arXiv subjects

Xiao-Ping Wang

Publications and source records attributed to Xiao-Ping Wang.

80 records · Page 5Linked to original sources

arXiv2011

$\mathscr{O}(100 GeV)$ Deci-weak $W^\prime/Z^\prime$ at Tevatron and LHC

Recently Tevatron released their measurements on invariant mass spectrum of electron/positron, as well as the di-jet arising from WW+WZ production with one W leptonically decay. Though the statistics is not significant, there are two bumps around 240 GeV and 120-160 GeV respectively. We proposed that the two bumps correspond to the extra light gauge bosons $Z^\prime$ and $W^\prime$, which couple with quarks with the deci-weak strength. In this brief report, we also simulated di-jet invariant mass distribution at the current running LHC.

hep-ph↗

arXiv2011

New Color-Octet Vector Boson Revisit

Motivated by CDF recent measurements on di-jet invariant mass spectrum where di-jet is associated production with charged leptons ($e/μ$) and missing energy, we re-examine the previous proposed massive color-octet axial-vector-like boson $Z_c$. Our simulation showed that the di-jet bump around 120-160 GeV can be induced by $Z_c$ with effective coupling $g_{Z_c q \bar q} =0.2 g_s$ (q represents the quark other than top and $g_s$ is the strong coupling constant). Moreover our numerical investigation indicated that the top quark forward-backward asymmetry $A_{FB}^t$ can be reproduced without distorting shape of differential cross section $dσ/d M_{t\bar t}$, provided that the $Z_c$ and top quark coupling is appropriately chosen ($g_{Z_c t \bar t} \simeq 4.5 g_{Z_c q \bar q}$). Our results also showed that the theoretical $A_{FB}^t$ as functions of $Δy$ and $M_{t\bar t}$ can be consistent with data within $1σ$ and $1.8σ$ respectively.

hep-ph↗

arXiv2006

A variational approach to the moving contact line hydrodynamics

In immiscible two-phase flows, contact line denotes the intersection of the fluid-fluid interface with the solid wall. When one fluid displaces the other, the contact line moves along the wall. A classical problem in continuum hydrodynamics is the incompatibility between the moving contact line and the no-slip boundary condition, as the latter leads to a non-integrable singularity. The recently discovered generalized Navier boundary condition (GNBC) offers an alternative to the no-slip boundary condition which can resolve the moving contact line conundrum. We present a variational derivation of the GNBC through the principle of minimum energy dissipation (entropy production), as formulated by Onsager for small perturbations away from the equilibrium. Through numerical implementation of a continuum hydrodynamic model, it is demonstrated that the GNBC can quantitatively reproduce the moving contact line slip velocity profiles obtained from molecular dynamics simulations. In particular, the transition from complete slip at the moving contact line to near-zero slip far away is shown to be governed by a power-law partial slip regime, extending to mesoscopic length scales. The sharp (fluid-fluid) interface limit of the hydrodynamic model, together with some general implications of slip versus no-slip, are discussed.

cond-mat.soft↗

arXiv2005

Molecular hydrodynamics of the moving contact line in two-phase immiscible flows

The ``no-slip'' boundary condition, i.e., zero fluid velocity relative to the solid at the fluid-solid interface, has been very successful in describing many macroscopic flows. A problem of principle arises when the no-slip boundary condition is used to model the hydrodynamics of immiscible-fluid displacement in the vicinity of the moving contact line, where the interface separating two immiscible fluids intersects the solid wall. Decades ago it was already known that the moving contact line is incompatible with the no-slip boundary condition, since the latter would imply infinite dissipation due to a non-integrable singularity in the stress near the contact line. In this paper we first present an introductory review of the problem. We then present a detailed review of our recent results on the contact-line motion in immiscible two-phase flow, from MD simulations to continuum hydrodynamics calculations. Through extensive MD studies and detailed analysis, we have uncovered the slip boundary condition governing the moving contact line, denoted the generalized Navier boundary condition. We have used this discovery to formulate a continuum hydrodynamic model whose predictions are in remarkable quantitative agreement with the MD simulation results at the molecular level. These results serve to affirm the validity of the generalized Navier boundary condition, as well as to open up the possibility of continuum hydrodynamic calculations of immiscible flows that are physically meaningful at the molecular level.

