54 research outputs found
A study of rheological limitations in rotary jet spinning of polymer nanofibers through modeling and experimentation
No full textThe recently popularized method of rotary jet spinning (RJS) or centrifugal spinning is investigated to evaluate the rheological limitations of polymer solutions and melts to optimal spinnability. The influence of Newtonian or non-Newtonian behavior of the polymer on spinnability is discussed. We observe that highly viscous polymers tend to block the die channels within a rotary jet spinneret and therefore suggest the use of relatively low Newtonian viscosities of between 1 and 10 Pa s for optimal fiber production. Computational fluid dynamics simulations are used in conjunction with experimental data to establish important processing parameters, such as typical shear rates in the device and optimal polymer melt or solution viscosities. A theoretical model for RJS is compared to measured fiber diameters. The comparison shows that although fiber diameters can be estimated very roughly in the case of polymer solutions, the prediction of fiber diameter in the case of polymer melts require further modeling work
Adaptive convex loss selection for m-estimation: adaptive rate and efficiency
No full textThis thesis delves into the classical linear regression problem, focusing on the estimation of the
unknown parameter β0 ∈ Rd given n random vectors (X1, Y1), . . . , (Xn, Yn) that follow the linear
model Yi = X⊤i β0 + ϵi. The covariates X1, . . . , Xn are independent and identically distributed
(i.i.d.) Rd-valued and independent of the i.i.d. real-valued errors ϵ1, . . . , ϵn with an unknown
distribution. We focus on the low-dimensional setting where the problem dimension d is fixed,
analysing the performance of estimators as n becomes large.
Historically, the use of estimators that minimise the sum of the squared error losses has been
widely adopted in solving regression problems. In the linear regression setting, the Gauss–Markov
theorem justifies the ordinary least squares (OLS) estimator only under limited conditions. We be-
lieve that it is the convexity of the L2 loss function, which makes the minimisation computationally
tractable, makes OLS attractive to the practitioners. This thesis thus focuses on the adaptive (data-
driven) selection of convex loss functions for better estimation in linear regression, in the hope of
challenging the default use of L2 and L1 loss functions in modern high-dimensional regression.
Chapter 1 explores the mean estimation of univariate distributions symmetric around their mean
under irregular error distributions, where rates faster than n−1/2 can be achieved. We construct an
estimator achieving optimal rates up to n−1/α for compactly supported error distributions, where
α ∈ (0, 2]. The method involves a convex M -estimator based on a convex loss function Lγ (x) =
ii |x|γ , where γ is selected in data-driven fashion. The proposed method is computationally efficient
can be extended to linear regression settings. This chapter is based on the join work with Min Xu
and Cun-Hui Zhang, Kao et al. (2022).
Chapter 2 addresses linear regression under smooth error densities with finite Fisher informa-
tion, where a faster than n−1/2 rate is unattainable. We focus on selecting convex loss functions
to minimise asymptotic covariance among convex M -estimators. We identify the optimal convex
loss function with the log-concave projection of the noise distribution with respect to the Fisher di-
vergence and construct an adaptive estimator achieving the minimal asymptotic covariance among
the convex M -estimators. The proposed procedure is computationally efficient. This work is based
on the joint work with Oliver Y. Feng, Min Xu, and Richard J. Samworth, Feng et al. (2024).
