arXiv subjects

Liang Sun

Publications and source records attributed to Liang Sun.

159 records · Page 9Linked to original sources

arXiv2012

BABAR Measurements on B->K(*)l+l- Rates and Rate Asymmetries

Based on 471 million BB pairs collected with the BABAR detector at the PEP-II e+e- collider, we perform a series of measurements on rare decays B->K(*)l+l-, where l+l- is either e+e- or mu+mu-. The measurements include total branching fractions, and partial branching fractions in six bins of di-lepton mass-squared. We also measure isospin asymmetries in the same six bins. Furthermore, we measure direct CP and lepton flavor asymmetries for di-lepton mass below and above the J/Psi resonance. Our measurements show good agreement with both Standard Model predictions and measurements from other experiments.

hep-ex↗

arXiv2012

Magnetic Impurities in Two-Dimensional Superfluid Fermi Gas with Spin-Orbit Coupling

We consider magnetic impurities in a two dimensional superfluid Fermi gas in the presence of spin-orbit coupling. By using the methods of t-matrix and Green's function, we find spin-orbit coupling has some dramatic impacts on the effects of magnetic impurities. For the single impurity problem, the number of bound states localized around the magnetic impurity is doubled. For the finite concentration $n$ of impurities, the energy gap is reduced and the density of states in the gapless region is greatly modified.

cond-mat.quant-gas↗

arXiv2011

General Temporal Instability Criteria For Stably Stratified Inviscid Flow

The temporal instability of stably stratified flow was investigated by analyzing the Taylor-Goldstein equation theoretically. According to this analysis, the stable stratification $N^2\geq0$ has a destabilization mechanism, and the flow instability is due to the competition of the kinetic energy with the potential energy, which is dominated by the total Froude number $Fr_t^2$. Globally, $Fr_t^2 \leq 1$ implies that the total kinetic energy is smaller than the total potential energy. So the potential energy might transfer to the kinetic energy after being disturbed, and the flow becomes unstable. On the other hand, when the potential energy is smaller than the kinetic energy ($Fr_t^2>1$), the flow is stable because no potential energy could transfer to the kinetic energy. The flow is more stable with the velocity profile $U'/U'''>0$ than that with $U'/U'''<0$. Besides, the unstable perturbation must be long-wave scale. Locally, the flow is unstable as the gradient Richardson number $Ri>1/4$. These results extend the Rayleigh's, Fjørtoft's, Sun's and Arnol'd's criteria for the inviscid homogenous fluid, but they contradict the well-known Miles-Howard theorem. It is argued here that the transform $F=ϕ/(U-c)^n$ is not suitable for temporal stability problem, and that it will lead to contradictions with the results derived from the Taylor-Goldstein equation. However, such transform might be useful for the study of the Orr-Sommerfeld equation in viscous flows.

physics.flu-dyn↗

arXiv2011

Long-wave instability and growth rate of the inviscid shear flows

In this paper, we studied the long-wave instability of the shear flows. When the wavenumber of perturbation is larger than the critical value, the flow is always neutrally stable. First, we obtain a new upper bound for the neutral wavenumber $k_1\leq (p^2-1)μ_1$, where $p>1$ and $μ_1$ is the smallest eigenvalue of Poincaré's problem. Second, we find a new upper bound for the imaginary part of the complex phase velocity $c_i \leq k_1 ΔU/\sqrt{μ_1}$, where $ΔU$ is the variance of the velocity. The new bound is finite for all $k>0$ similar to the Howard's semicircle theorem, while the previous ones by Craik and Banerjee et al would be infinity as $k\rightarrow 0$. Third, we find a new upper bound of growth rate $ω_i \leq (p-1) \sqrt{μ_1} ΔU$. All the new bounds are much more strict than the previous ones by Høiland, Howard, Craik and Banerjee et al. Our results also extend the inverse energy cascade theory by Kraichnan. As shear instability is due to long-wave instability, it implies that the truncation of long-waves may change the instability of shear flows.

physics.flu-dyn↗

arXiv2011

An exact axisymmetric spiral solution of incompressible 3D Euler equations

Spiral structure is one of the most common structures in the nature flows. A general steady spiral solution of incompressible inviscid axisymmetric flow was obtained analytically by applying separation of variables to the 3D Euler equations. The solution, depending on 3 parameters, describes the spiral path of the fluid material element on the Bernoulli surface, whereas some new exact solutions were obtained to be bounded within the whole region. The first one is a continued typhoon-like vortex solution, where there are two intrinsic length scales. One is the radius of maximum circular velocity $r_m$, the other is the radius of the vortex kernel $r_k=\sqrt{2}r_m$. The second one is a multi-planar solution, periodically in $z$-coordinate. Within each layer, the solution is a umbrella vortex similar to the first one. The third one is also a multi-planar solution in $z$-coordinate. In each layer, it is a combination of two independent solutions like the Rankine vortex, which is also finite but discontinued for either vertical or horizontal velocity. The fourth one is a multi-paraboloid vortex solution finite for $z$-coordinate but infinite for $r$-coordinate. Besides, some classical simple solutions (Rankine vortex, Batchelor vortex, Hill spherical vortex, etc.) were also obtained. The above explicit solutions can be applied to study the radial structure of the typhoon, tornados and mesoscale eddies. Both the solutions and approaches used here could also be applied to other complex flows by the Navier-Stokes equations.

