Search

We simulated the intermittent boundary-layer flashback (BLF) of hydrogen-enriched swirling flames using large-eddy simulation (LES) with the flame-surface-density (FSD) method. Three cases of intermittent BLF, characterized by periodic flame entry and exit of the mixing tube, are presented. The intermittent BLF characteristics varied with the hydrogen volume fraction. Small flame bulges entered and exited the mixing tube in low hydrogen-enrichment cases. The duration of intermittent BLF events and BLF depth increased as the hydrogen content increased. Meanwhile, a large flame tongue penetrating deeply upstream characterised the highest hydrogen-enrichment case.The mean BLF peak depths and standard deviations obtained through simulations aligned well with experimental data for low and moderate hydrogen-enrichment cases. However, LES-FSD underestimated the average BLF peak depth for the highest hydrogen-enrichment case.Analysis of the flow-flame interaction revealed two mechanisms underlying the intermittent BLF phenomena. The flame bulges' oscillation near the outlet is caused by the reverse flow induced by the recirculation zone. At the same time, the deep intermittent BLF occurrs due to the boundary layer separation induced by the large turbulent burning velocity, resulting from the synergy of turbulence and thermo-diffusive effects.

We propose a geometry-to-flow diffusion model that utilizes the input of obstacle shape to predict a flow field past the obstacle. The model is based on a learnable Markov transition kernel to recover the data distribution from the Gaussian distribution. The Markov process is conditioned on the obstacle geometry, estimating the noise to be removed at each step, implemented via a U-Net. A cross-attention mechanism incorporates the geometry as a prompt. We train the geometry-to-flow diffusion model using a dataset of flows past simple obstacles, including the circle, ellipse, rectangle, and triangle. For comparison, the CNN model is trained using the same dataset. Tests are carried out on flows past obstacles with simple and complex geometries, representing interpolation and extrapolation on the geometry condition, respectively. In the test set, challenging scenarios include a cross and characters `PKU'. Generated flow fields show that the geometry-to-flow diffusion model is superior to the CNN model in predicting instantaneous flow fields and handling complex geometries. Quantitative analysis of the model accuracy and divergence in the fields demonstrate the high robustness of the diffusion model, indicating that the diffusion model learns physical laws implicitly.

We study the QCD topology and axion properties at finite temperature and chemical potential in the framework of the two-flavor Nambu$-$Jona-Lasinio model. We find that the behaviors of the two lowest cumulants of the QCD topological charge distribution and axion properties are highly sensitive to the critical behavior of the chiral phase transition. In particular, the topological susceptibility and the axion mass follow the response of the chiral condensate to temperature and chemical potential, showing that both quantities decrease monotonically with the increment of temperature and/or chemical potential. However, it is important to note that the normalized fourth cumulant behaves differently depending on the temperature. At low temperatures, it is a non-monotonic function of the chemical potential, while at high temperatures, it monotonically decreases. Additionally, its value invariably approaches the asymptotic value of $b_2^{\text {inst }}=-1/12$, predicted by the dilute instanton gas model. We also observe that with the increase in chemical potential at relatively low temperatures, the axion self-coupling constant exhibits a sharp peak around the critical point, which can even be more than twice its vacuum value. After that, the self-coupling drops sharply to a much lower value than its vacuum value, eventually approaching zero in the high chemical potential limit. The finding that the axion self-coupling constant is significantly enhanced in high-density environments near the chiral phase transition could lead to the creation or enhancement of an axion Bose-Einstein condensate in compact astrophysical objects., Comment: 10 pages, 10 figures

We report the quantum computing of reacting flows by simulating the Hamiltonian dynamics. The scalar transport equation for reacting flows is transformed into a Hamiltonian system, mapping the dissipative and non-Hermitian problem in physical space to a Hermitian one in a higher-dimensional space. Using this approach, we develop the quantum spectral and finite difference methods for simulating reacting flows in periodic and general conditions, respectively. The present quantum computing algorithms offer a ``one-shot'' solution for a given time without temporal discretization, avoiding iterative quantum state preparation and measurement. We compare computational complexities of the quantum and classical algorithms. The quantum spectral method exhibits exponential acceleration relative to its classical counterpart, and the quantum finite difference method can achieve exponential speedup in high-dimensional problems. The quantum algorithms are validated on quantum computing simulators with the Qiskit package. The validation cases cover one- and two-dimensional reacting flows with a linear source term and periodic or inlet-outlet boundary conditions. The results obtained from the quantum spectral and finite difference methods agree with analytical and classical simulation results. They accurately capture the convection, diffusion, and reaction processes. This demonstrates the potential of quantum computing as an efficient tool for the simulation of reactive flows in combustion.

The strong coupling in the effective quark mass was usually taken as a constant in a quasiparticle model while it is, in fact, running with an energy scale. With a running coupling, however, the thermodynamic inconsistency problem appears in the conventional treatment. We show that the renormalization subtraction point should be taken as a function of the summation of the biquadratic chemical potentials if the quark's current masses vanish, in order to ensure full thermodynamic consistency. Taking the simplest form, we study the properties of up-down ($ud$) quark matter, and confirm that the revised quasiparticle model fulfills the quantitative criteria for thermodynamic consistency. Moreover, we find that the maximum mass of an $ud$ quark star can be larger than two times the solar mass, reaching up to $2.31M_{\odot}$, for reasonable model parameters. However, to further satisfy the upper limit of tidal deformability $\tilde{\Lambda}_{1.4}\leq 580$ observed in the event GW170817, the maximum mass of an $ud$ quark star can only be as large as $2.08M_{\odot}$, namely $M_{\text{max}}\lesssim2.08M_{\odot}$. In other words, our results indicate that the measured tidal deformability for event GW170817 places an upper bound on the maximum mass of $ud$ quark stars, but which does not rule out the possibility of the existence of quark stars composed of $ud$ quark matter, with a mass of about two times the solar mass., Comment: 10 pages, 8 figures

