A finite difference method for studying thermal deformation in two-dimensional micro scale metal thin films exposed to ultrashort pulsed lasers

Ultrashort-pulsed lasers have been attracting worldwide interest in science and engineering because the lasers with pulse durations on the order of sub-picoseconds to femtoseconds possess capabilities in limiting the undesirable spread of the thermal process zone in a heated sample during material processing at the microscale. Prevention of thermal damage is an important factor for success of ultrashort-pulsed lasers in real applications. The thermal damage induced by ultrashort pulses is intrinsically different from that induced by long-pulse or continuous lasers. It occurs after the heating pulse is over and involves the shattering of thin metal layers (without a clear signature of thermal damage by excessive temperature) rather than the melt damage caused by high temperatures. In this dissertation, by replacing the displacement components in the dynamic equations of motion using the velocity components, and employing a staggered grid, we develop a finite difference method for studying thermal deformation in two-dimensional films exposed to ultrashort-pulsed lasers, where the thin films are a single-layered thin film and a double-layered thin film with perfectly interfacial thermal contact and imperfectly interfacial thermal contact, respectively. The method is obtained based on the parabolic two-step heat transport equations. It accounts for the coupling effect between lattice temperature and strain rate, as well as for the hot electron blast effect in momentum transfer. The developed methodology allows us to avoid non-physical oscillations in the solution. Such oscillations have been an intrinsic feature of most numerical method proposed so far in the context of problem of interest. The development of physical-based, numerical-oscillation-free methods for thermal analysis of thin metal films subjected to heating of ultrashort-pulsed lasers represents challenging tools at the forefront of this practically important area of research. This method is tested for its applicability by investigating the temperature rise and deformation in (1) a single-layered gold thin film, (2) a double-layered gold and chromium thin film with perfect thermal contact at the interface, and (3) a double-layered gold and chromium thin film with imperfect thermal contact at the interface. Results show that there are no non-physical oscillations in the solutions, and the method is promising

Modeling on single-sided wind-driven natural ventilation

Buildings use 40% of the total primary energy in the United States, with a significant part of this energy being used for ventilation and cooling. Despite the large amount of energy used in buildings, reports have shown that the indoor air quality (IAQ) and thermal comfort are not satisfactory. The lowered productivity due to the bad IAQ could cause $125 billion loss per year and the sick building syndromes could cause$32 billion direct healthcare costs. Most of the problems related to IAQ are caused by insufficient fresh outdoor air supply or lack of maintenance with traditional mechanical ventilation systems. Natural ventilation is an alternative method to mechanical ventilation to reduce building energy use and improve indoor air quality. Natural ventilation can usually be classified into cross ventilation and single-sided ventilation. Cross ventilation is often favored for its larger air exchange rate than single-sided ventilation. However, in most cases, few buildings can achieve cross-ventilation due to the interior partitions, obstacles, and thicknesses. Therefore, single-sided ventilation is still of great importance in building design. However, the modeling of single-sided ventilation rate is difficult due to the bi-directional flow at the opening and the complex flow around buildings. The first part of this study is to develop a simple empirical model for buildings with simple openings. The model is able to accurately predict the mean ventilation rate and fluctuating ventilation rate caused by the pulsating flow and eddy penetration. This new model calculated the eddy penetration effect in the frequency domain based on Fast Fourier transform. We conducted Computational Fluid Dynamic (CFD) simulations with Large Eddy Simulation (LES) and used experimental data from other researchers to validate the new empirical model. The model predictions were generally within 25% error for simple opening. After we developed the model for simple openings, the second part of the research is to develop models for more complicated openings. In reality, only very few buildings use simple openings in their design, instead, the majority of the buildings use hopper, awning or casement windows. Therefore, based on the newly-developed model, we modified it to predict the ventilation rate for these windows types. In order to understand the flow characteristics around the complex openings, we used the CFD to generate database for various wind conditions. First, we validated the accuracy of the CFD LES model by conducting full-scale outdoor measurements and comparing against the CFD simulations. After validating the LES model, it was used to generate database to develop the semi-empirical models for hopper, awning and casement windows. Finally, the full-scale measured data was also compared with the proposed model predictions to validate the semi-empirical models. The comparison showed that the models were able to predict the ventilation rate generally within 30% error.^ After we developed the models for predicting single-sided, wind-driven ventilation rate, we evaluated the availability of natural ventilation in the future considering the impact of climate change. This research projected the future monthly weather based on HadCM3 Global Circulation Model (GCM) for 2020, 2050 and 2080 for three CO2 emission scenarios. To use the monthly weather data in energy simulation programs, we downscaled the monthly data to hourly data by Morphing method. Then we used the projected data to predict the future cooling and heating energy use in all seven climate zones in the U.S. for various commercial and residential buildings. We also coupled the newly-developed semi-empirical model with EnergyPlus to evaluate the natural ventilation potential in San Diego, San Francisco and Seattle, which are the representations of the typical climates where natural ventilation could be used. The results showed that the impact of climate change varied greatly depending on the geographic locations and building types. Also, the simulations showed that natural ventilation would still be acceptable by 2080 for San Francisco and Seattle even based on the worst case emission scenario, however, for San Diego or regions with warmer summer, natural ventilation could only be used for very limited time each year. Based on our study, the last step of this research is to seek potential approaches to utilize natural ventilation in hotter climates. One major limitation of natural ventilation is that it can only be used when outdoor is cool and may underperform during days with high outdoor temperatures. Mixed-mode cooling that combines natural ventilation and mechanical ventilation has the advantage of natural ventilation and mechanical cooling. To maximize the savings of mix-mode cooling, natural ventilation should be used as much as possible. In order to use natural ventilation mode during temporary hot weather, adequate amount of thermal mass with night cooling strategy would be one potential approach. However, the amount of thermal mass needed to be investigated to achieve cost-effective design. We conducted energy simulations with EnergyPlus to evaluate the impact of thermal mass on mixed-mode cooling energy savings. The results showed that electricity use can be reduced by 6-91% with mixed-mode ventilation compared to traditional mechanical cooling in different climates

