Nonlocal phonon thermal transport in graphene in hydrodynamic regime.

The hydrodynamic behavior of phonons is of particular interest and importance owing to the strong demand for highly thermal conductive materials. Thermal transport in hydrodynamic regime becomes essentially nonlocal, which can give rise to a number of new and counterintuitive phenomena. In this work, we present a direct numerical study of nonlocal phonon thermal transport in graphene ribbon with vicinity geometry based on the phonon Boltzmann transport equation with first-principles inputs. We demonstrate the viscosity-dominated hydrodynamic transport behaviors with two abnormal thermal transport phenomena: heat current whirlpools and negative nonlocal effect, which originate from phonon viscosity. Phonon viscosity produces the vorticity of shear flows, leading to the backflow of the heat current and the generation of negative nonlocal vicinity response. The system average temperature and the ribbon width as well as the relative positions of the heat sources play a pivotal role in the occurrence of heat current whirlpools and negative nonlocal temperature response. The present work provides solid evidence for phonon hydrodynamic transport in graphene and a potential avenue for experimental detection in the future.

Numerical prediction of transient electrohydrodynamic instabilities under an alternating current electric field and unipolar injection.

In this paper, a direct numerical simulation (DNS) of dielectric fluid flow subjected to unipolar injection under an alternating current (AC) electric field is carried out. The effect of frequency f of pulsed direct current (PDC) and AC on the transient evolution of electroconvection and their subcritical bifurcations are investigated in details. Electroconvection under PDC or AC tends to exhibit oscillating flow due to the periodic boundary condition of charge density and potential compared to the direct current (DC) case. The results demonstrate that under the PDC field, the linear criterion T c decreases with increasing frequency, while the nonlinear stability criterion T f is hardly affected. Under the AC field, a critical frequency f c  = 0.0316 is found, which separates electroconvection into two typical flow regimes-periodic flow regime (f < f c ) and inhibited flow regime (f ≥ f c )-depending on whether free charges can reach the collector electrode before electric field inversion. AC-electrohydrodynamics (EHD) systems promote various flow patterns with relatively lower voltage regimes than DC-EHD systems. These mechanisms of electroconvection under the PDC/AC field offer unique possibilities for fluid flow control in biological EHD-driven flow and portable EHD applications.

Lattice Boltzmann modeling of two-phase electrohydrodynamic flows under unipolar charge injection.

In this work, a two-dimensional droplet confined between two parallel electrodes under the combined effects of a nonuniform electric field and unipolar charge injection is numerically investigated using the lattice Boltzmann method (LBM). Under the non-Ohmic regime, the interfacial tension and electric forces at the droplet surface cooperate with the volumetric Coulomb force, leading to complex deformation and motion of the droplet while at the same time inducing a bulk electroconvective flow. After we validate the model by comparing with analytical solutions at the hydrostatic state, we perform a quantitative analysis on the droplet deformation factor D and bulk flow stability criteria T_{c} under different parameters, including the electric capillary number Ca, the electric Rayleigh number T, the permittivity ratio ɛ_{r}, and the mobility ratio K_{r}. It is found that the bulk flow significantly modifies the magnitude of D, which in turn decreases T_{c} of the electroconvective flow. For a droplet repelled by the anode, ɛ_{r}>1, an interesting linear relationship can be observed in the D-Ca curves. However, for a droplet attracted to the anode, ɛ_{r}<1, the system is potentially unstable. After first evolving into a quasisteady state, the droplet successively experiences steady flow, periodic flow, second steady flow, and oscillatory flow with increasing T. Moreover, discontinuities can be observed in the D-T curves due to the transitions of bulk flow.

Electroconvective instability near an ion-selective surface: A mesoscopic lattice Boltzmann study.

Direct numerical simulations of electroconvection instability near an ion-selective surface are conducted using a mesoscopic lattice Boltzmann method (LBM). An electrohydrodynamic model of ion transport and fluid flow is presented. We numerically solve the Poisson-Nernst-Planck equations for the electric field and the Navier-Stokes equations for the flow field. The results cover Ohmic, limiting, and overlimiting current regimes, and they are in good agreement with the asymptotic analytical solution for the relationship between current and voltage. The influences of different ion transport mechanisms on the voltage-current relationship are discussed. The results reveal that the electroconvection mechanism is as important as other ion transport mechanisms in electrohydrodynamic flow. By comparing the contribution of different regions in the numerical domain, we find that the flow in the extended space charge layer is dominated by electroconvection. We also study the influences of multiple driving parameters, and the electrohydrodynamic coupling constant plays a dominant role in triggering convective instability. The flow pattern and ion concentration distribution are described in detail. Moreover, the route of flow from steady state to periodic oscillation and then to chaos is discussed.

