Data-driven sub-grid model development for large eddy simulations of turbulence

Turbulence modeling remains an active area of research due to its significant impact on a diverse set of challenges such as those pertaining to the aerospace and geophysical communities. Researchers continue to search for modeling strategies that improve the representation of high-wavenumber content in practical computational fluid dynamics applications. The recent successes of machine learning in the physical sciences have motivated a number of studies into the modeling of turbulence from a data-driven point of view. In this research, we utilize physics-informed machine learning to reconstruct the effect of unresolved frequencies (i.e., small-scale turbulence) on grid-resolved flow-variables obtained through large eddy simulation. In general, it is seen that the successful development of any data-driven strategy relies on two phases - learning and a-posteriori deployment. The former requires the synthesis of labeled data from direct numerical simulations of our target phenomenon whereas the latter requires the development of stability preserving modifications instead of a direct deployment of learning predictions. These stability preserving techniques may be through prediction modulation - where learning outputs are deployed via an intermediate statistical truncation. They may also be through the utilization of model classifiers where the traditional $L_2$-minimization strategy is avoided for a categorical cross-entropy error which flags for the most stable model deployment at a point on the computational grid. In this thesis, we outline several investigations utilizing the aforementioned philosophies and come to the conclusion that sub-grid turbulence models built through the utilization of machine learning are capable of recovering viable statistical trends in stabilized a-posteriori deployments for Kraichnan and Kolmogorov turbulence. Therefore, they represent a promising tool for the generation of closures that may be utilized in flows that belong to different configurations and have different sub-grid modeling requirements

Labour supply: a review of alternative approaches

This chapter surveys existing approaches to modeling labor supply and identifies important gaps in the literature that could be addressed in future research. The discussion begins with a look at recent policy reforms and labor market facts that motivate the study of labor supply. The analysis then presents a unifying framework that allows alternative empirical formulations of the labor supply model to be compared and their resulting elasticities to be interpreted. This is followed by critical reviews of alternative approaches to labor-supply modeling. The first review assesses the difference in-differences approach and its relationship to natural experiments. The second analyzes estimation with non-linear budget constraints and welfare-program participation. The third appraises developments of family labor-supply models including both the standard unitary and collective labor-supply formulations. The fourth briefly explores dynamic extensions of the labor supply model, characterizing how participation decisions, learning-by-doing, human capital accumulation and habit formation affect the analysis of the lifecycle model. At the end of each of the four broad reviews, we summarize a selection of the recent empirical findings. The concluding section asks whether the developments reviewed in this chapter place us in a better position to answer the policy-reform questions and to interpret the trends in participation and hours with which we began this review. q1999 Elsevier Science B.V. All rights reserved.

Labor supply models: unobserved heterogeneity, nonparticipation and dynamics

This chapter is concerned with the identification and estimation of models of labor supply. The focus is on the key issues that arise from unobserved heterogeneity, nonparticipation and dynamics. We examine the simple ‘static’ labor supply model with proportional taxes and highlight the problems surrounding nonparticipation and missing wages. The difference in differences approach to estimation and identification is developed within the context of the labour supply model. We also consider the impact of incorporating nonlinear taxation and welfare programme participation. Family labor supply is looked at from botht e unitary and collective persepctives. Finally we consider intertemporal models focusing on the difficulties that arise with participation and heterogeneity

Explorations into the inertial and integral scales of homogeneous axisymmetric turbulence

A flow generator is described in which homogeneous axisymmetric turbulent air flows with varying and fully controllable degrees of anisotropy, including the much studied isotropic case, are generated by the combined agitations produced by 32 acoustic mixers focusing at the center of the system. The axisymmetric turbulence in a central volume of the size of the inertial scale is shown to have negligible mean and shear. The Taylor Reynolds number is about 480. The influence of large scale anisotropy on the turbulence is examined from three aspects, namely the velocity structure functions, the velocity correlation functions, and the integral lengths. The directional dependence of two different second order transverse structure functions, in which one of them has separations stretched along the axis of symmetry of the turbulence and the other one normal to it, is studied. It is shown that the inertial range scaling exponents, determined using the extended-self-similarity procedure, and the Kolmogorov constants of the two structure functions are unaffected by the direction in which the structure functions are measured. Finally, it is found that, except in the isotropic case, the second order transverse velocity correlation functions deviate from each other at the large scale with increasing anisotropy. A self-similarity argument similar to one found in the study of critical phenomena is proposed. It is shown that the argument leads to a power-law relationship between the large scale velocity fluctuation and the correlation length, with an exponent that depends on the inertial range scaling exponent of the turbulence. The data collapse predicted by the self-similarity hypothesis is verified. It is demonstrated that the value of the power-law exponent is consistent with the value of the inertial range scaling exponent.Comment: PhD thesis, 201 pages, 56 figures, 17 table

