小当量地下爆炸的质点速度模型

在第四纪砂砾层中进行了系列公斤至百公斤级的地下封闭爆炸,研究了小当量化学爆炸地震波传播规律。在近场,地震波持时很短,水平振幅强于垂向振幅。质点速度随当量呈现指数增加,水平向指数为1.09,垂直向指数为0.77,质点速度随距离呈现指数衰减,水平向和垂直向指数分别为2.07与1.57。Sadauskas模型、显函模型和双极模型都能定量描述小当量地下爆炸地震波质点速度变化,但他们之间的反演精度存在差异,双极模型的残差最小。换言之,采用双极模型反演的数据更接近实际。     

Exact solutions for discrete velocity models

A new class of exact solutions for discrete kinetic models is presented. It is shown that these solutions can be used to solve both initial and boundary value problems of rarefied gas dynamics.     

Discrete models leading to a weakly dissipative korteweg-de vries equation

It is shown how a simple one-dimensional non-linear lattice model in which damping is present leads to a continuum approximation which is a generalization of the Korteweg-de Vries equation. The effect of the damping in the discrete model is to introduce a damping term in the continuum approximation which depends only on a power of the undifferentiated dependent variable. Numerical results are presented which show that such damping is extremely weak and that dispersive effects predominate.     

Discrete and continuous models for in plane loaded random elastic brickwork

The aim of this paper is to study non-periodic masonries – typical of historical buildings – by means of a perturbation approach and to evaluate the effect of a random perturbation on the elastic response of a periodic masonry wall. The random masonry is obtained starting from a periodic running bond pattern. A random perturbation on the horizontal positions of the vertical interfaces between the blocks which form the masonry wall is introduced. In this way, the height of the blocks is uniform, while their width in the horizontal direction is random. The perturbation is limited such as each block has still exactly 6 neighboring blocks. In a first discrete model, the blocks are modeled as rigid bodies connected by elastic interfaces (mortar thin joints). In other words, masonry is seen as a “skeleton” in which the interactions between the rigid blocks are represented by forces and moments which depend on their relative displacements and rotations. A second continuous model is based on the homogenization of the discrete model. Explicit upper and lower bounds on the effective elastic moduli of the homogenized continuous model are obtained and compared to the well-known effective elastic moduli of the regular periodic masonry. It is found that the effective moduli are not very sensitive to the random perturbation (less than 10%). At the end, the Monte Carlo simulation method is used to compare the discrete random model and the continuous model at the structural level (a panel undergoing in plane actions). The randomness of the geometry requires the generation of several samples of size L of the discrete masonry. For a sample of size L, the structural discrete problem is solved using the same numerical procedure adopted in [Cecchi, A., Sab, K., 2004. A comparison between a 3D discrete model and two homogenized plate models for periodic elastic brickwork, International Journal of Solids Structures 41 (9–10), 2259–2276] and the average solution over the samples gives an estimation which depends on L. As L increases, an asymptotic limit is reached. One issue is to find the minimum size for L and to compare the asymptotic average solution to the one obtained from the continuous homogenized model.     

Quantitative measurements with small models of ships in a circulating water channel

The distributions of velocity and their fluctuations, the steady wave of free surface and the presence of air bubbles can significantly influence the extent to which useful resistance measurements can be obtained from ship models in circulating water channels. This paper describes a. circulating water channel and experimental techniques which allowed accurate measurements of the resistance of small models and the extrapolation of the results to ship hulls.     

Specified discharge velocity models for numerical simulations of laminar vortex rings

We numerically and theoretically investigate the flow generated at the exit section of a piston/cylinder arrangement that is generally used in experiments to produce vortex rings. Accurate models for the velocity profile in this section (also called specified discharge velocity, SDV models) are necessary in (i) numerical simulations of laminar vortex rings that do not compute the flow inside the cylinder and (ii) in slug-models that provide a formula for the total circulation of the flow. Based on the theoretical and numerical analysis of the flow evolution in the entrance region of a pipe, we derive two new and easy to implement SDV models. A first model takes into account the unsteady evolution of the centerline velocity, while the second model also includes the time variation of the characteristics of the boundary layer at the exit plane of the vortex generator. The models are tested in axisymmetric direct numerical simulations of vortex rings. As distinguished from classical SDV model, the new models allow to accurately reproduce the characteristics of the flow. In particular, the time evolution of the total circulation is in good agreement with experimental results and previous numerical simulations including the vortex generator. The second model also provides a more realistic time evolution of the vortex ring circulation. Using the classical slug-model and the new correction for the centerline velocity, we finally derive a new and accurate analytical expression for the total circulation of the flow.     

