Worst-case optimal investment with a random number of crashes

We study a portfolio optimization problem in a market which is under the threat of crashes. At random times, the investor receives a warning that a crash in the risky asset might occur. We construct a strategy which renders the investor indifferent about an immediate crash of maximum size and no crash at all. We then verify that this strategy outperforms every other trading strategy using a direct comparison approach. We conclude with numerical examples and calculating the costs of hedging against crashes.     

Effects of gas bubbles on the concentration profiles and conversion efficiency of three-dimensional packed-bed electrodes

The effects of evolving gas bubbles on the concentration profile and conversion efficiency of three-dimensional packed-bed electrode was simulated for the first time, taking account of material balance, bubble, ohmic, kinetic, and mass transfer effects. The model produced different dimensionless groups and parameters that control the behavior of the packed-bed electrode under potentiostatic conditions. The effects of the different groups on the conversion efficiency, concentration, polarization, and current profiles were studied. Higher conversion efficiency were obtained at higher values of ω , lower values of δ and higher values of the bubble group, ξ (). Gas bubble formation retarded the operation at higher conversion efficiency. In presence of gas bubble, lower values of δ were required to obtain as much as conversion efficiency obtained when the gas bubble formation is absent. Also, the bubble formation retarded the operation at lower flow rates as it causes lower obtainable current and non-uniform distributions of the currents. A case study was introduced for understanding the separate important operating conditions, e.g., flow rate. Account of gas bubble effects on the concentration profiles for such system is crucial.     

Generation of Superstable,Monodisperse Microbubbles Using a pH‐Driven Assembly of Surface‐Active Particles

Bubbling to the surface : Microscale gas bubbles can be generated in a microfluidic device by simultaneously injecting CO₂ and a dispersion of particles whose hydrophobicity increases as the pH value decreases. The CO₂ dissolves rapidly out of the bubbles, which shrink, and render the dispersion increasingly acidic. This drives the particles to the bubble surface where they form a type of “armor” against further dissolution (see picture).

     

Spatial distribution of sonoluminescence and sonochemiluminescence generated by cavitation bubbles in 1.2 MHz focused ultrasound field

An intensified charge coupled device (ICCD) camera was used to observe the spatial distribution of sonoluminescence (SL) and sonochemiluminescence (SCL) generated by cavitation bubbles in a 1.2 MHz focused ultrasound (FU) field in order to investigate the mechanisms of acoustic cavitation under different sonication conditions for FU therapeutic applications.It was found that SL emissions were located in the post-focal region. When the intensity of SL and SCL increased as the power rose, the growth of SCL was much higher than that of SL. In the post-focal region, the SCL emissions moved along specific paths and formed branch-like streamers. At the beginning of the ultrasound irradiation, cavitation bubbles generated SCL in both the pre-focal and the post-focal region. When the electrical power or the sonication time increased, the SCL in the post-focal region increased and became higher than that in the pre-focal region. The intensity of SCL in the focal region is usually the weakest because of “oversaturation”.The spatial distribution of SCL near a tissue boundary differed from that obtained in free fields. It organized into special structures under different acoustic amplitudes. When the electrical power was relatively low, the SCL emission was conical shape which suggested a standing wave formation at the tissue-fluid boundary. When the electrical power exceeded a certain threshold, only a bright spot could be captured in the focus. The cavitation bubbles which centralized in the focus concentrated energy and hindered the formation of standing waves. With rising electrical power at high levels, besides a bright spot in the focus, there were some irregular light spots in pre-focal region, which indicated some cavitation bubbles or small bubble clusters achieved the threshold of SCL and induced the reaction with the luminol solution.     

