空心板材固有频率的求解及优化设计

用扩展Ｄｉｒａｃ函数一表示空心板材在刚度以及质量密度上由空洞而引起的非连续变化，从而不连续函数的求解问题为连续函数的求解问题，继而用Ｇａｌｅｒｋｉｎ法对其固有频率进行了求解，得到了适应于任意大小、任意分布空调的空心板材的固有频率计算公式。计算结果显示：空心板材的固有频率不仅与板材的几何尺寸有关，而且还与各空调的位置即分布方式有关，除此之外还与混凝土的Ｐｏｉｓｓｏｎ比有关。     

Influence of slip boundary on flow separation and drag of flow past bluff body at high Reynolds numbers北大核心CSCD

［Objectives］Flow separation increases the drag and noise of underwater vehicles, and influences the controllability of their control surfaces. Therefore, the influence of slip caused by superhydrophobic surfaces on drag reduction and flow separation is studied. ［Methods］A partial slip boundary condition is developed, and the flow around a circular cylinder and foil with a slip boundary at high Reynolds numbers are numerically simulated. ［Results］The results show that the when the slip length increases, the flow around the cylinder goes through three stages: the turbulent Kármán vortex street, laminar Kármán vortex street and non-separation Stokes flow. The drag coefficient increases first and then decreases, and the vortex shedding frequency increases. For flow around a foil, the separation position moves downstream until the separation region disappears when the slip length increases, and the drag coefficient decreases while the lift coefficient increases. ［Conclusions］The results of this study show that for flow past bluff body at high Reynolds number, the slip boundary can control flow separation and reduce drag effectively, providing technical support for the application of superhydrophobic surfaces for the flow control of underwater vehicle appendages. © 2022 Journal of Clinical Hepatology. All rights reserved.     

Visibility mapping in New York's coastal zone: A case study of alternative methods

Abstract

Most scenery evaluation methodologies incorporate visibility extent. In New York's coastal zone, visibility mapping for planning and project studies is complicated by low topographic relief, and diverse configurations of water edge and upland surface character. A generalized visibility model has been developed, including macrolandscape, observer, and sight‐line recording. Based on a prior shorescape study of New York's coast, the Lake Ontario Port Bay site was selected to test alternative visibility methods. Within the study site, four “landscape control points”; were chosen which provided extensive views of representative land and water forms, and surface types. For each control point, four families of visibility mapping approaches were applied: primary (field observation); secondary (topographic maps and vertical air photo analysis); tertiary (physical topographic model); and quaternary (digital terrain computer model). Major study conclusions are: the methods require different resources; all methods readily produced visibility maps; all methods, except secondary, could be used to produce perspective scenes for subsequent content evaluation; all methods except primary omitted one or more elements of the general visibility model; all methods should incorporate field work due to critical viewer environment conditions; sensitivity analysis in each method produced ambiguous zones; each view map was different, particularly in the background. An integrated, multi‐approach strategy would appear desirable for most planning and project applications.     

Formulating and solving the network design problem by decomposition

The optimal transportation network design problem is formulated as a convex nonlinear programming problem and a solution method based on standard traffic assignment algorithms is presented. The technique can deal with network improvements which introduce new links, which increase the capacity of existing links, or which decrease the free-flow (uncongested) travel time on existing links (with or without simultaneously increasing link capacity). Preliminary computational experience with the method demonstrates that it is capable of solving very large problems with reasonable amounts of computer time.     

Commuting costs and the residential mobility of job changers

In traffic and transport research, attention is given to the relevance of location patterns of activities to moving behaviour, the inverse causality being mostly left out of account. This paper considers what influence (changes in) travel costs have on moving behaviour and residential choice. The analysis has been carried out for employed people who change jobs. The residential choice has been split into a marginal probability of moving and a conditional destination choice. Both choices appear to be influenced significantly by travel‐cost variables.     

Crashworthiness design based on a simplified deceleration pulse

This paper proposes a procedure to improve the design of an automobile crashworthiness using the deceleration pulse in a simplified form as a design variable. A complete vehicle in a full frontal crash was simulated to find its deceleration pulse by finite element method. Based on this deceleration pulse, sled tests were performed, also in a virtual environment. Comparisons between the real deceleration pulse and a simplified pulse were made based on the HIC₁₅ produced. The simplified pulse is developed by dividing the pulse in three phases, each with a constant level of deceleration. Simulations were made to minimize the HIC₁₅ changing parameters in the restraint system and in the deceleration pulse. An expression was found to relate HIC₁₅ and the first phase of the deceleration pulse. A design case using this expression is presented. The benefits of using the pulse as a design variable along with the restraint system are accounted.     

On appropriate microstate descriptions in entropy modelling

Using an information-theoretic approach to entropy modelling, expressions for the possible number of microstates are derived from the sets of choices available to households and firms in given location/transport planning situations. In planning applications, the urban space is usually subdivided into discrete zones, and various classes of householders and firms are grouped into separate activities. Nevertheless, the location sites within zones may vary considerably in accessibility, price and quality, and amenity v. cost trade-offs between location and travel many differ markedly between individuals within the same activity group. The inclusion of such variations within a random utility framework is demonstrated to be equivalent to maximizing entropy with the microstates disaggregated to the level where these variations occur. Using constraints based on spatial conditions, observed behaviour and planning policy, entropy maximization is used to determine the most probable macrostate according to each choice set specification. The resulting distributions are compared and their revelance discussed.     

Visitor Preferences for Public Beach Amenities and Beach Restoration in South Florida

Coastal erosion threatens many sandy beaches and the ecological, economic, social and cultural amenities they provide. The problem is especially chronic in South Florida. A frequent solution for beach restoration involves sand replacement, or nourishment, but is temporary, expensive, and has usually been funded by governmental sources. However, as such agencies reduce their share and require more local funding, beach nourishment must rely on other funding sources, including beach recreationists. Our study characterized three South Florida beaches and probed visitor willingness-to-pay for beach nourishment. We found that even beaches within close proximity attract different user types. Users are amenable to higher fees if they lead to greater resource protection.     

Transport reviews prize

Bombay, the commercial capital of India, is suffering from traffic thrombosis. Bombay used to be an attractive city some three decades ago and was commonly referred to as ‘Bombay the Beautiful’. The picture is now, however, the opposite extreme.

The way Bombay has grown, an underground railway is no doubt justified. Unfortunately, the economy may not be able to afford it for many years to come. Low cost measures have therefore to be adopted like (a) development of land use with a view to controlling the traffic demand itself; (b) discouraging further growth in and around the existing congested central business district; (c) establishment of a counter‐magnet; (d) conversion of the mono‐centric pattern to a poly‐nucleated structure; (e) better pedestrian facilities; (f) intermediate public transport; (g) traffic management measures to maximize available road capacities; (h) bus priority measures; (i) creating an east‐west axis for activities to relieve the present north‐south axis.

This would form the core of transport strategy. Capital intensive rail projects will face severe shortage of funds. An additional road/rail link has also been suggested to the mainland. The congestion in Bombay can be relieved if the master plan is prepared for an integrated system. Traffic and town planning must go hand in hand with priority to traffic planning.