Why does the high-speed train look like this?

在过去十年中,中国高速列车迎来了爆炸式的发展,本文就高速列车主要涉及到的空气动力学问题如气动阻力、交会压力波、隧道压力波、气动噪声进行了阐述,并就这些问题的影响因素和规律给出了介绍.     

What does it take to launch self-confined jets from astrophysical accretion discs?

Well collimated jets are observed from young stellar objects (YSO), active galactic nuclei (AGN) and some galactic compacts objects (X-ray Binaries). These jets are powerful in the sense that they seem to carry a sizable fraction of the power released in the underlying accretion disc. The most complete and reliable jet models are actually disc winds, launched from the innermost disc regions: accretion and ejection are naturally interdependent, through the action of large scale magnetic fields anchored in the disc.In the last decade, disc winds have been quite successfully compared to observed YSO jets. Indeed, these jets are well monitored and provide several valuable diagnostics (plasma temperature, density and velocity stratification) that put stringent constraints on all available jet models. However, even if disc winds provide a satisfactory “first order” explanation for YSO jets, they cannot explain alone the whole phenomenology of these jets and some interaction with the central object must be taken into account.Disc wind models rely on the presence of a large scale vertical field close to equipartition. It will be shown that fulfilling such a condition is not an easy task. A complex interplay between advection and turbulent diffusion of the field is probably the cause of long term variability, as observed in X-ray binaries for instance. We also show, for the first time, that the outer parts in accretion discs could be highly magnetized. This may be the reason of ubiquitous massive, low velocity winds observed in YSOs, AGN and XrBs.     

The compressible Coanda wall jet—an experimental study of jet structure and breakaway

An underexpanded jet issuing from a convergent slot and blowing over a surface of convex streamwise curvature was studied experimentally. The jet was confined between side walls, with the slot aspect ratio varying between 40 and 6, but tests showed that in the area of interest close to the slot the flow was effectively two-dimensional. The ratio of slot width to the radius of curvature of the downstream surface varied between 0.05 and 0.33. The main techniques used were Schlieren and shadowgraph to show the jet structure, and surface flow visualization which revealed areas of separation and reattachment. Surface static pressures were also measured on the curved surface. The curved jet proved to have a shock cell structure similar to that of a plane jet. However, the cell structure disappeared more rapidly as the outer shear layer grew more quickly due to the destabilizing effect of the curvature on the turbulence in the shear layer. Even at modest upstream jet pressures, a separated region on the Coanda surface became evident. This region was characterized by a stagnant constant pressure part followed by a region of strongly reversed flow before reattachment took place. The separation was caused by the compression at the end of the first shock cell, with reattachment taking place where expansion in the second cell started. The separated region grew rapidly as the upstream pressure was increased, until, finally, reattachment failed to occur and the jet suddenly broke away from the surface. This work is related to studies of the Coanda flare, where the jet is axisymmetric. The high level of turbulence causes rapid entrainment of air and so gives us clean combustion. However there should be more general application to devices that use the Coanda effect, varying from fluidic devices to blown jet flaps on wings.     

What does head movement tell about the minimum number of mechanical degrees of freedom in quiet human stance?

In this study, we checked experimentally whether anterior–posterior accelerations of the head during quiet human stance are usually below or above known thresholds of the otolith sensor. Thereto, we measured head kinematics with high spatial resolution. Furthermore, we used both these experimental data and computer simulations of two double inverted pendulum (DIP) models in order to verify the validity of DIP models in general. The results are clear cut. First, not only are acceleration thresholds regularly exceeded about once a second but also are velocity thresholds exceeded, albeit probably less frequently. Second, COM and head movement predicted by interwoven DIP model dynamics can not reproduce the mean measured amplitudes at once. Thus, neither the formerly promoted single inverted pendulum nor any DIP model can causally explain the dynamics of quiet human stance. Instead, we suggest to factor in at least three mechanical degrees of freedom. Due to a couple of reasons discussed, the triple inverted pendulum (TIP) model seems to be a promising abstraction implying potential to better understand the dynamics of quiet human stance.     