iiiPh.D.Includes bibliographical reference
The effects of superimposed tilt and lower body negative pressure on anterior and posterior cerebral circulations
No full textSteady-state tilt has no effect on cerebrovascular reactivity to increases in the partial pressure of end-tidal carbon dioxide (PETCO2). However, the anterior and posterior cerebral circulations may respond differently to a variety of stimuli that alter central blood volume, including lower body negative pressure (LBNP). Little is known about the superimposed effects of head-up tilt (HUT; decreased central blood volume and intracranial pressure) and headdown tilt (HDT; increased central blood volume and intracranial pressure), and LBNP on cerebral blood flow (CBF) responses. We hypothesized that (a) cerebral blood velocity (CBV; an index of CBF) responses during LBNP would not change with HUT and HDT, and (b) CBV in the anterior cerebral circulation would decrease to a greater extent compared to posterior CBV during LBNP when controlling PETCO2. In 13 male participants, we measured CBV in the anterior (middle cerebral artery, MCAv) and posterior (posterior cerebral artery, PCAv) cerebral circulations using transcranial Doppler ultrasound during LBNP stress (−50 mmHg) in three body positions (45°HUT, supine, 45°HDT). PETCO2 was measured continuously and maintained at constant levels during LBNP through coached breathing. Our main findings were that (a) steady-state tilt had no effect on CBV responses during LBNP in both the MCA (P = 0.077) and PCA (P = 0.583), and (b) despite controlling for PETCO2, both the MCAv and PCAv decreased by the same magnitude during LBNP in HUT (P = 0.348), supine (P = 0.694), and HDT (P = 0.407). Here, we demonstrate that there are no differences in anterior and posterior circulations in response to LBNP in different body positions.Library OA FundFinancial support for the initial building of the apparatus (supplies) was provided by Mount Royal University, Faculty of Science and Technology, Laboratory Support Centre, and most of the funding for data collection (student salaries) by a Government of Alberta STEP Grant (MMT and RJS), an NSERC USRA (MMT), and the MRU Petro-Canada Young Investigator award (TAD). Funds for covering the costs to publish in open access were provided by Mount Royal University Library
Listening to narrative speech after aphasic stroke: The role of the left anterior temporal lobe
No full textThe dorsal bank of the primate superior temporal sulcus (STS) is a polysensory area with rich connections to unimodal sensory association cortices. These include auditory projections that process complex acoustic information, including conspecific vocalizations. We investigated whether an extensive left posterior temporal (Wernicke\u27s area) lesion, which included destruction of early auditory cortex, may contribute to impaired spoken narrative comprehension as a consequence of reduced function in the anterior STS, a region not included within the boundary of infarction. Listening to narratives in normal subjects activated the posterior-anterior extent of the left STS, as far forward as the temporal pole. The presence of a Wernicke\u27s area lesion was associated with both impaired sentence comprehension and a reduced physiological response to heard narratives in the intact anterior left STS when compared to aphasic patients without temporal lobe damage and normal controls. Thus, in addition to the loss of language function in left posterior temporal cortex as the direct result of infarction, posterior ablation that includes primary and early association auditory cortex impairs language function in the intact anterior left temporal lobe. The implication is that clinical studies of language on stroke patients have underestimated the role of left anterior temporal cortex in comprehension of narrative speech. \ua9 The Author 2005. Published by Oxford University Press. All rights reserved
A scalable high-energy diode-pumped solid state laser for laser-plasma interaction science and applications
No full textPredictive Control for Alleviation of Gust Loads on Very Flexible Aircraft
No full text11/03/14 meb. Author retains copyright as beforeIn this work the dynamics of very flexible aircraft are described by a set of non-linear, multi-disciplinary equations of motion. Primary structural components are represented by a geometrically-exact composite beam model which captures the large dynamic deformations of the aircraft and the interaction between rigid-body and elastic degrees-of-freedom. In addition, an implementation of the unsteady vortex-lattice method capable of handling arbitrary kinematics is used to capture the unsteady, three-dimensional flow-eld around the aircraft as it deforms. Linearization of this coupled nonlinear description, which can in general be about a nonlinear reference state, is performed to yield relatively high-order linear time-invariant state-space models. Subsequent reduction of these models using standard balanced truncation results in low-order models suitable for the synthesis of online, optimization-based control schemes that incorporate actuator constraints. Predictive controllers are synthesized using these reduced-order models and applied to nonlinear simulations of the plant dynamics where they are shown to be superior to equivalent optimal linear controllers (LQR) for problems in which constraints are active