Essays on the macroeconomic management of foreign aid flows in Africa

The main motivation of this thesis is to contribute to the literature on the macroeconomic effects of foreign aid flows. It consists of four empirical papers, investigating the two main channels through which aid flows impact the recipient economy: (i) the fiscal sector, and (ii) the real exchange rate. The first paper is concerned with the impact of aid on government expenditure, domestic revenues and borrowing. It uses a traditional fiscal response framework with annual data for Ethiopia. The second paper also focuses on the fiscal sector but uses a recently compiled quarterly fiscal dataset and the cointegrated vector autoregression methodology. The main result arising from both papers is the strong correlation between aid inflows and domestic borrowing, possibly as a strategy to smooth unpredictable and volatile aid inflows. Aid is positively correlated with government expenditures, but there is little evidence of tax displacement. There is also evidence of aid heterogeneity, as grants and loans induce different effects. The third paper assesses the impact of foreign aid on the Ethiopian real exchange rate, which is a common measure of external competitiveness. It uses a quarterly macroeconomic dataset and applies two distinct methodologies: (i) single-equation cointegration models, and (ii) an unobserved components model. The results do not provide support for the ‘Dutch disease’ hypothesis. The fourth paper investigates the extent to which foreign aid is ‘absorbed’ and ‘spent’. The empirical analysis uses a panel of 25 African low-income countries and applies recently developed panel cointegration techniques. The findings suggest that aid is fully spent while absorption is higher than previously estimated

The Fifth NASA/DOD Controls-Structures Interaction Technology Conference, part 2

This publication is a compilation of the papers presented at the Fifth NASA/DoD Controls-Structures Interaction (CSI) Technology Conference held in Lake Tahoe, Nevada, March 3-5, 1992. The conference, which was jointly sponsored by the NASA Office of Aeronautics and Space Technology and the Department of Defense, was organized by the NASA Langley Research Center. The purpose of this conference was to report to industry, academia, and government agencies on the current status of controls-structures interaction technology. The agenda covered ground testing, integrated design, analysis, flight experiments and concepts

The years of high econometrics: A short history of the generation that reinvented economic

This book is an essay in biography and its subject matter is the collective effort of that brilliant generation of economists who aspired to transform economics into a rigorous science. The powerful econometric movement took shape in the 1930s, the years of high theory – the concept that Shackle used to describe the period of the inception of the Keynesian revolution, a period that cannot be thoroughly understood unless both movements are contrasted. In a sense, both the Keynesian revolution and the econometric revolution shared the same motivation: to extend the empirical capacity of economics, broadening its analytical scope and strengthening its capacity for designing a control policy. As the story unfurls, it becomes obvious that the young econometricians with Keynesian leanings were more radically engaged in such a task than the Cambridge circle itself, and this was the profound reason for a great deal of the harsh criticism and disappointment that they faced. Furthermore, the acceptance of the epistemological primacy of a very peculiar type of simple mathematical formalism contributed to the marginalisation of some of the major theoretical alternatives developed in the first half of the century. Evidence shows that the endorsement of the urgent political agenda for action against unemployment and the dangers of war were instrumental in determining the victory of a specific mathematical drive, and that the econometric programme as it came to be conceived in these incipient years was shaped by this movement. As a consequence of its impact, econometrics became a tool for the reconstruction of neoclassical economics, which sought to be redescribed in the language of mathematical formalism and statistical inference and estimation, and simultaneously responsible for the decay of heterodox alternatives elsewhere. In that sense, modern economics was a tributary of that success.info:eu-repo/semantics/publishedVersio