Global behaviour corresponding to the absolute instability of the rotating-disc boundary layer

A study is carried out of the linear global behaviour corresponding to the absolute instability of the rotating-disc boundary layer. It is based on direct numerical simulations of the complete linearized Navier–Stokes equations obtained with the novel velocity–vorticity method described in Davies & Carpenter (2001). As the equations are linear, they become separable with respect to the azimuthal coordinate, $\theta$. This permits us to simulate a single azimuthal mode. Impulse-like excitation is used throughout. This creates disturbances that take the form of wavepackets, initially containing a wide range of frequencies. When the real spatially inhomogeneous flow is approximated by a spatially homogeneous flow (the so-called parallel-flow approximation) the results ofthe simulations are fully in accordance with the theory of Lingwood (1995). If the flow parameters are such that her theory indicates convective behaviour the simulations clearly exhibit the same behaviour. And behaviour fully consistent with absolute instability is always found when the flow parameters lie within the theoretical absolutely unstable region. The numerical simulations of the actual inhomogeneous flow reproduce the behaviour seen in the experimental study of Lingwood (1996). In particular, there is close agreement between simulation and experiment for the ray paths traced out by the leading and trailing edges of the wavepackets. In absolutely unstable regions the short-term behaviour of the simulated disturbances exhibits strong temporal growth and upstream propagation. This is not sustained for longer times, however. The study suggests that convective behaviour eventually dominates at all the Reynolds numbers investigated, even for strongly absolutely unstable regions. Thus the absolute instability of the rotating-disc boundary layer does not produce a linear amplified global mode as observed in many other flows. Instead the absolute instability seems to be associated with transient temporal growth, much like an algebraically growing disturbance. There is no evidence of the absolute instability giving rise to a global oscillator. The maximum growth rates found for the simulated disturbances in the spatially inhomogeneous flow are determined by the convective components and are little different in the absolutely unstable cases from the purely convectively unstable ones. In addition to the study of the global behaviour for the usual rigid-walled rotating disc, we also investigated the effect of replacing an annular region of the disc surface with a compliant wall. It was found that the compliant annulus had the effect of suppressing the transient temporal growth in the inboard (i.e. upstream) absolutely unstable region. As time progressed the upstream influence of the compliant region became more extensive

The initial development of the general non-isoenergetic compressible free shear layer

Numerical solution of partial differential equations governing intial development of general nonisoenergetic compressible free shear laye

Matching service development to mental health needs: a case study of a rural county

The development of Somerset's mental health services from 1991 to 1996 involved the closure of the county's last long-stay hospital, and its replacement by more local in-patient provision and an expansion of community services. The process and outcome of this change is examined, drawing upon (a) an externally-commissioned evaluation in 1994-6 and (b) a comprehensive mental health needs assessment in 1996. The findings from these studies indicate that the development of accommodation for people with severe and long-term mental health problems has been dominated by the needs of long-stay residents in the old institutions. This led initially to geographical inequities and a shortage of supported accommodation for others with severe and enduring mental health problems. However, developments during this period also suggest a local capacity to detect and respond to unmet needs in this group

Growth mechanisms of perturbations in boundary layers over a compliant wall

The temporal modal and nonmodal growth of three-dimensional perturbations in the boundary-layer flow over an infinite compliant flat wall is considered. Using a wall-normal velocity/wall-normal vorticity formalism, the dynamic boundary condition at the compliant wall admits a linear dependence on the eigenvalue parameter, as compared to a quadratic one in the canonical formulation of the problem. This greatly simplifies the accurate calculation of the continuous spectrum by means of a spectral method, thereby yielding a very effective filtering of the pseudospectra as well as a clear identification of instability regions. The regime of global instability is found to be matching the regime of the favorable phase of the forcing by the flow on the compliant wall so as to enhance the amplitude of the wall. An energy-budget analysis for the least-decaying hydroelastic (static-divergence, traveling-wave-flutter and near-stationary transitional) and Tollmien--Schlichting modes in the parameter space reveals the primary routes of energy flow. Moreover, the flow exhibits a slower transient growth for the maximum growth rate of a superposition of streamwise-independent modes due to a complex dependence of the wall-boundary condition with the Reynolds number. The initial and optimal perturbations are compared with the boundary-layer flow over a solid wall; differences and similarities are discussed. Unlike the solid-wall case, viscosity plays a pivotal role in the transient growth. A slowdown of the maximum growth rate with the Reynolds number is uncovered and found to originate in the transition of the fluid-solid interaction from a two-way to a one-way coupling. Finally, a term-by-term energy budget analysis is performed to identify the key contributors to the transient growth mechanism