Analytical solution for the general mechanochemical corrosion of an ideal elastic–plastic thick-walled tube under pressure

The paper presents analytical solutions for the equal-rate mechanochemical wear of an ideal elastic–plastic thick-walled cylindrical tube subjected to any combination of internal and external pressure. The rates of corrosion at the inner and outer surfaces are supposed to be proportional to the equivalent tensile stress at the surface involved when it exceeds a given threshold. Furthermore, the corrosion rate can decay exponentially with time. The obtained solutions allow to assess the time of the initial yielding at the bore of the tube and the time of fully plastic yielding. Calculations showed that the time of plastic-zone propagation throughout the tube wall can be much greater than the length of the pure elastic stage. The proposed analytical solutions are to be used for design purposes and as benchmark solutions for numerical analysis.     

Why does necking ignore notches in dynamic tension?

Recent experimental work has revealed that notched tensile specimens, subjected to dynamic loading, may fail by growing a neck outside of the notched region. This apparent lack of sensitivity to a classical stress concentration case was reported but not explained or modeled.The present paper combines experimental and numerical work to address this issue. Specifically, it is shown that the dynamic tensile failure locus is dictated by both the applied velocity boundary condition and the material mechanical properties, specifically strain-rate sensitivity and strain-rate hardening.It is shown that at sufficiently high impact velocities, the flows stress in the notch vicinity becomes quite higher than in the rest of the specimen, so that while the former resists deformation, it transfers the load to the latter. The result will be the formation of a local neck and failure away from the notch.This effect is shown to be active when the material properties are perturbed only at the local level, as in the case of machining of the notch, which in itself may again be sufficient to stabilize the structure under local failure until a neck forms elsewhere.While the physical observations are quite counterintuitive with respect to the engineering views of stress concentrator's effect, the present work rationalizes those observations and also provides information for the designers of dynamically tensioned structures that may contain notches or similar flaws.     

What is behind the plastic strain rate?

The plastic strain rate plays a central role in macroscopic models on elasto-viscoplasticity. In order to discuss the concept behind this quantity, we propose, first, a kinetic toy model to describe the dynamics of sliding layers representative of plastic deformation of single crystalline metals. The dynamic variable is given by the distribution function of relative strains between adjacent layers, and the plastic strain rate emerges as the average hopping rate between energy wells. We demonstrate the behavior of this model under different deformations and how it captures the elastic-to-plastic transition. Second, the kinetic toy model is reduced to a closed evolution equation for the average of the relative strain, allowing us to make a direct link to macroscopic theories. It is shown that the constitutive relation for the plastic strain rate does not only depend on the stress, but also on the macroscopic applied deformation rate, contrary to common practice.     

How does a woodpecker work? An impact dynamics approach

To understand how a woodpecker is able accelerate its head to such a high velocity in a short amount of time,a multi-rigid-segment model of a woodpecker’s body is established in this study.Based on the skeletal specimen of the woodpecker and several videos of woodpeckers pecking,the parameters of a three-degree-of-freedom system are determined.The high velocity of the head is found to be the result of a whipping effect,which could be affected by muscle torque and tendon stiffness.The mechanism of whipping is analyzed by comparing the response of a hinged rod to that of a rigid rod.Depending on the parameters,the dynamic behavior of a hinged rod is classified into three response modes.Of these,a high free-end velocity could be achieved in mode II.The model is then generalized to a multihinge condition,and the free-end velocity is found to increase with hinge number,which explains the high free-end velocity resulting from whipping.Furthermore,the effects of some other factors,such as damping and mass distribution,on the velocity are also discussed.     

Is vorticity-banding due to an elastic instability?

A possible mechanism for the vorticity-banding instability is proposed on the basis of experiments with colloidal rod-like particles that exhibit an isotropic–nematic phase transition. The proposed mechanism is similar to the well-known elastic instability for polymer systems that is due to nonuniform elastic deformation of polymer chains as a result of gradients in the local shear rate (the Weissenberg effect). However, the role of polymer chains is now played by inhomogeneities that exist in systems exhibiting vorticity banding. For the rod-like colloidal system investigated here, inhomogeneities are formed during the early stages of phase separation. Nonuniform deformation of these inhomogeneities are thus proposed to lead to hoop stresses which give rise to banded structures where there is secondary, weakly rolling flow within each of the bands. Many of the features found experimentally for the rod-like colloidal system can be understood on the basis of this proposed mechanism. For different types of systems that also show vorticity banding, inhomogeneities can be identified, which might lead to vorticity banding for the same reasons as for the rod-like colloidal systems studied here.     