Discrete non-linear adaptive data driven control based upon simultaneous perturbation stochastic approximation

A novel adaptive data driven control strategy is proposed for general discrete non-linear systems. The controller is designed based upon the Simultaneous Perturbation Stochastic Approximation (SPSA) method, and is constructed through use of a Function Approximator (FA), which is fixed as a neural network here. In this novel control strategy, the parametric estimation is designed to be adaptive with convergence analysis, and the control ability has been greatly improved. The proposed control method is finally applied into the non-linear tracking problems, as well as near-optimal control problems for discrete-time non-linear systems. Simulation comparison tests were conducted on typical non-linear plants, through which, the convergence and feasibility of the proposed adaptive data driven control strategy are well demonstrated.     

Distribution of the velocity profiles of the liquid phase in a gas-liquid flow with small gas contents

Artilce [1] gives the results of measurement of the friction at the wall of a channel under bubble conditions, in a wide range of Reynolds numbers. It is shown that the concept of laminar flow conditions has no meaning when it is applied to the flow of a two-phase mixture, since, even with very small Reynolds numbers, the level of the pulsations of the friction is high, and the spectrum of the pulsations of the friction is continuous. In this case, the mean friction is much greater than the calculated; here the value of the resistance coefficient is not a single-valued function of the Reynolds number. The present article gives the results of measurement of the velocity profiles of the liquid phase, carried out using an electro diffusion method. It is shown that, with Reynolds numbers corresponding to turbulent flow conditions, the profile of the velocity in a two-phase mixture is close to turbulent and does not depend on the gas content.     

Introduction of small velocity and pressure variation into a stationary compressible fluid

Summary This work formulates the aerodynamic time-dependent Coulomb and Biot-Savart laws with inherent transmission retardation. It generalizes the theory by Baskin et al. (for only the isentropic propagation of small vortex disturbances) to deal with the propagation of small disturbances caused by a distributed system of both vortices and variation in specific entropy of a thermodynamically simple compressible fluid. The proof here uses an alternative, more appealing derivation with the help of a novel wave-cone transformation (which changes Kirchhoff's retarded potentials into Newtonian potentials). The results suggest the possibility of using a focused laser spot to simulate the effect of a source in a compressible fluid; the source may even move at a supersonic speed to create a shock wave. They also suggest a time-dependent, three-dimensional formula for lifts caused by small disturbances that is in agreement with Blasius' formula for two-dimensional, time-independent, inviscid flows.

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Störung einer stationären, kompressiblen Flüssigkeit durch kleine Geschwindigkeits- und Druckänderungen

Übersicht Formuliert werden die zeitabhängigen, aerodynamischen Gesetze von Coulomb und Biot-Savart mit inhärenter Übertragungsverzögerung. Damit wird die Theorie von Baskin u. a. für isentrope Fortpflanzung kleiner Wirbelstörungen verallgemeinert auf die Fortpflanzung kleiner Störungen, die durch eine Verteilung von Wirbeln und von Änderungen der spezifischen Entropie einer thermodynamisch einfachen, kompressiblen Flüssigkeit verursacht werden. Mit Hilfe einer neuartigen Wellenkegel-Transformation, welche die Kirchhoff-Potentiale mit Verzögerung in Newtonsche Potentiale über führt, wird der Beweis hier in einer alternativen Form hergeleitet. Die Ergebnisse zeigen die Möglichkeit, durch einen fokusierten Laserfleck den Effekt einer Quelle in der kompressiblen Flüssigkeit zu simulieren, wobei die Quelle zur Erzeugung einer Schockwelle sogar mit Überschallgeschwindigkeit wandern kann. Aufgrund der Resultate wird eine zeitabhängige, dreidimensionale Formel für den Auftrieb infolge kleiner Störungen vorgeschlagen, die mit Blasius' Formel für zweidimensionale, zeitunabhängige zähigkeitslose Strömungen in Einklang steht.