Enhancement and Manipulation of Near-Field Thermal Radiation Using Hybrid Hyperbolic Polaritons.

Owing to a high electromagnetic confinement and a strong photonic density of states, hyperbolic surface plasmon polaritons (HSPPs) provide a fascinating promise for applications in thermal photonics. In this work, we theoretically predict a possibility for the improvement of the near-field radiative heat transfer on the basis of tailoring the electromagnetic state of hyperbolic metasurfaces by the uniaxial hyperbolic substrate. By using the photonic tunneling coefficient and the polaritons dispersion, we present a comprehensive study of the hybrid effect of the hyperbolic substrate on HSPPs. We find that due to the hybrid effect of the hyperbolic substrate, the anisotropy surface state of hyperbolic metasurfaces would undergo significant deformations and even topological transition. Moreover, we systematically exhibit the evolution of such hybrid hyperbolic mode with different thicknesses of the hyperbolic substrate and analyze the thickness effect on radiative properties of the hybrid system. It is shown that the resulting heat transfer with the assistance of the hybrid hyperbolic mode by optimizing the substrate parameters is many times stronger than that of monolayer hyperbolic metasurface at the same vacuum gap. Taken together, our results provide a platform to tailor 2D hyperbolic plasmons as a potential strategy toward passive or active control of the near-field heat transfer, and the hybrid hyperbolic mode presented here may facilitate the system design for near-field energy harvesting, thermal imaging, and radiative cooling applications.

Management and postoperative use of double-cannula irrigation-drainage tube in cervical necrotizing fasciitis: a Chinese single-institution experience of 46 patients.

PURPOSE: This study aimed to analyze a Chinese institution's experience with managing cervical necrotizing fasciitis (CNF) and observe the effects of a new therapeutic approach for postoperative drainage system. METHODS: A retrospective study was established including a total of 46 CNF patients who underwent surgical debridement between April 2006 and April 2018. Analyses of demographic data, etiology, comorbidity, microbiology, complications, treatment methods, duration of treatment, and treatment outcomes were obtained. RESULTS: There were 16 kinds of microbes cultured in 29 patients. Diabetic patients were more commonly infected by microbes (P < 0.05). There was a significant reduction in the number of operative time (P < 0.05) and length of hospitalization (P < 0.01) with postoperative therapy of double-cannula irrigation-drainage (DCID) system. CONCLUSION: CNF management includes controlling for comorbidities especially glycemic control and reasonable utilization of antibiotics and aggressive postoperative therapy. DCID system can effectively reduce operative frequency and duration of hospitalization.

Efficient lattice Boltzmann method for electrohydrodynamic solid-liquid phase change.

Melting in the presence of electrohydrodynamic (EHD) flow driven by the Coulomb force in dielectric phase change material is numerically studied. A model is developed for the EHD flow in the solid-liquid phase change process. The fully coupled equations including mechanical equations, electrical equations, energy equations, and the continuity equations in the solid-liquid interface are solved using a unified lattice Boltzmann model (LBM). Firstly, the numerical model is validated by several cases in the hydrostatic state, and all LBM results are found to be highly consistent with analytical solutions. Besides, our LBM code is able to reproduce the step changes in the distribution of charge density and electric field due to the discontinuous distribution of physical properties at the interface. Then, a systematical investigation is conducted on various nondimensional parameters, including electric Rayleigh number T, Prandtl number Pr, and Stefan number St. Results are presented for the transient evolutions of temperature, fluid flow, charge density fields, and liquid fraction. Four flow stages in the melting process together with three kinds of flow instabilities are observed. It is found that the electric field has significant influence on the melting, especially at high T and Pr and low St. Over the tested cases, a maximum melting time saving of around 50% is found.

Multiscale solutions of radiative heat transfer by the discrete unified gas kinetic scheme.

The radiative transfer equation (RTE) has two asymptotic regimes characterized by the optical thickness, namely, optically thin and optically thick regimes. In the optically thin regime, a ballistic or kinetic transport is dominant. In the optically thick regime, energy transport is totally dominated by multiple collisions between photons; that is, the photons propagate by means of diffusion. To obtain convergent solutions to the RTE, conventional numerical schemes have a strong dependence on the number of spatial grids, which leads to a serious computational inefficiency in the regime where the diffusion is predominant. In this work, a discrete unified gas kinetic scheme (DUGKS) is developed to predict radiative heat transfer in participating media. Numerical performances of the DUGKS are compared in detail with conventional methods through three cases including one-dimensional transient radiative heat transfer, two-dimensional steady radiative heat transfer, and three-dimensional multiscale radiative heat transfer. Due to the asymptotic preserving property, the present method with relatively coarse grids gives accurate and reliable numerical solutions for large, small, and in-between values of optical thickness, and, especially in the optically thick regime, the DUGKS demonstrates a pronounced computational efficiency advantage over the conventional numerical models. In addition, the DUGKS has a promising potential in the study of multiscale radiative heat transfer inside the participating medium with a transition from optically thin to optically thick regimes.

Treatment choice by patients with obstructive sleep apnea: data from two centers in China.

BACKGROUND: Standard management has been recommended for obstructive sleep apnea (OSA) by several guidelines, but patient choice in the practical setting is unclear. METHODS: A survey nested in two prospective cohort studies of OSA (enrollment: 2001-2010) in China. The last interview was conducted between July 2014 and May 2015, using a comprehensive 10-point questionnaire administered in a face-to-face or telephone interview, and assessed (I) whether the participant had received any OSA treatment; (II) why he or she had decided for or against treatment; (III) what treatment was received; (IV) whether the participant used continuous positive airway pressure (CPAP) or OA daily; and (V) the perceived efficacy of therapy. RESULTS: A total of 4,097 subjects with a mean age of 45 years [37-55] responded to this survey, with a response rate of 79.4% (4,097/5,160); 2,779 subjects (67.8%) did not receive any treatment: 1,485 (53.4%) believed that their condition was not serious, despite severe OSA in 53.7% of the patients. A multivariate regression showed that the decision to receive treatment was associated with: age between 45-59 years [odds ratio (OR) 0.805, 95% CI: 0.691-0.936; P<0.001], female gender (OR 0.492, 95% CI: 0.383-0.631; P<0.001), severe OSA (OR 1.92, 95% CI: 1.01-3.64; P<0.001), hypertension (OR 1.414, 95% CI: 1.209-1.654; P<0.001) and diabetes (OR 1.760, 95% CI: 1.043-2.972; P=0.034). In subjects receiving treatment (n=1,318), 50.9% reported negative perceptions about the treatments. CONCLUSIONS: Nearly two thirds of Chinese patients choose not to receive treatment after OSA diagnosis, and nearly half are negative about their treatments for OSA. This requires clinical attention, and warrants further study in different geographic settings.

Transient/time-dependent radiative transfer in a two-dimensional scattering medium considering the polarization effect.

Transient/time-dependent radiative transfer in a two-dimensional scattering medium is numerically solved by the discontinuous finite element method (DFEM). The time-dependent term of the transient vector radiative transfer equation is discretized by the second-order central difference scheme and the space domain is discretized into non-overlapping quadrilateral elements by using the discontinuous finite element approach. The accuracy of the transient DFEM model for the radiative transfer equation considering the polarization effect is verified by comparing the time-resolved Stokes vector component distributions against the steady solutions for a polarized radiative transfer problem in a two-dimensional rectangular enclosure filled with a scattering medium. The transient polarized radiative transfer problems in a scattering medium exposed to an external beam and in an irregular emitting medium are solved. The distributions of the time-resolved Stokes vector components are presented and discussed.

Transient polarized radiative transfer analysis in a scattering medium by a discontinuous finite element method.

Transient (time-dependent) polarized radiative transfer in a scattering medium exposed to an external collimated beam illumination is conducted based on the time-dependent polarized radiative transfer theory. The transient term, which persists the nanosecond order time and cannot be ignored for the time-dependent radiative transfer problems induced by a short-pulsed beam, is considered as well as the polarization effect of the radiation. A discontinuous finite element method (DFEM) is developed for the transient vector radiative transfer problem and the derivation of the discrete form of the governing equation is presented. The correctness of the developed DFEM is first verified by comparing the DFEM solutions with the results from the literature. The DFEM is then applied to study the transient polarized radiative transfer induced by a pulsed beam. The time-dependent Stokes vector components are calculated, plotted and analyzed as functions of the axis coordinate and discrete direction. Effects of the diffuse/specular boundary and the incident beam polarization state with respect to the Stokes vector components are further analyzed for cases of different boundary reflection modes and incident beam illuminations.

Analysis of transient radiative transfer induced by an incident short-pulsed laser in a graded-index medium with Fresnel boundaries.