We model the boundary-layer flashback (BLF) of CH$_4$/H$_2$/air swirling flames via large-eddy simulations with the flame-surface-density method (LES-FSD), in particular, at high pressures. A local displacement speed model tabulating the stretched flame speed is employed to account for the thermo-diffusive effects, flame surface curvature, and heat loss in LES-FSD. The LES-FSD well captures the propagation characteristics during the BLF of swirling flames. In the LES-FSD for lean CH$_4$/H$_2$/air flames at 2.5 bar, the critical equivalence ratio for flashback decreases with the increasing hydrogen volume fraction, consistent with the experiments. This is due to the improved modeling of effects of the flame stretch and heat loss on the local displacement speed. We also develop a simple model to predict the BLF limits of swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 14 datasets of CH$_4$/H$_2$/air swirling flame experiments, with the hydrogen volume fractions in fuel from 50% to 100%. The present model well estimates the flashback limits in various operating conditions., Comment: 29 pages, 16 figures

Due to nonholonomic dynamics, the motion planning of nonholonomic robots is always a difficult problem. This letter presents a Discrete States-based Trajectory Planning(DSTP) algorithm for autonomous nonholonomic robots. The proposed algorithm represents the trajectory as x and y positions, orientation angle, longitude velocity and acceleration, angular velocity, and time intervals. More variables make the expression of optimization and constraints simpler, reduce the error caused by too many approximations, and also handle the gear shifting situation. L-BFGS-B is used to deal with the optimization of many variables and box constraints, thus speeding up the problem solving. Various simulation experiments compared with prior works have validated that our algorithm has an order-of-magnitude efficiency advantage and can generate a smoother trajectory with a high speed and low control effort. Besides, real-world experiments are also conducted to verify the feasibility of our algorithm in real scenes. We will release our codes as ros packages., Comment: 8 pages, 9 figures

Correcting scan-positional errors is critical in achieving electron ptychography with both high resolution and high precision. This is a demanding and challenging task due to the sheer number of parameters that need to be optimized. For atomic-resolution ptychographic reconstructions, we found classical refining methods for scan positions not satisfactory due to the inherent entanglement between the object and scan positions, which can produce systematic errors in the results. Here, we propose a new protocol consisting of a series of constrained gradient descent (CGD) methods to achieve better recovery of scan positions. The central idea of these CGD methods is to utilize a priori knowledge about the nature of STEM experiments and add necessary constraints to isolate different types of scan positional errors during the iterative reconstruction process. Each constraint will be introduced with the help of simulated 4D-STEM datasets with known positional errors. Then the integrated constrained gradient decent (iCGD) protocol will be demonstrated using an experimental 4D-STEM dataset of the 1H-MoS2 monolayer. We will show that the iCGD protocol can effectively address the errors of scan positions across the spectrum and help to achieve electron ptychography with high accuracy and precision.

The magnetic field and density behaviors of various thermodynamic quantities of strange quark matter under compact star conditions are investigated in the framework of the thermodynamically self-consistent quasiparticle model. For individual species, a larger number density $n_i$ leads to a larger magnetic field strength threshold that align all particles parallel or antiparallel to the magnetic field. Accordingly, in contrast to the finite baryon density effect which reduces the spin polarization of magnetized strange quark matter, the magnetic field effect leads to an enhancement of it. We also compute the sound velocity as a function of the baryon density and find the sound velocity shows an obvious oscillation with increasing density. Except for the oscillation, similar to the zero-magnetic field case that the sound velocity grows with increasing density and approaches the conformal limit $V_s^2=1/3$ at high densities from below., Comment: 10 pages, 7 figures; Manuscript has been accepted for publication as a Regular Article in Chinese Physics C

Boson peak, the excess low energy excitations in the terahertz regime, is one of the most unique features of disordered systems and has been linked to many anomalous properties of glass materials. The nature and structural origin of the boson peak remain elusive and have been debated for more than a half century mainly due to the lack of real-time and real-space experimental insights of the dynamic phenomenon. In this work we employed femtosecond MeV ultrafast electron diffraction to characterize the atomic dynamics of metallic glasses in real time. The experiment reveals collective atomic oscillations, presented in elastic electron scattering and atomic pair distribution functions, within the boson peak frequency range of 1.0-1.8 THz in both reciprocal and real space. It was found that the oscillation frequency has reciprocal dependence on interatomic pair distances and the corresponding wave velocity experimentally affirms the transverse acoustic wave nature of the boson peak. The observed strong correlation between THz acoustic vibrations and coherent electron scattering provides compelling evidence that the boson peak originates from the collective transverse vibrational modes of structurally ordered atoms in the disordered system.

Glasses are featured with a disordered amorphous structure, being opposite to crystals that are constituted by periodic lattices. In this study we report that the exceptional thermodynamic and kinetic stability of an ultra-stable binary ZrCu metallic glass, fabricated by high-temperature physical vapor deposition, originates from ubiquitous crystal-like medium range order (MRO) constituted by Voronoi polyhedron ordering with well-defined local translational symmetry beyond nearest atomic neighbors. The crystal-like MRO significantly improves the thermodynamic and kinetic stability of the glass, which is in opposition to the conventional wisdom that crystal-like order deteriorates the stability and forming ability of metallic glasses. This study unveils the structural origin of ultra-stable metallic glasses and shines a light on the intrinsic correlation of local atomic structure ordering with glass transition of metallic glasses.