Analysis of model relations for turbulent heat diffusion as applied to the calculation of wake flows

An experimental test of the known approximations is made and a new relation is proposed for turbulent diffusion of heat or of a passive admixture.Translated from Inzhererno-Fizicheskii Zhurnal, Vol. 51, No. 6, pp. 896–902, December, 1986.     

Pathpair: a fast available bandwidth estimation tool with the asymptotic one way delay comparison model

Significant research has been dedicated to active probing methods used to estimate the end-to-end available bandwidth (avail-bw) of a network path. The measurement speed of an avail-bw estimation tool is vital to real-time network services such as transport control protocol (TCP) slow start, path selection for P2P applications and video streaming services and so on. In addition, a fast avail-bw estimation tool sends less probing traffic to a measured network. Pathload, a one way delay (OWD) based tool, has been widely regarded as one of the most accurate tools, but its estimation speed is often 10 or more seconds per measurement. A fast OWD-based avail-bw estimation scheme has been explored. This scheme is based on the proposed asymptotic OWD comparison model, and an estimation tool called Pathpair has been developed. Validated in a real Internet environment, experimental results show that with Pathpair the measurement speed has been significantly improved (1-3 s per measurement), while preserving accuracy compared with other major tools like Pathload and Spruce.     

Strict and effective bounds in goal‐oriented error estimation applied to fracture mechanics problems solved with XFEM

In this work, we analyze a method that leads to strict and high‐quality local error bounds in the context of fracture mechanics. We investigate in particular the capability of this method to evaluate the discretization error for quantities of interest computed using the extended finite element method (XFEM). The goal‐oriented error estimation method we are focusing on uses the concept of constitutive relation error along with classical extraction techniques. The main innovation in this paper resides in the methodology employed to construct admissible fields in the XFEM framework, which involves enrichments with singular and level set basis functions. We show that this construction can be performed through a generalization of the classical procedure used for the standard finite element method. Thus, the resulting goal‐oriented error estimation method leads to relevant and very accurate information on quantities of interest that are specific to fracture mechanics, such as mixed‐mode stress intensity factors. The technical aspects and the effectiveness of the method are illustrated through two‐dimensional numerical examples. Copyright © 2009 John Wiley & Sons, Ltd.     

Analysis of optimal centroid estimation applied to Shack-Hartmann sensing

The problem of estimating the centroid of an incoherently imaged point with a CCD array is analyzed. An exact analysis is presented that uses the actual short-exposure function at the CCD instead of the traditional Gaussian approximation. The analysis shows that, for Poisson noise, the centroid variance depends on the CCD size and that truncation effects play a significant part in determining the optimum CCD size. The effects of this on a wave-front reconstruction formed by a Shack-Hartmann sensor are described.     

Adaptive wavelets applied to the analysis of nonlinear systems with chaotic dynamics

We consider an approach to the analysis of nonstationary processes based on the application of wavelet basis sets constructed using segments of the analyzed time series. The proposed method is applied to the analysis of time series generated by a nonlinear system with and without noise.     

Extended kinetic theory applied to inclined granular flows: role of boundaries

We compare the predictions of extended kinetic theory (EKT), where the roles of surface friction and correlation in fluctuation velocities are taken into account, with discrete element simulations of steady, fully-developed, inclined flows of identical spheres over bumpy bases, in the presence and absence of flat, frictional sidewalls. We show that the constitutive relation for the pressure of EKT must be modified in the proximity of the boundary, because of the influence of excluded volume and shielding associated with collisions of particles with the boundary itself. We also note that currently available boundary conditions for flows over bumpy planes in kinetic theory underestimate the energy dissipation. These two observations explain the lack of agreement of EKT with the simulations, in terms of the maximum angles of inclination for which steady, fully-developed flows are possible. That is, for some high angles of inclination, EKT does not have solutions, while steady flows are predicted in DEM. However, whenever a solution to the system of differential equations of EKT does exist, the predicted distributions of velocity, solid volume fraction and granular temperature satisfactorily match the numerical measurements. The incompressible, algebraic approximation of EKT, which ignores the conduction of energy in the energy balance, admits solutions for a wider range of angles of inclination, as in the simulations, but fails to reproduce the quantitative and qualitative behaviour of solid volume fraction and granular temperature in the two conductive layers at the top and bottom of the flow. When frictional sidewalls are added to the domain, we show that the spanwise ratio of shear stress to pressure is linearly distributed in the dense core region of the flow, confirming that the sidewalls exert, on average, a Coulomb-like resistance to the flow with an effective friction coefficient which is less than half the actual particle-wall friction.     

