 / Chapter 6.3.1 --- Microclimatic characteristics --- p.167 / Chapter 6.3.2 --- Invertebrate communities --- p.168 / Chapter 6.4 --- Discussion --- p.173 / Chapter 6.5 --- Conclusions --- p.178 / Chapter Chapter 7 --- General Conclusions --- p.180 / Chapter 7.1 --- Overall Discussion --- p.180 / Chapter 7.2 --- Limitation of the Present Study and Future Research --- p.184 / Chapter 7.3 --- Contributions to Our Knowledge --- p.188 / Reference --- p.192 / Appendices --- p.220

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 160-168).

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This research reports on the use of the resilience assessment approach to evaluate the social-ecological systems (SES) at the NWU (Vaal) for a nine month period from March 2013 to November 2013. The research aims to determine the merits of the use of a resilience assessment approach to determine the resilience of the NWU (Vaal) in respect of social-ecological systems. Objectives of the research include the identification of disturbing events to environmental resilience, to construct a conceptual social-ecological systems model of the NWU (Vaal) in order to determine thresholds relevant to resilience and to determine if the resilience assessment approach can be applied at the NWU (Vaal). Data was collected in the form of a literature review, which aided with the construction of a time line for the focal social-ecological system; document reviews, interviews with relevant role players, observation by the researcher and the use of the assessment approach to construct the social-ecological system model. The research findings include identification of possible disturbances and thresholds relevant to resilience of the social-ecological system of the NWU (Vaal). The construction of a social-ecological system model of the NWU (Vaal) aids with proposals to incorporate a stewardship approach which enhance resilience of the focal social-ecological system. / M (Environmental Management), North-West University, Potchefstroom Campus, 2015

This research reports on the use of the resilience assessment approach to evaluate the social-ecological systems (SES) at the NWU (Vaal) for a nine month period from March 2013 to November 2013. The research aims to determine the merits of the use of a resilience assessment approach to determine the resilience of the NWU (Vaal) in respect of social-ecological systems. Objectives of the research include the identification of disturbing events to environmental resilience, to construct a conceptual social-ecological systems model of the NWU (Vaal) in order to determine thresholds relevant to resilience and to determine if the resilience assessment approach can be applied at the NWU (Vaal). Data was collected in the form of a literature review, which aided with the construction of a time line for the focal social-ecological system; document reviews, interviews with relevant role players, observation by the researcher and the use of the assessment approach to construct the social-ecological system model. The research findings include identification of possible disturbances and thresholds relevant to resilience of the social-ecological system of the NWU (Vaal). The construction of a social-ecological system model of the NWU (Vaal) aids with proposals to incorporate a stewardship approach which enhance resilience of the focal social-ecological system. / M (Environmental Management), North-West University, Potchefstroom Campus, 2015

<p>Both optimal disturbances and optimal control are studied by means of numerical simulations for the case of the flat-plate boundary-layer flow. The optimisation method is the Lagrange multiplier technique where the objective function is the kinetic energy of the flow perturbations and the constraints involve the linearised Navier–Stokes equations. We consider both the optimal initial condition leading to the largest growth at finite times and the optimal time-periodic forcing leading to the largest asymptotic response. The optimal disturbances for spanwise wavelengths of the order of the boundary layer thickness are streamwise vortices exploiting the lift-up mechanism to create streaks. For long spanwise wavelengths it is the Orr mechanism combined with the amplification of oblique wave packets that is responsible for the disturbance growth. Control is applied to the bypass-transition scenario with high levels of free-stream turbulence. In this scenario low frequency perturbations enter the boundary layer and streamwise elongated disturbances emerge due to the non-modal growth. These so-called streaks are growing in amplitude until they reach high enough energy levels and breakdown into turbulent spots via their secondary instability. When control is applied in the form of wall blowing and suction, within the region that it is active, the growth of the streaks is delayed, which implies a delay of the whole transition process. Additionally, a comparison with experimental work is performed demonstrating a remarkable agreement in the disturbance attenuation once the differences between the numerical and experimental setup are reduced.</p><p> </p><p> </p>