Effects of Nose Bluntness and Shock-Shock Interactions on Blunt Bodies in Viscous Hypersonic Flows

A numerical study has been conducted to investigate the effects of blunt leading edges on the viscous flow field around a hypersonic vehicle such as the proposed National Aero-Space Plane. Attention is focused on two specific regions of the flow field. Analysis of these flow regions is required to accurately predict the overall flow field as well as to get necessary information on localized zones of high pressure and intense heating. The forebody is modeled by slender cones and ogives with spherically blunted nose. A combination of Navier-Stokes and parabolized Navier-Stokes equations is used to compute the flow field. The influence of entropy layer thickness on the extent of the leading edge effects is also considered. The extent of downstream effects of leading edge thickness are determined at Mach numbers of 10 and 20 for cone angles of 5°, 10°, and 20°. Three values of nose bluntness are considered with the smallest nose blunting (0.0025m) representing the sharp cone/ogive. For the flow region around the inlet the forebody shock can interact either with the blunt cowl leading edge shock or with the shock produced by the blunt leading edges of the swept sidewall compression surfaces of the inlet. For the interaction at the cowl leading edge, the forebody shock is assumed planar and the cowl is modeled by a two-dimensional cylindrically blunted wedge of infinite width. Use of the full Navier-Stokes equations is made on the cowl forebody and the thin-layer Navier-Stokes equations are suitably modified for space marching on the cowl afterbody. The results of the study show that the flow around the cowl is significantly altered by the impinging shock. The peak value of pressure is found to be nine times and heating rates eight times the stagnation point value for unimpinged case at Mach 8.03. The peak values were slightly lower for Mach 5.94 calculations. A three-dimensional thin-layer Navier-Stokes code has been used to calculate the flow field. The peak pressure for this case is found to be 2.25 times and the peak heating three times the unimpinged stagnation values. The results of the study are compared with the available experimental and numerical results

Genetic Basis of Self-Incompatibility in the Lichen-Forming Fungus Lobaria pulmonaria and Skewed Frequency Distribution of Mating-Type Idiomorphs: Implications for Conservation

Fungal populations that reproduce sexually are likely to be genetically more diverse and have a higher adaptive potential than asexually reproducing populations. Mating systems of fungal species can be self-incompatible, requiring the presence of isolates of different mating-type genes for sexual reproduction to occur, or self-compatible, requiring only one. Understanding the distribution of mating-type genes in populations can help to assess the potential of self-incompatible species to reproduce sexually. In the locally threatened epiphytic lichen-forming fungus Lobaria pulmonaria (L.) Hoffm., low frequency of sexual reproduction is likely to limit the potential of populations to adapt to changing environmental conditions. Our study provides direct evidence of self-incompatibility (heterothallism) in L. pulmonaria. It can thus be hypothesized that sexual reproduction in small populations might be limited by an unbalanced distribution of mating-type genes. We therefore assessed neutral genetic diversity (using microsatellites) and mating-type ratio in 27 lichen populations (933 individuals). We found significant differences in the frequency of the two mating types in 13 populations, indicating a lower likelihood of sexual reproduction in these populations. This suggests that conservation translocation activities aiming at maximizing genetic heterogeneity in threatened and declining populations should take into account not only presence of fruiting bodies in transplanted individuals, but also the identity and balanced representation of mating-type genes

Long-term consequences of disturbances on reproductive strategies of the rare epiphytic lichen Lobaria pulmonaria: clonality a gift and a curse

The effect of disturbance on symbiotic organisms such as lichens is particularly severe. In case of heterothallic lichen-forming fungi, disturbances may lead to unbalanced gene frequency and patchy distribution of mating types, thus inhibiting sexual reproduction and imposing clonality. The impact of disturbance on reproductive strategies and genetic diversity of clonal systems has so far received little attention. To infer the effects of disturbances on mating-type allele frequencies and population structure, we selected three populations in the Parc Jurassien Vaudois (Switzerland), which were affected by uneven-aged forestry, intensive logging and fire, respectively. We used microsatellite markers to infer genetic diversity, allelic richness and clonal diversity of the epiphytic lichen Lobaria pulmonaria and used L. pulmonaria-specific MAT1-1 and MAT1-2 markers to analyse the frequency and distribution of mating types of 889 individuals. Our study shows that stand-replacing disturbances affect the mating-type frequency and distribution, thus compromising the potential for sexual reproduction. The fire-disturbed area had a significantly lower genetic and genotypic diversity and a higher clonality. Furthermore, the majority of compatible mating pairs in this area were beyond the effective vegetative dispersal range of the species. We conclude that stand-replacing disturbances lead to lower chances of sex and symbiont reshuffling and thus have long-lasting negative consequences on the reproductive strategies and adaptive potential of epiphytic lichen symbiose