Comparison of two non-convex mixed-integer nonlinear programming algorithms applied to autoregressive moving average model structure and parameter estimation

In this article, the stochastic modelling approach proposed by Box and Jenkins is treated as a mixed-integer nonlinear programming (MINLP) problem solved with a mesh adaptive direct search and a real-coded genetic class of algorithms. The aim is to estimate the real-valued parameters and non-negative integer, correlated structure of stationary autoregressive moving average (ARMA) processes. The maximum likelihood function of the stationary ARMA process is embedded in Akaike's information criterion and the Bayesian information criterion, whereas the estimation procedure is based on Kalman filter recursions. The constraints imposed on the objective function enforce stability and invertibility. The best ARMA model is regarded as the global minimum of the non-convex MINLP problem. The robustness and computational performance of the MINLP solvers are compared with brute-force enumeration. Numerical experiments are done for existing time series and one new data set.     

缩尺模型实验在交通噪声影响评价中的应用

本文综述了缩尺模型实验在交通噪声对环境的影响评价的应用，模型实验的基本原理，及实现交通噪声的模拟方法。计算机模拟也是一种预报交通噪声传播特性的方法，它和模型实验相辅相成，在实验中二者应紧密结合。最后指出缩尺模型实验中应注意的问题。     

Optimization and simulation of traffic flows in the case of evacuating urban areas

In cases where urban areas need to be evacuated, a large number of vehicles must be routed through narrow street networks effectively. This paper combines two aspects of this problem. On the one hand, the traffic flows must be routed through the road network in such a way that e.g. the time required to get all out is as small as possible. Hence, we have to solve an optimization problem to find out which roads shall be used in what direction. On the other hand, even if routes are defined, traffic flows are dynamic in nature and phenomena related to the traffic jam are to be examined by simulation. By defining an optimization-based simulation procedure, we get a heuristic for the overall problem. As an example we compute and simulate an evacuation plan for a neighborhood in the city of Duisburg, Germany, to demonstrate the applicability of our approach to real-world sized problems.     

Adaptive mesh refinement applied to the scalar transported PDF equation in a turbulent jet

This paper describes a novel solution method for the transported probability density function (PDF) equation for scalars (compositions). In contrast to conventional solution methods based on the Monte Carlo approach, we use a finite‐volume method combined with adaptive mesh refinement (AMR) applied in both physical and compositional space. The obvious advantage of this over a uniform grid is that fine meshes are only used where the solution requires high resolution. The efficiency of the method is demonstrated by a number of tests involving a turbulent jet flow with up to two scalars (both reacting and non‐reacting). We find that the AMR calculation can be at a fraction of the computer cost of a uniform grid calculation with the same accuracy. Copyright © 2010 John Wiley & Sons, Ltd.     

Adaptive polynomial chaos expansions applied to statistics of extremes in nonlinear random vibration

This paper presents a new module towards the development of efficient computational stochastic mechanics. Specifically, the possibility of an adaptive polynomial chaos expansion is investigated. Adaptivity in this context refers to retaining, through an iterative procedure, only those terms in a representation of the solution process that are significant to the numerical evaluation of the solution. The technique can be applied to the calculation of statistics of extremes for nongaussian processes. The only assumption involved is that these processes be the response of a nonlinear oscillator excited by a general stochastic process. The proposed technique is an extension of a technique developed by the second author for the solution of general nonlinear random vibration problems. Accordingly, the response process is represented using its Karhunen-Loeve expansion. This expansion allows for the optimal encapsulation of the information contained in the stochastic process into a set of discrete random variables. The response process is then expanded using the polynomial chaos basis, which is a complete orthogonal set in the space of second-order random variables. The time dependent coefficients in this expansion are then computed by using a Galerkin projection scheme which minimizes the approximation error involved in using a finite-dimensional subspace. These coefficients completely characterize the solution process, and the accuracy of the approximation can be assessed by comparing the contribution of successive coefficients. A significant contribution of this paper is the development and implimentation of adaptive schemes for the polynomial chaos expansion. These schemes permit the inclusion of only those terms in the expansion that have a significant contribution.     

A postponement model to determine the customisation degree applied to the notebook computer industry

Facing an increasingly intense and competitive environment, the information industry must design a global service chain for its self-development and also to make its global development more efficient. The application of the postponement concept has become an efficient method to help enterprises reach this goal. With the aim of building and analysing a postponement structure from the viewpoint of added value and demand uncertainty, this study constructed a multi-objective postponement model. The model was then analysed, using three quantifying objects: the overall cost, product types and the average assembly time; and two decision variables: the variety of parts and inventory quantity. In an indeterminate function, both the application demanding frequency and demanded quantity submit to a Poisson distribution and Normal distribution. Considering the profit model, the manufacturer can decide on the most suitable degree of customisation according to market situations, suppliers at the time of production and most important, the manufacturer's profit. This study uses a notebook computer manufacturer as an example to carry out empirical analysis. The manufacturer was permitted to decide the optimal product types depending on its financial status. Postponement techniques were then provided as a reference to the manufacturer to help it maximise profits.     

