Energy spectra of developed superfluid turbulence

Turbulence spectra in superfluids are modified by the nonlinear energy dissipation caused by the mutual friction between quantized vortices and the normal component of the liquid. We have found a new state of fully developed turbulence, which occurs in some range of two Reynolds parameters characterizing the superfluid flow. This state displays both the Kolmogorov-Obukhov 5/3-scaling law E_k ∝ k^?5/3 and a new “3-scaling law” E_k ∝ k^?3, each in a well-separated range of k.     

Nonlinear responses of the periodic structure composed of single negative materials

We investigate the nonlinear responses of the one-dimensional periodic structure containing alternate single negative materials. The transmission property of this nonlinear periodic structure is studied and the bistable behavior is found. Similar as in the linear periodic structure, the transmission property of this nonlinear structure in the zero phase (zero-?) gap region is found to be relatively insensitive to the incident angle from 0° to 30°, in comparison with that in the Bragg gap. Especially, the transmissions associated with the zero-? gap reach the peaks at almost the same input intensity for different incident angles. Another merit of the transmission property in the zero-? gap is that a lower threshold is needed to achieve the bistability than in the Bragg gap. The electric field distribution in the system is further studied. We observe the zero-? gap soliton and compare it with the usual Bragg gap soliton. The differences between these two kinds of solitons are analyzed, and the zero-? gap soliton is shown to be insensitive to the incident angle. In the end, we find that the transmission and the soliton in the zero-? gap are also weakly dependent on the loss and the scaling in contrast to that in the Bragg gap.     

Properties suggesting H3+-type clusters in some metallic hydrides

The p-c-T relationships for the hydrogen-rich phase of some metallic hydrides are calculated supposing the formation of H₃⁺ -type clusters in the metal lattice in equilibrium with the hydrogen gas. The experimental data for the hydride phase of LaNi₅, Pd, Nb (H?Nb? 0.75) and U are consistent with this model. Other properties reported in literature which seem to support this model are discussed.     

A comparative study of turbulence models for non-premixed swirl-stabilized flames

ABSTRACT

The accuracy of turbulent swirl-stabilized flame simulation strongly depends on the choice of turbulence model. In this study, four 3D unsteady turbulence closures, including large eddy simulation, scale-adaptive simulation, and two detached eddy simulation variants, along with four RANS models, including RNG k??, SST k?ω, transition SST, and RSM, are examined for moderate- and high-swirl case studies. It is observed that the scale-adaptive simulation provides the most accurate results for almost all variables and both swirl conditions in the reactive flow. Only the 3D unsteady models predict the vortex breakdown bubble and flame attachment state correctly. However, based on our error analysis, the flow and composition fields predicted by the RANS models are in acceptable agreement with the experimental fields, especially the ones of transition SST when higher swirl number cases or minor species concentration are of interest. Moreover, it is concluded that the viscosity ratio criterion is a better measure of the local LES quality than the turbulent kinetic energy ratio, and the accuracy of a hybrid simulation may be much more dependent on the ability of the model to operate close to the RANS mode where the grid resolution is not sufficient for a resolving simulation than the fraction of the resolved kinetic energy. Finally, the propriety of the base (RANS) model of a DES for the application of interest is important, such that DES with realizable k?? outperforms the commonly used DES with SST k?ω model.     

Surface pressure characteristics of a highly loaded turbine blade at design and off-design conditions; a CFD methodology

The flow field passing through a highly loaded low pressure (LP) turbine cascade is numerically investigated at design and off-design conditions. The Field Operation And Manipulation (OpenFOAM) platform is used as the computational Fluid Dynamics (CFD) tool. In this regard, the influences of grid resolution on the results of k-ε, k-ω, and large-eddy simulation (LES) turbulence models are investigated and compared with those of experimental measurements. A numerical pressure undershoot is appeared near the end of blade pressure surface which is sensitive to grid resolution and flow turbulence modeling. The LES model is able to resolve separation on both coarse and fine grid resolutions. In addition, the off-design flow condition is modeled by negative and positive inflow incidence angles. The numerical experiments show that a separation bubble generated on blade pressure side is predicted by LES. The total pressure drop has also been calculated at incidence angles between -20° and +8°. The minimum total pressure drop is obtained by k-ω and LES at design point.     

