Tiltrotor CFD part II: aerodynamic optimisation of tiltrotor blades

This paper presents aerodynamic optimisation of tiltrotor blades with high-fidelity computational fluid dynamics. The employed optimisation framework is based on a quasi-Newton method, and the required high-fidelity flow gradients were computed using a discrete adjoint solver. Single-point optimisations were first performed, to highlight the contrasting requirements of the helicopter and aeroplane flight regimes. It is then shown how a trade-off blade design can be obtained using a multi-point optimisation strategy. The parametrisation of the blade shape allowed to modify the twist and chord distributions, and to introduce a swept tip. The work shows how these main blade shape parameters influence the optimal performance of the tiltrotor in helicopter and aeroplane modes, and how a compromise blade shape can increase the overall tiltrotor performance. Moreover, in all the presented cases, the accuracy of the adjoint gradients resulted in a small number of flow evaluations for finding the optimal solution, thus indicating gradient-based optimisation as a viable tool for modern tiltrotor design

Magnetic hour-glass dispersion and its relation to high-temperature superconductivity in iron-tuned Fe1+y_{1+y}Te0.7_{0.7}Se0.3_{0.3}

High-temperature superconductivity remains arguably the largest outstanding enigma of condensed matter physics. The discovery of iron-based high-temperature superconductors has renewed the importance of understanding superconductivity in materials susceptible to magnetic order and fluctuations. Intriguingly they show magnetic fluctuations reminiscent of the superconducting (SC) cuprates, including a 'resonance' and an 'hour-glass' shaped dispersion, which provide an opportunity to new insight to the coupling between spin fluctuations and superconductivity. Here we report inelastic neutron scattering data on Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$ using excess iron concentration to tune between a SC ($y=0.02$) and a non-SC ($y=0.05$) ground states. We find incommensurate spectra in both samples but discover that in the one that becomes SC, a constriction towards a commensurate hourglass shape develop well above $T_c$. Conversely a spin-gap and concomitant spectral weight shift happen below $T_c$. Our results imply that the hourglass shaped dispersion is most likely a pre-requisite for superconductivity, whereas the spin-gap and shift of spectral weight are consequences of superconductivity. We explain this observation by pointing out that an inwards dispersion towards the commensurate wave-vector is needed for the opening of a spin gap to lower the magnetic exchange energy and hence provide the necessary condensation energy for the SC state to emerge

Angular momentum distribution of hot gas and implications for disk galaxy formation

We study the angular momentum profiles both for dark matter and for gas within virialized halos, using a statistical sample of halos drawn from cosmological hydrodynamics simulations. Three simulations have been analyzed, one is the ``non-radiative'' simulation, and the other two have radiative cooling. We find that the gas component on average has a larger spin and contains a smaller fraction of mass with negative angular momentum than its dark matter counterpart in the non-radiative model. As to the cooling models, the gas component shares approximately the same spin parameter as its dark matter counterpart, but the hot gas has a higher spin and is more aligned in angular momentum than dark matter, while the opposite holds for the cold gas. After the mass of negative angular momentum is excluded, the angular momentum profile of the hot gas component approximately follows the universal function originally proposed by Bullock et al. for dark matter, though the shape parameter $\mu$ is much larger for hot gas and is comfortably in the range required by observations of disk galaxies. Since disk formation is related to the distribution of hot gas that will cool, our study may explain the fact that the disk component of observed galaxies contains a smaller fraction of low angular momentum material than dark matter in halos.Comment: 30 pages, 12 figures, 4 tables, accepted for publication in Ap

Galaxy Evolution, Deep Galaxy Counts and the Near-IR Cosmic Infrared Background

Accurate synthetic models of stellar populations are constructed and used in evolutionary models of stellar populations in forming galaxies. Following their formation, the late type galaxies are assumed to follow the Schmidt law for star formation, while early type galaxies are normalized to the present-day fundamental plane relations assumed to mimic the metallicity variations along their luminosity sequence. We then compute predictions of these models for the observational data at early epochs for various cosmological parameters $\Omega, \Omega_\Lambda$ and $H_0$. We find good match to the metallicity data from the damped $L_\alpha$ systems and the evolution of the luminosity density out to $z\simeq 1$. Likewise, our models provide good fits for low values of $\Omega$ to the deep number counts of galaxies in all bands where data is available; this is done without assuming existence of extra populations of galaxies at high $z$. Our models also match the data on the redshift distribution of galaxy counts in $B$ and $K$ bands. We compute the predicted mean levels and angular distribution of the cosmic infrared background produced from the early evolution of galaxies. The predicted fluxes and fluctuations are still below the current observational limits, but not by a large factor. Finally, we find that the recent detection of the diffuse extragalactic light in the visible bands requires for our models high redshift of galaxy formation, $z_f \geq$(3-4); otherwise the produced flux of the extragalactic light at optical bands exceeds the current observational limits.Comment: Accepted to Ap

