A computational study of oscillatory extinction of spherical diffusion flames

The transient behavior of burner-supported spherical diffusion flames was studied in the transport-induced limit of low mass flow rate and the radiation-induced limit of high mass flow rate which characterize the isola response of flame extinction. Oscillatory instability was observed near both steady-state extinction limits. The oscillation typically grows in amplitude until it becomes large enough to extinguish the flame. The oscillatory behavior was numerically observed using detailed chemistry and transport for methane (50%CH₄/50%He into 21%O₂/79%He) and hydrogen (100% H₂ into 21%O₂/79%He) diffusion flames where the fuel was issued from a point source, and helium was selected as an inert to increase the Lewis number, facilitating the onset of oscillation. In both methane and hydrogen flames, the oscillation always leads to extinction, and no limit cycle behavior was found. The growth rate of the oscillation was found to be slow enough under certain conditions to allow the flame to oscillate for over 450 s, suggesting that such oscillations can possibly be observed experimentally. For the hydrogen flames, however, the frequency of oscillation near the transport-induced limit is much larger, approximately 60 Hz as compared to 0.35 Hz for the methane flame, and the maximum amplitude of temperature oscillations was about 5 K. The distinctively different structures of the hydrogen and methane flames suggest that while both instabilities are thermal-diffusive in origin, oscillations in the hydrogen flames resemble those of premixed flames, while oscillations in the methane flames are non-premixed in character.     

Material Selection for Cryogenic Support Structures

Design specifications for the support structures of low temperature instrumentation often call for low thermal conductivity between temperature stages, high stiffness, and specific load bearing capabilities. While overall geometric design plays an important role in both overall stiffness and heat conduction between stages, material selection can affect a structure’s properties significantly. In this contribution, we suggest and compare several alternative materials to the current standard materials for building cryogenic support structures.     

Explicit strategies for incompressible fluid‐structure interaction problems: Nitsche type mortaring versus Robin–Robin coupling

We discuss explicit coupling schemes for fluid‐structure interaction problems where the added mass effect is important. In this paper, we show the close relation between coupling schemes by using Nitsche's method and a Robin–Robin type coupling. In the latter case, the method may be implemented either using boundary integrals of the stresses or the more conventional discrete lifting operators. Recalling the explicit method proposed in Comput. Methods Appl. Mech. Engrg. 198(5‐8):766–784, 2009, we make the observation that this scheme is stable under a hyperbolic type CFL condition, but that optimal accuracy imposes a parabolic type CFL conditions because of the splitting error. Two strategies to enhance the accuracy of the coupling scheme under the hyperbolic CFL‐condition are suggested, one using extrapolation and defect‐correction and one using a penalty‐free non‐symmetric Nitsche method. Finally, we illustrate the performance of the proposed schemes on some numerical examples in two and three space dimensions. Copyright © 2013 John Wiley & Sons, Ltd.     

Tailoring the Mechanical Properties of High‐Aspect‐Ratio Carbon Nanotube Arrays using Amorphous Silicon Carbide Coatings

The porous nature of carbon nanotube (CNT) arrays allows for the unique opportunity to tailor their mechanical response by the infiltration and deposition of nanoscale conformal coatings. Here, we fabricate novel photo‐lithographically defined CNT pillars that are conformally coated with amorphous silicon carbide (a‐SiC) to strengthen the interlocking of individual CNTs at junctions using low pressure chemical vapor deposition (LPCVD). We further quantify the mechanical response by performing flat‐punch nanoindentation measurements on coated CNT pillars with various high‐aspect‐ratios. We discovered new mechanical failure modes of coated CNT pillars, such as “bamboo” and brittle‐like composite rupture as coating thickness increases. Furthermore, a significant increase in strength and modulus is achieved. For CNT pillars with high aspect ratio (1:10) and coating thickness of 21.4 nm, the compressive strength increases by an order of magnitude of 3, towards 1.8 GPa (from below 1 MPa for uncoated CNT pillars) and the elastic modulus increases towards 125 GPa. These results show that our coated CNT pillars, which can serve as vertical interconnects and 3D super‐capacitors, can be transformed into robust high‐aspect‐ratio 3D‐micro architectures with semiconductor device compatible processes.     