Survey and Evaluation of Supersonic Base Flow Theories

Survey and evaluation of supersonic base flow theorie

Risk Attitudes and Well-Being in Latin America

A common premise in both the theoretical and policy literatures on development is that people remain poor because they are too impatient to save and too risk averse to take the sort of chances needed to accumulate wealth. The empirical literature, however, suggests that this assumption is far from proven. We report on field experiments designed to address many of the issues confounding previous analyses of the links between risk preferences and well-being. Our sample includes more than 3,000 participants who were drawn representatively from six Latin American cities: Bogotá, Buenos Aires, Caracas, Lima, Montevideo, San José. In addition to the experiment which reveals interesting cross-country differences, participants completed an extensive survey that provides data on a variety of well-being indicators and a number of important controls. Focusing on risk preferences, we find little evidence of robust links between risk aversion and well-being. However, when we analyze the results of three treatments designed to better reflect common choices made under uncertainty, we see that these, more subtle, instruments correlate better with well-being, even after controlling for a variety of other important factors like the accumulation of human capital and access to credit.risk aversion, ambiguity aversion, loss aversion, risk pooling, well-being, Latin America

Experiments and Economic Development: Lessons from Field Labs in the Developing World

Along with the traditional primitives of economic development (material preferences, technology, and endowments), there is a growing interest in exploring how psychological and sociological factores (e.g., bounded rationality, norms, or social preferences) also influence economic decisions, the evolution of institutions, and outcomes. Simultaneously, a vast literature has arisen arguing that economic experiments are important tools in identifying and quantifying the role of institutions, socialnorms and preferences on behavior and outcomes. Reflecting on our experience conducting experiments in the field over more than five years, we survey the growing literature at the intersection of these two research areas. Our review has four components. In the introduction we set the stage identifying a set of behavioral factors that seem to be central for understanding growth and economic development./ We then divide the existing literature in two piles: standard experiments conducted in the field and on how to econometrically identify sociological factors in experimental data. We conclude by suggesting topics for future research.experimental economics, behavioral economics, institutions, social preferences, poverty, development

Identifying safety strategies for on-farm grain bins using risk analysis

The potential for grain bin accidents exists each year on Arkansas farms and farms across the nation. The trend toward increasing utilization of on-farm grain drying and storage could lead to an increase in grain bin accidents. The sharp contrast between a safe, efficient operation and one that leads to injury or death can be represented as sets of farmer-decisions and subsequent chance events. A model was constructed to define the risk associated with grain bin entry and inbin activity so that safety interventions could be identified and implemented to reduce the probability of injury and death. A survey was distributed to Arkansas grain farmers to gather data on the level of safety education, storage techniques, operations management, and other parameters. The data collected from the survey provided quantitative input of many of the model’s probability-distribution functions. Using a fault tree (with parallel modes of failure) in conjunction with a Monte Carlo simulation technique, we evaluated six safety intervention strategies and identified the one with the greatest potential for reducing the risk of serous injury or death. As part of senior design in biological engineering, plans are underway to design and test a probe that can locate and break bridged grain (a common risk factor in grain bin management) while working outside the bin on the ground

Network Architecture and Mutual Monitoring in Public Goods Experiments

Recent experiments show that public goods can be provided at high levels when mutual monitoring and costly punishment are allowed. All these experiments, however, study monitoring and punishment in a setting where all agents can monitor and punish each other (i.e., in a complete network). The architecture of social networks becomes important when individuals can only monitor and punish the other individuals to whom they are connected by the network. We study several non-trivial network architectures that give rise to their own distinctive patterns of behavior. Nevertheless, a number of simple, yet fundamental, properties in graph theory allow us to interpret the variation in the patterns of behavior that arise in the laboratory and to explain the impact of network architecture on the efficiency and dynamics of the experimental outcomes.experiment, networks, public good, monitoring, punishment

Cooperation, Trust and Social Capital in Southeast Asian Urban Slums

We conduct experiments in urban slums to measure trust and cooperation and to see how behavior varies with demographic factors and associational measures of social capital. Overall, we find high contribution rates among Thai and Vietnamese participants in a voluntary contribution game, and we see that many participants are willing to signal their disapproval of free riding despite it being costly to do so. At the individual level, we find that behavior varies with many demographic factors and with many associational factors. However, these correlations often differ significantly between our two locations, indicating the role of culture, defined broadly.Cooperation, trust, social disapproval, Thailand, Vietnam