Loss of star forming gas in SDSS galaxies

Using the star formation rates from the SDSS galaxy sample, extracted using the MOPED algorithm, and the empirical Kennicutt law relating star formation rate to gas density, we calculate the time evolution of the gas fraction as a function of the present stellar mass. We show how the gas-to-stars ratio varies with stellar mass, finding good agreement with previous results for smaller samples at the present epoch. For the first time we show clear evidence for progressive gas loss with cosmic epoch, especially in low-mass systems. We find that galaxies with small stellar masses have lost almost all of their cold baryons over time, whereas the most massive galaxies have lost little. Our results also show that the most massive galaxies have evolved faster and turned most of their gas into stars at an early time, thus strongly supporting a downsizing scenario for galaxy evolution.Comment: 29 pages, 9 figures, ApJ, accepte

On the detectability of gravitational waves background produced by gamma ray bursts

In this paper we discuss a new strategy for the detection of gravitational radiation likely emitted by cosmological gamma ray burst. Robust and conservative estimates lead to the conclusion that the uncorrelated superimposition of bursts of gravitational waves can be detected by interferometric detectors like VIRGO or LIGO. The expected signal is predicted to carry two very distinctive signatures: the cosmological dipole anisotropy and a characteristic time scale in the auto correlation spectrum, which might be exploited, perhaps with ad hoc modifications and/or upgrading of the planned experiments, to confirm the non-instrumental origin of the signal.Comment: 9 pages, 2 figures, LATEX2e, Accepted for pubblications as a Letter to the Editor in Journal of Physics G: Nuclear and Particle Physic

Second-Generation Objects in the Universe: Radiative Cooling and Collapse of Halos with Virial Temperatures Above 10^4 Kelvin

The first generation of protogalaxies likely formed out of primordial gas via H2-cooling in cosmological minihalos with virial temperatures of a few 1000K. However, their abundance is likely to have been severely limited by feedback processes which suppressed H2 formation. The formation of the protogalaxies responsible for reionization and metal-enrichment of the intergalactic medium, then had to await the collapse of larger halos. Here we investigate the radiative cooling and collapse of gas in halos with virial temperatures Tvir > 10^4K. In these halos, efficient atomic line radiation allows rapid cooling of the gas to 8000 K; subsequently the gas can contract nearly isothermally at this temperature. Without an additional coolant, the gas would likely settle into a locally gravitationally stable disk; only disks with unusually low spin would be unstable. However, we find that the initial atomic line cooling leaves a large, out-of-equilibrium residual free electron fraction. This allows the molecular fraction to build up to a universal value of about x(H2) = 10^-3, almost independently of initial density and temperature. We show that this is a non--equilibrium freezeout value that can be understood in terms of timescale arguments. Furthermore, unlike in less massive halos, H2 formation is largely impervious to feedback from external UV fields, due to the high initial densities achieved by atomic cooling. The H2 molecules cool the gas further to about 100K, and allow the gas to fragment on scales of a few 100 Msun. We investigate the importance of various feedback effects such as H2-photodissociation from internal UV fields and radiation pressure due to Ly-alpha photon trapping, which are likely to regulate the efficiency of star formation.Comment: Revised version accepted by ApJ; some reorganization for clarit

Ab initio evaluation of the charge-ordering in α′NaV2O5\alpha^\prime NaV_2O_5

We report {\it ab initio} calculations of the charge ordering in $\alpha^\prime NaV_2O_5$ using large configurations interaction methods on embedded fragments. Our major result is that the $2p_y$ electrons of the bridging oxygen of the rungs present a very strong magnetic character and should thus be explicitly considered in any relevant effective model. The most striking consequence of this result is that the spin and charge ordering differ substantially, as differ the experimental results depending on whether they are sensitive to the spin or charge density.Comment: 4 page

Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances

We identify an accretion disk atmosphere and corona from the high resolution X-ray spectrum of Hercules X-1, and we determine its detailed physical properties. More than two dozen recombination emission lines (from Fe XXVI at 1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were detected in a 50 ks observation with the Chandra High-Energy Transmission Grating Spectrometer (HETGS). They allow us to measure the density, temperature, spatial distribution, elemental composition, and kinematics of the plasma. We exclude HZ Her as the source of the recombination emission. We compare accretion disk model atmospheres with the observed spectrum in order to constrain the stratification of density and ionization, disk atmosphere area, elemental composition, and energetics. The atmospheric spectrum observed during the low state is photoionized by the main-on X-ray continuum, indicating that the disk is observed edge-on during the low state. We infer the mean number of scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism modeled by Sako (2003) to enhance N VII emission via a line overlap with O VIII. The line optical depth diagnostics are consistent with a flattened atmosphere. Our spectral analysis, the disk atmosphere model, and the presence of intense N VII and N VI lines (plus N V in the UV), confirm the over-abundance of nitrogen relative to other metals, which was shown to be indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap