5,684 research outputs found
A Unified Fitting of HI and HeII Ly\alpha Transmitted Flux of QSO HE2347 with LCDM Hydrodynamic Simulations
Using cosmological hydrodynamic simulations of the LCDM model, we present a
comparison between the simulation sample and real data sample of HI and HeII
Ly\alpha transmitted flux in the absorption spectra of the QSO HE2347-4342. The
LCDM model is successful in simultaneously explaining the statistical features
of both HI and HeII Ly\alpha transmitted flux. It includes: 1.) the power
spectra of the transmitted flux of HI and HeII can be well fitted on all scales
> 0.28h^{-1} Mpc for H, and > 1.1h^{-1} Mpc for He; 2.) the Doppler parameters
of absorption features of HeII and HI are found to be turbulent-broadening; 3.)
the ratio of HeII to HI optical depths are substantially scattered, due to the
significant effect of noise. A large part of the \eta-scatter is due to the
noise in the HeII flux. However, the real data contain more low-\eta events
than simulation sample. This discrepancy may indicate that the mechanism
leading extra fluctuations upon the simulation data, such as a fluctuating UV
radiation background, is needed. Yet, models of these extra fluctuations should
satisfy the constraints: 1.) if the fluctuations are Gaussian, they should be
limited by the power spectra of observed HI and HeII flux; 2.) if the
fluctuations are non-Gaussian, they should be limited by the observed
non-Gaussian features of the HI and HeII flux.Comment: 21 pages, 7 figs, ApJ in pres
Power spectrum and intermittency of the transmitted flux of QSOs Ly-alpha absorption spectra
Using a set of 28 high resolution, high signal to noise ratio (S/N) QSO
Ly-alpha absorption spectra, we investigate the non-Gaussian features of the
transmitted flux fluctuations, and their effect upon the power spectrum of this
field. We find that the spatial distribution of the local power of the
transmitted flux on scales k >= 0.05 s/km is highly spiky or intermittent. The
probability distribution functions (PDFs) of the local power are long-tailed.
The power on small scales is dominated by small probability events, and
consequently, the uncertainty in the power spectrum of the transmitted flux
field is generally large. This uncertainty arises due to the slow convergence
of an intermittent field to a Gaussian limit required by the central limit
theorem (CLT). To reduce this uncertainty, it is common to estimate the error
of the power spectrum by selecting subsamples with an "optimal" size. We show
that this conventional method actually does not calculate the variance of the
original intermittent field but of a Gaussian field. Based on the analysis of
intermittency, we propose an algorithm to calculate the error. It is based on a
bootstrap re-sampling among all independent local power modes. This estimation
doesn't require any extra parameter like the size of the subsamples, and is
sensitive to the intermittency of the fields. This method effectively reduces
the uncertainty in the power spectrum when the number of independent modes
matches the condition of the CLT convergence.Comment: 26 pages (incl. figures). Accepted for publication in MNRA
Nonlinear Hall response in the driving dynamics of ultracold atoms in optical lattices
We propose that a nonlinear Hall response can be observed in Bloch
oscillations of ultracold atoms in optical lattices under the condition of
preserved time-reversal symmetry. In the short-time limit of Bloch oscillations
driven by a direct current (dc) field, the nonlinear Hall current dominates,
being a second-order response to the external field strength. The associated
Berry curvature dipole, which is a second-order nonlinear coefficient of the
driving field, can be obtained from the oscillation of atoms. In an alternating
current (ac) driving field, the nonlinear Hall response has a double frequency
of the driving force in the case of time-reversal symmetry.Comment: 7 pages, 6 figure
Aeroacoustic noise reduction design of a landing gear structure based on wind tunnel experiment and simulation
In the process of aircraft landing, the aerodynamic noise of the landing gear constitutes an appreciable part of the airframe noise. Therefore it is important to dedicate research efforts to study of aerodynamic noise of landing gear and its structural parts. Acoustic wind tunnel test on landing gear is designed to measure aerodynamic noise of structural parts of landing gear such as pillar and torque arm. Aerodynamic noise spectrum characteristic and radiation directive characteristic of structural parts in different velocities are established. The effect of flow velocity to noise is analyzed. Two noise reduction designs are proposed in the paper. The effect of the relative position of pillar and torque arm to structural noise is considered based on simulations and testing. Simulation method to assess the noise reduction effect of torque arm shape modification is adopted. The results demonstrate that structural noise can be appreciably reduced by placing torque arm behind the pillar as well as by modifying the shape of the torque arm. In total, the study holds reference value to the ongoing research activities on aerodynamic noise of landing gear and design method for low noise operation of the gear
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