Spatial patterns and intensity of the surface storm tracks in CMIP5 models
No full textAuthor Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 4965-4981, doi:10.1175/JCLI-D-16-0228.1.To improve the understanding of storm tracks and western boundary current (WBC) interactions, surface storm tracks in 12 CMIP5 models are examined against ERA-Interim. All models capture an equatorward displacement toward the WBCs in the locations of the surface storm tracks’ maxima relative to those at 850 hPa. An estimated storm-track metric is developed to analyze the location of the surface storm track. It shows that the equatorward shift is influenced by both the lower-tropospheric instability and the baroclinicity. Basin-scale spatial correlations between models and ERA-Interim for the storm tracks, near-surface stability, SST gradient, and baroclinicity are calculated to test the ability of the GCMs’ match reanalysis. An intermodel comparison of the spatial correlations suggests that differences (relative to ERA-Interim) in the position of the storm track aloft have the strongest influence on differences in the surface storm-track position. However, in the North Atlantic, biases in the surface storm track north of the Gulf Stream are related to biases in the SST. An analysis of the strength of the storm tracks shows that most models generate a weaker storm track at the surface than 850 hPa, consistent with observations, although some outliers are found. A linear relationship exists among the models between storm-track amplitudes at 500 and 850 hPa, but not between 850 hPa and the surface. In total, the work reveals a dual role in forcing the surface storm track from aloft and from the ocean surface in CMIP5 models, with the atmosphere having the larger relative influence.JFB was partially supported by the NOAA Climate Program Office’s Modeling, Analysis, Predictions, and Projections program (Grant NA15OAR4310094). Y-OK was supported by NSF Division of Atmospheric and Geospace Science Climate and Large-scale Dynamics Program (AGS-1355339), NASA Physical Oceanography Program (NNX13AM59G), and DOE Office of Biological and Environmental Research Regional and Global Climate Modeling Program (DE-SC0014433). RJS was supported by DOE Office of Biological and Environmental Research (DE-SC0006743) and NSF Directorate for Geosciences Division of Ocean Sciences (1419584),2017-10-0
Mathematical modelling of polyamine metabolism in bloodstream-form trypanosoma brucei: An application to drug target identification
No full text© 2013 Gu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are creditedThis article has been made available through the Brunel Open Access Publishing Fund.We present the first computational kinetic model of polyamine metabolism in bloodstream-form Trypanosoma brucei, the causative agent of human African trypanosomiasis. We systematically extracted the polyamine pathway from the complete metabolic network while still maintaining the predictive capability of the pathway. The kinetic model is constructed on the basis of information gleaned from the experimental biology literature and defined as a set of ordinary differential equations. We applied Michaelis-Menten kinetics featuring regulatory factors to describe enzymatic activities that are well defined. Uncharacterised enzyme kinetics were approximated and justified with available physiological properties of the system. Optimisation-based dynamic simulations were performed to train the model with experimental data and inconsistent predictions prompted an iterative procedure of model refinement. Good agreement between simulation results and measured data reported in various experimental conditions shows that the model has good applicability in spite of there being gaps in the required data. With this kinetic model, the relative importance of the individual pathway enzymes was assessed. We observed that, at low-to-moderate levels of inhibition, enzymes catalysing reactions of de novo AdoMet (MAT) and ornithine production (OrnPt) have more efficient inhibitory effect on total trypanothione content in comparison to other enzymes in the pathway. In our model, prozyme and TSHSyn (the production catalyst of total trypanothione) were also found to exhibit potent control on total trypanothione content but only when they were strongly inhibited. Different chemotherapeutic strategies against T. brucei were investigated using this model and interruption of polyamine synthesis via joint inhibition of MAT or OrnPt together with other polyamine enzymes was identified as an optimal therapeutic strategy.The work was carried out under a PhD programme partly funded by Prof. Ray Welland, School of Computing Science, University of Glasgo
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