Critical Momentum Research Articles

Auto-Stack - Implementing a European Automotive Fuel Cell Stack Cluster

Currently, Europe is lacking competitive fuel cell stack development for automotive application. While some OEMs have their captive development activities in the US and Canada, others are not yet prioritizing the technology. The supply chain and research activities are fragmented and suffer from the associated strategic uncertainties. A lack of investment and consequently a lack of critical mass and momentum in automotive stack development is the consequence of the status quo. With this background, the Auto-Stack project combined key European players including automotive OEMs, component suppliers and research organizations in a structured approach to facilitate the development and commercialization of automotive fuel cells in Europe. The consortium assessed ways to identify and reduce the critical barriers for better collaboration between stakeholders and generate a more attractive business case for a European automotive stack industry during pre-commercial and early commercial phases. Activities included the development of a common OEM specification and stack platform concept, analysis of the potential for meeting the mid-term technical and cost targets by the European supply chain, the assessment of synergies with other applications and finally options for facilitating economies of scale.

Experimental Study on Spray Characteristics of Gas-Centered Swirl Coaxial Injectors

Gas-centered swirl coaxial injectors have become an important subject of study for staged combustion rocket engines with hydrocarbon fuels. While these injectors are employed successfully in rocket engines, it is very rare to find the related research results as applicable to design data. An experimental study on spray characteristics of gas-centered swirl coaxial injectors has been performed. The effects of momentum flux ratio and recess length on the spray characteristics have been investigated by cold flow tests with a photographic technique. The liquid intact length L, which profoundly affects the global spray characteristics, decreases as the momentum flux ratio M increases. The critical momentum flux ratio Mc is introduced to identify the flow patterns as internal or external mixing in the injectors. Concerning the effect of the recess length lR, it is shown that the spray cone angle and the drop size decrease as lR increases.

HIGH ENERGY PROCESSES IN THE VICINITY OF THE KERR'S BLACK HOLE HORIZON

Two particle collisions close to the horizon of the rotating nonextremal black hole are analyzed. It is shown that high energy of the order of the Grand Unification scale in the centre of mass of colliding particles can be obtained when there is a multiple collision – the particle from the accretion disc gets the critical momentum in first collision with the other particle close to the horizon and then there is a second collision of the critical particle with the ordinary one. High energy occurs due to a great relative velocity of two particles and a large Lorentz factor. The dependence of the relative velocity on the distance to horizon is analyzed, the time of movement from the point in the accretion disc to the point of scattering with large energy as well as the time of back movement to the Earth are calculated. It is shown that they have reasonable order.

Completely Inelastic Collision to Confirm Maximum Punch Pressure Force of Cold Pressure Welding

The maximum punch pressure force (TMPPF) of cold welding pressure affects not only the performance of the welding bonding but also the amount of the area of welding bonding. If punch force is more bigger, it may be take place cracks, stress concentration and welding concave pits, which affect application of materials and increase incidence of faults. The process that punch pressure of cold welding pressure applied on sheet materials to attain welding bonding is simplified beam in cold pressure welding. Under normal circumstances it is an important method to TMPPF by the manner of experiment, but the method to attain TMPPF is limited. In the paper completely inelastic collision theory is applied to explain the process of cold welding pressure, the theory of completely inelastic collision offers principle theory to attain TMPPF. In this paper, two supposes are put forward, on the basis of two supposes the critical velocity is attained. At the same time, critical kinetic momentum or critical kinetic energy will be attained. On the basis of the law of conservation of energy, energy balance equation is attained, which will drop in calculating difficulty of the non-linear process of cold pressure welding and decrease calculated amount. In this paper on the basis of the process of collision all phases are analyzed. Indeed elastic-plastic deformation phase is completely inelastic collision phase, and from point of view of collision to calculate energy loss. At the same time, to suppose other plate is utter stiff can attain critical velocity, thus, to achieve the equation of TMPPF.

Bluestocking Studies: the State of the Field - and into the Future

Although Sylvia Myers’s groundbreaking book, The Bluestocking Circle, was published in 1990, the field of Bluestocking Studies did not gain critical momentum until some 12 years ago, when two developments (the demolition of the ‘domestic thesis’ and the English translation of Habermas’s Structural Transformation of the Public Sphere) had opened the way for a new approach. It was now realized that the Bluestockings were not mere salon ‘hostesses’ and harmless dabblers in men’s intellectual culture; rather they were innovative agents in 18th-century culture and society and were full participants in, indeed fashioners of, the Enlightenment. The present essay reviews the scholarly literature, praising advances where appropriate and censuring retrograde tendencies when necessary. It criticizes the ahistorical imposition of 20th-century feminism onto the Blues, and it challenges certain orthodoxies, empty cliches, and facile generalizations. It argues that the Bluestockings were not a ‘circle’ and Elizabeth Montagu was not ‘Queen of the Blues’. The Bluestockings were rather a loosely bounded system of separate but interrelated groups of persons or influences. Looking toward the future, the way to move ahead in Bluestocking Studies is to go back ad fontes– to the sources.

