3,514 research outputs found
Effects of very high turbulence on convective heat transfer
The effects of high-intensity, large-scale turbulence on turbulent boundary-layer heat transfer are studied. Flow fields were produced with turbulence intensities up to 40% and length scales up to several times the boundary layer thickness. In addition, three different types of turbulence will be compared to see whether they have the same effect on the boundary layer. The three are: the far field of a free jet, flow downstream of a grid, and flow downstream of a simulated gas turbine combustor. Each turbulence field will be characterized by several measures: intensity (by component), scale, and spectrum. Heat transfer will be measured on a 2.5 m long, 0.5 m wide flat plate using the energy-balance technique. The same plate will be used in each of the four flow fields; a low-turbulence tunnel for baseline data, and the three flow situations mentioned
Gravitational Waves in the Nonsymmetric Gravitational Theory
We prove that the flux of gravitational radiation from an isolated source in
the Nonsymmetric Gravitational Theory is identical to that found in Einstein's
General Theory of Relativity.Comment: 10 Page
Aspects of noncommutative (1+1)-dimensional black holes
We present a comprehensive analysis of the spacetime structure and
thermodynamics of dimensional black holes in a noncommutative
framework. It is shown that a wider variety of solutions are possible than the
commutative case considered previously in the literature. As expected, the
introduction of a minimal length cures singularity pathologies
that plague the standard two-dimensional general relativistic case, where the
latter solution is recovered at large length scales. Depending on the choice of
input parameters (black hole mass , cosmological constant ,
etc...), black hole solutions with zero, up to six, horizons are possible. The
associated thermodynamics allows for the either complete evaporation, or the
production of black hole remnants.Comment: 24 pages, 12 figures, some comments added, conclusions not modified,
version matching that published on PR
Geometry of manifolds with area metric: multi-metric backgrounds
We construct the differential geometry of smooth manifolds equipped with an
algebraic curvature map acting as an area measure. Area metric geometry
provides a spacetime structure suitable for the discussion of gauge theories
and strings, and is considerably more general than Lorentzian geometry. Our
construction of geometrically relevant objects, such as an area metric
compatible connection and derived tensors, makes essential use of a
decomposition theorem due to Gilkey, whereby we generate the area metric from a
finite collection of metrics. Employing curvature invariants for multi-metric
backgrounds we devise a class of gravity theories with inherently stringy
character, and discuss gauge matter actions.Comment: 34 pages, REVTeX4, journal versio
Thermodynamic ground states of platinum metal nitrides
The thermodynamic stabilities of various phases of the nitrides of the
platinum metal elements are systematically studied using density functional
theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal
structures, in which the metal ions occupy simple tetragonal lattice sites,
have lower formation enthalpies at ambient conditions than any previously
proposed structures. The region of stability with respect to those structures
extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal
structures at this pressure are thermodynamically stable also with respect to
phase separation. The fact that the local density and generalized gradient
approximations predict different values of the absolute formation enthalpies as
well different relative stabilities between simple tetragonal and the pyrite or
marcasite structures are further discussed.Comment: 5 pages, 4 figure
Potential impacts of teleworking on transport systems
Reports the proceedings of a meeting, hosted by the University of Westminster as part of the Chartered Institute of Logistics and Transport's (CILT) Transport Planning Forum programme, that included a presentation on the impact of teleworking on the UK's transport systems. Explains that the presentation drew on a range of information sources, including: data published in the annual reports from the National Travel Survey (NTS) in Britain; a study undertaken jointly by the University of Westminster, University College London, and Imperial College London, which examined organizational perceptions of teleworking, analysed NTS data, and evaluated some possible outcomes; and a study by David Moffat, which surveyed a sample of staff at the Department for Transport (DfT) in London and regional Government Offices (GOs) to identify their present extent of teleworking, and attitudes towards its wider use. Reveals that the trend toward greater teleworking is not dramatic, although growth is evident, both in full-time and part-week working at home
