新概念缔造新生活--美国NBBJ设计理念简介

NBBJ正在通过其丰富多元的经验和专业技能,为业主,使用者和社会提供能够创造更多价值的创新设计。     

Robustness studies for H∞ feedback control in a structural acoustic model with periodic excitation

Results from an extensive study on the robustness of an H^∞ compensator for a 2-D structural acoustic model are presented. The effects of frequency uncertainties in an exogenous signal are studied for both the case where the signal is contained in the controller formulation and the case where it is excluded. Delays are inserted in the input and/or output signals and their effect on the controller performance is recorded. A comparison between the standard LQG/Kalman filter and the H^∞/Min-Max compensator reveals no significant differences in the overall controller performance. Modifications in the controller structure are studied to see whether loss of information (the tracking variable) that must be calculated a priori would result in performance degradation. This study provides valuable insight into the computational and implementational issues that arise when dealing with the control of large and complex systems that are governed by partial differential equations.     

Femtosecond-laser sharp shaping of millimeter-scale geometries with vertical sidewalls

As femtosecond (fs) laser machining advances from micro/nanoscale to macroscale, approaches capable of machining macroscale geometries that sustain micro/nanoscale precisions are in great demand. In this research, an fs laser sharp shaping approach was developed to address two key challenges in macroscale machining (i.e. defects on edges and tapered sidewalls). The evolution of edge sharpness (edge transition width) and sidewall tapers were systematically investigated through which the dilemma of simultaneously achieving sharp edges and vertical sidewalls were addressed. Through decreasing the angle of incidence (AOI) from 0?to?5?, the edge transition width could be reduced to below 10 μm but at the cost of increased sidewall tapers. Furthermore, by analyzing lateral and vertical ablation behaviors, a parameter-compensation strategy was developed by gradually decreasing the scanning diameters along depth and using optimal laser powers to produce non-tapered sidewalls. The fs laser ablation behaviors were precisely controlled and coordinated to optimize the parameter compensations in general manufacturing applications. The AOI control together with the parameter compensation provides a versatile solution to simultaneously achieve vertical sidewalls as well as sharp edges of entrances and exits for geometries of different shapes and dimensions. Both mm-scale diameters and depths were realized with dimensional precisions below 10 μm and surface roughness below 1 μm. This research establishes a novel strategy to finely control the fs laser machining process, enabling the fs laser applications in macroscale machining with micro/nanoscale precisions.     

Effects of extraction conditions and alkali type on yield and composition of wheat straw hemicellulose

The utilization of various alkaline regimes for the optimal extraction and isolation of hemicellulose and cellulose from wheat straw was extensively examined. Factors in-vestigated include varying concentrations of one alkali (KOH) and H₃BO₃, varying temperature and time of extraction, and varying the nature of the alkali: calcium hy-droxide, sodium hydroxide, lithium hydroxide, and liquid ammonia were examined in this context. For example, a preferred extraction of hemicellulose from holocellulose preparations utilized a solution of 24% KOH/2% H₃BO₃ at 20°C for 2 h. This produced yields for hemicellulose and cellulose of 34.23 and 35.96%, respectively. The neutral sugar composition of the various hemicellulose fractions was found to vary slightly with treatment regime. In all hydrolysates of hemicellulose preparations, xylosé was by far the predominant sugar, comprising around 80% of the material. Minor constituents were arabinose, galactose, glucose, and uronic acids. The composition of phenolic acids and aldehydes in extracted wheat straw hemicellulose was also studied. The average molecular weights of the hemicellulose isolates ranged from 12,000 for the 30% KOH/2% H₃BO₃ (20°C, 2 h) extract to 27,000 for the extract obtained using 5% KOH/2% H₃BO₃ (20°C, 2 h). © 1996 John Wiley & Sons, Inc.     

A collision detection method for multicast transmissions in CSMA/CA networks

Compared to unicast traffic, multicast is not protected by any ARQ mechanism in 802.11 networks: collisions with other multicast and unicast transmissions are not detected and senders will not adapt to the contention situation by backing off. This results in an unreliable service for multicast transmissions. We propose early multicast collision detection (EMCD), an algorithm with the purpose of increasing the reliability of multicast transmissions in the MAC layer of an IEEE 802.11 network. A multicast sender using it will introduce an early pause in a transmission, perform a clear channel assessment (CCA), and if a collision is detected abort the transmission after a fixed time and schedule a retransmission. This allows for detecting collisions with both multicast and unicast transmissions but also adapting to the contention situation. A probabilistic analysis is provided showing that EMCD is more efficient than ordinary multicast and can be made even more efficient by tuning parameters. Simulations show that EMCD leads to increased reliability for multicast transmissions. Copyright © 2006 John Wiley & Sons, Ltd.     

