45,752 research outputs found
Roughness effects in turbulent forced convection
We conducted direct numerical simulations (DNSs) of turbulent flow over
three-dimensional sinusoidal roughness in a channel. A passive scalar is
present in the flow with Prandtl number , to study heat transfer by
forced convection over this rough surface. The minimal channel is used to
circumvent the high cost of simulating high Reynolds number flows, which
enables a range of rough surfaces to be efficiently simulated. The near-wall
temperature profile in the minimal channel agrees well with that of the
conventional full-span channel, indicating it can be readily used for
heat-transfer studies at a much reduced cost compared to conventional DNS. As
the roughness Reynolds number, , is increased, the Hama roughness
function, , increases in the transitionally rough regime before
tending towards the fully rough asymptote of , where
is a constant that depends on the particular roughness geometry and
is the von K\'arm\'an constant. In this fully rough
regime, the skin-friction coefficient is constant with bulk Reynolds number,
. Meanwhile, the temperature difference between smooth- and rough-wall
flows, , appears to tend towards a constant value,
. This corresponds to the Stanton number (the temperature
analogue of the skin-friction coefficient) monotonically decreasing with
in the fully rough regime. Using shifted logarithmic velocity and temperature
profiles, the heat transfer law as described by the Stanton number in the fully
rough regime can be derived once both the equivalent sand-grain roughness
and the temperature difference are known. In
meteorology, this corresponds to the ratio of momentum and heat transfer
roughness lengths, , being linearly proportional to ,
the momentum roughness length [continued]...Comment: Accepted (In press) in the Journal of Fluid Mechanic
Induced Lorentz- and CPT-violating Chern-Simons term in QED: Fock-Schwinger proper time method
Using the Fock-Schwinger proper time method, we calculate the induced
Chern-Simons term arising from the Lorentz- and CPT-violating sector of quantum
electrodynamics with a term. Our
result to all orders in coincides with a recent linear-in- calculation
by Chaichian et al. [hep-th/0010129 v2]. The coincidence was pointed out by
Chung [Phys. Lett. {\bf B461} (1999) 138] and P\'{e}rez-Victoria [Phys. Rev.
Lett. {\bf 83} (1999) 2518] in the standard Feynman diagram calculation with
the nonperturbative-in- propagator.Comment: 11 pages, no figur
Lorentz and CPT Violating Chern-Simons Term in the Formulation of Functional Integral
We show that in the functional integral formalism the (finite) coefficient of
the induced, Lorentz- and CPT-violating Chern-Simons term, arising from the
Lorentz- and CPT-violating fermion sector, is undetermined.Comment: 5 pages, no figure, RevTe
Asymptotic Symmetries of Rindler Space at the Horizon and Null Infinity
We investigate the asymptotic symmetries of Rindler space at null infinity
and at the event horizon using both systematic and ad hoc methods. We find that
the approaches that yield infinite-dimensional asymptotic symmetry algebras in
the case of anti-de Sitter and flat spaces only give a finite-dimensional
algebra for Rindler space at null infinity. We calculate the charges
corresponding to these symmetries and confirm that they are finite, conserved,
and integrable, and that the algebra of charges gives a representation of the
asymptotic symmetry algebra. We also use relaxed boundary conditions to find
infinite-dimensional asymptotic symmetry algebras for Rindler space at null
infinity and at the event horizon. We compute the charges corresponding to
these symmetries and confirm that they are finite and integrable. We also
determine sufficient conditions for the charges to be conserved on-shell, and
for the charge algebra to give a representation of the asymptotic symmetry
algebra. In all cases, we find that the central extension of the charge algebra
is trivial.Comment: 37 pages, 4 figures. Version 3: New Section 5 adde
Ising thin films with modulations and surface defects
Properties of magnetic films are studied in the framework of Ising models. In
particular, we discuss critical phenomena of ferromagnetic Ising films with
straight lines of magnetic adatoms and straight steps on the surface as well as
phase diagrams of the axial next-nearest neighbour Ising (ANNNI) model for thin
films exhibiting various spatially modulated phases.Comment: 6 pages, 4 figures include
Progress toward the development of dual junction GaAs/Ge solar cells
Large area GaAs/Ge cells offer substantial promise for increasing the power output from existing silicon solar array designs and for providing an enabled technology for missions hitherto impossible using silicon. Single junction GaAs/Ge cells offer substantial advantages in both size, weight, and cost compared to GaAs cells but the efficiency is limited to approximately 19.2 to 20 percent AMO. The thermal absorptance of GaAs/Ge cells is also worse than GaAs/GaAs cells (0.88 vs 0.81 typ.) due to the absorption in the Ge substrate. On the other hand dual junction GaAs/Ge cells offer efficiencies up to ultimately 24 percent AMO in sizes up to 8 x 8 cm but there are still technological issues remaining to achieve current matching in the GaAs and Ge cells. This can be achieved through tuned antireflection (AR) coatings, improved quality of the GaAs growth, improved quality Ge wafers and the use of a Back Surface Field (BSF)/Back Surface Reflector (BSR) in the Ge cell. Although the temperature coefficients of efficiency and voltage are higher for dual junction GaAs/Ge cells, it has been shown elsewhere that for typical 28 C cell efficiencies of 22 percent (dual junction) vs 18.5 percent (single junction) there is a positive power tradeoff up to temperatures as high as 120 C. Due to the potential ease of fabrication of GaAs/Ge dual junction cells there is likely to be only a small cost differential compared to single junction cells
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