Investigation on Rijke pipe＇s acoustic characteristics by numerical simulation： modeling the pulsing flow field coupled the inner of pipe with its outer space

Based on the results of fluid dynamics, heat transfer and acoustics, a Computational Fluid Dynamics （CFD） method was utilized to study the acoustic characteristics and self-excited pulsation mechanism inside a Rijke pipe. To avoid settling the irrational boundary conditions of the finite-amplitude standing wave in the Rijke thermo-acoustic system, the simulation modeling in the flow field, which coupled the inner of pipe with its outer space, was carried out to replace the traditional way in form of internal flow field numerical investigations. A hypothesis for heat source in energy equation including the relationship on unsteady heat of air around heat source, oscillation pressure and oscillation velocity was presented. To reflect the essence of Rijke pipe, simulation on self-excited oscillation was conducted by means of its own pulsation of pressure, velocity and temperature. This method can make the convergence process steady and effectively avoid divergence. The physical phenomenon of the self-excited Rijke pipe was analyzed. Moreover, the mechanisms on the Rijke pipe＇s self-excited oscillation were explained. Based on this method, comparative researches on the acoustic characteristic of the Rijke pipe with different size and different shape of nozzle were performed. The simulation results agreed with the experimental data satisfactorily. The results show that this numerical simulation can be used to study the sound pressure of nozzle for the engineering application of Rijke pipes.     

Effect of Ce-doped MnO<Subscript>2</Subscript> catalyst on the lithium-air batteryperformance in the ambient atmosphere

MnO₂ doped with Ce was hydrothermally synthesized and the as-made breathable waterproof membrane used outside the cathode was prepared for improving the lithium-air battery performance in air. The samples were characterized by scanning electron microscopy (SEM), energy dispersive spectrum analysis (EDS), charge–discharge cycle tests, charge–discharge cycle tests of limited capacity, and electrochemical impedance spectroscopy (EIS) tests. The result showed that Ce _x Mn_1-x O₂ can effectively reduce the charge overpotential of the cathode. The charge and discharge electrical potential difference of Ce_0.1Mn_0.9O₂ was only 700 mV while MnO₂’s was 2100 mV. And Ce_0.1Mn_0.9O₂ that exhibited high discharge capacity of 400 mAh g^?1 in air had a stable discharge platform of 2.5 V and then the more obvious charge phenomenon appeared after 3.5 V. The excellent catalysis, the effect of cathode catalytic materials named Ce _x Mn_1-x O₂, may attribute to the decrease of reaction potential energy of oxygen reduction reaction and oxygen evolution reaction.     

An $$O(s^r)$$ O ( s r ) -resolution ODE framework for understanding discrete-time algorithms and applications to the linear convergence of minimax problems

Mathematical Programming - There has been a long history of using ordinary differential equations (ODEs) to understand the dynamics of discrete-time algorithms (DTAs). Surprisingly, there are still...     

Establishment and practical application of the electron transfer model in lithium-air batteries

The electron transfer failure model is proposed to analyze the effects of discharge products on the air electrode performance in lithium-air batteries. In order to understand how the air electrode products are obtained during the discharge cycle, three kinds of carbon-based air electrode are constructed, and discharged products are analyzed by powder XRD and SEM techniques. It can be concluded that the air electrode is covered with the products, indicating that the discharge products closely depended on the surface states of carbon materials. EIS studies show that the value of R _ct value changes due to the deposition of discharge products on carbon materials. Finally, all present studies show that the air electrode failure is mainly caused by the difficulty of charge transfer.     

The landscape of the proximal point method for nonconvex–nonconcave minimax optimization

Mathematical Programming - Minimax optimization has become a central tool in machine learning with applications in robust optimization, reinforcement learning, GANs, etc. These applications are...     

