114,456 research outputs found
Shuttle rocket booster computational fluid dynamics
Additional results and a revised and improved computer program listing from the shuttle rocket booster computational fluid dynamics formulations are presented. Numerical calculations for the flame zone of solid propellants are carried out using the Galerkin finite elements, with perturbations expanded to the zeroth, first, and second orders. The results indicate that amplification of oscillatory motions does indeed prevail in high frequency regions. For the second order system, the trend is similar to the first order system for low frequencies, but instabilities may appear at frequencies lower than those of the first order system. The most significant effect of the second order system is that the admittance is extremely oscillatory between moderately high frequency ranges
Development of a computerized analysis for solid propellant combustion instability with turbulence
A multi-dimensional numerical model has been developed for the unsteady state oscillatory combustion of solid propellants subject to acoustic pressure disturbances. Including the gas phase unsteady effects, the assumption of uniform pressure across the flame zone, which has been conventionally used, is relaxed so that a higher frequency response in the long flame of a double-base propellant can be calculated. The formulation is based on a premixed, laminar flame with a one-step overall chemical reaction and the Arrhenius law of decomposition with no condensed phase reaction. In a given geometry, the Galerkin finite element solution shows the strong resonance and damping effect at the lower frequencies, similar to the result of Denison and Baum. Extended studies deal with the higher frequency region where the pressure varies in the flame thickness. The nonlinear system behavior is investigated by carrying out the second order expansion in wave amplitude when the acoustic pressure oscillations are finite in amplitude. Offset in the burning rate shows a negative sign in the whole frequency region considered, and it verifies the experimental results of Price. Finally, the velocity coupling in the two-dimensional model is discussed
Epitaxial Growth of an n-type Ferromagnetic Semiconductor CdCr2Se4 on GaAs(001) and GaP(001)
We report the epitaxial growth of CdCr2Se4, an n-type ferromagnetic
semiconductor, on both GaAs and GaP(001) substrates, and describe the
structural, magnetic and electronic properties. Magnetometry data confirm
ferromagnetic order with a Curie temperature of 130 K, as in the bulk material.
The magnetization exhibits hysteretic behavior with significant remanence, and
an in-plane easy axis with a coercive field of ~125 Oe. Temperature dependent
transport data show that the films are semiconducting in character and n-type
as grown, with room temperature carrier concentrations of n ~ 1 x 10^18 cm-3.Comment: 12 pages, 3 figure
Regularization, Renormalization and Range: The Nucleon-Nucleon Interaction from Effective Field Theory
Regularization and renormalization is discussed in the context of low-energy
effective field theory treatments of two or more heavy particles (such as
nucleons). It is desirable to regulate the contact interactions from the outset
by treating them as having a finite range. The low energy physical observables
should be insensitive to this range provided that the range is of a similar or
greater scale than that of the interaction. Alternative schemes, such as
dimensional regularization, lead to paradoxical conclusions such as the
impossibility of repulsive interactions for truly low energy effective theories
where all of the exchange particles are integrated out. This difficulty arises
because a nonrelativistic field theory with repulsive contact interactions is
trivial in the sense that the matrix is unity and the renormalized coupling
constant zero. Possible consequences of low energy attraction are also
discussed. It is argued that in the case of large or small scattering lengths,
the region of validity of effective field theory expansion is much larger if
the contact interactions are given a finite range from the beginning.Comment: 7 page
Element-by-element factorization algorithms for heat conduction
Element-by-element solution strategies are developed for transient heat conduction problems. Results of numerical tests indicate the effectiveness of the procedures proposed. The small database requirements and attractive architectural features of the algorithms suggest considerable potential for solving large scale problems
Erratum: Dynamics and scaling in a quantum spin chain material with bond randomness
Follow-up neutron measurements, performed on a sample much larger than the
one used in the original study, show that in the energy range 0.5-45 meV the
magnetic excitations in BaCu2SiGeO7 are indistinguishable from those in
conventional (disorder-free) quantum S=1/2 chains. Scrutinizing the previous
data, we found that the analysis was affected by a poorly identified structured
background and an additional technical mistake in the data reduction.Comment: This is a complete withdrawal of the original paper, also published
as in Phys. Rev. Lett 93, 077206 (2004). One page, one figur
Exotic Smooth Structures on Small 4-Manifolds
Let M be either CP^2#3CP^2bar or 3CP^2#5CP^2bar. We construct the first
example of a simply-connected symplectic 4-manifold that is homeomorphic but
not diffeomorphic to M.Comment: 11 page
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