Transient radiative transfer induced by a short-pulsed laser in a one-dimensional graded-index medium is investigated by the discontinuous finite element method (DFEM). The boundaries of the medium are Fresnel reflectors, and the incident pulse is considered as the combination of the collimated and the diffuse parts after its first interaction with the medium. The correctness and accuracy of the DFEM solutions for time-resolved reflectance and transmittance are first validated by comparisons with the results obtained by the Monte Carlo method, and the DFEM is then employed to investigate the transient radiative transfer in a graded-index medium with Fresnel boundaries. Effects of the refractive index distributions, the pulse width, the optical thickness, and the scattering phase functions on the transient radiative signals are examined. Several meaningful trends on the time-resolved reflectance and transmittance are observed and analyzed.

Lattice Boltzmann model for a steady radiative transfer equation.

A complete lattice Boltzmann model (LBM) is proposed for the steady radiative transfer equation (RTE). The RTE can be regarded as a pure convection equation with a source term. To derive the expressions for the equilibrium distribution function and the relaxation time, an artificial isotropic diffusion term is introduced to form a convection-diffusion equation. When the dimensionless relaxation time has a value of 0.5, the lattice Boltzmann equation (LBE) is exactly applicable to the original steady RTE. We also perform a multiscale analysis based on the Chapman-Enskog expansion to recover the macroscopic RTE from the mesoscopic LBE. The D2Q9 model is used to solve the LBE, and the numerical results obtained by the LBM are comparable to the results obtained by other methods or analytical solutions, which demonstrates that the proposed model is highly accurate and stable in simulating multidimensional radiative transfer. In addition, we find that the convergence rate of the LBM depends on the transport properties of RTE: for diffusion-dominated RTE with a large optical thickness, the LBM shows a second-order convergence rate in space, while for convection-dominated RTE with a small optical thickness, a lower convergence rate is observed.

Vector radiative transfer in a multilayer medium by natural element method.

The vector radiative transfer problem in a vertically multilayer scattering medium with spatial changes in the index of refraction is solved by the natural element method (NEM). The top boundary of the multilayer medium is irradiated by a collimated beam. In our model, the angular space is discretized by the discrete ordinates approach, and the spatial discretization is conducted by the Galerkin weighted residuals approach. In the solution procedure, the collimated component for the Stokes parameters is first solved by NEM, and then it is embedded into the vector radiative transfer equation for the diffuse component as a source term. To keep the consistency of the directions in all the layers, angular interpolation of the Stokes parameters at the interfaces is adopted. The NEM approach for the collimated component is first validated. Then, the classical coupled atmosphere-water system irradiated by different states of collimated beam is examined to verify the numerical performance of the method. Numerical results show that the NEM is accurate, flexible, and effective in solving polarized radiative transfer in a multilayer medium. Finally, polarized radiative transfer in a four-layer system is investigated and analyzed.

Lattice Boltzmann model for Coulomb-driven flows in dielectric liquids.

In this paper, we developed a unified lattice Boltzmann model (LBM) to simulate electroconvection in a dielectric liquid induced by unipolar charge injection. Instead of solving the complex set of coupled Navier-Stokes equations, the charge conservation equation, and the Poisson equation of electric potential, three consistent lattice Boltzmann equations are formulated. Numerical results are presented for both strong and weak injection regimes, and different scenarios for the onset and evolution of instability, bifurcation, and chaos are tracked. All LBM results are found to be highly consistent with the analytical solutions and other numerical work.

Local radial basis function meshless scheme for vector radiative transfer in participating media with randomly oriented axisymmetric particles.

A local radial basis function meshless scheme (LRBFM) is developed to solve polarized radiative transfer in participating media containing randomly oriented axisymmetric particles in which radial basis functions augmented with polynomial basis are employed to construct the trial functions, and the vector radiative-transfer equation based on the discrete-ordinates approach is discretized directly by collocation method. The LRBFM belongs to a class of truly meshless methods that do not need any mesh or any numerical integration scheme. Performances of the LRBFM are verified with analytical solutions and other numerical results reported earlier in the literature via five various test cases. The predicted angular distribution of brightness temperature and Stokes vector by the LRBFM agree very well with the benchmark. It is demonstrated that the LRBFM is accurate to solve vector radiative transfer in participating media with randomly oriented axisymmetric particles.

Short-pulsed laser transport in two-dimensional scattering media by natural element method.

The natural element method (NEM) is extended to solve transient radiative transfer (TRT) in two-dimensional semitransparent media subjected to a collimated short laser irradiation. The least-squares (LS) weighted residuals approach is employed to spatially discretize the transient radiative heat transfer equation. First, for the case of the refractive index matched boundary, LSNEM solutions to TRT are validated by comparison with results reported in the literature. Effects of the incident angle on time-resolved signals of transmittance and reflectance are investigated. Afterward, the accuracy of this algorithm for the case of the refractive index mismatched boundary is studied. Finally, the LSNEM is extended to study the TRT in a two-dimensional semitransparent medium with refractive index discontinuity irradiated by the short pulse laser. The effects of scattering albedo, optical thickness, scattering phase function, and refractive index on transmittance and reflectance signals are investigated. Several interesting trends on the time-resolved signals are observed and analyzed.

Polarized radiative transfer in an arbitrary multilayer semitransparent medium.

Polarized radiative transfer in a multilayer system is an important problem and has wide applications in various fields. In this work, a Monte Carlo (MC) model is developed to simulate polarized radiative transfer in a semitransparent arbitrary multilayer medium with different refractive indices in each layer. Two kinds of polarization mechanisms are considered: scattering by particles and reflection and refraction at the Fresnel surfaces or interfaces. The MC method has an obvious superiority in that complex mathematical derivations can be avoided in solving the polarization by Fresnel reflection and refraction in an arbitrary multilayer system. We define the vector radiative transfer matrix (VRTM), which describes the polarization characteristics of radiative transfer, and obtain four elements of Stokes vector using the VRTM. The results for the two-layer model by MC method are compared against those for coupled atmosphere-ocean model by the discrete-ordinate method available in the literature, which validates the correctness of the MC multilayer model of polarized radiative transfer. Finally, the results for three-layer, five-layer, and ten-layer models are presented in graphical form. Results show that in the multilayer system, total reflections occurring at the surfaces/interfaces have significant effects on the polarized radiative transfer, which causes abrupt changes or fluctuations like waves in the curves of the Stokes vector.

Approximate solution to vector radiative transfer in gradient-index medium.

Within a gradient-index medium, the radiative rays propagate in curved paths, which makes polarized states change continuously and the solution to the radiative transfer be thus more complex and difficult. In this paper, an arbitrary multilayer model is developed to approximately simulate vector (polarized) radiative transfer in a gradient-index plane-parallel medium. The gradient-index medium is divided into an arbitrary number of sublayers, and each sublayer has a uniform refractive index and two virtual Fresnel's interfaces where only transmission (refraction) is considered. Thus the polarization caused by the curved propagation of lights is approximated by that resulting from refraction on the interfaces. Radiative transfer with consideration of polarization caused by particle scattering and refraction (reflection) on the interfaces (surfaces) in the multilayer model is solved by the MC method. The grid independence of results obtained by the multilayer model for vector radiative transfer in gradient-index medium shows that the convergent solution of Stokes vector will be achieved provided that the sublayer number is large enough. The results for apparent emissivity of gradient-index medium and Stokes vector for two-layer medium are compared well with those in published literatures. Finally, we investigate polarized behaviors of radiative transfer in Rayleigh scattering slabs with linear and sinusoidal gradient indexes and present angular distributions of Stokes vector. Results show that total reflection inside the gradient-index medium resulting from the curved paths traveled by the photons affects greatly the angular distribution characteristics of Stokes vector.

Associated predictors of therapeutic response to uvulopharyngopalatoplasty for severe obstructive sleep apnea hypopnea syndrome.

The aim of this study was to determine the associated factors affecting the outcome of uvulopharyngopalatoplasty (UPPP) in patients with severe obstructive sleep apnea hypopnea syndrome (OSAHS), and to investigate whether cephalometric measurements were predictive of the therapeutic response to UPPP in patients with severe OSAHS. We retrospectively studied 51 consecutive patients who underwent revised UPPP with uvula preservation (H-UPPP), or Z-palatopharyngoplasty (ZPPP) for severe OSAHS [apnea-hypopnea index (AHI) >30]. All patients were evaluated using physical examination, Epworth Sleepiness Scale (ESS), cephalometry, and nocturnal polysomnography (PSG) before surgery and at 6-12 months after surgery. Based on the success criteria defined as an AHI of <20 and a decrease >50 %, the overall success rate was 45.1 %. The preoperative distance from the posterior border of the uvula to the middle pharyngeal wall (U-MPW) was significantly longer in the responder group than in the nonresponder group, when considering the whole group or the H-UPPP group alone. Among all study subjects, U-MPW and change in body mass index (△BMI) were the significant predictors of surgical success. U-MPW was the key predictor for H-UPPP surgical success, whereas mandibular plane angle (MPA) and Friedman stage were the key predictors for ZPPP surgical success. In conclusion, U-MPW was a significant predictor of UPPP surgical success. Patients with U-MPW >10 mm who are unwilling to receive nasal continuous positive airway pressure (CPAP) therapy might be suitable candidates for UPPP surgery.