Large eddy simulation of a compressor cascade and the influence of spanwise domain

A controlled diffusion compressor cascade is studied using large eddy simulation (LES). The aim of this study is to assess the capability of LES to be used in an industrial context. The Reynolds number is approximately 700 000 based on chord length and inlet velocity. A 'thin-slice' representation of the cascade is used as the reference grid, and the influence of a narrow span is studied by comparison simulations with a domain that has a span five times larger than the thin-slice grid. While the instantaneous flow-fields of the thin-slice and wide-domain simulations are qualitatively similar, the thin-slice simulations suffer from flow confinement problems caused by the imposition of the narrow span. The non-unity axial velocity density ratio of the flow enforces the use of inviscid wall spanwise boundaries, which have a parasitic influence on the development of the flow in the thin-slice simulations. The resultant data obtained from the thin-slice simulations are therefore compromised and the computed loss estimation is considered unreliable. However, when comparing mean quantities such as surface pressure and boundary layer growth the narrow does give reasonable predictions. While the inviscid spanwise walls also affect the flow near the boundaries in the wide domain simulations, there is sufficient region of span from which reliable flow data and loss estimations can be obtained. For blade flows at off-design conditions, a span of 20 per cent of the blade chord is sufficient to give good agreement with experimental data. This incurs a computational cost that may be too high to incorporate parametric LES studies into the design cycle of turbomachinery components with current computers

Large eddy simulation of a controlled diffusion compressor cascade

In this research a Controlled Diffusion (CD) compressor cascade stator blade is simulated at a Reynolds number of similar to 700,000, based on inflow velocity and chord length, using Large Eddy Simulation (LES). A wide range of flow inlet angles are computed, including conditions near the design angle, and at high negative and positive incidence. At all inlet angles the surface pressure distributions are well-predicted by the LES. Near the design angle the computed suction side boundary layer thickness agrees well with experimental data, whilst the pressure side boundary layer is poorly predicted due to the inability of LES to capture natural boundary layer transition on the present grid. A good estimation of the loss is computed near the design angle, whilst at both high positive and negative incidences the loss is less well predicted owing to discrepancies between the computed and experimental boundary layer thickness. At incidences above the design angle a laminar separation bubble forms near the leading edge of the suction surface, which undergoes a transition to turbulence. Similar behaviour is noted on the pressure surface at negative incidence. At high negative incidence contra-rotating vortex pairs are found to form around the leading edge in response to an unsteady stagnation line across the span of the blade. Such structures are not apparent in time-averaged statistical data due to their highly-transient nature

Subsidence and carbon loss in drained tropical peatlands

Biogeosciences931053-107

Some general properties of the renormalized stress-energy tensor for static quantum states on (n+1)-dimensional spherically symmetric black holes

We study the renormalized stress-energy tensor (RSET) for static quantum states on (n+1)-dimensional, static, spherically symmetric black holes. By solving the conservation equations, we are able to write the stress-energy tensor in terms of a single unknown function of the radial co-ordinate, plus two arbitrary constants. Conditions for the stress-energy tensor to be regular at event horizons (including the extremal and ``ultra-extremal'' cases) are then derived using generalized Kruskal-like co-ordinates. These results should be useful for future calculations of the RSET for static quantum states on spherically symmetric black hole geometries in any number of space-time dimensions.Comment: 9 pages, no figures, RevTeX4, references added, accepted for publication in General Relativity and Gravitatio

On the formation of a Hawking-radiation photosphere around microscopic black holes

We show that once a black hole surpasses some critical temperature $T_{crit}$, the emitted Hawking radiation interacts with itself and forms a nearly thermal photosphere. Using QED, we show that the dominant interactions are bremsstrahlung and electron-photon pair production, and we estimate $T_{crit} \sim m_{e}/\alpha^{5/2}$, which when calculated more precisely is found to be $T_{crit} \approx$45 GeV. The formation of the photosphere is purely a particle physics effect, and not a general relativistic effect, since the the photosphere forms roughly $\alpha^{-4}$ Schwarzschild radii away from the black hole. The temperature $T$ of the photosphere decreases with distance from the black hole, and the outer surface is determined by the constraint $T\sim m_{e}$ (for the QED case), since this is the point at which electrons and positrons annihilate, and the remaining photons free stream to infinity. Observational consequences are discussed, and it is found that, although the QED photosphere will not affect the Page-Hawking limits on primordial black holes, which is most important for 100MeV black holes, the inclusion of QCD interactions may significantly effect this limit, since for QCD we estimate $T_{crit}\sim \Lambda_{QCD}$. The photosphere greatly reduces possibility of observing individual black holes with temperatures greater than $T_{crit}$, since the high energy particles emitted from the black hole are processed through the photosphere to a lower energy, where the gamma ray background is much higher. The temperature of the plasma in the photosphere can be extremely high, and this offers interesting possibilities for processes such as symmetry restoration.Comment: Latex, 16 pages, 3 postscript figures, submitted to PRD. Also available at http://fnas08.fnal.gov

Method to compute the stress-energy tensor for the massless spin 1/2 field in a general static spherically symmetric spacetime

A method for computing the stress-energy tensor for the quantized, massless, spin 1/2 field in a general static spherically symmetric spacetime is presented. The field can be in a zero temperature state or a non-zero temperature thermal state. An expression for the full renormalized stress-energy tensor is derived. It consists of a sum of two tensors both of which are conserved. One tensor is written in terms of the modes of the quantized field and has zero trace. In most cases it must be computed numerically. The other tensor does not explicitly depend on the modes and has a trace equal to the trace anomaly. It can be used as an analytic approximation for the stress-energy tensor and is equivalent to other approximations that have been made for the stress-energy tensor of the massless spin 1/2 field in static spherically symmetric spacetimes.Comment: 34 pages, no figure

Noise Kernel and Stress Energy Bi-Tensor of Quantum Fields in Hot Flat Space and Gaussian Approximation in the Optical Schwarzschild Metric

Continuing our investigation of the regularization of the noise kernel in curved spacetimes [N. G. Phillips and B. L. Hu, Phys. Rev. D {\bf 63}, 104001 (2001)] we adopt the modified point separation scheme for the class of optical spacetimes using the Gaussian approximation for the Green functions a la Bekenstein-Parker-Page. In the first example we derive the regularized noise kernel for a thermal field in flat space. It is useful for black hole nucleation considerations. In the second example of an optical Schwarzschild spacetime we obtain a finite expression for the noise kernel at the horizon and recover the hot flat space result at infinity. Knowledge of the noise kernel is essential for studying issues related to black hole horizon fluctuations and Hawking radiation backreaction. We show that the Gaussian approximated Green function which works surprisingly well for the stress tensor at the Schwarzschild horizon produces significant error in the noise kernel there. We identify the failure as occurring at the fourth covariant derivative order.Comment: 21 pages, RevTeX

Expulsion of Magnetic Flux Lines from the Growing Superconducting Core of a Magnetized Quark Star

The expulsion of magnetic flux lines from a growing superconducting core of a quark star has been investigated. The idea of impurity diffusion in molten alloys and an identical mechanism of baryon number transport from hot quark-gluon-plasma phase to hadronic phase during quark-hadron phase transition in the early universe, micro-second after big bang has been used. The possibility of Mullins-Sekerka normal-superconducting interface instability has also been studied.Comment: Thoroughly revised version. Accepted for Astrophysics & Space Scienc

Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV

We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive Pi0 production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The cross section was measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T < 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of Pi0's in their parent jets was found to be around 0.7 for electromagnetically triggered events.Comment: 6 pages, 3 figures, submitted to Phys. Rev. D (RC