Film cooling on a convex wall: Heat transfer and hydrodynamic measurements for full and partial coverage
Turbine-blade cooling is an important issue for high-efficiency turbine engines, and discrete-hole injection is widely used as a cooling method. In the present study, detailed measurements were made of the heat transfer and hydrodynamics of a film-cooled flow on a convex wall, both for full and partial coverage. Two important parameters were altered: the blowing ratio, m, and the number of rows of injection holes. Three values of m were tested: m = 0.2, 0.4, and 0.6. In the blown region, m = 0.4 results in the lowest Stanton numbers of the three blowing ratios tested. This indicates that the value of m = 0.4 is near optimum on the convex wall from the point of view of cooling effect by injection. In the recovery region, Stanton numbers gradually approach the no injection values. Although the heat-transfer behavior during recovery from injection looks relatively complicated, the behavior of Stanton number can be explained in terms of two mechanisms: recovery from the thermal effect of injection and recovery from the turbulence augmentation. This interpretation of the data is supported by the hydrodynamic and temperture-profile measurements. For partial blowing cases, the data follow the full-coverage values inside the blown region. In the unblown region, both in the curved and in the flat plate, the effect of the number of blown rows is clearly seen. Hydrodynamic boundary-layer profiles were measured with the aid of a triple hot-water probe. Three mean-velocity components and six turbulence quantities were simultaneously measured, and inside the blown region strong three-dimensionality was observed
Turbulent boundary layer heat transfer experiments: Convex curvature effects, including introduction and recovery
Heat transfer rates were measured through turbulent and transitional boundary layers on an isothermal, convexly curved wall and downstream flat plate. The effect of convex curvature on the fully turbulent boundary layer was a reduction of the local Stanton numbers 20-50% below those predicted for a flat wall under the same circumstances. The recovery of the heat transfer rates on the downstream flat wall was extremely slow. After 60 cm of recovery length, the Stanton number was still typically 15-20% below the flat wall predicted value. Various effects important in the modeling of curved flows were studied separately. These are: (1) the effect of initial boundary layer thickness; (2) the effect of freestream velocity; (3) the effect of freestream acceleration; (4) the effect of unheated starting length; and (5) the effect of the maturity of the boundary layer. Regardless of the initial state, curvature eventually forced the boundary layer into an asymptotic curved condition. The slope, minus one, is believed to be significant
Invariant length scale in relativistic kinematics - Lessons from Dirichlet branes
We show that Dirac-Born-Infeld theory possesses a hidden invariance that
enhances the local O(1,p) Lorentz symmetry on a Dirichlet p-brane to an O(1,p)
x O(1,p) gauge group, encoding both an invariant velocity and acceleration (or
length) scale. This enlarged gauge group predicts consequences for the
kinematics of observers on Dirichlet branes, with admissible accelerations
being bounded from above. An important lesson beyond string theory is that a
fundamental length scale can be implemented into the kinematics of general
relativity, whilst preserving both space-time as a smooth manifold and local
Lorentz symmetry, contrary to common belief. We point out consequences for
string phenomenology, classical gravity and atomic physics.Comment: 4 pages, to be published in Phys Lett
Experimental data and model for the turbulent boundary layer on a convex, curved surface
Experiments were performed to determine how boundary layer turbulence is affected by strong convex curvature. The data gathered on the behavior of the Reynolds stress suggested the formulation of a simple turbulence model. Data were taken on two separate facilities. Both rigs had flow from a flat surface, over a convex surface with 90 deg of turning and then onto a flat recovery surface. The geometry was adjusted so that, for both rigs, the pressure gradient along the test surface was zero. Two experiments were performed at delta/R approximately 0.10, and one at weaker curvature with delta/R approximately 0.05. Results show that after a sudden introduction of curvature the shear stress in the outer part of the boundary layer is sharply diminished and is even slightly negative near the edge. The wall shear also drops off quickly downstream. When the surface suddenly becomes flat again, the wall shear and shear stress profiles recover very slowly towards flat wall conditions. A simple turbulence model, which was based on the theory that the Prandtl mixing length in the outer layer should scale on the velocity gradient layer, was shown to account for the slow recovery
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