What is the Correct Definition of Average Pressure?

Use of a correct definition of average pressure is important in numerical modeling of oil reservoirs and aquifers, where the simulated domain can be very large. Also, the average pressure needs to be defined in the application of pore-network modeling of (two-phase) flow in porous media, as well as in the (theoretical) upscaling of flow equations. Almost always the so-called intrinsic phase-volume average operator, which weighs point pressure values with point saturation values, is employed. Here, we introduce and investigate four other potentially plausible averaging operators. Among them is the centroid-corrected phase-average pressure, which corrects the intrinsic phase-volume average pressure for the distance between the centroid of the averaging volume and the phase. We consider static equilibrium of two immiscible fluids in a homogeneous, one-dimensional, vertical porous medium domain under a series of (static) drainage conditions. An important feature of static equilibrium is that the total potential (i.e., the sum of pressure and gravity potentials) is constant for each phase over the whole domain. Therefore, its average will be equal to the same constant. It is argued that the correct average pressure must preserve the fact that fluid potentials are constant. We have found that the intrinsic phase-volume average pressure results in a gradient in the total phase potential, i.e., the above criterion is violated. In fact, only the centroid-corrected operator satisfies this criterion. However, at high saturations, use of the centroid-corrected average can give rise to negative values of the difference between the average nonwetting and wetting phase pressures. For main drainage, differences among various averaging operators are significantly less because both phases are present initially, such that the difference between the centroids of phases, and the middle of the domain are relatively small.     

What are the Longest Ropes on the Unit Sphere?

We consider the variational problem of finding the longest closed curves of given minimal thickness on the unit sphere. After establishing the existence of solutions for any given thickness between 0 and 1, we explicitly construct for each given thickness \({\Theta_n:= {\rm sin}\, \pi/(2n),}\) \({n\in\mathbb{N}}\), exactly \({\varphi(n)}\) solutions, where \({\varphi}\) is Euler’s totient function from number theory. Then we prove that these solutions are unique, and also provide a complete characterisation of sphere filling curves on the unit sphere; that is of those curves whose spherical tubular neighbourhood completely covers the surface area of the unit sphere exactly once. All of these results carry over to open curves as well, as indicated in the last section.     

What role for public health in genetics and vice versa?

Some epidemiologists and geneticists claim that integrating genetics into public health policies and programs is necessary and unavoidable. OBJECTIVE: To examine the extent to which further integration of public health and genetics is warranted. METHODS: Synthesis of the literature in four areas: research, genetic services, regulation, and education. The analysis is limited to human genetics. RESULTS: Public support for basic genetic research has and will continue to lead to new applications and to further understanding of human origins and dispersions. Some applied research, particularly for genetic risk factors for common complex diseases, has low yield and is better supported by private funds. The only genetic service for which a public health role is paramount is newborn screening. With the patenting of genes, and the proliferation of commercial interests in genetic tests and directly advertising them to the public, regulation by public health agencies is increasingly important. As most genetic testing and other services will be provided in the personal health care system, education about genetics is best left to the educational and medical systems. Public health practitioners should be aware of the limitations of genetic tests. CONCLUSIONS: There is little need for further integration of genetic services and education into public health especially in countries in which public and private health services are dichotomized. Newborn screening and follow-up, however, are most safely and effectively provided under public health auspices. The most important area for strengthening the public health role is in the regulation of genetic tests and other genetic services provided primarily by the private sector. Continued support for basic genetic research is needed.     

