Accretion disks are ubiquitous in astrophysics. They are found in active galactic nuclei, around newly formed stars, around compact stellar objects, like black holes, neutron stars etc. When the ambient matter with sufficient initial angular momentum falls towards a central massive object, forming a disk shaped astrophysical structure, it is called an accretion disk. There are both ionized and neutral disks depending on their temperatures. Generally, in accretion disks, Gravitational force is balanced by the centrifugal force (due to the presence of angular momentum of the matter) and the forces due to gas pressure, radiation pressure and advection. Now, the matter to be accreted needs to lose angular momentum. For most of the accretion disks, the mass of the central object is much higher than the mass of the disk, giving rise to a dynamics governed by a central force. Therefore we can neglect the effect of self-gravity of the disk. Balancing the Newtonian gravitational force and centrifugal force leads to a Keplerian rotation profile of the accreting matter with the angular velocity ∼ r−3/2, where r is the distance from the central object. The Keplerian disk model is extremely useful to explain several flow classes (e.g. emission of soft X-ray in disks around stellar mass black holes). Due to the presence of differential rotation and hence shear viscosity, the matter can slowly lose its angular momentum and falls towards the central object. In this way, the accreting matter in the disk releases its gravitational potential energy and gives rise to luminosity that we observe. However, the molecular viscosity originated from the microscopic physics (due to the collisions between molecules) of the disk matter is not sufficient to explain the observed luminosity or accretion rate. For example, it can be shown that the temperature arisen from the dissipation of energy due to molecular viscosity (which is around 50000K for optical depth τ = 100) is much less than the temperature observed in these systems (around 107K). In my thesis, I have addressed the famous problem of infall of matter in astrophysical accretion disks. In general, the emphasis is given on the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, which are extensively seen in astrophysics, are Rayleigh-stable, but must be turbulent in order to explain observed data (observed temperature, as described above). Since the molecular viscosity is negligible in these systems, for a very large astrophysical length scale, Shakura and Sunyaev argued for turbulent viscosity for energy dissipation and hence to explain the infall of matter towards the central object. This idea is particularly attractive because of its high Reynolds number (Re ∼ 1014). However, the Keplerian disks, which are relevant to many astrophysical applications, are remarkably Rayleigh stable. Therefore, linear perturbation apparently cannot induce the onset of turbulence, and consequently cannot provide enough viscosity to transport matter inwards. The primary theme of my thesis is, how these accretion disks can be made turbulent in the first place to give rise to turbulent viscosity. With the application of Magnetorotational Instability (MRI) to Keplerian disks, Balbus and Hawley showed that initial seed, weak magnetic fields can lead to the velocity and magnetic field perturbations growing exponentially. Within a few rotation times, such exponential growth could reveal the onset of turbulence. Since then, MRI has been a widely accepted mechanism to explain origin of instability and hence transport of matter in accretion disks. Note that for flows having strong magnetic fields, where the magnetic field is tightly coupled with the flow, MRI is not expected to work. Hence, it is very clear that the MRI is bounded in a small regime of parameter values when the field is also weak. It has been well established by several works that transient growth (TG) can reveal nonlinearity and transition to turbulence at a sub-critical Re. Such a sub-critical transition to turbulence was invoked to explain colder, purely hydrodynamic accretion flows, e.g. quiescent cataclysmic variables, proto-planetary and star-forming disks, the outer region of the disks in active galactic nuclei etc. Baroclinic instability is another plausible source for vigorous turbulence in colder accretion disks. Note that while hotter flows are expected to be ionized enough to produce weak magnetic fields therein and subsequent MRI, colder flows may remain to be practically neutral in charge and hence any instability and turbulence therein must be hydrodynamic. However, in the absence of magnetic effects, the Coriolis force does not allow any significant TG in accretion disks in three dimensions, independent of Re, while in pure two dimensions, TG could be large at large Re. However, a pure two-dimensional flow is a very idealistic case. Nevertheless, in the presence of magnetic field, even in three dimensions, TG could be very large (Coriolis effects could not suppress the growth). Hence, in a real three-dimensional flow, it is very important to explore magnetic TG. However, as mentioned above, the charge neutral Rayleigh-stable astrophysical flows have hardly any magnetic field (e.g. protoplanetary disks, quiescent cataclysmic variables etc.). Also, the hydrodynamic Rayleigh-stable Taylor-Couette flows and plane Couette flows in the laboratory experiments are seen to be turbulent without the presence of any magnetic field, while they are shown to be stable in linear stability analysis. It is a century old unsolved problem to explain hydrodynamically, the linear instability of Couette flows and other Rayleigh-stable Flows, which are observed to be turbulent, starting from laboratory experiments to astrophysical observations. Therefore, as in one hand, the hydrodynamic instability of the astrophysical accretion flows and laboratory shear flows (e.g. Rayleighstable Taylor-Couette flow, plane Couette