The cohesive frictional crack model applied to the analysis of the dam-foundation joint

The mechanical behaviour of dam-foundation joints plays a key role in concrete dam engineering since it is the weakest part of the structure and therefore the evolutionary crack process occurring along this joint determines the global load-bearing capacity. The reference volume involved in the above mentioned process is so large that it cannot be tested in a laboratory: structural analysis has to be carried on by numerical modelling. The use of the asymptotic expansions proposed by Karihaloo and Xiao [13] at the tip of a crack with normal cohesion and Coulomb friction can overcome the numerical difficulties that appear in large scale problems when the Newton-Raphson procedure is applied to a set of equilibrium equations based on ordinary shape functions (Standard Finite Element Method). In this way it is possible to analyze problems with friction and crack propagation under the constant load induced by hydro-mechanical coupling. For each position of the fictitious crack tip, the condition K₁=K₂=0 allows us to obtain the external load level and the tangential stress at the tip. If the joint tangential strength is larger than the value obtained, the solution is acceptable, because the tensile strength is assumed negligible and the condition K₁=0 is sufficient to cause the crack growth. Otherwise, the load level obtained can be considered as an overestimation of the critical value and a special form of contact problem has to be solved along the fictitious process zone. For the boundary condition analyzed (ICOLD benchmark on gravity dam model), after an initial increasing phase, the water lag remains almost constant and the maximum value of load carrying capacity is achieved when the water lag reaches its constant value.     

Estimating the number of accidents at intersections from a knowledge of the traffic flows on the approaches

The increase in the number of accidents at intersections, relative to non-junction accidents, may be explained by the basic fact that an increase in the number of vehicles on the road is generally accompanied by an increase in the number of collisions, which rise at a faster rate than single-vehicle accidents. Generally, more than 50% of the collisions occur at intersections. Data from a number of countries studied support the above statement and show that over the years, the number of intersection accidents has increased at a faster rate than other accidents. The study includes a statistical analysis of the general trends in the number of intersection accidents, their severity, and an analysis of types of accidents at intersections. A model was developed which enables the estimation of the expected number of accidents at individual intersections. It was found that vehicle exposure can be used as the basis for estimation. The exposure, denned as the number of occasions for accidents, was calculated through the sum of the products of flow at the 24 points where vehicle paths cross or merge. This measure of exposure, expressed as a traffic flow index, showed good correlation with the number of accidents. On the assumption that the number of accidents at an intersection, in a given time interval, is Poisson distributed, significance tests were made comparing the actual number of accidents with the expected according to the vehicle exposure index. Such comparisons are useful in the determination of accident “black spots”.     

An experimental comparison in the uncertainty estimation affecting wavelet-based signal analysis by means of the IEC-ISO guide and the random-fuzzy approaches

The aim of this paper is a comparative estimation of the measurement uncertainty associated with the results of complex measurements, such as the evaluation of the wavelet transform of signals affected by transient disturbances. Uncertainty estimation is performed by means of two different methods, namely 1) the approach suggested by the International Electrotechnical Commission-International Organization for Standardization (IEC-ISO) "Guide to the Expression of Uncertainty in Measurement" (GUM) and 2) the one, recently proposed in the literature, based on the use of random-fuzzy variables. The comparison is performed starting from actual signals generated and acquired by means of a suitable test system.     

Experimental verification of models of triple mixed correlation between velocity and temperature as applied to the calculation of jet flows

The optimum values of the constants in the well-known approximation of the moment u ₂ ² are determined from the results of experimental investigations of nonisothermal wakes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 3, pp. 362–369, March, 1987.     

Application of the Lambert W function to steady shearing flows of the Papanastasiou model

Using the Lambert W function, the constitutive relation of the Papanastasiou model is inverted so that the second invariant of the first Rivlin-Ericksen tensor can be expressed as a function of the second invariant of the extra stress tensor. In steady shearing flows, this results in the magnitude of the shear rate becoming a function of the magnitude of the shear stress. Since the distribution of the latter is known explicitly in channel, Poiseuille and Couette flows, one can investigate the nature of analytical solutions in these flows. It is shown that explicit answers are found for channel and Poiseuille flows only, with the Couette flow requiring a numerical solution in general. From the channel flow results, it is obvious that there is a great amount of congruence between the predictions of the Papanastasiou model and the Bingham fluid. In turn, this lends further confidence to the application of the Papanastasiou model to study the flows of Bingham fluids.