On the heat in the nozzle of the industrial pneumatic pulsator

An industrially applicable nozzle is the subject of this study. The nozzle is an auxiliary equipment of a pneumatic pulsator system for unclogging outlets of silos which store loose materials. The aim is to determine the amount of heat which is generated during one work cycle of the system. Investigation in this field has not been carried out so far, and the present-day designing process is significantly based on heuristic knowledge. The heat is calculated by using results of a numerical simulation. The Finite Volume Method has been used with a thermodynamically ideal gas model. The airflow is assumed to be transient, compressible and supersonic, and it is driven by a time-varying pressure difference. There is an estimation of discretization error of the numerical results carried out in order to confirm the reliability of the solution. The error estimation shows that the results lie in the vicinity of the exact solution of the governing equations. Instantaneous results of the simulation indicate a locally flow which intensifies flow parameters in a similar way as the converging-diverging nozzles do. The value of the total heat generated during gas conversion within the nozzle is negative; thus, the nozzle could be cooled during its functioning.     

About the numerical simulation of the dynamics of granular media and the definition of the mean stress tensor

To define the stresses in a granular medium considered as a continuum, Weber has calculated a tensor at a point by averaging the contact forces in the vicinity. However this tensor is not symmetric. Besides, the Weber formula neglects the body forces, which does not allow it to describe dynamical problems. In the present work, it is proposed to use a general expression of the mean stress tensor based not only on the contact reactions but also on the body forces. It is proved that this tensor is automatically symmetric and invariant by translation. Next, the pertinence of this approach is illustrated by an analytical example. It can be clearly ascertained that accounting for the contribution of the gravity and inertia effects is essential to ensure the symmetry of the stress tensor, according to Cauchy theory. We then propose to extend this definition of the mean stress tensor to a space–time analysis. An average in time as well appears to be pertinent. Finally, numerical simulations with a large number of grains are performed using the software ‘MULTICOR' developed in our laboratories and based on the contact dynamics and the bipotential approach. The attention is particularly focused on the motion of ensiled matters.     

On the sensitivity of the vibrational response of the human head

A parametric study based on finite element analysis is performed in order to investigate the sensitivity of the eigenfrequency spectrum of the human head system upon variation of its constitutive material properties. This study stems from the ever increasing medical interest connected to early diagnosis of brain edema and the lack of existing accurate and non-invasive diagnostic methods to achieve it. The present computational work aims to resolve the question of whether or not such a spectrum shifting is detectable with current experimental procedures. The human head is modeled as a prolate spheriod consisting of confocal shell representing the scalp-skull system, in contact with the subarachnoidal space which surrounds the brain. The skull is modeled as an isotropic elastic material whereas the brain is assumed to be a frequency dependent viscoelastic body.     

Study on the interaction between the shape and the temperature field of the melt in the electromagnetic shaping process

Based on analyses for the electromagnetic pressure on the melt and the heat induced in the melt, the ratio of heat to pressure Q₀/P_m is defined, to give the relationship between Q_o/P_m and the thickness a, the electromagnetic parameter (μγ) of the melt and the electric current frequency/under the electromagnetic confinement and shaping process. If Q_o/P_m is large, any adjustment to the melt shape will easily cause a variation of the temperature in the melt. In this situation, there appears to be a more sensitive interaction between the shape and the temperature field and a more narrow adjustment range for the process. Experiments on thin plate samples with a cross-section of 6 mm × 18 mm are done with two kinds of induction coils. The results show that when a coil with a trumpet inside wall is used and the positions of the melt top and the S/L interface are properly selected, the melt periphery is nearly vertical and the temperature gradient ahead of the S/L interface is high. Under these conditions, a more stable and wider coupling between the shape and the temperature field is continuously maintained and samples with a smooth surface and unidirectional crystals are successfully obtained.     