flow etc.) has to be understood, on the other hand, the magnetohydrodynamic (MHD) instability of the hotter flows has also to be investigated to understand the nature of MHD instability clearly, whether it arises due to MRI or TG. I have investigated the effect of stochastic noise (which is generated by the shearing motion of the disk layers) on the hydrodynamics and magnetohydrodynamics of accretion disks and explain how stochastic noise can make accretion Disks turbulent. It is found that such stochastically driven flows exhibit large temporal and spatial correlations of perturbations, and hence large energy dissipations of perturbation with time, which presumably generates instability and turbulence. I have also given in my thesis, a plausible resolution of the hydrodynamic turbulence problem of the accretion flows and laboratory shear flows (as discussed above) from pure hydrodynamics, invoking the idea of Brownian motion of particles. I have shown that in any shear flow, very likely, the stochastic noise is generated due to thermal fluctuations. Therefore, the shear flows must be studied including the effect of stochastically driving force and hence the governing equations should not be deterministic. Incorporating the effects of noise in the study of the above mentioned shear flows, I have shown in my thesis that hydrodynamic Rayleigh-stable flows and plane Couette flows can be linearly unstable. I have also investigated the importance of transient growth over magnetorotational instability (MRI) to produce turbulence in accretion disks. Balbus and Hawley asserted that the MRI is the fastest weak field instability in accretion disks. However, they used only the plane wave perturbations to study the instability problem. I have shown that for the flows with high Reynolds number, which are indeed the case for astrophysical accretion disks, transient growth can make the system nonlinear much faster than MRI and can be a plausible primary source of turbulence, using the shearing mode perturbations. Therefore, this thesis provides a plausible resolution of hydrodynamic turbulence observed in astrophysical accretion disks and some laboratory shear flows, such as, Rayleigh-stable Taylor-Couette flows and plane Couette flows. Moreover, this thesis also provides a clear understanding of MHD turbulence for astrophysical accretion disks.

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Les performances en mathématiques sont associées à de nombreux enjeux dans notre société, des enjeux de nature individuelle et des enjeux de nature sociétale. Malgré le fait que les élèves québécois réussissent très bien sur la scène internationale en mathématiques, dans le cadre des évaluations à grande échelle en éducation, les analyses selon le genre, elles, font état d’écarts préoccupants. En effet, à plusieurs des cycles de ces grandes enquêtes, les filles ont obtenu des résultats statistiquement inférieurs à ceux des garçons. L’anxiété mathématique, pourrait expliquer, à tout le moins en partie, les écarts de performances observés entre les filles et les garçons (Stoet et al., 2016). En effet, les recherches menées sur le sujet depuis les années 70 révèlent que, de manière générale, chez les adolescents et les adultes, les filles ont tendance à se dire davantage anxieuses à l’égard des mathématiques que les garçons (p.ex. Else-Quest et al., 2010; Hyde et al., 1990; Stoet et al. 2016). Également, dans ces mêmes groupes d’âge, une corrélation linéaire négative est observée entre le niveau d’anxiété mathématique des individus et leurs performances en mathématiques (p.ex. Barroso et al.,2021; Hembree, 1990; Ma, 1999; Zhang et al., 2019). En 2003 et en 2012, le PISA s’est intéressé à l’anxiété mathématique. En effet, comme les mathématiques ont constitué le domaine majeur d’évaluation lors de ces deux enquêtes, l’anxiété mathématique a été documentée, à ces occasions, au même titre que bon nombre d’autres facteurs non-cognitifs liés aux performances dans le domaine. Le portrait canadien, issu du PISA de 2003 et de 2012, supporte et renforce l’idée selon laquelle l’anxiété mathématique pourrait contribuer aux écarts de performance observés entre les garçons et les filles, en mathématiques, au Québec. Devant l’absence de portrait québécois en regard du phénomène, et souhaitant, à terme, fournir des leviers en vue de réduire les écarts de performances observés entre les filles et les garçons québécois, nous énonçons l’objectif général de recherche comme suit : quantifier les écarts d’anxiété mathématique entre les filles et les garçons, étudier le lien anxiété mathématique/performances en mathématiques et ensuite, identifier des facteurs ayant le potentiel de réduire les écarts d’anxiété mathématique observés, chez les élèves québécois francophones de 15 ans, à partir d’une analyse secondaire des données du PISA de 2003 et de 2012. Afin de répondre à cet objectif général de recherche, nous définissons trois objectifs spécifiques de recherche. Chacun d’eux est traité dans un des trois articles de cette thèse par articles. Le premier objectif spécifique de notre recherche vise à identifier les considérations méthodologiques inhérentes aux données issues du PISA et à proposer des techniques d’analyse qui permettent de les traiter, adéquatement. Cet objectif spécifique est traité dans le premier article, un article de nature méthodologique. Les deuxième objectif spécifique de notre recherche vise à quantifier les écarts d’anxiété mathématique entre les garçons et les filles francophones de 15 ans du Québec, à partir d’une analyse secondaire des données du PISA de 2003 et de 2012, et à étudier le lien négatif anxiété mathématique/performances en mathématiques, chez ces mêmes élèves. Cet objectif spécifique est traité dans le second article de la thèse, un article de nature empirique, qui prend appui sur les fondements méthodologiques proposés dans l’article 1. Le troisième objectif spécifique de notre recherche