Lipase-catalyzed synthesis of triolein-based sunscreens in supercritical CO2

Novozym^® 435-catalyzed transesterification of ethyl 4-hydroxy-3-methoxy cinnamate (ethyl ferulate, EF) with triolein to form the ultraviolet (UV)-absorbing lipids monoferuloylmonooleoyl-glycerol (FMO) and feruloyl-dioleoyl-glycerol (FDO) has been conducted using supercritical CO₂ (SC?CO₂) batch reactions. The alcoholysis of 0.1 M EF with 0.1 M 1-octanol in SC?CO₂ to form octyl ferulate was used as a model reaction to optimize pressure and temperature conditions. Conditions ranging from 45 to 80°C and 10.3 to 34.5 MPa (1,500 to 5,000 psi) were tested with a maximal conversion of 53% of the EF being achieved at 13.8 MPa (2,000 psi) and 80°C after 24 h. These optimized conditions applied to the transesterification of EF with triolein effected a combined FMO and FDO yield of 69%. Triolein exhibits higher solubilities in SC?CO₂ at higher pressures; therefore, the transesterification was performed at 80°C over a range of pressures from 13.8 to 34.5 MPa (2,000 to 5,000 psi). Results showed that a maximal yield of 74% of FMO and FDO was reached at 80°C and 24.1 MPa (3,500 psi) after 48 h. Compared to the FMO and FDO synthesis conducted neat or in toluene, the synthesis of the UV-absorbing lipids in SC?CO₂ affords higher yields within a shorter amount of time. Therefore, the transesterification of EF with triolein in a SC?CO₂ batch reaction is a viable route to UV-absorbing lipids that could be used as active ingredients in sunscreen formulations.     

Flow visualization of wave-type vortex generators having inline fin-tube arrangement

This study presents visual observation of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Three samples of fin-and-tube heat exchanger having inline arrangements are examined, including one plain fin and two wave-type vortex generators. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and a very large secondary flow circulation is seen between the first and second row. This flow re-circulation phenomenon is almost disappeared with the presence of proposed vortex generators. The presence of vortex generators significantly increase the vortrical motions of the horseshoe vortices hitting on the tubes. A much better mixing characteristics is seen by introducing the vortex generators. The frictional penalty of the proposed vortex generators are about 25-55% higher than that of the plain fin geometry. The penalty of pressure drops of the proposed vortex generators relative to plain fin geometry is relatively insensitive to change of Reynolds number.     

Artificial neural network prediction indicators of density functional theory metal hydride models

The metal hydride is a capable candidate for mobile storage for hydrogen-powered vehicles. An artificial neural network (ANN) has proved useful for many applications, and capable of much more in discovery of new materials. Because of its ability to generalize from examples presented to it, an ANN is a powerful tool for discovering new metal hydride combinations. An ANN can deduce quantitative structure property relationships for metal hydrides. The ANN found correlations between fundamental electronic and energy values modeled ab initio and several experimental parameters. Some of the properties successfully predicted with good correlation are: entropy, enthalpy, temperature at 1 atm of pressure, pressure at 25 °C, and the percent weight of hydrogen stored. The marriage of ANN to computational modeling produces good predictions for many important properties of metal hydrides.     

Conversion of oils to monoglycerides by glycerolysis in supercritical carbon dioxide media

Glycerolysis of soybean oil was conducted in a supercritical carbon dioxide (SC-CO₂) atmosphere to produce monoglycerides (MG) in a stirred autoclave at 150–250°C, over a pressure range of 20.7–62.1 MPa, at glycerol/oil molar ratios between 15–25, and water concentrations of 0–8% (wt% of glycerol). MG, di-, triglyceride, and free fatty acid (FFA) composition of the reaction mixture as a function of time was analyzed by supercritical fluid chromatography. Glycerolysis did not occur at 150°C but proceeded to a limited extent at 200°C within 4 h reaction time; however, it did proceed rapidly at 250°C. At 250°C, MG formation decreased significantly (P<0.05) with pressure and increased with glycerol/oil ratio and water concentration. A maximum MG content of 49.2% was achieved at 250°C, 20.7 MPa, a glycerol/oil ratio of 25 and 4% water after 4 h. These conditions also resulted in the formation of 14% FFA. Conversions of other oils (peanut, corn, canola, and cottonseed) were also attempted. Soybean and cottonseed oil yielded the highest and lowest conversion to MG, respectively. Conducting this industrially important reaction in SC-CO₂ atmosphere offered numerous advantages, compared to conventional alkalicatalyzed glycerolysis, including elimination of the alkali catalyst, production of a lighter color and less odor, and ease of separation of the CO₂ from the reaction products.     

Fast joining of composite pipes using UV curing FRP composites

In this paper, twelve butt-weld composite pipe joints were prepared. Among them, six were prepared using ultraviolet (UV) curing fiber reinforced plastics (FRPs) and six were prepared using ambient environment curing FRPs as controls. Filament wound composite pipes were used. Each section of pipe was 304.8 mm long with a 101.6 mm inner diameter. The wet lay-up technique was used to prepare the test samples. The curing time for the UV curing samples was 40 minutes, while the curing time was 24 hours for the control samples. Both internal pressure tests and four-point bending tests were conducted on the prepared samples. The test results show that the UV curing FRP wrapped composite pipe joints achieved nearly the same bending strength as the control samples. However, the internal pressure rating achieved by the UV curing FRP coupled joints was lower than that achieved by the control samples. A finite element analysis was conducted to understand the mechanism for the lowered internal pressure rating. Based on the test and the finite element analysis results, the UV curing FRP can be used in joining composite truss structures and composite frame structures. Suggestions were made to increase the internal pressure rating of UV curing FRP coupled composite pipes. Polym. Compos. 25:298–306, 2004. © 2004 Society of Plastics Engineers.