A Study on the Feasibility of Measuring the Emissivity with the Laser-Flash Method

The laser-flash method is a fast, widely used and well established technique to measure the thermal diffusivity. Since its introduction in the 1960s, it was proposed to expand this technique to the measurement of heat capacity and emissivity. Currently, the measurement of spectral emissivity at high temperatures is connected with relatively large uncertainties, although the spectral emissivity is an essential parameter for applications, e.g., in the lamp industry and fusion research. In this work, a theoretical study is presented on the possibility of emissivity measurements using the laser-flash method. Two mathematical approaches are discussed which solve the problem, that a measured temperature rise—necessary to calculate the emissivity—itself depends on the emissivity. It is shown that both methods have a negligible arithmetic error, making them applicable to be used in future work.     

On the formation of the stircast structure

The conditions of solidification in a stirred bulk liquid are investigated to explain the non-dendritic microstructure of stircast alloys. A model of stirred solidification is presented, which allows a comparison of the solidification behaviour of metal alloys and organic analogues. This shows that nucleation and growth of solid in e bulk liquid is facilitated under the influence of stirring, provided the Prandtl-number is greater than unity. It is shown further that the solute gradient ahead of the solid-liquid interface of floating particles in a bulk liquid is reduced by the fluid flow. Combined with the thermal properties, and in analogy with the constitutional supercooling criterion, A is shown that solid growth in metals is likely to be cellular in an early stage of the solidification, In contrast, in stirred organic analogues, the solidification is dendritic in the early stage.     

Determination of the SI Volt at the PTB

Work on an electrostatic measuring system for the determination of the SI volt and the conversion factor KV at the PTB is reported. The system consists of a so-called voltage balance and a generator for the dc measuring voltage. There is a need for the development of such a device with an uncertainty below 1 part in 106 because measurements with existing voltage balances, and results obtained by other methods show discrepancies of several parts in 106. In order to reduce the uncertainty and to simplify the apparatus a special measuring method was developed. The voltage/force transducer of the PTB voltage balance is formed from two coaxial cylindrical electrodes and the generated force is substituted for the force of gravity on a weight of 2 g. The electrode voltage is composed of a constant part of 10186 V, derived in a 1000-fold stepup from 10 standard cells, and a much smaller variable part used for balancing the scale beam by means of a control loop. Taking the root sum of squares the total relative uncertainty of the SI volt and of KV is expected to be less than 4 parts in 107.     

On the autonomy of the process region

Two methods are examined for accurately calculating stress intensity factors in two and three dimensions: the stiffness derivative technique and the J-integral method. In two dimensions the J-integral is expressed as an area integral, whereas in three dimensions it is a volume integral. With both techniques, a virtual crack extension is introduced. Although the expressions employed for each method are quite different, it is proven that when written analytically for finite element calculation, they reduce to identical expressions.Numerical calculations are carried out in both two and three dimensions. As a result of the equivalence of the two methods, close numerical agreement is expected. For two-dimensional geometries and a penny shaped crack in a finite height cylinder, there is at least five significant figure agreement between solutions determined by both methods. For an elliptical crack embedded in a plate, the agreement is generally to four significant figures.     

Understanding the unit in the Rasch model

The purpose of this paper is to explain the role of the unit implicit in the dichotomous Rasch model in determining the multiplicative factor of separation between measurements in a specified frame of reference. The explanation is provided at two complementary levels: first, in terms of the algebra of the model in which the role of an implicit, multiplicative constant is made explicit; and second, at a more fundamental level, in terms of the classical definition of measurement in the physical sciences. The Rasch model is characterized by statistical sufficiency, which arises from the requirement of invariant comparisons within a specified frame of reference. A frame of reference is defined by a class of persons responding to a class of items in a well-defined response context. The paper shows that two or more frames of reference may have different implicit units without destroying sufficiency. Understanding the role of the unit permits explication of the relationship between the Rasch model and the two parameter logistic model. The paper also summarises an approach that can be used in practice to express measurements across different frames of reference in the same unit.     