Superheating field of superconductors within Ginzburg-Landau theory

We study the superheating field of a bulk superconductor within Ginzburg-Landau theory, which is valid near the critical temperature. We calculate, as functions of the Ginzburg-Landau parameter $\ensuremath{\kappa}$, the superheating field ${H}_{\mathrm{sh}}$ and the critical momentum ${k}_{c}$ characterizing the wavelength of the instability of the Meissner state to flux penetration. By mapping the two-dimensional linear stability theory into a one-dimensional eigenfunction problem for an ordinary differential equation, we solve the problem numerically. We demonstrate agreement between the numerics and analytics, and show convergence to the known results at both small and large $\ensuremath{\kappa}$. We discuss the implications of the results for superconducting rf cavities used in particle accelerators.

Modeling on the Momentum and Heat/Mass Transfer Characteristics of an Argon Plasma Jet Issuing into Air Surroundings and Interacting with a Counter-Injected Argon Jet

Modeling study is performed to reveal the momentum and heat/mass transfer characteristics of a turbulent or laminar plasma reactor consisting of an argon plasma jet issuing into ambient air and interacting with a co-axially counter-injected argon jet. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent regimes, respectively. Modeling results presented include the streamline, isotherm and argon mass fraction distributions for the cases with different jet-inlet parameters and different distances between the counter-injected jet exit and the plasma torch exit. It is shown that there exists a quench layer with steep temperature gradients inside the reactor; a great amount of ambient air is always entrained into the plasma reactor; and the flow direction of the entrained air, the location and shape of the quench layer are dependent on the momentum flux ratio of the plasma jet to the counter-injected cold gas. Two quite different flow patterns are obtained at higher and lower momentum flux ratios, and thus there exists a critical momentum flux ratio to separate the different flow patterns and to obtain the widest quench layer. There exists a high argon concentration or even ‘air-free’ channel along the reactor axis. No appreciable difference is found between the turbulent and laminar plasma reactors in their overall plasma parameter distributions and the quench layer locations, but the values of the critical momentum flux ratio are somewhat different.

Reflexivity in the medical encounter: contributions from post‐modern systemic practice

In this article, I attempt to identify the forms of reflexivity that take place in the medical encounter and the possible contributions that the understanding of reflexivity in systemic thinking can make to medical practice. I start by reviewing some of the literature on the doctor–patient relationship as the necessary foundation to think about and understand reflective practice in medicine. I then argue that, as the medical profession comes under scrutiny from a variety of different individuals and groups, as well as from itself, a process of reconciliation between modern and postmodern thinking, as it has been suggested in the systemic literature, might help get a better grasp of the complex predicament and dilemmas of doctors in modern society. The notion of reflexivity, the ability of any system to critically think about itself by promoting a conversation between the experts and the subject of their expertise is probably the essence of the critical momentum of postmodernism and should be an essential professional attribute for all doctors.

HIGH ENERGY PROCESSES IN THE VICINITY OF THE KERR'S BLACK HOLE HORIZON

Two particle collisions close to the horizon of the rotating nonextremal black hole are analyzed. It is shown that high energy of the order of the Grand Unification scale in the centre of mass of colliding particles can be obtained when there is a multiple collision – the particle from the accretion disc gets the critical momentum in first collision with the other particle close to the horizon and then there is a second collision of the critical particle with the ordinary one. High energy occurs due to a great relative velocity of two particles and a large Lorentz factor. The dependence of the relative velocity on the distance to horizon is analyzed, the time of movement from the point in the accretion disc to the point of scattering with large energy as well as the time of back movement to the Earth are calculated. It is shown that they have reasonable order.

Struggling for the New Role for Business Controller

The recent discussion on changes in the controller's role has mainly focused on the national and organizational level culture aspects of that professional role. While earlier studies have demonstrated how the role changes have been stimulated by corporate culture, IT systems, new accounting technologies and interprofessional competition, we contend that such changes cannot be achieved without active individuals in organizations. Our study is informed by the concept of institutional entrepreneurship, concentrating on changes in institutions, such as professional roles, which are achieved by an active agency. Thus we concentrate on the individual level by tracing how the role of a single controller was changed from that of 'bean counter' towards that of business partner. We illustrate how new cost accounting techniques and modern ICT (ERP) serve as critical momentum for this role change, and how the aspiring controller must creatively combine multiple competencies in order to change the institutionalized role. Organizational roles and other social institutions defending accounting practices may change and are changed only as a result of active agencies. Therefore, in order to understand the conditions for these changes, we need to pay more attention to what active individuals are doing when bringing about these changes. More attention should paid to the political and discursive action on accountants.