physics.flu-dyn↗

arXiv2010

Eigen-Based Transceivers for the MIMO Broadcast Channel with Semi-Orthogonal User Selection

This paper studies the sum rate performance of two low complexity eigenmode-based transmission techniques for the MIMO broadcast channel, employing greedy semi-orthogonal user selection (SUS). The first approach, termed ZFDPC-SUS, is based on zero-forcing dirty paper coding; the second approach, termed ZFBF-SUS, is based on zero-forcing beamforming. We first employ new analytical methods to prove that as the number of users K grows large, the ZFDPC-SUS approach can achieve the optimal sum rate scaling of the MIMO broadcast channel. We also prove that the average sum rates of both techniques converge to the average sum capacity of the MIMO broadcast channel for large K. In addition to the asymptotic analysis, we investigate the sum rates achieved by ZFDPC-SUS and ZFBF-SUS for finite K, and show that ZFDPC-SUS has significant performance advantages. Our results also provide key insights into the benefit of multiple receive antennas, and the effect of the SUS algorithm. In particular, we show that whilst multiple receive antennas only improves the asymptotic sum rate scaling via the second-order behavior of the multi-user diversity gain; for finite K, the benefit can be very significant. We also show the interesting result that the semi-orthogonality constraint imposed by SUS, whilst facilitating a very low complexity user selection procedure, asymptotically does not reduce the multi-user diversity gain in either first (log K) or second-order (loglog K) terms.

cs.IT↗

arXiv2009

Stabilities of Parallel Flow and Horizontal Convection

In the first part, the stability of two-dimensional parallel flow is discussed. A more restrictively general stability criterion for inviscid parallel flow is obtained analytically. In the second part, we report the numerical simulations of the partial-penetrating flow in horizontal convection within a squire cavity tank at high Rayleigh numbers $10^7<Ra<10^{10}$. The fast estab but slowly steadied flow is simulated, where a shallow and closed circulation cell is obtained numerically as partial-penetrating flow for the first time, which is consistent with the experiment.

physics.flu-dyn↗

arXiv2009

Impacts of Typhoons on the Kuroshio Large Meander: Observation Evidences

The formation of the Kuroshio large meander in summer 2004 was investigated by using the cruise data, Argo profiles data, and satellite remote sensing data. We validated the point that cyclonic eddy contributes to the large meander. Besides, the impacts of typhoons on Kuroshio meanders were studied. From 29 July to 4 August, the typhoons stirred the ocean and upwelled the deep water, which enhanced the existed cyclonic eddy, and immediately made a drastic meander of the Kuroshio. Moreover, the unexpected typhoons in June 2004 also contributed to the initial meander at Tokara Strait. The result suggests an alternative meander mechanism of Kuroshio path via typhoon-eddy-Kuroshio interactions. It is argued that typhoons accompanied with cyclonic eddies, might play crucial roles in meanders of the Kuroshio. This will provide a more comprehensive understanding of the dynamics of the west boundary flows like the Kuroshio and the Gulf Stream, and will be useful in eddy-resolution models.

physics.ao-ph↗

arXiv2008

Different responses of the upper ocean to typhoon Namtheun

The responses of the upper ocean to typhoon Namtheun in July 2004 are investigated by sea surface measurements and vertical profiles. Pre-typhoon ocean environment played an important role in this case. There were two extreme cooling regions located at cyclonic eddy A and typhoon wake B. Although the magnitudes of SST cooling at A and B were similar, other physical and biophysical responses were quite different. Combining multi-satellite data with vertical profile data, it is found that the upwelling dominated the responses at A and the vertical mixing dominated the responses at B. This study implies that to insight into the ocean surface responses to typhoon, the subsurface dynamics need to be analyzed via both the in situ and satellite-based observations, and the physical and biological models.

physics.ao-ph↗

arXiv2008

Onset of unsteady horizontal convection in rectangle tank at $Pr=1$

The horizontal convection within a rectangle tank is numerically simulated. The flow is found to be unsteady at high Rayleigh numbers. There is a Hopf bifurcation of $Ra$ from steady solutions to periodic solutions, and the critical Rayleigh number $Ra_c$ is obtained as $Ra_c=5.5377\times 10^8$ for the middle plume forcing at $Pr=1$, which is much larger than the formerly obtained value. Besides, the unstable perturbations are always generated from the central jet, which implies that the onset of instability is due to velocity shear (shear instability) other than thermally dynamics (thermal instability). Finally, Paparella and Young's [J. Fluid Mech. 466 (2002) 205] first hypotheses about the destabilization of the flow is numerically proved, i.e. the middle plume forcing can lead to a destabilization of the flow.

physics.flu-dyn↗

arXiv2007

Phase transition of a one-dimensional Ising model with distance-dependent connections

The critical behavior of Ising model on a one-dimensional network, which has long-range connections at distances $l>1$ with the probability $Θ(l)\sim l^{-m}$, is studied by using Monte Carlo simulations. Through studying the Ising model on networks with different $m$ values, this paper discusses the impact of the global correlation, which decays with the increase of $m$, on the phase transition of the Ising model. Adding the analysis of the finite-size scaling of the order parameter $[< M>]$, it is observed that in the whole range of $0<m<2$, a finite-temperature transition exists, and the critical exponents show consistence with mean-field values, which indicates a mean-field nature of the phase transition.

nlin.PS↗

arXiv2007

The sensitivity and stability of the ocean's thermohaline circulation to finite amplitude perturbations

Within a simple model context, the sensitivity and stability of the thermohaline circulation to finite amplitude perturbations is studied. A new approach is used to tackle this nonlinear problem. The method is based on the computation of the so-called Conditional Nonlinear Optimal Perturbation (CNOP) which is a nonlinear generalization of the linear singular vector approach (LSV). It is shown that linearly stable thermohaline circulation states can become nonlinearly unstable and the properties of the perturbations with optimal nonlinear growth are determined. An asymmetric nonlinear response to perturbations exists with respect to the sign of finite amplitude freshwater perturbations, on both thermally dominated and salinity dominated thermohaline flows. This asymmetry is due to the nonlinear interaction of the perturbations through advective processes.

physics.ao-ph↗

arXiv2006

Essence of inviscid shear instability: a point view of vortex dynamics

The essence of shear instability is fully revealed both mathematically and physically. A general sufficient and necessary stable criterion is obtained analytically within linear context. It is the analogue of Kelvin-Arnol'd theorem, i.e., the stable flow minimizes the kinetic energy associated with vorticity. Then the mechanism of shear instability is explored by combining the mechanisms of both Kelvin-Helmholtz instability (K-H instability) and resonance of waves. It requires both concentrated vortex and resonant waves for the instability. The waves, which have same phase speed with the concentrated vortex, have interactions with the vortex to trigger the instability. We call this mechanism as "concentrated vortex instability". The physical explanation of shear instability is also sketched. Finally, some useful criteria are derived from the theorem. These results would intrigue future works to investigate the other hydrodynamic instabilities.

physics.flu-dyn↗

arXiv2006

General stability criterion of inviscid parallel flow

A more restrictively general stability criterion of two-dimensional inviscid parallel flow is obtained analytically. First, a sufficient criterion for stability is found as either $-μ_1<\frac{U''}{U-U_s}<0$ or $0<\frac{U''}{U-U_s}$ in the flow, where $U_s$ is the velocity at inflection point, $μ_1$ is the eigenvalue of Poincaré's problem. Second, this criterion is generalized to barotropic geophysical flows in $β$ plane. Based on the criteria, the flows are are divided into different categories of stable flows, which may simplify the further investigations. And the connections between present criteria and Arnol'd's nonlinear criteria are discussed. These results extend the former criteria obtained by Rayleigh, Tollmien and Fjørtoft and would intrigue future research on the mechanism of hydrodynamic instability.

physics.flu-dyn↗

arXiv2005

General stability criterion of two-dimensional inviscid parallel flow

General stability criterions of two-dimensional inviscid parallel flow are obtained analytically for the first time. First, a criterion for stability is found as $\frac{U''}{U-U_s}>-μ_1$ everywhere in the flow, where $U_s$ is the velocity at inflection point, $μ_1$ is eigenvalue of Poincaré's problem. Second, we also prove a principle that the flow is stable, if and only if all the disturbances with $c_r=U_s$ are neutrally stable. Finally, following this principle, a criterion for instability is found as $\frac{U''}{U-U_s}<-μ_1$ everywhere in the flow. These results extend the former theorems obtained by Rayleigh, Tollmien and Fjørtoft and will lead future works to investigate the mechanism of hydrodynamic instability.

physics.flu-dyn↗