cond-mat.soft↗

arXiv2005

Hydrodynamic slip boundary condition at chemically patterned surfaces: A continuum deduction from molecular dynamics

We investigate the slip boundary condition for single-phase flow past a chemically patterned surface. Molecular dynamics (MD) simulations show that modulation of fluid-solid interaction along a chemically patterned surface induces a lateral structure in the fluid molecular organization near the surface. Consequently, various forces and stresses in the fluid vary along the patterned surface. Given the presence of these lateral variations, a general scheme is developed to extract hydrodynamic information from MD data. With the help of this scheme, the validity of the Navier slip boundary condition is verified for the chemically patterned surface, where a local slip length can be defined. Based on the MD results, a continuum hydrodynamic model is formulated using the Navier-Stokes equation and the Navier boundary condition, with a slip length varying along the patterned surface. Steady-state velocity fields from continuum calculations are in quantitative agreement with those from MD simulations. It is shown that, when the pattern period is sufficiently small, the solid surface appears to be homogeneous, with an effective slip length that can be controlled by surface patterning. Such a tunable slip length may have important applications in nanofluidics.

cond-mat.soft↗

arXiv2004

Driven cavity flow: from molecular dynamics to continuum hydrodynamics

Molecular dynamics (MD) simulations have been carried out to investigate the slip of fluid in the lid driven cavity flow where the no-slip boundary condition causes unphysical stress divergence. The MD results not only show the existence of fluid slip but also verify the validity of the Navier slip boundary condition. To better understand the fluid slip in this problem, a continuum hydrodynamic model has been formulated based upon the MD verification of the Navier boundary condition and the Newtonian stress. Our model has no adjustable parameter because all the material parameters (density, viscosity, and slip length) are directly determined from MD simulations. Steady-state velocity fields from continuum calculations are in quantitative agreement with those from MD simulations, from the molecular-scale structure to the global flow. The main discovery is as follows. In the immediate vicinity of the corners where moving and fixed solid surfaces intersect, there is a core partial-slip region where the slippage is large at the moving solid surface and decays away from the intersection quickly. In particular, the structure of this core region is nearly independent of the system size. On the other hand, for sufficiently large system, an additional partial-slip region appears where the slippage varies as $1/r$ with $r$ denoting the distance from the corner along the moving solid surface. The existence of this wide power-law region is in accordance with the asymptotic $1/r$ variation of stress and the Navier boundary condition.

physics.flu-dyn↗

arXiv2004

Power-Law Slip Profile of the Moving Contact Line in Two-Phase Immiscible Flows

Large scale molecular dynamics (MD) simulations on two-phase immiscible flows show that associated with the moving contact line, there is a very large $1/x$ partial-slip region where $x$ denotes the distance from the contact line. This power-law partial-slip region is verified in large-scale adaptive continuum simulations based on a local, continuum hydrodynamic formulation, which has proved successful in reproducing MD results at the nanoscale. Both MD and continuum simulations indicate the existence of a universal slip profile in the Stokes-flow regime, well described by $v^{slip}(x)/V_w=1/(1+{x}/{al_s})$, where $v^{slip}$ is the slip velocity, $V_w$ the speed of moving wall, $l_s$ the slip length, and $a$ is a numerical constant. Implications for the contact-line dissipation are discussed.

physics.flu-dyn↗

arXiv2003

Molecular scale contact line hydrodynamics of immiscible flows

From extensive molecular dynamics simulations on immiscible two-phase flows, we find the relative slipping between the fluids and the solid wall everywhere to follow the generalized Navier boundary condition, in which the amount of slipping is proportional to the sum of tangential viscous stress and the uncompensated Young stress. The latter arises from the deviation of the fluid-fluid interface from its static configuration. We give a continuum formulation of the immiscible flow hydrodynamics, comprising the generalized Navier boundary condition, the Navier-Stokes equation, and the Cahn-Hilliard interfacial free energy. Our hydrodynamic model yields interfacial and velocity profiles matching those from the molecular dynamics simulations at the molecular-scale vicinity of the contact line. In particular, the behavior at high capillary numbers, leading to the breakup of the fluid-fluid interface, is accurately predicted.

cond-mat.soft↗