Numerical Simulation of Flameless Premixed Combustion with an Annular Nozzle in a Recuperative Furnace

This paper reports an investigation of Computational Fluid Dynamics (CFD) on the influence of injection momentum rate of premixed air and fuel on the flameless Moderate or Intense Low oxygen Dilution (MILD) combustion in a recuperative furnace. Details of the furnace flow velocity, temperature, O 2, CO 2 and NO x concentrations are provided. Results obtained suggest that the flue gas recirculation plays a vital role in establishing the premixed MILD combustion. It is also revealed that there is a critical momentum rate of the fuel-air mixture below which MILD combustion does not occur. Moreover, the momentum rate appears to have less significant influence on conventional global combustion than on MILD combustion.

Importance of Initial Momentum Rate and Air-Fuel Premixing on Moderate or Intense Low Oxygen Dilution (MILD) Combustion in a Recuperative Furnace

This paper reports an investigation on the influences of air−fuel injection momentum rate and the air−fuel premixing on the moderate or intense low oxygen dilution (MILD) combustion in a 20-kW recuperative furnace. Various patterns of partially and fully premixed reactants have proven experimentally to work extremely well in the present furnace. H2 recorded for a variety of equivalence ratios at a firing rate of 10 kW. The present numerical study suggests that there is a critical momentum rate of the inlet fuel−air mixture below which the MILD combustion cannot occur. Also, it is revealed, both experimentally and numerically, that, above the critical rate, both the inlet fuel−air mixedness and momentum rate impose insignificant influence on the stability of and emissions from the MILD combustion.

Collective modes and superflow instabilities of strongly correlated Fermi superfluids

We study the superfluid phase of the one-band attractive Hubbard model of fermions as a prototype of a strongly correlated s-wave fermion superfluid on a lattice. We show that the collective mode spectrum of this superfluid exhibits, in addition to the long wavelength sound mode, a sharp roton mode over a wide range of densities and interaction strengths. We compute the sound velocity and the roton gap within a generalized random phase approximation (GRPA) and show that the GRPA results are in good agreement, at strong coupling, with a spin-wave analysis of the appropriate strong-coupling pseudospin model. We also investigate, using this two-pronged approach, the breakdown of superfluidity in the presence of a supercurrent. We find that the superflow can break down at a critical flow momentum via several distinct mechanisms--depairing, Landau instabilities or dynamical instabilities--depending on the dimension, the interaction strength and the fermion density. The most interesting of these instabilities is a charge modulation dynamical instability which is distinct from previously studied dynamical instabilities of Bose superfluids. The charge order associated with this instability can be of two types: (i) a commensurate checkerboard modulation driven by softening of the roton mode at the Brillouin zone corner, or, (ii)more » an incommensurate density modulation arising from superflow-induced finite momentum pairing of Bogoliubov quasiparticles. We elucidate the dynamical phase diagram showing the critical flow momentum of the leading instability over a wide range of fermion densities and interaction strengths and point out implications of our results for experiments on cold atom fermion superfluids in an optical lattice.« less

Experimental investigation of ejections and sweeps involved in particle suspension

Experiments involving particle‐laden flows were conducted to study interactions between ejections and suspended particles. Three types of beads and four hydraulic conditions are tested (10,000 < Re < 30,000). A two‐dimensional Particle Image Velocimetry technique was applied to measure turbulent characteristics and particle motion simultaneously. The study also focused on the type and intensity of turbulent structures involved in bead suspension. Applying quadrant analysis to the flow in the vicinity of each moving bead, observations showed that upward particle movement is strongly correlated to the presence of ejections in the vicinity of the particle. For a given experiment, all particles surrounded by ejections with an instantaneous value of momentum flux greater than a critical value move upward with a positive vertical velocity. Once made dimensionless by the particle buoyant weight per unit area, the critical momentum flux is weakly dependent on the flow Reynolds number Re. From the statistical analysis a conceptual model of particle motion within an ejection is proposed.

Learning in a Mobile Age

The launch of the International Journal of Mobile and Blended Learning is one of several indicators that mobile learning globally is reaching a critical and sustainable momentum and identity. The past six or seven years have seen a host of pilots and initiatives across sectors and across countries and these have established firstly that mobile learning takes learning to individuals, communities and countries where access to learning was challenging or problematic and secondly that mobile learning enhances, enriches and extends how learning is understood. Environmental factors have meant that this development has been haphazard. The mobile learning community is now faced with broader challenges of scale, durability, equity, embedding and blending in addition to the earlier and more specific challenges of pedagogy and technology, but these developments take place in the context of societies where mobile devices, systems and technologies have a far wider impact than just mobile learning as it is currently conceived. This paper looks at the definition and evolution of mobile learning as the starting point for a discussion of this wider impact.