On the stability of the Rendulic-Roscoe clay

A flow rule proposed for virgin clays by Poorooshasb and Yong (1982) utilizes the concept of intersecting yield surfaces in conjunction with the Rendulic-Roscoe criterion. According to this criterion, during a loading process the successive states experienced by a clay element trace a unique surface in the state space. In the present paper the proposed flow rule is examined in the light of Drucker's work hardening postulate and a necessary condition for ‘stability’ is derived. Furthermore, a theorem which may be used to prove the uniqueness of the solution for a certain class of problems is stated and proved. Finally, the question of extremum is discussed and it is shown that a variational form can be obtained only under a special, yet important, circumstance, i.e., the rapid loading of the system.     

On the Sensitivity of the EOQ to Errors in the Forecast of Demand

The economic order quantity (EOQ) has long been used by practitioners as a rule of thumb for setting order quantities on the basis of the commonly held assumption that the EOQ is a very robust result. That is, as many authors have stated, the EOQ appears to be quite insensitive to errors in the specification of the appropriate cost parameters and forecast of demand. This paper demonstrates that the EOQ is indeed insensitive to errors in the cost parameters, but, contrary to the conventional wisdom, the EOQ can be quite sensitive to forecast errors whenever the lead time is nonzero. This important caveat is illustrated by a number of examples.     

On the estimation of the toughness of surface microcracks

Hardness testing is a method frequently used for evaluating the resistance of body surfaces to the influence of contact loading. The Vickers hardness test is applied for this purpose in a case when the material is sufficiently ductile and no cracks occur in the corners of the indents. Testing of the surface hardness of such materials as glass and ceramics on the basis of the Vickers testing method must take into account the energy that is spent on crack spreading. This paper describes a more exact and accurate method for evaluating resistance to microcrack formation on the surface of a material. To evaluate the conditions for crack spreading, it is necessary to test a specimen under loading in bulk. The suggested procedure involves a bent strip. As bending stresses are known, from the differences in crack length along and across the strip, the resistance of the material surface to crack propagation is determined.     

Modeling the inherent optical properties of the ocean based on the detailed composition of the planktonic community

We describe an approach to modeling the ocean's inherent optical properties (IOPs) that permits extensive analyses of IOPs as the detailed composition of suspended particulate matter is varied in a controlled manner. Example simulations of the IOP model, which includes 18 planktonic components covering a size range from submicrometer viruses and heterotrophic bacteria to microplanktonic species of 30-mum cell diameter, are discussed. Input data to the model include the spectral optical cross sections on a per particle basis and the particle-number concentration for each individual component. This approach represents a significant departure from traditional IOP and bio-optical models in which the composition of seawater is described in terms of a few components only or chlorophyll concentration alone. The simulations illustrate how the separation and understanding of the effects of various types of particle present within a water body can be achieved. In an example simulation representing an oligotrophic water body with a chlorophyll a concentration of 0.18 mg m(-3), the planktonic microorganisms altogether are the dominant particulate component in the process of light absorption, but their relative contribution to light scattering is smaller than that of nonliving particles. A series of simulations of water bodies with the same chlorophyll a concentration but dominated by different phytoplankton species shows that composition of the planktonic community is an important source of optical variability in the ocean.     

On the estimation of the toughness of surface microcracks

Hardness testing is a method frequently used for evaluating the resistance of body surfaces to the influence of contact loading. The Vickers hardness test is applied for this purpose in a case when the material is sufficiently ductile and no cracks occur in the corners of the indents. Testing of the surface hardness of such materials as glass and ceramics on the basis of the Vickers testing method must take into account the energy that is spent on crack spreading. This paper describes a more exact and accurate method for evaluating resistance to microcrack formation on the surface of a material. To evaluate the conditions for crack spreading, it is necessary to test a specimen under loading in bulk. The suggested procedure involves a bent strip. As bending stresses are known, from the differences in crack length along and across the strip, the resistance of the material surface to crack propagation is determined.     