     

Dominant parameters for maximum velocity induced by body-force models for plasma actuators

This study investigates the relationship between body-force fields and maximum velocity induced in quiescent air for development of a simple body-force model of a plasma actuator. Numerical simulations are conducted with the body force near a wall. The spatial distribution and temporal variation of the body force are a Gaussian distribution and steady actuation, respectively. The dimensional analysis is performed to derive a reference velocity and Reynolds number based on the body-force distribution. It is found that the derived Reynolds number correlates well with the nondimensional maximum velocity induced in quiescent conditions when the center of the Gaussian distribution is fixed at the wall. Additionally, two flow regimes are identified in terms of the Reynolds number. Considering the variation of the center of gravity of force fields, another Reynolds number is defined by introducing a new reference length. The nondimensional maximum velocity is found to be scaled with the latter Reynolds number, i.e., the maximum induced velocity in quiescent conditions is determined from three key parameters of the force field: the total induced momentum per unit time, the height of the center of gravity, and the standard deviation from it. This scaling turns out to be applicable to existing body-force models of the plasma actuator, despite the force distributions different from the Gaussian distribution. Comparisons of velocity profiles with experimental data validate the results and show that the flow induced by a plasma actuator can be simulated with simple force distributions by adjustment of the key body-force parameters.     

Lagrangian evaluations of viscous models for velocity gradient dynamics in non-stationary turbulence

Simple autonomous dynamical models of velocity gradients are found to be useful in understanding the essential physics of non-linear turbulent processes. Such models can also be employed as closure models for the Lagrangian PDF methods of turbulence computations. The pressure Hessian and the viscous processes incumbent in the exact velocity gradient evolution equation are non-local in nature. Several models have been proposed for these processes. In this work, we focus specifically on two models meant for the incumbent viscous process: the linear Lagrangian diffusion model (LLDM) and the recent fluid deformation closure model (RFDM). Performance of both the models have indeed been examined earlier, but most evaluations have been restricted to statistical stationary flow fields. In this work, we subject these models to further scrutiny. Our evaluation procedure (i) uses direct numerical simulation data of decaying isotropic (non-stationary) turbulence, (ii) follows identified fluid particles (the so-called Lagrangian evolution), (iii) uses both compressible and nearly incompressible flow fields. In nearly incompressible regime, the RFD model is found to be satisfactory, while the LLDM model overestimates viscous effects at late times. In the compressible regime, both the models show inadequacies. For compressible flows, we propose an alternative modelling strategy which shows improvement over both LLD and RFD models.     

冲击风稳态流场CFD模拟及三维风速经验模型研究

建筑结构抗风设计中相当多地区的设计风速,往往参考当地气象观测到的雷暴冲击风的最大风速值,但雷暴冲击风的风场与目前抗风设计采用的大气边界层流场相差悬殊,使得设计风速与设计风场并不匹配.本文将在CFD计算结果与已有文献资料进行比较以验证CFD计算模型及参数准确性的基础上,进行了风场特性的全面研究,主要通过改变冲击风描述参数,来研究雷暴冲击风水平风速及竖向风速的分布特性变化情况,研究成果较为完整的描述了冲击风风场特性.最后提出了用于结构抗风设计的水平风速及竖向风速经验模型,较为简明地反映了雷暴冲击风三维风速的空间分布情况,并得到了CFD模拟、实测以及试验结果的验证.该经验模型为雷暴冲击风下的抗风设计计算提供了理论基础.     

Accuracy of out-of-plane vorticity measurements derived from in-plane velocity field data

 A study of the errors in out-of-plane vorticity (ω _z ) calculated using a local χ² fitting of the measured velocity field and analytic differentiation has been carried out. The primary factors of spatial velocity sampling separation and random velocity measurement error have been investigated. In principle the ω _z error can be decomposed into a bias error contribution and a random error contribution. Theoretical expressions for the transmission of the random velocity error into the random vorticity error have been derived. The velocity and vorticity field of the Oseen vortex has been used as a typical vortex structure in this study. Data of different quality, ranging from exact velocity vectors of analytically defined flow fields (Oseen vortex flow) sampled at discrete locations to computer generated digital image frames analysed using cross-correlation DPIV, have been investigated in this study. This data has been used to provide support for the theoretical random error results, to isolate the different sources of error and to determine their effect on ω _z measurements. A method for estimating in-situ the velocity random error is presented. This estimate coupled with the theoretically derived random error transmission results for the χ² vorticity calculation method can be used a priori to estimate the magnitude of the random error in ω _z . This random error is independent of a particular flow field. The velocity sampling separation is found to have a profound effect on the precise determination of ω _z by introducing a bias error. This bias error results in an underestimation of the peak vorticity. Simple equations, which are based on a local model of the Oseen vortex around the peak vorticity region, allowing the prediction of the ω _z bias error for the χ² vorticity calculation method, are presented. An important conclusion of this study is that the random error transmission factor and the bias error cannot be minimised simultaneously. Both depend on the velocity sampling separation, but with opposing effects. The application of the random and bias vorticity error predictions are illustrated by application to experimental velocity data determined using cross-correlation DPIV (CCDPIV) analysis of digital images of a laminar vortex ring. Received: 31 October 1997/Accepted: 6 February 1998     

Identification of vortex pairs in aircraft wakes from sectional velocity data

The dynamics of multiple-vortex wake systems behind aircraft endangering air traffic can be assessed also from physical modelling. Large-scale laboratory investigations of multiple-vortex systems have been performed in a free-flight laboratory and in a water towing tank. Specialized PIV measurements provide time-resolved flow velocity fields normal to the wake axis. The applicability of various ∇u-based vortex identification schemes to planar velocity data is addressed and demonstrated for unequal-strength co- and counter-rotating vortex pairs. Large vortices shed off the wing tips and flaps are identified employing a ∇u-based criterion. Their cooperative mechanisms of generation and decay are evidenced from iso-surfaces of squared swirling strength and from further characteristic vortex parameters.

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利用频响函数对阻尼结构进行模型修正的方法

基于频响函数和模型缩聚技术提出了一个动力结构模型修正方法。与传统频响函数法相比,由于引入了阻尼刚度比,减少了修正参数,提高了计算效率。并且针对传统频响函数法测试维数过高的缺点,引入模型缩聚技术,降低了模型测量维数的要求。数值算例证明了本方法的有效性与可行性。本方法可以对有阻尼的模型进行修正,克服了模态修正法的缺点;利用模型缩聚减小了测试点的数目,引进单元阻尼刚度比。与传统的频响函数修正法相比,减少了修正未知数,提高计算效率,并且修正结果也较准确。     

Comparison of experimental data with results of some drying models for regularly shaped products

This paper presents an experimental and theoretical investigation of drying of moist slab, cylinder and spherical products to study dimensionless moisture content distributions and their comparisons. Experimental study includes the measurement of the moisture content distributions of slab and cylindrical carrot, slab and cylindrical pumpkin and spherical blueberry during drying at various temperatures (e.g., 30, 40, 50 and 60°C) at specific constant velocity (U = 1 m/s) and the relative humidity φ = 30%. In theoretical analysis, two moisture transfer models are used to determine drying process parameters (e.g., drying coefficient and lag factor) and moisture transfer parameters (e.g., moisture diffusivity and moisture transfer coefficient), and to calculate the dimensionless moisture content distributions. The calculated results are then compared with the experimental moisture data. A considerably high agreement is obtained between the calculations and experimental measurements for the cases considered. The effective diffusivity values were evaluated between 0.741 × 10⁻⁵ and 5.981 × 10⁻⁵ m²/h for slab products, 0.818 × 10⁻⁵ and 6.287 × 10⁻⁵ m²/h for cylindrical products and 1.213 × 10⁻⁷ and 7.589 × 10⁻⁷ m²/h spherical products using the Model-I and 0.316 × 10⁻⁵–5.072 × 10⁻⁵ m²/h for slab products, 0.580 × 10⁻⁵–9.587 × 10⁻⁵ m²/h for cylindrical products and 1.408 × 10⁻⁷–13.913 × 10⁻⁷ m²/h spherical products using the Model-II.