3,501 research outputs found
RANS Equations with Explicit Data-Driven Reynolds Stress Closure Can Be Ill-Conditioned
Reynolds-averaged Navier--Stokes (RANS) simulations with turbulence closure
models continue to play important roles in industrial flow simulations.
However, the commonly used linear eddy viscosity models are intrinsically
unable to handle flows with non-equilibrium turbulence. Reynolds stress models,
on the other hand, are plagued by their lack of robustness. Recent studies in
plane channel flows found that even substituting Reynolds stresses with errors
below 0.5% from direct numerical simulation (DNS) databases into RANS equations
leads to velocities with large errors (up to 35%). While such an observation
may have only marginal relevance to traditional Reynolds stress models, it is
disturbing for the recently emerging data-driven models that treat the Reynolds
stress as an explicit source term in the RANS equations, as it suggests that
the RANS equations with such models can be ill-conditioned. So far, a rigorous
analysis of the condition of such models is still lacking. As such, in this
work we propose a metric based on local condition number function for a priori
evaluation of the conditioning of the RANS equations. We further show that the
ill-conditioning cannot be explained by the global matrix condition number of
the discretized RANS equations. Comprehensive numerical tests are performed on
turbulent channel flows at various Reynolds numbers and additionally on two
complex flows, i.e., flow over periodic hills and flow in a square duct.
Results suggest that the proposed metric can adequately explain observations in
previous studies, i.e., deteriorated model conditioning with increasing
Reynolds number and better conditioning of the implicit treatment of Reynolds
stress compared to the explicit treatment. This metric can play critical roles
in the future development of data-driven turbulence models by enforcing the
conditioning as a requirement on these models.Comment: 35 pages, 18 figure
Quantum memory and non-demolition measurement of single phonon state with nitrogen-vacancy centers ensemble
In diamond, the mechanical vibration induced strain can lead to interaction
between the mechanical mode and the nitrogen-vecancy (NV) centers. In this
work, we propose to utilize the strain induced coupling for the quantum
non-demolition (QND) single phonon measurement and memory in diamond. The
single phonon in a diamond mechanical resonator can be perfectly absorbed and
emitted by the NV centers ensemble (NVE) with adiabatically tuning the
microwave driving. An optical laser drives the NVE to the excited states, which
have much larger coupling strength to the mechanical mode. By adiabatically
eliminating the excited states under large detuning limit, the effective
coupling between the mechanical mode and the NVE can be used for QND
measurement of the single phonon state. Under realistic experimental
conditions, we numerically simulate the scheme. It is found that the fidelity
of the absorbing and emitting process can reach a much high value. The overlap
between the input and the output phonon shapes can reach .Comment: 7 pages, 3 figure
Stellar weak-interaction rates for -process waiting-point nuclei from projected shell model
We propose a projected shell model (PSM) for description of stellar
weak-interaction rates between even-even and odd-odd nuclei with extended
configuration space where up to six-quasiparticle (qp) configurations are
included, and the stellar weak-interaction rates for eight -process
waiting-point (WP) nuclei, Ge, Se, Kr, Sr,
Zr, Mo, Ru and Pd, are calculated and analyzed for
the first time within the model. Higher-order qp configurations are found to
affect the underlying Gamow-Teller strength distributions and the corresponding
stellar weak-interaction rates. Under -process environments with high
temperatures and densities, on one hand, thermal population of excited states
of parent nuclei tends to decrease the stellar decay rates. On the
other hand, the possibility of electron capture (EC) tends to provide
increasing contribution to the rates with temperature and density. The
effective half-lives of WP nuclei under the -process peak condition are
predicted to be reduced as compared with the terrestrial case, especially for
Ge and Se
The coevolution of overconfidence and bluffing in the resource competition game
Resources are often limited, therefore it is essential how convincingly competitors present their claims
for them. Beside a player’s natural capacity, here overconfidence and bluffing may also play a decisive
role and influence how to share a restricted reward. While bluff provides clear, but risky advantage,
overconfidence, as a form of self-deception, could be harmful to its user. Still, it is a long-standing
puzzle why these potentially damaging biases are maintained and evolving to a high level in the
human society. Within the framework of evolutionary game theory, we present a simple version of
resource competition game in which the coevolution of overconfidence and bluffing is fundamental,
which is capable to explain their prevalence in structured populations. Interestingly, bluffing seems
apt to evolve to higher level than corresponding overconfidence and in general the former is less
resistant to punishment than the latter. Moreover, topological feature of the social network plays an
intricate role in the spreading of overconfidence and bluffing. While the heterogeneity of interactions
facilitates bluffing, it also increases efficiency of adequate punishment against overconfident behavior.
Furthermore, increasing the degree of homogeneous networks can trigger similar effect. We also
observed that having high real capability may accommodate both bluffing ability and overconfidence
simultaneously
Nuclear spectrum from projected shell model (I): allowed one-to-one transition
Nuclear spectrum and the corresponding (anti-)neutrino spectrum play
important roles in many aspects of nuclear astrophysics, particle physics,
nuclear industry and nuclear data. In this work we propose a projected shell
model (PSM) to calculate the level energies as well as the reduced one-body
transition density (ROBTD) by the Pfaffian algorithm for nuclear
decays. The calculated level energies and ROBTD are inputed to the Beta
Spectrum Generator (BSG) code to study the high precision spectrum of
allowed one-to-one transitions. When experimental level energies are adopted,
the calculated spectrum by ROBTD of the PSM deviates from the one by
the extreme simple particle evaluation of the BSG by up to , reflecting
the importance of nuclear many-body correlations. When calculated level
energies are adopted, the calculated spectrum shows sensitive
dependence on the reliability of calculated level energies. The developed
method for ROBTD by the PSM will also be useful for study of the
first-forbidden transitions, the isovector spin monopole resonance etc. in a
straightforward way
(2 +1)-dimensional Duffin-Kemmer-Petiau oscillator under a magnetic field in the presence of a minimal length in the noncommutative space
Using the momentum space representation, we study the (2 +1)-dimensional
Duffin-Kemmer-Petiau oscillator for spin 0 particle under a magnetic field in
the presence of a minimal length in the noncommutative space. The explicit form
of energy eigenvalues are found, the wave functions and the corresponding
probability density are reported in terms of the Jacobi polynomials.
Additionally, we also discuss the special cases and depict the corresponding
numerical results
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