30,991 research outputs found
A compressible near-wall turbulence model for boundary layer calculations
A compressible near-wall two-equation model is derived by relaxing the assumption of dynamical field similarity between compressible and incompressible flows. This requires justifications for extending the incompressible models to compressible flows and the formulation of the turbulent kinetic energy equation in a form similar to its incompressible counterpart. As a result, the compressible dissipation function has to be split into a solenoidal part, which is not sensitive to changes of compressibility indicators, and a dilational part, which is directly affected by these changes. This approach isolates terms with explicit dependence on compressibility so that they can be modeled accordingly. An equation that governs the transport of the solenoidal dissipation rate with additional terms that are explicitly dependent on the compressibility effects is derived similarly. A model with an explicit dependence on the turbulent Mach number is proposed for the dilational dissipation rate. Thus formulated, all near-wall incompressible flow models could be expressed in terms of the solenoidal dissipation rate and straight-forwardly extended to compressible flows. Therefore, the incompressible equations are recovered correctly in the limit of constant density. The two-equation model and the assumption of constant turbulent Prandtl number are used to calculate compressible boundary layers on a flat plate with different wall thermal boundary conditions and free-stream Mach numbers. The calculated results, including the near-wall distributions of turbulence statistics and their limiting behavior, are in good agreement with measurements. In particular, the near-wall asymptotic properties are found to be consistent with incompressible behavior; thus suggesting that turbulent flows in the viscous sublayer are not much affected by compressibility effects
On quantization of weakly nonlinear lattices. Envelope solitons
A way of quantizing weakly nonlinear lattices is proposed. It is based on
introducing "pseudo-field" operators. In the new formalism quantum envelope
solitons together with phonons are regarded as elementary quasi-particles
making up boson gas. In the classical limit the excitations corresponding to
frequencies above linear cut-off frequency are reduced to conventional envelope
solitons. The approach allows one to identify the quantum soliton which is
localized in space and understand existence of a narrow soliton frequency band.Comment: 5 pages. Phys. Rev. E (to appear
Spontaneous Magnetization of Solid Quark-cluster Stars
Pulsar-like compact stars usually have strong magnetic fields, with the
strength from to Gauss on surface. How such strong
magnetic fields can be generated and maintained is still an unsolved problem,
which is, in principle, related to the interior structure of compact stars,
i.e., the equation of state of cold matter at supra-nuclear density. In this
paper we are trying to solve the problem in the regime of solid quark-cluster
stars. Inside quark-cluster stars, the extremely low ratio of number density of
electrons to that of baryons and the screening effect from
quark-clusters could reduce the long-range Coulomb interaction between
electrons to short-range interaction. In this case, the Stoner's model could
apply, and we find that the condition for ferromagnetism is consistent with
that for validity of Stoner's model. Under the screened Coulomb repulsion, the
electrons inside the stars could spontaneously magnetized and become
ferromagnetic, and hence would contribute non-zero net magnetic momentum to the
whole star. We conclude that, for most cases in solid quark-cluster stars, the
amount of net magnetic momentum, which is proportional to the amount of
unbalanced spins and depends on the number density of
electrons , could be significant with non-zero . The net
magnetic moments of electron system in solid quark-cluster stars could be large
enough to induce the observed magnetic fields for pulsars with
to Gauss.Comment: 7 pages, 1 figure. Accepted by Chinese Physics
A Polytropic Model of Quark Stars
A polytropic quark star model is suggested in order to establish a general
framework in which theoretical quark star models could be tested by
observations. The key difference between polytropic quark stars and the
polytropic model studied previously for normal (i.e., non-quarkian) stars is
related to two issues: (i) a constant term representing the contribution of
vacuum energy may be added in the energy density and the pressure for a quark
star, but not for a normal star; (ii) the quark star models with non-vanishing
density at the stellar surface are not avoidable due to the strong interaction
between quarks. The first one implies that the vacuum inside a quark star is
different from that outside, while the second one is relevant to the effect of
color confinement. The polytropic equations of state are stiffer than that
derived in conventional realistic models (e.g., the bag model) for quark
matter, and pulsar-like stars calculated with a polytropic equation of state
could then have high maximum masses (> 2 M_sun). Quark stars can also be very
low massive, and be still gravitationally stable even if the polytropic index,
n, is greater than 3. All these would result in different mass-radius
relations, which could be tested by observations. In addition, substantial
strain energy would develop in a solid quark star during its accretion/spindown
phase, and could be high enough to take a star-quake. The energy released
during star-quakes could be as high as ~ 10^{47} ergs if the tangential
pressure is ~ 10^{-6} higher than the radial one.Comment: 17 pages, 4 figures, last version accepted for publication in
Astroparticle Physic
Dissipative chaotic scattering
We show that weak dissipation, typical in realistic situations, can have a
metamorphic consequence on nonhyperbolic chaotic scattering in the sense that
the physically important particle-decay law is altered, no matter how small the
amount of dissipation. As a result, the previous conclusion about the unity of
the fractal dimension of the set of singularities in scattering functions, a
major claim about nonhyperbolic chaotic scattering, may not be observable.Comment: 4 pages, 2 figures, revte
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