Infinitesimal Stability of the Equilibrium States of?an?Incompressible, Isotropic Elastic Tube Under?Pressure

The infinitesimal stability of the equilibrium states of an arbitrary incompressible, isotropic and homogeneous elastic cylindrical shell in a pure radial expansion under a constant inflation pressure is studied for both thick- and thin-walled shells. The classical criterion of infinitesimal stability yields a general stability theorem relating the frequency and pressure response and reveals that points at which the pressure is stationary define the domain of unstable or neutrally stable states. All results are expressed in terms of a general shear response function, and specific results are provided for the Mooney-Rivlin, Gent and Ogden models, the second having limited extensibility, the last including experimental data. Every static state of a Mooney-Rivlin tube is stable so long as the pressure is less than an asymptotic limit that increases with the thickness. Otherwise, only the Ogden model exhibits static states of instability for all long cylindrical tubes of thickness less than a transitional value above which all static states are infinitesimally stable. A long cylindrical cavity in all three unbounded models is stable for all pressures. All results are illustrated graphically.     

Bifurcation boundary analysis as a nonlinear damage detection feature: does it work?

Many systems in engineering and science are inherently nonlinear and require damage detection. For such systems, nonlinear damage detection methods may be useful. A bifurcation boundary analysis method as a new nonlinear damage detection tool was previously introduced in the literature to track bifurcation boundary changes due to damages over a small region of an aeroelastic panel model. Results of this method based upon a finite difference solution showed higher sensitivities to the small amount of damage than methods based upon linear models. In this paper, four methods including Finite Difference, Finite Element (FEM), Rayleigh-Ritz and Galerkin Approach are used to further investigate the sensitivity of the bifurcation boundary for damage detection. Results of the FEM and Rayleigh-Ritz method agree with each other and also show that the sensitivity of the bifurcation boundary to damage is much less than what previously reported when using a finite difference solution method.     

Collision operators for Stark line shapes: When does the unified theory fail?

It is shown that the unified theory for Stark broadening leads to incorrect results when correlated emitter–perturber collisions are present. With a kinetic theory treatment, we propose an extension of the model able to account for such correlations. The treatment presents analogies with renormalization techniques and the resulting collision operator has a structure similar to that obtained within the unified theory. We illustrate the applicability of the model through calculations of hydrogen line shapes in ideal cases.     

What are gene patents and why are people worried about them?

This article examines what it means to patent a gene. Numerous ethical concerns have been raised about the effects of such patents on clinical medical practice as well as on research and development. We describe what kinds of inventions are covered by human gene patents, give several examples and summarize the small body of empirical research performed in the US examining the effects of these patents. There is little evidence that early fears about gene patenting placing substantial restraints on research and clinical medicine have come to fruition. Nonetheless, there are areas of concern, and policy makers, physicians and the public should be alert to ensure that the net social benefits of patenting human genes are maintained.     

Fourier-BEM: or What To Do if No Fundamental Solution is Available?

The application of boundary element methods (BEM) to soil-structure interaction problems is still restricted to cases where fundamental solutions are known. Hence, a large number of engineering problems cannot be solved by the BEM. Therefore, an alternative approach is presented here which establishes new boundary integral equations (BIEs) for the computation of the entries of the BEM matrices by means of the spatial Fourier transform.For these alternative BIEs, we need only the transform of the fundamental solution and not the fundamental solution itself. The former is always available as long as the underlying differential operator is linear and has constant coefficients. The approach is possible for all variants of the BEM. For Galerkin approaches, the double integrations over the boundary panels are replaced by single integrations over the infinite domain.     

What do dry granular flows tell us about dense non-Brownian suspension rheology?

Recent years have seen significant progress in our understanding of the rheology of dry granular materials. A scale invariance of equations of motion in the rigid grain limit has helped identifying dimensionless quantities which govern flow. After a review of recent results on dry granular materials, we show how this same dimensional analysis carries over to the case of dense, non-Brownian suspensions. Our review is based on compiled data from various sets of numerical simulations, using both molecular and contact dynamics. It covers the breakdown of kinetic theory, which arises when contact forces dominate collisional forces in inertial flows, and the approach to dense flows up to the quasi-static limit. We show that simple invariance arguments permit to clarify the conditions of occurrence of viscous and inertial scaling in suspensions, and show in particular that both may occur in dense flows even in the presence of significant contact forces, up to the jamming limit. Some implications of the properties of steady uniform quasistatic granular flow under constant pressure for very dense suspensions near the maximum concentration are also discussed.