Non-local Attention Optimized Deep Image Compression

This paper proposes a novel Non-Local Attention Optimized Deep Image Compression (NLAIC) framework, which is built on top of the popular variational auto-encoder (VAE) structure. Our NLAIC framework embeds non-local operations in the encoders and decoders for both image and latent feature probability information (known as hyperprior) to capture both local and global correlations, and apply attention mechanism to generate masks that are used to weigh the features for the image and hyperprior, which implicitly adapt bit allocation for different features based on their importance. Furthermore, both hyperpriors and spatial-channel neighbors of the latent features are used to improve entropy coding. The proposed model outperforms the existing methods on Kodak dataset, including learned (e.g., Balle2019, Balle2018) and conventional (e.g., BPG, JPEG2000, JPEG) image compression methods, for both PSNR and MS-SSIM distortion metrics

Equivariant Transporter Network

Transporter Net is a recently proposed framework for pick and place that is able to learn good manipulation policies from a very few expert demonstrations. A key reason why Transporter Net is so sample efficient is that the model incorporates rotational equivariance into the pick module, i.e. the model immediately generalizes learned pick knowledge to objects presented in different orientations. This paper proposes a novel version of Transporter Net that is equivariant to both pick and place orientation. As a result, our model immediately generalizes place knowledge to different place orientations in addition to generalizing pick knowledge as before. Ultimately, our new model is more sample efficient and achieves better pick and place success rates than the baseline Transporter Net model.Comment: Project Website: https://haojhuang.github.io/etp_page

Research on the vibration damping performance of hydro-pneumatic suspension of mine dump truck

The working environment of mine dump truck is relatively harsh, thus the suspension system with poor performance will directly affect driver's driving comfort and physical and mental health. An oil cylinder is one of the key components of the hydro-pneumatic suspension system, however, the high friction levels between cylinder and piston would lead to ‘friction locking’ phenomenon in the practical application. In order to improve the vibration damping performance of hydro-pneumatic suspension system and enhance the comfortability of whole vehicle, a 110 t mine dump truck was studied by theoretical and experimental method. The results of this study show that increasing the length of cylinder guide can effectively decrease the cylinder vibration transmission rate and improve the driving comfortability of vehicle

CoRec: An Easy Approach for Coordination Recognition

In this paper, we observe and address the challenges of the coordination recognition task. Most existing methods rely on syntactic parsers to identify the coordinators in a sentence and detect the coordination boundaries. However, state-of-the-art syntactic parsers are slow and suffer from errors, especially for long and complicated sentences. To better solve the problems, we propose a pipeline model COordination RECognizer (CoRec). It consists of two components: coordinator identifier and conjunct boundary detector. The experimental results on datasets from various domains demonstrate the effectiveness and efficiency of the proposed method. Further experiments show that CoRec positively impacts downstream tasks, improving the yield of state-of-the-art Open IE models.Comment: Accepted by EMNLP 2023 Main Conference (oral presentation

The Magnetic Properties of 1111-type Diluted Magnetic Semiconductor (La1−x_{1-x}Bax_{x})(Zn1−x_{1-x}Mnx_{x})AsO in the Low Doping Regime

We investigated the magnetic properties of (La$_{1-x}$Ba$_{x}$)(Zn$_{1-x}$Mn$_{x}$)AsO with $x$ varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of $x$ $\leq$ 0.01, the system remains paramagnetic down to the lowest measurable temperature of 2 K. Only when the doping level increases to $x$ = 0.02 does the ferromagnetic ordering appear. Our analysis indicates that antiferromagnetic exchange interactions dominate for $x$ $\leq$ 0.01, as shown by the negative Weiss temperature fitted from the magnetization data. The Weiss temperature becomes positive, i.e., ferromagnetic coupling starts to dominate, for $x$ $\geq$ 0.02. The Mn-Mn spin interaction parameter $\mid$$2J/k_B$$\mid$ is estimated to be in the order of 10 K for both $x$ $\leq$ 0.01 (antiferromagnetic ordered state) and $x$ $\geq$ 0.02 (ferromagnetic ordered state). Our results unequivocally demonstrate the competition between ferromagnetic and antiferromagnetic exchange interactions in carrier-mediated ferromagnetic systems.Comment: 9 pages, 3 figure