Design of a low band gap oxide ferroelectric: Bi6_6Ti4_4O17_{17}

A strategy for obtaining low band gap oxide ferroelectrics based on charge imbalance is described and illustrated by first principles studies of the hypothetical compound Bi$_6$Ti$_4$O$_{17}$, which is an alternate stacking of the ferroelectric Bi$_4$Ti$_3$O$_{12}$. We find that this compound is ferroelectric, similar to Bi$_4$Ti$_3$O$_{12}$ although with a reduced polarization. Importantly, calculations of the electronic structure with the recently developed functional of Tran and Blaha yield a much reduced band gap of 1.83 eV for this material compared to Bi$_4$Ti$_3$O$_{12}$. Therefore, Bi$_6$Ti$_4$O$_{17}$ is predicted to be a low band gap ferroelectric material

Magnetic reconnection and Blandford-Znajek process around rotating black holes

We provide a semi-analytic comparison between the Blandford-Znajek (BZ) and the magnetic reconnection power for accreting black holes in the curved spacetime of a rotating black hole. Our main result is that for a realistic range of astrophysical pa- rameters, the reconnection power may compete with the BZ power. The field lines anchored close to or on the black hole usually evolve to open field lines in general rel- ativistic magnetohydrodynamic (GRMHD) simulations. The BZ power is dependent on the black hole spin while magnetic reconnection power is independent of it for the near force-free magnetic configuration with open field lines adopted in our theoretical study. This has obvious consequences for the time evolution of such systems particu- larly in the context of black hole X-ray binary state transitions. Our results provide analytical justification of the results obtained in GRMHD simulations

Gene abundance linked to climate zone: Parallel evolution of gene content along elevation gradients in lichenized fungi

IntroductionIntraspecific genomic variability affects a species' adaptive potential toward climatic conditions. Variation in gene content across populations and environments may point at genomic adaptations to specific environments. The lichen symbiosis, a stable association of fungal and photobiont partners, offers an excellent system to study environmentally driven gene content variation. Many of these species have remarkable environmental tolerances, and often form populations across different climate zones. Here, we combine comparative and population genomics to assess the presence and absence of genes in high and low elevation genomes of two lichenized fungi of the genus Umbilicaria. MethodsThe two species have non-overlapping ranges, but occupy similar climatic niches in North America (U. phaea) and Europe (U. pustulata): high elevation populations are located in the cold temperate zone and low elevation populations in the Mediterranean zone. We assessed gene content variation along replicated elevation gradients in each of the two species, based on a total of 2050 individuals across 26 populations. Specifically, we assessed shared orthologs across species within the same climate zone, and tracked, which genes increase or decrease in abundance within populations along elevation. ResultsIn total, we found 16 orthogroups with shared orthologous genes in genomes at low elevation and 13 at high elevation. Coverage analysis revealed one ortholog that is exclusive to genomes at low elevation. Conserved domain search revealed domains common to the protein kinase superfamily. We traced the discovered ortholog in populations along five replicated elevation gradients on both continents and found that the number of this protein kinase gene linearly declined in abundance with increasing elevation, and was absent in the highest populations. DiscussionWe consider the parallel loss of an ortholog in two species and in two geographic settings a rare find, and a step forward in understanding the genomic underpinnings of climatic tolerances in lichenized fungi. In addition, the tracking of gene content variation provides a widely applicable framework for retrieving biogeographical determinants of gene presence/absence patterns. Our work provides insights into gene content variation of lichenized fungi in relation to climatic gradients, suggesting a new research direction with implications for understanding evolutionary trajectories of complex symbioses in relation to climatic change

A First-Principles Approach to Insulators in Finite Electric Fields

We describe a method for computing the response of an insulator to a static, homogeneous electric field. It consists of iteratively minimizing an electric enthalpy functional expressed in terms of occupied Bloch-like states on a uniform grid of k points. The functional has equivalent local minima below a critical field E_c that depends inversely on the density of k points; the disappearance of the minima at E_c signals the onset of Zener breakdown. We illustrate the procedure by computing the piezoelectric and nonlinear dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm