Computation of nozzle flow fields using the PARC2D Navier-Stokes code

Supersonic nozzles which operate at low Reynolds numbers and have large expansion ratios have very thick boundary layers at their exit. This leads to a very strong viscous/inviscid interaction upon the flow within the nozzle and the traditional nozzle design techniques which correct the inviscid core with a boundary layer displacement do not accurately predict the nozzle exit conditions. A full Navier-Stokes code (PARC2D) was used to compute the nozzle flow field. Grids were generated using the interactive grid generator code TBGG. All computations were made on the NASA MSFC CRAY X-MP computer. Comparison was made between the computations and in-house wall pressure measurements for CO2 flow through a conical nozzle having an area ratio of 40. Satisfactory agreement existed between the computations and measurements for a stagnation pressure of 29.4 psia and stagnation temperature of 1060 R. However, agreement did not exist at a stagnation pressure of 7.4 psia. Several reasons for the lack of agreement are possible. The computational code assumed a constant gas gamma whereas gamma for CO2 varied from 1.22 in the plenum chamber to 1.38 at the nozzle exit. Finally, it is possible that condensation occurred during the expansion at the lower stagnation pressure

Can emerging African Stock Markets improve their informational efficiency by formally harmonising and integrating their operations?

Despite experiencing rapid growth in their number and size, African stock markets remain highly segmented, small, illiquid and technologically bankrupt, severely affecting their informational efficiency. On this basis, with specific focus on the weak-form of the efficient markets hypothesis, we attempt to empirically ascertain whether African stock markets can improve their informational efficiency by formally harmonising and integrating their operations using a new robust non-parametric variance-ratios test in addition to its parametric alternative. On average, we find that irrespective of the diagnostic used, all the 24 African continent-wide indices applied returns’ display better normal distribution properties than those of the 8 individual national stock price indices examined. We record evidence of statistically significant improvements in the informational efficiency of the African continent-wide stock price indices over the individual national stock price indices used irrespective of the test statistic applied. The potential improvement in efficiency to be gained is much higher in economic sectors indices than in size and regional indices. Finally, consistent with prior evidence, (eg., Wright, 2000; Belaire-Franch and Opong, 2005, Ntim, et al., 2007), the results of the Lo and MacKinlay (1988) parametric variance-ratios test are ambiguous. By contrast, the ranks and signs alternative offer consistent results throughout.African stock markets, Integration, Efficiency, Variance-ratios, Ranks and signs

Supernova 1987A: Rotation and a Binary Companion

In this paper we provide a possible link between the structure of the bipolar nebula surrounding SN1987A and the properties of its progenitor star. A Wind Blwon Bubble (WBB) scenario is emplyed, in which a fast, tenuous wind from a Blue Supergiant expands into a slow, dense wind, expelled during an earlier Red Supergiant phase. The bipolar shapre develops due to a pole-to-equator density contrast in the slow wind (ie, the slow wind forms a slow torus). We use the Wind Compressed Disk (WCD) model of Bjorkman & Cassinelli (1992) to determine the shape of the slow torus. In the WCD scenario, the shape of the torus is determined by the rotation of the progenitor star. We then use a self-similar semi-analytical method for wind blown bubble evolution to determine the shape of the resulting bipolar nebula. We find that the union of the wind-compressed-disk and bipolar-wind-blown- bubble models allows us to recover the salient properties of SN1987A's circumstellar nebula. In particular, the size, speed and density of SN1987A's inner ring are easily reproduced in our calculations. An exploration of parameter space shows the the red supergiant progenitor must be been rotating at > 0.3 of its breakup speed. We conclude that the progenitor was most likely spun up by a merger with a binary companion. Using a simple model for the binary merger we find that the companion is likely to have had a mass > 0.5 M_sun.Comment: 30 pages, 4 figure

Stochastic Wobble of Accretion Discs and Jets from Turbulent Rocket Torques

Models of accretion discs and their associated outflows often incorporate assumptions of axisymmetry and symmetry across the disc plane. However, for turbulent discs these symmetries only apply to averaged quantities and do not apply locally. The local asymmetries can induce local imbalances in outflow power across the disc mid-plane, which can in turn induce local tilting torques. Here we calculate the effect of the resulting stochastic torques on disc annuli that are a consequence of standard mean field accretion disc models. The torques induce a random walk of the vector perpendicular to the plane of each averaged annulus. This random walk is characterized by a radially dependent diffusion coefficient which we calculate for small angle tilt. We use the coefficient to calculate a radially dependent time scale for annular tilt and associated jet wobble. The wobble time depends on the square of the wander angle so the age of a given system determines the maximum wobble angle. We apply this to examples of blazars, young stellar objects and binary engines of pre-planetary nebulae and microquasars. It is noteworthy that for an averaging time $t_w\sim 3$ days, we estimate a wobble angle for jets in SS433 of $\theta\sim 0.8$ degrees, not inconsistent with observational data. In general the non-periodic nature of the stochastic wobble could distinguish it from faster periodic jet precession.Comment: 12 pages, accepted by MNRA

