On the Global Convergence of Continuous-Time Stochastic Heavy-Ball Method for Nonconvex Optimization

We study the convergence behavior of the stochastic heavy-ball method with a small stepsize. Under a change of time scale, we approximate the discrete method by a stochastic differential equation that models small random perturbations of a coupled system of nonlinear oscillators. We rigorously show that the perturbed system converges to a local minimum in a logarithmic time. This indicates that for the diffusion process that approximates the stochastic heavy-ball method, it takes (up to a logarithmic factor) only a linear time of the square root of the inverse stepsize to escape from all saddle points. This results may suggest a fast convergence of its discrete-time counterpart. Our theoretical results are validated by numerical experiments.Comment: accepted at IEEE International Conference on Big Data in 201

Fungal Endophytes and Their Ecological Roles in Cotton: Evaluation of Potential Antagonistic Activity against Plant Parasitic Nematodes and Insect Herbivores

Results showed that both endophytes could negatively affect root-knot nematode infection and reproduction in cotton under greenhouse conditions. Further, endophytic C. globosum was also shown to negatively affect the fecundity of both cotton aphids and beet armyworms in greenhouse trials. C. globosum as an endophyte in cotton also affected the development rates and growth of beet armyworm larvae. This was the first study to demonstrate the negative effects of a since fungal endophyte, C. globosum, on insect herbivores feeding above-ground as well as plant parasitic nematodes feeding below-ground, using the same host plant species. Across two years of field trials evaluating efficacy of the endophytes against nematodes, no significant effects of either P. lilacinum or C. globosum were detected on root-knot or reniform nematode populations. However, positive effects on cotton plant growth and yields were observed in some treatments combinations of endophyte genotype, seed treatment and plant genotype at some sites, indicating the importance of ontext dependency in determining the outcome of cotton-endophyte-nematode interactions in the field. The results of this study indicate that the presence of fungal endophytes in crops can be manipulated and many have the potential to be incorporated as part of an IPM strategy to protect plants against both insect herbivores and plant parasitic nematodes. This novel approach may help provide an environmentally-sound and sustainable tool for pest management in agricultural systems in which the application of pesticides is currently the most commonly utilized control tactic

Modeling of Welding-Induced Distortion Effects on Fatigue Behaviors of Lightweight Shipboard Structures

In the design and construction of modern lightweight shipboard structures, wide-spread welding-induced distortions have become a major structural producibility issue and an increasing structural integrity concern over secondary bending stresses caused by interactions of distortions with cyclic service loads. The goal of this dissertation is to develop an effective methodology for evaluating the secondary bending effects caused by complex welding-induced distortions on fatigue behaviors of lightweight structures. A novel analytical approach based on a divide-and-conquer approach is taken to obtain the solutions to complex distortion problems in closed-forms through an assembly of its solution parts achievable through a decomposition technique. A notional load method for providing analytical treatment of distortion curvature effects on fatigue behaviors of lightweight shipboard structures within the context of beam theory is first presented. Using this method, closed-form analytical formulae can be developed for analyzing secondary bending stresses caused by nonlinear interactions between several common distortion types and remotely applied load. Then, an analytical method for computing the secondary bending stresses at weld locations caused by both axial and angular misalignments without curvatures. The model enables a consistent definition of each type of misalignment commonly observed in practice. As such, the secondary bending stresses caused by misalignments at each weld toe location can be appropriately combined for fatigue evaluation purposes. All closed-form analytical solutions derived are validated by direct finite element computations in various cases. Moreover, the developed analytical solutions are used for interpreting fatigue test data of welded components with misalignments and distortion curvatures. An excellent agreement is achieved not only between thin plate lab specimens and full-scale stiffened panels but also with the traction structural stress-based master S-N curve scatter band adopted by ASME Div. 2 since 2007, further validating their effectiveness in fatigue evaluation of welded structures exhibiting general forms of misalignments and distortion curvatures. These new closed-form solutions offer some significant insights not only on what types of distortions are more detrimental to fatigue performance than others but also on the validity limits of the empirical equations stipulated in current Codes and Standards. In addition, parameterized limits can now be clearly stated on conditions when straightening effects should be considered based on the closed-form solutions. Finally, a general distortion mode decomposition-and-assembly procedure is presented. By introducing a consistent reference framework, complex distortions regarding both butt-welded joints and fillet-welded joints in panel structures can be readily decomposed into various elementary distortion modes studied in this dissertation. The final assembly of the constituent secondary stress solutions is accomplished through superposition. To facilitate real-world engineering applications and support future adoptions of Codes and Standards, the closed-form formulae are presented in tabular form for following the workflow of the proposed decomposition-assembly procedure. Two examples are provided for illustrating how the procedure and closed-form solutions are used in real engineering applications. In summary, this dissertation presents a series of novel analytical treatments for computing secondary bending stresses caused by various elementary distortion modes, accompanied by a comprehensive distortion decomposition-and-assembly procedure based on a consistent framework. These new solutions offer a comprehensive suite of tools to engineers and researchers for a consistent and effective treatment of secondary stresses caused by distortion types unique to lightweight shipboard structures in performing fatigue evaluations.PHDNaval Architecture & Marine EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/168039/1/wqzhou_1.pd

ALECE: An Attention-based Learned Cardinality Estimator for SPJ Queries on Dynamic Workloads (Extended)

For efficient query processing, DBMS query optimizers have for decades relied on delicate cardinality estimation methods. In this work, we propose an Attention-based LEarned Cardinality Estimator (ALECE for short) for SPJ queries. The core idea is to discover the implicit relationships between queries and underlying dynamic data using attention mechanisms in ALECE's two modules that are built on top of carefully designed featurizations for data and queries. In particular, from all attributes in the database, the data-encoder module obtains organic and learnable aggregations which implicitly represent correlations among the attributes, whereas the query-analyzer module builds a bridge between the query featurizations and the data aggregations to predict the query's cardinality. We experimentally evaluate ALECE on multiple dynamic workloads. The results show that ALECE enables PostgreSQL's optimizer to achieve nearly optimal performance, clearly outperforming its built-in cardinality estimator and other alternatives.Comment: VLDB 202

Directional Spin Wave in Spin-Torque Oscillators Induced by Interfacial Dzyaloshinskii–Moriya Interaction

Spin torque oscillators (STOs) are currently of great interest due to its wide tunable frequencies, low energy consumption and high quality factors compared with traditional oscillators. Here, we report the characteristics of the nanocontact-(NC-)STO in the presence of interfacial Dzyaloshinskii-Moriya interaction (DMI), using micromagnetic simulations. We find that the DMI can decrease the STO frequency by around 2 GHz. More importantly, the DMI is able to break the isotropy of the spin-wave spectrum and turn the emitted microwave into directional spin-wave beams potentially facilitating the synchronization of multiple STOs

Can Teaching Enthusiasm Partially Predict the Reading Attainment of Low-income Students in Secondary Schools in England?

The poverty achievement gap in early reading is a persistent issue in England and around the world, potentially disadvantaging poor students and their further study. This new study employs student-perceived teaching enthusiasm and reading attitudes to help explain the poverty attainment gap. The sample was 5,242 15-year-old participants in PISA 2018 from 175 secondary schools in England. Path analysis is used to investigate the potential effect of teaching enthusiasm on the reading attainment of low-income students. The findings indicate that family socioeconomic status remains an important predictor of students' reading achievement. Students from economically privileged families tend to rate teaching enthusiasm more highly and express a positive reading attitude, which can partially explain the poverty attainment gap in reading. Therefore, teachers might be able to enhance low-income students’ reading outcomes and close the poverty attainment gap a little through enhanced teaching enthusiasm to cultivate students’ positive reading attitudes

Numerical Analysis on a Perforated Muffler Applied in the Discharge Chamber of a Twin Screw Refrigeration Compressor Based on Fluid-Acoustic Coupling Method

The twin screw compressor has been widely used in the refrigeration systems due to advantages such as compact structure, stable operation, high efficiency and good adaptability. Intermittent gas flow generates gas pulsation that cause serious problems such as structural vibration and noise in the twin screw refrigeration compressor. Because the mechanical noise can be controlled well with the improvement of machining and assembly accuracy, the aerodynamic noise induced by gas pulsation even has become the main noise source of the twin screw refrigeration compressor. In order to reduce the pressure pulsation, a broadband perforated panel muffler applied in the discharge chamber of the twin screw refrigeration compressor is proposed based on the noise spectrum and flow characteristics of the compressor. In order to obtain the noise spectrum of the twin screw refrigeration compressor, the pressure fluctuation in discharge chamber based on a three-dimensional CFD simulation model is calculated, and the acoustical model is established based on fluid-acoustic coupling method. Then the impacts of different structural parameters on the performance of a perforated panel muffler are investigated, including perforation rate, perforation diameter and panel thickness. Through the optimization of the perforated muffler, a better reduction effect of broadband noise can be achieved. Results of fluid-acoustic coupled analysis can provide guidance on the design and optimization of the perforated muffler and noise reduction of the twin screw refrigeration compressor