vise à identifier des facteurs qui permettent d’expliquer les écarts d’anxiété mathématique observés entre les garçons et les filles francophones du Québec et qui ont le potentiel de guider, à terme, la mise en œuvre d’interventions visant à réduire les écarts observés. Répondre à cet objectif spécifique fait l’objet du troisième article de la thèse. Cet article prend également appui sur les fondements méthodologiques exposés dans l’article 1. Des résultats issus de l’article 2, il ressort que : 1) en moyenne, les filles francophones de 15 ans du Québec font état d’un niveau d’anxiété mathématique statistiquement plus élevé que les garçons et 2) le lien entre l’anxiété mathématique et les performances en mathématiques est un lien négatif et équivalent entre les garçons et les filles francophones du Québec. Des résultats issus de l’article 3, il ressort que les écarts d’anxiété mathématique observés entre les filles et les garçons québécois francophones ayant participé au PISA de 203 et de 2012 pourraient s’expliquer par : 1) un concept de soi en mathématiques plus faible chez les filles et 2) un lien direct genre/anxiété mathématique. Des implications scientifiques et pratiques de ces résultats découlent des recommandations en vue de réduire les écarts d’anxiété mathématique observés entre les filles et les garçons francophones du Québec, de même que des avenues de recherche à explorer dans le futur. Ces recommandations et avenues de recherches sont présentées au terme de la thèse. / Performance in mathematics is linked to a number of issues in our society, both individual and societal. Even though Quebec students are doing very well internationally in mathematics, in large-scale educational assessments, gender-based analyses reveal disturbing gaps. Over the course of several cycles of these large-scale surveys, girls achieved statistically lower results than boys. Mathematics anxiety could at least partially explain the performance gaps observed between girls and boys (Stoet et al., 2016). Indeed, research conducted since the 1970s has shown that, in general, in adolescents and adults, girls tend to report greater anxiety about mathematics than boys (e.g.Else-Quest et al., 2010; Hyde et al., 1990; Stoet et al. 2016). Furthermore, in these same age groups, there is a negative linear correlation between individuals' level of anxiety in mathematics and their performance in mathematics (e.g.Hembree, 1990; Ma, 1999; Zhang et al., 2019). In 2003 and 2012, PISA turned its attention to mathematics anxiety. Indeed, since mathematics was the main domain of assessment in both surveys, mathematical anxiety was documented on these occasions, along with many other non-cognitive factors related to performance in the domain. The Canadian portrait, from both the 2003 and 2012 PISA surveys, supports and reinforces the idea that mathematics anxiety could explain, at least in part, the observed performance gaps between boys and girls in mathematics in Quebec. Given the absence of a Quebec portrait of the phenomenon, and wishing, in the long run, to provide levers for reducing the observed performance gaps between Quebec girls and boys, we state the general research objectives as follows: 1) quantify the gaps between girls and boys with respect to mathematics anxiety, 2) study the link between mathematics anxiety and mathematical performance, and finally 3) identify factors that may reduce the gaps observed in mathematical anxiety among 15-year-old French-speaking students in Québec, based on a secondary analysis of the 2003 and 2012 PISA data. 7 In order to meet this overall research objective, three specific research objectives are identified. Each of these is discussed in one of the three articles of the present thesis per article. The first specific objective of our research is to identify the methodological considerations inherent to the PISA data and to propose analytical techniques that will allow us to adequately address them. This specific objective is discussed in the first article, a methodology article. The second specific objective of our research is to quantify the differences in mathematical anxiety between 15-year-old French-speaking boys and girls in Quebec, based on a secondary analysis of the 2003 and 2012 PISA data, and to investigate the negative relation between mathematical anxiety and mathematical performance in these same students. This specific objective is discussed in the second article of the thesis, an empirical article, which is based on the methodological bases proposed in Article 1. The third specific objective of our research is to identify factors that contribute to explaining the differences observed in mathematical anxiety between French-speaking boys and girls in Quebec and have the potential to guide, over the long run, the implementation of interventions aimed at reducing the observed gaps. The third paper in the thesis focuses on this specific objective. This article is also based on the methodological bases set out in Article 1. The results of Article 2 show that: 1) on average, French-speaking 15-year-old girls in Quebec report a statistically higher level of mathematical anxiety than boys and 2) the relationship between mathematical anxiety and mathematical performance is negative and equivalent between French-speaking boys and girls in Quebec. From the results of Article 3, it appears that the differences in mathematical anxiety observed between Quebec French-speaking girls and boys who participated in PISA 2003 and 2012 can be explained by: 1) a lower perception of mathematical competence among girls and 2) a direct gender/mathematical anxiety link. The scientific and practical implications of these results lead to recommendations to reduce the differences observed in mathematical anxiety among French-speaking girls and boys in Quebec, as well as lines of future research. These recommendations and lines of research are presented at the end of the thesis.