The subscale orbital fluid transfer experiment

The Subscale Orbital Fluid Transfer Experiment (SOFTE) is a planned Shuttle Orbiter fluid transfer experiment. CASP (Center for Advanced Space Propulsion) performed certain aspects of the conceptual design of this experiment. The CASP work consisted of the conceptual design of the optical system, the search for alternative experimental fluids, the determination of the flow meter specifications and the examination of materials to use for a bladder that will empty one of the tanks in the experiment

Can emerging African Stock Markets improve their informational efficiency by formally harmonising and integrating their operations?

Despite experiencing rapid growth in their number and size, African stock markets remain highly segmented, small, illiquid and technologically bankrupt, severely affecting their informational efficiency. On this basis, with specific focus on the weak-form of the efficient markets hypothesis, we attempt to empirically ascertain whether African stock markets can improve their informational efficiency by formally harmonising and integrating their operations using a new robust non-parametric variance-ratios test in addition to its parametric alternative. On average, we find that irrespective of the diagnostic used, all the 24 African continent-wide indices applied returns’ display better normal distribution properties than those of the 8 individual national stock price indices examined. We record evidence of statistically significant improvements in the informational efficiency of the African continent-wide stock price indices over the individual national stock price indices used irrespective of the test statistic applied. The potential improvement in efficiency to be gained is much higher in economic sectors indices than in size and regional indices. Finally, consistent with prior evidence, (eg., Wright, 2000; Belaire-Franch and Opong, 2005, Ntim, et al., 2007), the results of the Lo and MacKinlay (1988) parametric variance-ratios test are ambiguous. By contrast, the ranks and signs alternative offer consistent results throughout

A stacking-fault based microscopic model for platelets in diamond

We propose a new microscopic model for the $\{001\}$ planar defects in diamond commonly called platelets. This model is based on the formation of a metastable stacking fault, which can occur because of the ability of carbon to stabilize in different bonding configurations. In our model the core of the planar defect is basically a double layer of three-fold coordinated $sp^2$ carbon atoms embedded in the common $sp^3$ diamond structure. The properties of the model were determined using {\it ab initio} total energy calculations. All significant experimental signatures attributed to the platelets, namely, the lattice displacement along the $[001]$ direction, the asymmetry between the $[110]$ and the $[1\bar{1}0]$ directions, the infrared absorption peak $B^\prime$, and broad luminescence lines that indicate the introduction of levels in the band gap, are naturally accounted for in our model. The model is also very appealing from the point of view of kinetics, since naturally occurring shearing processes will lead to the formation of the metastable fault.Comment: 5 pages, 4 figures. Submitted for publication on August 2nd, 200

Genetic structure of Anopheles gambiae populations on islands in northwestern Lake Victoria, Uganda

BACKGROUND: Alternative means of malaria control are urgently needed. Evaluating the effectiveness of measures that involve genetic manipulation of vector populations will be facilitated by identifying small, genetically isolated vector populations. The study was designed to use variation in microsatellite markers to look at genetic structure across four Lake Victoria islands and two surrounding mainland populations and for evidence of any restriction to free gene flow. METHODS: Four Islands (from 20–50 km apart) and two surrounding mainland populations (96 km apart) were studied. Samples of indoor resting adult mosquitoes, collected over two consecutive years, were genotyped at microsatellite loci distributed broadly throughout the genome and analysed for genetic structure, effective migration (Nem) and effective population size (Ne). RESULTS: Ne estimates showed island populations to consist of smaller demes compared to the mainland ones. Most populations were significantly differentiated geographically, and from one year to the other. Average geographic pair-wise FST ranged from 0.014–0.105 and several pairs of populations had Ne m < 3. The loci showed broad heterogeneity at capturing or estimating population differences. CONCLUSION: These island populations are significantly genetically differentiated. Differences reoccurred over the study period, between the two mainland populations and between each other. This appears to be the product of their separation by water, dynamics of small populations and local adaptation. With further characterisation these islands could become possible sites for applying measures evaluating effectiveness of control by genetic manipulation

Immune sensing of Candida albicans requires cooperative recognition of mannans and glucans by lectin and Toll-like receptors

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