arXiv subjects

Qian Mao

Publications and source records attributed to Qian Mao.

7 recordsLinked to original sources

arXiv2026

An improved fully one-sided diffuse-interface immersed boundary method with target-value reconstruction for compressible flows

Although one-sided spreading has been shown to improve the near-wall accuracy of diffuse-interface immersed boundary methods (DIBMs), the effect of its asymmetric kernel support on the effective boundary location remains insufficiently understood. In this work, a detailed analysis of the one-sided spreading operator reveals an inward displacement of the effective boundary relative to the geometric boundary. To compensate for this displacement, a target-value reconstruction strategy is developed to ensure consistency between the values imposed at the effective boundary and the prescribed conditions at the geometric boundary. The strategy is incorporated into the fully one-sided diffuse-interface immersed boundary method (FODIBM) and applies to both Dirichlet and Neumann boundary conditions. Although confined to the target-value evaluation step, the modification substantially improves boundary-condition enforcement with negligible additional computational cost. Coupled with a hybrid lattice Boltzmann solver, the improved method consistently reduces L_2 and L_{\infty} error norms across different grid resolutions while retaining approximately second-order grid convergence. The no-slip and isothermal boundary-condition errors are reduced by 77% and 85%, respectively. Simulations involving various two- and three-dimensional geometries further show improved predictions relative to both the conventional DIBM and the original FODIBM. The results agree well with body-fitted reference solutions and experimental data, demonstrating accurate and computationally efficient simulations of compressible flows around complex geometries.

physics.flu-dyn↗

arXiv2026

A fully one-sided diffuse-interface immersed boundary method for wall-modeled large-eddy simulation

Diffuse-interface immersed boundary methods (DIBMs) provide a simple and robust approach for simulating flows involving complex geometries. However, their inherent diffusion effect can contaminate the near-wall flow field and significantly degrade wall-shear-stress prediction in wall-modeled large-eddy simulation (WMLES). To address this limitation, we develop a WMLES approach based on a fully one-sided diffuse-interface immersed boundary method (FODIBM). By performing interpolation and spreading exclusively inside the immersed body, the proposed method removes the cross-boundary diffusion effect that adversely affects wall modeling in conventional DIBMs. A wall-shear-stress enforcement strategy is developed by coupling the wall-parallel immersed-boundary forcing with the wall shear stress predicted by an explicit wall model. In addition, a tau-model based on the modeled turbulent shear-stress tensor is introduced to preserve the total shear-stress balance below the reference height. The method is first validated in high-Reynolds-number turbulent channel flows, showing good agreement with DNS data for the mean velocity, Reynolds shear stress, and skin-friction coefficient. Sensitivity studies with respect to grid resolution, reference height, wall inclination angle, and Reynolds number demonstrate the robustness of the method. Compared with the conventional DIBM, the proposed method substantially improves the overall prediction accuracy, particularly at low reference heights. The approach is further assessed for turbulent flow over a NACA23012 airfoil, where the predicted pressure distribution and lift coefficient agree well with experimental data.

physics.flu-dyn↗

arXiv2026

Fluid transport by a single active filament in a three-dimensional two-phase flow

Micro-scale cilia play a vital role in mucociliary clearance (MCC) in the human respiratory airways. In this numerical study, we examine fluid transport driven by the active beating of a single filament immersed in a three-dimensional two-phase flow. The cilium is modeled as an elastic filament actuated by a time-varying basal angle. The two-phase flow is resolved using the Shan-Chen model in a lattice Boltzmann solver, while the two-way coupling between the filament and the fluid is treated by the immersed boundary method. Pathological conditions such as cystic fibrosis and chronic obstructive pulmonary disease are associated with drastic alterations of MCC properties, including changes in periciliary layer (PCL) thickness and the viscosity ratio between the PCL and the mucus layer (ML). Here, we systematically investigate the effects of these parameters, along with filament bending stiffness, on the beating pattern and fluid transport. Within the parameter ranges investigated, a moderate PCL thickness and viscosity ratio, together with high bending stiffness, tend to yield higher net flow rate and transport efficiency. The underlying hydrodynamic mechanisms are characterized through analyses of the beating pattern, filament dynamics, energy partition, and flow-field evolution. Two competing mechanisms are identified: the drag-elastic force balance and the viscous diffusion of momentum. Furthermore, quantitative relationships are established between flow rate and beating pattern, expressed in terms of tip amplitude and beating asymmetry.