3-D numerical analysis of EHD turbulent flow and mono-disperse charged particle transport and collection in a wire-plate ESP

The present study attempts to develop a detailed numerical approach and a simulation procedure to predict the motion of gas, ions and particles inside a simple parallel plate channel containing a single corona wire. A hybrid Finite Element (FEM)-Flux Corrected Transport (FCT)-Finite Volume (FVM) method is used: the FEM–FCT numerical algorithm is applied for modeling the steady-state corona discharge, while the turbulent gas flow and the particle motion under electrostatic forces are modeled using the commercial CFD code FLUENT. Calculations for the gas flow are carried out by solving the Reynolds-averaged Navier–Stokes equations and turbulence is modeled using the k–? turbulence model. An additional source term is added to the gas flow equation to include the effect of the electric field, obtained by solving a coupled system of the electric field and charge transport equations using User-Defined Functions (UDFs). The particle phase is simulated based on the Lagrangian approach, where a large number of particles is traced with their motion affected by the gas flow and electrostatic forces using the Discrete Phase Model (DPM) in FLUENT. The developed model is useful to gain insight into the particle collection phenomena that take place inside an ESP.     

Veneziano model for nucleon nucleon and nucleon antinucleon scattering. I

The thermal conductivity has been measured on two niobium samples of different purity (ratio of mean free path to coherence length 220 and 10, respectively), in the mixed state, with heat current parallel (k _∥) and normal (k _⊥) to the magnetic field. The experimental results are compared with theories; the range of validity is discussed. Maki's relation (k-k _n) ～ (H _c2 -H)^1/2 could be verified only for the purer sample in the field range (H _c2 -H)?140 Oe. The relationsk _∥/k ₁>1 forT?T _c andk _∥/k ₁<1 forT?T _c were found to be valid for the whole range of fields in the mixed state.     

Multi-scale k-distribution model for gas mixtures in hypersonic nonequilibrium flows

A k-distribution model is presented for gas mixtures in thermodynamic nonequilibrium, containing strongly radiating atomic species N and O together with molecular species of N₂, N₂⁺, NO and O₂. In the VUV range of the spectrum there is strong absorption of atomic radiation by bands of N₂. For this spectral range, a multi-scale model is presented, where RTEs are solved separately for each emitting species and overlap with other species is treated in an approximate way. Methodology for splitting the gas mixture into scales and evaluation of the overlap parameter between different scales is presented. The accuracy of the new model is demonstrated by solving the radiative transfer equation along the stagnation line flow field of the Crew Exploration Vehicle (CEV).     

Plasmon dispersion in Beryllium

We have measured the plasmon dispersion in polycrystalline Beryllium at wave vectors up tok=1.29 Å^?1 with electron energy loss spectroscopy. The plasmon energy is 18.8±0.1 eV, the fitted quadratic dispersion coefficient is α=0.36±0.03 for 0<k<1.29 Å^?1 and α _L =0.52+0.04 fork<0.55 Å^?1. We compare our results with the plasmon dispersion in other simple metals and point out discrepancies with the theories of the homogeneous electron gas.     

DYNAMIC RESPONSE OF WIND-EXCITED BUILDING USING CFD

Computational wind engineering as a new branch of computational fluid dynamics (CFD) has been developed recently to evaluate the interaction between wind and buildings numerically. In the present study, a systematic examination of wind effects on tall buildings and flow condition around buildings has been carried out using commercially available CFD software FLUENT 5. Both renormalization group (RNG) k-ε method and large eddy simulation (LES) with the Smagorinsky model are adopted as turbulence models and the results are compared with the wind-tunnel measurements. The weighted amplitude wave superposition (WAWS) method is used to generate atmospheric wind turbulence. The RNG k-ε method can predict the vortex shedding phenomenon well when compared with experiments for uniform flow input, but fails to predict the shedding frequency accurately for fluctuating incoming flow. On the other hand, the LES model shows reasonably good agreement with experiment in predicting vortex-shedding phenomenon for both uniform and fluctuating flows at inlet. Random-vibration based theory is employed for estimating r.m.s. response of tall buildings and the results compared well with the experimental results for a square building.     

Phenomenology of two-dimensional stably stratified turbulence under large-scale forcing

In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k^?3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k^?3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k^?11/5 form at large scales, to a steeper approximate k^?3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k^?1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k^?2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k^?1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux.     