On the classification of the low energy excitations in the doped antiferromagnet

A long-wavelength, low energy hamiltonian is derived to describe the dynamic of a single hole in a quantum antiferromagnet in two or higher spatial dimensions. In thisexactly solvable limit anew kind of symmetry is important to classify the elementary spin excitations of a single hole in the t — J model. This symmetry is hidden at finite size or at short wavelength. The resulting classification has important consequences for understanding the physics of doped antiferromagnets. Fermi liquid like s = 1/2 and charge one excitations are still well defined in a quantum antiferromagnet, but are not a complete set to describe the low energy physics.     

On the mechanism of the Ranque–Hilsch effect

A model of flow in a Ranque vortex tube is suggested. It is based not on the thermal interaction between hot and cold flows, but rather on a mechanical one. It is shown that to describe the Ranque–Hilsch effect it is necessary, along with the radial flow, to take into account the uptake or addition of mass, as well as to ensure a smoother conjugation between a forced and a peripheral vortices, demanding the continuity not only of the tangential velocity component, but also of its first derivative with respect to the radius. In this case, the motion in the vortex tube is considered as a system of vortex flows and vortex sources interacting between themselves.     

Batching and scheduling to minimize the makespan in the two-machine flowshop

In this paper, we consider a class of batching and scheduling problems in the two-machine flowshop where one of the machines is a discrete processor and the other one is a batch processor. The jobs are processed separately on the discrete processor and processed in batches on the batch processor. The processing time of a batch is equal to the total processing time of the jobs contained in it, and the completion time of a job in a batch is defined as the completion time of the batch containing it. A constant setup time is incurred whenever a batch is formed on the batch processor. The problem is to find the optimal batch compositions and the optimal schedule of the batches so that the makespan is minimized. All problems in this class are shown to be NP-complete in the ordinary sense. We also identify some polynomially solvable cases by introducing their corresponding solution methods.     

火源横向位置对隧道烟气分布的影响

依托工程实际,采用数值模拟方法研究隧道火源位于不同车道时,隧道临界风速、温度、烟雾分布扩散规律。结果表明:纵向通风速度不大于临界通风速度时,火源下游段隧道棚顶温度呈e函数下降,当通风速度大于临界通风速度时,该规律不再适用。火源位于侧壁面时,由于隧道侧壁面的限制作用,其近火源区烟气运移呈“之”字形向隧道出口处蔓延,隧道棚顶不同纵向温度线呈滚动式交替变化。     

Estimation of the Proportion of the Variance Explained by Regression,When the Number of Parameters in the Model May Depend on the Sample Size

The two-sided bivariate t-test for simultaneous comparison of two treatments with a control is studied. The problem of allocating a given number of observations to the treatments and the control is discussed, and it is concluded that, in the absence of preferences, a general purpose rule would be to choose the sample sizes equal. A table is presented which gives the sample size required to ensure that the null hypothesis will be rejected with at least a prescribed probability, given the significance level and the standardized differences between the treatments and the control. Similar tables are provided for use when the object is to ensure that both treatments means will be found significantly different from the control mean with at least a given probability, or to ensure that a specified treatment will be found significant with at least a given probability.     

抽油杆柱纵向振动共振条件的质疑与力学模型的修正

在研究抽油杆柱纵向振动特性时，将抽油杆柱简化为上端固定于随悬点运动的基础之上、下端受液体负荷激励的力学模型。研究了单级抽油杆柱纵向自由振动的固有频率以及抽油杆柱对各种激励的稳态响应。建立了单级杆柱第i阶固有频率pi和基频p0之间的关系，即pi=(2i-1)p0，指出了单级抽油杆柱产生共振的条件是激励频率等于基频的奇数倍，证明了APIRPllL有杆抽油系统的设计计算推荐方法中关于激励频率等于偶数倍基频时，单级杆柱将产生共振的结论是错误的，并指出了导致这一错误结论的原因是所采用的力学模型是错误的。通过对抽油杆柱动态特性的计算机仿真，进一步证明了本文结论的正确性。