The Rise of the ^|^ldquo;Liberal Majority^|^rdquo; in 1960s America

The purpose of this paper is to examine how the so-called “Liberal” position arose concerning the issue of ethnic minorities in 1960s America. Some earlier research has seen “Liberal” as only the opposition to “Conservative”, and explained that the “Liberal” position in America was established in the presidential election of 1964 in which the main issue was civil rights policy. According to such research, as the civil rights movement became radical in the late 1960s, the “Liberal” began to collapse. But in this paper, to grasp the “Liberal” more appropriately, I focus on the relation and difference between “majority” and “minority” inside the “Liberal”, and examine how “Liberals in the majority” responded to claims of minority. From this perspective, I will make it clear the radicalization of civil rights movement was not the beginning of the collapse of the “Liberal” but rather the critical momentum for the establishment of “Liberals in the majority”, as it were a “Liberal Majority”.

Classification of Turbulence Modification by Dispersed Spheres Using a Novel Dimensionless Number

A novel dimensionless parameter, the particle moment number Pa, was derived using dimensional analysis of the particle-laden Navier-Stokes equations, in order to understand the underlying physics of turbulence modification by particles. A set of 80 previous experimental measurements where the turbulent kinetic energy was modified by particles was examined, and all results could clearly be divided into three groups in Re-Pa mappings. The turbulence attenuation region was observed between the augmentation regions with two critical particle momentum numbers.

On the onset of incipient fluidization

The onset of incipient fluidization is investigated theoretically and simulated by a computational fluid dynamics (CFD) procedure. The onset of incipient instability in a particle bed is preceded by stable gas diffusion in the interstices and is caused by a critical momentum force that may overcome the inertia of the particles. The critical momentum force is provided by the critical superficial gas velocity U c in the form of critical mass flux of diffusion. It is found that the first movement of particles may be predicted by a critical transient Rayleigh number determined by a critical superficial velocity equals to the minimum fluidization velocity, U mf. The onset of incipient fluidization was found to occur at a critical transient Rayleigh number of 3.1, which is close to the lowest theoretical value for buoyancy convection in a porous medium bounded by free surfaces. Consequently the onset times of incipient fluidization may be predicted accurately. The finding has been found to be supported by the present CFD study, past experiments and simulations in the literature.

Deconstructing triplet nucleon-nucleon scattering

Nucleon-nucleon scattering in spin-triplet channels is analyzed within an effective field theory where one-pion exchange is treated nonperturbatively. Justifying this requires the identification of an additional low-energy scale in the strength of that potential. Short-range interactions are organized according to the resulting power counting, in which the leading term is promoted to significantly lower order than in the usual perturbative counting. In each channel there is a critical momentum above which the waves probe the singular core of the tensor potential and the new counting is necessary. When the effects of one- and two-pion exchange have been removed using a distorted-wave Born approximation, the residual scattering in waves with L{ =}3 is consistent with the perturbative counting, at least for energies up to 300 MeV. This pattern is in agreement with estimates of the critical momenta in these channels.

Meissner effect in superconductors with a finite pair momentum

The features of the Meissner effect in superconductors with a finite pairing momentum are analyzed. Response to a weak magnetic field is calculated for various cases covering a pair momentum range from q ≪ Δ/v0 to q ∼ p0, including q = Δ0/v0 (v0 is the velocity on the Fermi surface and Δ0 is the order parameter at zero temperature; the system of units where ħ = 1 is used). The response of a superconductor carrying the transport current at a temperature close to the critical temperature Tc is determined. It is shown that, at a certain critical momentum (current), the response parallel to the momentum vanishes and the London length is infinite. The response perpendicular to the momentum remains unchanged. The response of the superconductor in the current state at zero temperature is calculated. A new contribution to the paramagnetic current is found, and its mechanism is determined. This contribution can be large for high momenta q ∼ p0. The Meissner effect is analyzed in detail for the state proposed by Larkin and Ovchinnikov, Zh. Eksp. Teor. Fiz. 47, 1136 (1964) [Sov. Phys. JETP 20, 762 (1964)], as well as by Fulde and Ferrel, Phys. Rev. A 135, 550 (1964). It is shown that the response parallel to the vector q is nonzero and diamagnetic. On the contrary, the response perpendicular to the momentum vanishes at the optimal momentum q0. The sensitivity of the Meissner effect to the fine features of the superconducting state such as the quasiparticle spectrum, coherent factors, etc. is demonstrated.