physics.flu-dyn↗

arXiv2025

Epitaxial formation of ultrathin HfO2 on graphene by sequential oxidation

We demonstrate the formation of epitaxial, ultrathin hafnia (HfO2) on graphene. Monoclinic hafnia (m-HfO2) forms as the end of a series of sequential oxidation reactions. Starting from Hf metal grown epitaxially on graphene, oxidation leads first to an amorphous suboxide (a-HfOx), then to a crystalline, hexagonal suboxide (h-HfOx) in epitaxial relationship with the substrate, and finally to m-HfO2 that is also epitaxial. We use scanning transmission electron microscopy to characterize the epitaxial relationships and to investigate the structure of h-HfOx. We propose a series of displacive transformations that relate the different crystalline phases and are consistent with the observed epitaxial relationships with the graphene substrate. ReaxFF based reactive molecular dynamics simulations confirm our model of the oxide phase sequencing, and illustrate the role of graphene in promoting oxide crystallization. Our results suggest a way to achieve heteroepitaxial integration of high-performance, crystalline dielectrics with two dimensional (2D) semiconductors with an atomically sharp interface, which is also relevant to hafnia phase engineering.

cond-mat.mtrl-sci↗

arXiv2020

Review of Machine-Learning Methods for RNA Secondary Structure Prediction

Secondary structure plays an important role in determining the function of non-coding RNAs. Hence, identifying RNA secondary structures is of great value to research. Computational prediction is a mainstream approach for predicting RNA secondary structure. Unfortunately, even though new methods have been proposed over the past 40 years, the performance of computational prediction methods has stagnated in the last decade. Recently, with the increasing availability of RNA structure data, new methods based on machine-learning technologies, especially deep learning, have alleviated the issue. In this review, we provide a comprehensive overview of RNA secondary structure prediction methods based on machine-learning technologies and a tabularized summary of the most important methods in this field. The current pending issues in the field of RNA secondary structure prediction and future trends are also discussed.

q-bio.BM↗

arXiv2020

Polyacrylonitrile/Graphene Nanocomposite: Towards the Next Generation of Carbon Fibers

Carbon Fibers (CFs) are the key solution for the future lightweight vehicle with enhanced fuel efficiency and reduced emissions owing to their ultrahigh strength to weight ratio. However, the high cost of the current dominant PAN-based CFs hinders their application. The use of low-cost alternative precursors may overcome this issue. Unfortunately, low-cost CFs derived from cheaper single component precursors suffer from poor mechanical properties. Developing composite CFs by adding nanoadditives is very promising for low-cost CFs. Therefore, a fundamental understanding of carbonization condition impacts and polymer/additives conversion mechanisms during whole CF production are essential to develop low-cost CFs. In this work, we have demonstrated how the carbonization temperature affects the PAN/graphene CFs properties by performing a series of ReaxFF based molecular dynamics simulations. We found that graphene edges along with the nitrogen and oxygen functional groups have a catalytic role and act as seeds for the graphitic structure growth. Our MD simulations unveil that the addition of the graphene to PAN precursor modifies all-carbon membered rings in CFs and enhances the alignments of 6-member carbon rings in carbonization which leads to superior mechanical properties compare to PAN-based CFs. These ReaxFF simulation results are validates by experimental structural and mechanical characterizations. Interestingly, mechanical characterizations indicate that PAN/graphene CFs carbonized at 1250 C demonstrate 90.9% increase in strength and 101.9% enhancement in Young's modulus compare to the PAN-based CFs carbonized at 1500 C. The superior mechanical properties of PAN/graphene CFs at lower carbonization temperatures offers a path to both energy savings and cost reduction by decreasing the carbonization temperature and could provide key insights for the development of low-cost CFs.

cond-mat.mtrl-sci↗

arXiv2019

Theoretical analysis and kinetic modeling on hydrogen addition and abstraction by H radical of 1,3-cyclopentadiene and the associated chain-branching reactions

Cyclopentadiene (CPD) is an important intermediate in the combustion of fuel and the formation of aromatics. In the present study, the kinetics and thermodynamic properties for hydrogen abstraction and addition with CPD, and the related reactions including the isomerization and decomposition on the C5H7 potential energy surface were systematically investigated by theoretical calculations. High-level ab initio calculations were adopted to obtain the stationary points on the potential energy surfaces of CPD + H. Phenomenological rate coefficients for temperature- and pressure-dependent reactions in the full potential energy surface were calculated by solving the time-dependent multiple-well RRKM/master equation, and the hydrogen abstraction reactions were based on the conventional transition state theory. In terms of the hydrogen abstraction reactions, the hydrogen abstraction from the saturated carbon atom in CPD is found to be the dominant channel. For the hydrogen addition and the associated reactions on the C5H7 PES, the allylic and vinylic cyclopentenyl radicals and C2H2 + C3H5 were found to be the most important channels and reactivity-promoting products, respectively. The previously neglected role of open-chain intermediates in the evaluation of the reaction kinetics has been suggested and the corresponding rate constants have been recommended for inclusion in the modeling of the H+ c-C5H6 reaction. Results indicate that the transformation from to straight-chain C5H7 is kinetically unfavorable due to the high strain energy of the 3-membered ring structure of the isomerization transition state. Moreover, the thermodynamic data and the calculated rate coefficients for both H atom abstraction and addition were incorporated into the kinetics model to examine the impact of the computed pressure-dependent kinetics of C5H6 + H reactions on model predictions.

physics.comp-ph↗