271 research outputs found
A consistent and conservative diffuse-domain lattice Boltzmann method for multiphase flows in complex geometries
Modeling and simulation of multiphase flows in complex geomerties are
challenging due to the complexity in describing the interface topology changes
among different phases and the difficulty in implementing the boundary
conditions on the irregular solid surface. In this work, we first developed a
diffuse-domain (DD) based phase-field model for multiphase flows in complex
geometries. In this model, the irregular fluid region is embedded into a larger
and regular domain by introducing a smooth characteristic function. Then, the
reduction-consistent and conservative phase-field equation for the multiphase
field and the consistent and conservative Navier-Stokes equations for the flow
field are reformulated as the diffuse-domain based consistent and conservative
(DD-CC) equations where some additional source terms are added to reflect the
effects of boundary conditions. In this case, there is no need to directly
treat the complex boundary conditions on the irregular solid surface, and
additionally, based on a matched asymptotic analysis, it is also shown that the
DD-CC equations can converge to the original governing equations as the
interface width parameter tends to zero. Furthermore, to solve the DD-CC
equations, we proposed a novel and simple lattice Boltzmann (LB) method with a
Hermite-moment-based collision matrix which can not only keep consistent and
conservation properties, but also improve the numerical stability with a
flexible parameter. With the help of the direct Taylor expansion, the
macroscopic DD-CC equations can be recovered correctly from the present LB
method. Finally, to test the capacity of LB method, several benchmarks and
complex problems are considered, and the numerical results show that the
present LB method is accurate and efficient for the multiphase flows in complex
geomerties.Comment: 22 pages, 9 figure
Resonance instability of primordial gravitational waves during inflation in Chern-Simons gravity
We investigate axion inflation where the gravitational Chern-Simons term is
coupled to a periodic function of the inflaton. We find that tensor
perturbations with different polarizations are amplified in different ways by
the Chern-Simons coupling. Depending on the model parameters, the resonance
amplification results in a parity-violating peak or a board plateau in the
energy spectrum of gravitational waves, and the sharp cutoff in the infrared
region constitutes a characteristic distinguishable from stochastic
gravitational wave backgrounds produced by matter fields in Einstein gravity.Comment: 16 pages, 4 figure
Explaining Pulsar Timing Array Observations with Primordial Gravitational Waves in Parity-Violating Gravity
The pulsar timing array (PTA) collaborations have recently suggested the
presence of a gravitational wave background at nano-Hertz frequencies. In this
paper, we explore potential inflationary interpretation of this signal within
the context of a simple and health parity-violating gravity model termed the
Nieh-Yan modified Teleparallel Gravity. Through this model, two inflationary
scenarios are evaluated, both yielding significant polarized primordial
gravitational waves (PGWs) that align well with the results from PTA
observations. Furthermore, the resulting PGWs can display strong circular
polarization and significant anisotropies in the PTA frequency band, which are
distinct features to be verified by observations of both PTA and the cosmic
microwave background.The detection of such a distinctive background of PGWs is
expected to provide strong evidence supporting our scenarios and insights into
inflationary dynamics and gravity theory.Comment: 9 pages, 8 figure
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