氦离子注入4H-SiC晶体的纳米硬度研究

4H-SiC晶体经能量为100 keV,剂量为3×10¹⁶ cm^-2的氦离子高温（500 K）注入后,再在773—1273 K温度范围内进行了退火处理,最后使用纳米压痕仪测量了样品注入面的硬度.测试结果表明,在500—1273 K温度范围内样品的硬度随退火温度升高呈现先增大后减小再增大的趋势,其中773 K退火样品的硬度增大明显.分析认为,退火样品的硬度变化是由退火过程中缺陷复合与氦泡生长导致样品内部的Si—C键密度、键长和键角改变引起的. 关键词： SiC 注入 氦泡 纳米压痕     

基于粒子成像测速技术的水中气泡界面的光学性质研究

基于粒子成像测速的流体可视化技术，根据菲涅耳公式计算了入射到水中气泡界面的光强，得出点光源连续两次反射或折射的光强具有等比数列的规律，光线在气泡界面反射、折射4次就变成完全偏振光且光线几乎消失.当入射角避开35°左右时，即便有一定宽度的线光源入射到气泡界面，第2次折射出气泡的光强与线光源的宽度无关，类似一条光线入射所产生的光强，给出了面光源在界面所产生的光强的二重积分表达式.线光源所产生的界面光强理论值、面光源产生的光强数值解与实验值都较为符合. 关键词： 水中气泡 界面 光强 粒子成像测速     

Applications of Defocusing DPIV to Bubbly Flow Measurement

Defocusing digital particle image velocimetry (DDPIV) was used to investigate a bubbly flow in the wake of a hydrofoil. DDPIV is a three component volumetric velocimetry technique that operates at full video rate. Complex, three‐dimensional, and time‐dependent flows can be measured. To measure the bubble sizes, an extension to DDPIV was made to infer bubble sizes from their intensities. Both bubble size distributions and bubble velocity fields were simultaneously measured. Results indicate that DDPIV can reliably measure bubble sizes in the range of 100 microns, as well as resolving their aggregate motion.     

两种气泡混合的声空化

将非线性声波方程和改进的Rayleigh-Plesset方程联立可以描述空化环境中的声场及相应的气泡动力学特征. 用时域有限差分方法模拟了圆柱形容器内两种气泡相互混合时的空化情况. 在烧杯内的稳态背景声场形成过程中, 瓶壁耗散吸收扮演了重要的角色. 在稳态背景声场的基础上, 分析了混合气泡与声场的相互作用、气泡之间的相互作用、混合情况下的频谱特性. 结果表明: 两种气泡平衡半径都不太大时, 气泡与声场的相互作用不强, 声场及气泡的行为也比较规律; 相反, 当其中一种气泡平衡半径相对比较大时, 声场与气泡具有较强的非线性相互作用, 声场及气泡的行为表现出复杂的特性.     

Environment-friendly surface cleaning using micro-nano bubbles

Cleaning a surface using a solution containing a large number of micro to nano scale bubbles has significant advantage regarding environmental protection. This review first briefly introduces the cleaning mechanism of micro-nano bubbles (MNBs), including physical and chemical effects. Then the applications of MNBs in cleaning of metal parts, precision parts, cultural relics or food are introduced. After that, coupled cleaning method of ultrasound and bubbles is introduced. Finally, the characterization methods for the cleaning effect are introduced, which mainly focuses on the changes of physico-chemical properties (mass or cleaning area, infiltration, colony number and light scattering intensity) of the cleaned parts or that (like conductivity) of the solvent. It is believed that MNBs technology will be applied in a broader range of surface cleaning applications.     

Effective medium method of slightly compressible elastic media permeated with air-filled bubbles

An effective medium method is developed for the slightly compressible elastic media permeated with air-filled bubbles, according to the nonlinear oscillation of the bubble, which happens when compressional wave travels through the porous media. The effective Lame coefficients of the porous medium and the nonlinear elastic wave equation are deduced, based on the fact that the micro-unit of the effective medium should have the same stress and strain as the micro-unit of the porous media. The linearized properties obtained by this method are in good agreement with the results of Gaunaurd’s classic theory [Gaunaurd G.C. and überall H., J. Acoust. Soc. Am., 1978, 63: 1699–1711]. Furthermore, the nonlinear coefficient, which is an important property of the porous media, can also be acquired by this method. __________ Translated from Acta Acustica, 2006 (in Chinese) (in press)