7H-三呋咱并[3,4-b:3',4'-f:3',4'-d]氮杂环庚三烯及衍生物的合成与表征

以3,4-双(3'-氨基呋咱-4'-基)呋咱(BATF)为原料,经Caro’s acid氧化制备了3,4-双(3'-硝基呋咱-4'-基)呋咱(BNTF),收率为93%;基于BNTF分子中的NO2反应性,与水合肼、氨水等发生亲核取代、环化反应,合成了含能化合物7H-三呋咱并[3,4-b:3',4'-f:3',4'-d]氮杂环庚三烯(HTFAZ)和7-氨基-三呋咱并[3,4-b:3',4'-f:3',4'-d]氮杂环庚三烯(ATFAZ).在硝酸-醋酐体系中氧化HTFAZ得到新化合物7,7'-二(呋咱[3,4-b:3,4'-d:3',4'-f]氮杂环庚三烯)(BTFZ),采用红外光谱、核磁共振、元素分析和质谱等进行了结构表征.培养了HTFAZ单晶,X射线单晶衍射分析表明:HTFAZ晶体结构属单斜晶系,空间群为P2(1)/n,a=0.7252(15)nm,b=0.6458(14)nm,c=1.6814(4)nm,V=0.7781(3)nm3,Z=4,Dc=1.871 g·cm-3,F(000)=440,R1=0.0353,w R2=0.0895.开展了HTFAZ,ATFAZ和BTFZ的物化、爆轰性能和热性能研究,其中HTFAZ密度1.871 g·cm―3、生成焓733.7kJ·mol-1、熔点193.1℃、分解点345.1℃、爆速8416.5 m·s-1、爆压为32.9GPa;ATFAZ密度1.788 g·cm-3、生成焓831.2kJ·mol-1、熔点204~207℃、分解点323.7℃、爆速8300 m·s-1、爆压为30 GPa;BTFZ密度1.862 g·cm-3、生成焓1752.4kJ·mol-1、分解点280.2℃、爆速7924.5 m·s-1、爆压为28.42 GPa.     

New computational guarantees for solving convex optimization problems with first order methods,via a function growth condition measure

Motivated by recent work of Renegar (A framework for applying subgradient methods to conic optimization problems, arXiv:1503.02611v2, 2015) we present new computational methods and associated computational guarantees for solving convex optimization problems using first-order methods. Our problem of interest is the general convex optimization problem \(f^* = \min _{x \in Q} f(x)\), where we presume knowledge of a strict lower bound \(f_{\mathrm{slb}}< f^*\). [Indeed, \(f_{\mathrm{slb}}\) is naturally known when optimizing many loss functions in statistics and machine learning (least-squares, logistic loss, exponential loss, total variation loss, etc.) as well as in Renegar’s transformed version of the standard conic optimization problem arXiv:1503.02611v2, 2015; in all these cases one has \(f_{\mathrm{slb}}= 0 < f^*\).] We introduce a new functional measure called the growth constant G for \(f(\cdot )\), that measures how quickly the level sets of \(f(\cdot )\) grow relative to the function value, and that plays a fundamental role in the complexity analysis. When \(f(\cdot )\) is non-smooth, we present new computational guarantees for the Subgradient Descent Method and for smoothing methods, that can improve existing computational guarantees in several ways, most notably when the initial iterate \(x^0\) is far from the optimal solution set. When \(f(\cdot )\) is smooth, we present a scheme for periodically restarting the Accelerated Gradient Method that can also improve existing computational guarantees when \(x^0\) is far from the optimal solution set, and in the presence of added structure we present a scheme using parametrically increased smoothing that further improves the associated computational guarantees.     

内外流场耦合法数值研究Rijke管声学特性

基于流体力学、传热学和声学研究成果,采用内外流场耦合建模的方法研究Rijke管脉动内流场,解决了大振幅驻波热声场管口边界条件的处理和出现数值发散的问题。从能量平衡的角度出发,提出非定常的热流量与声压、质点振速之间关系的假设。依靠自身参量,如气体的脉动压力、脉动流速和温度波动,来激发Rijke管振荡.模拟了Rijke管的声学特性,分析了Rijke管自激励振荡的机理。运用该模拟方法对不同尺寸、不同管口形状的Rijke管进行了数值实验,模拟结果与实验结果较好吻合,能捕捉Rijke管的管口声学特征量。     

Faster first-order primal-dual methods for linear programming using restarts and sharpness

Mathematical Programming - First-order primal-dual methods are appealing for their low memory overhead, fast iterations, and effective parallelization. However, they are often slow at finding high...