SPATIAL RESPONSES TO FORDIST AND POST-FORDIST ACCUMULATION AND REGULATION

ABSTRACT This paper deals with the political geography of regional economic development under Fordism and its transition to post-Fordism. It summarises international, national and regional trends in Fordist accumulation and regulation. Second, it indicates haw Fordism dissolves and is replaced by a so tar insufficiently perceived and defined "post-Fordism." A general sketch of the political and economic geography for this, perhaps, new stage in capitalist development is put forward. Third, the authors discuss which elements in different institutional and neo-Marxian theories of the state are the most helpful to understand the role of the state under Fordism and the transition to post-Fordism.     

Rupture Pathway of Phosphatidylcholine Liposomes on Silicon Dioxide

We have investigated the pathway by which unilamellar POPC liposomes upon adsorption undergo rupture and form a supported lipid bilayer (SLB) on a SiO₂ surface. Biotinylated lipids were selectively incorporated in the outer monolayer of POPC liposomes to create liposomes with asymmetric lipid compositions in the outer and inner leaflets. The specific binding of neutravidin and anti-biotin to SLBs formed by liposome fusion, prior to and after equilibrated flip-flop between the upper and lower monolayers in the SLB, were then investigated. It was concluded that the lipids in the outer monolayer of the vesicle predominantly end up on the SLB side facing the SiO₂ substrate, as demonstrated by having maximum 30–40% of lipids in the liposome outer monolayer orienting towards the bulk after forming the SLB.     

Algorithmic Graph Minor Theory: Improved Grid Minor Bounds and Wagner’s Contraction

We explore three important avenues of research in algorithmic graph-minor theory, which all stem from a key min-max relation between the treewidth of a graph and its largest grid minor. This min-max relation is a keystone of the Graph Minor Theory of Robertson and Seymour, which ultimately proves Wagner’s Conjecture about the structure of minor-closed graph properties. First, we obtain the only known polynomial min-max relation for graphs that do not exclude any fixed minor, namely, map graphs and power graphs. Second, we obtain explicit (and improved) bounds on the min-max relation for an important class of graphs excluding a minor, namely, K _3,k -minor-free graphs, using new techniques that do not rely on Graph Minor Theory. These two avenues lead to faster fixed-parameter algorithms for two families of graph problems, called minor-bidimensional and contraction-bidimensional parameters, which include feedback vertex set, vertex cover, minimum maximal matching, face cover, a series of vertex-removal parameters, dominating set, edge dominating set, R-dominating set, connected dominating set, connected edge dominating set, connected R-dominating set, and unweighted TSP tour. Third, we disprove a variation of Wagner’s Conjecture for the case of graph contractions in general graphs, and in a sense characterize which graphs satisfy the variation. This result demonstrates the limitations of a general theory of algorithms for the family of contraction-closed problems (which includes, for example, the celebrated dominating-set problem). If this conjecture had been true, we would have had an extremely powerful tool for proving the existence of efficient algorithms for any contraction-closed problem, like we do for minor-closed problems via Graph Minor Theory.     

Sensor placement for fault isolation in linear differential-algebraic systems

An algorithm is proposed for computing which sensor additions make a diagnosis requirement specification regarding fault detectability and isolability attainable for a given linear differential-algebraic model. Restrictions on possible sensor locations can be given, and if the diagnosis specification is not attainable with any available sensor addition, the algorithm provides the solutions that maximize specification fulfillment. Previous approaches with similar objectives have been based on the model structure only. Since the proposed algorithm utilizes the analytical expressions, it can handle models where structural approaches fail.