Coupled fluid–structure solver: The case of shock wave impact on monolithic and composite material plates

An unstructured adaptive mesh flow solver, a finite element structure solver and a moving mesh algorithm were implemented in the numerical simulation of the interaction between a shock wave and a structure. In the past, this interaction is mostly considered as one-way in the sense that the shock causes a transient load on the structure while it is reflected uneffected by the impact. A fully coupled approach was implemented in the present work which can account for the effects associated with a mutual interaction. This approach included a compressible flow Eulerian solver of second order accuracy in finite volume formulation for the fluid and a Langargian solver in finite element formulation for the solid structure. A novel implementation of advancing front moving mesh algorithm was made possible with the introduction of a flexible and efficient quad-edge data structure. Adaptive mesh refinement was introduced into the flow solver for improved accuracy as well. Numerical results are further validated by theoretical analysis, experimental data and results from other numerical simulations. Grid dependency study was performed and results showed that the physical phenomena and quantities were independent of the numerical grid chosen in the simulations. The results illuminated complicated flow phenomena and structure vibration patterns, which in order to be detected experimentally require capabilities beyond those of the current experimental techniques. The numerical simulations also successfully modelled the aero-acoustic damping effects on the structure, which do not exist in previous numerical models. Further analysis of the results showed that the mutual interaction is not linear and that the non-linearity arises because the wave propagation in the fluid is not linear and it cascades a non-linear and non-uniform loading on the plate. Non-linearity intensifies when the plate is vibrating at high frequency while the wave propagation speed is low.     

Limit cycle in a bound exciton recombination model in non-equilibrium semiconductors

A non-equilibrium semiconductor model involving the processes of photogeneration of electron-hole pairs (e-h) (rate G), stimulated creation of excitons from e-h (rate constant C) and decay of excitons on recombination centres (rate constant k) is analyzed in this paper for steady states and limit cycle behaviour. Considering the exciton decay to be similar to enzymatic processes in chemical reactions obeying a Michaelis-Menten law, and choosing units such that k = 1 = N, where N is the concentration of recombination centres, the model represents a 2-parameter (C and G) 2-dimensional (exciton and electron-hole concentrations x, n) dynamical system with a unique steady state (x₀,n₀) which is unstable in the region (l ? G)³?4C, the equality sign corresponding to the bifurcation curve in parameter space. In the region (l ? G)³ > 4C the system displays a unique stable limit cycle which is obtained in analytical form by employing a two-time-scales method for parameters in the neighbourhood of the bifurcation curve. The limit cycles are tilted ellipses with angular frequency $\text{}\text{?}$gw of the order of 10⁶ s^?1. In a realistic semiconductor situation G$\text{\$}\text{?}$10^?3.     

Galvanomagnetic effects in graphite—II: The influence of trigonal warping of the constant energy surfaces on σxx(Bz)

The dispersion relationship for the electrons near the region of band overlap in graphite corresponds to the case where the constant energy surfaces are strongly warped in the k_x?k_y plane with three-fold symmetry (trigonal warping). The effects of this warping on the galvanomagnetic tensor component σ_xx(B_z) are examined. In the present calculation the trigonal warping is treated using a simplified mathematical model. Implications with respect to the calculation of the density of free carriers in graphite are discussed.     

Advection of a passive vector field by the Gaussian velocity field with finite correlations in time

Using the field theoretic renormalization group technique the model of a passive vector field advected by an incompressible turbulent flow is investigated up to the second order of the perturbation theory (two-loop approximation). The turbulent environment is given by statistical fluctuations of the velocity field that has a Gaussian distribution with zero mean and defined noise with finite correlations in time. Two-loop analysis of all possible scaling regimes in general d-dimensional space is done in the plane of exponents ? ? η, where ? characterizes the energy spectrum of the velocity field in the inertial range E ∝ k ^{1 ? 2ε}, and η is related to the correlation time at the wave number k which is scaled as k ^{?2 + η}. It is shown that the scaling regimes of the present model of vector advection have essentially different properties than the scaling regimes of the corresponding model of passively advected scalar quantity. The results demonstrate the fact that within the present model of passively advected vector field the internal tensor structure of the advected field can have nontrivial impact on the diffusion processes deep inside in the inertial interval of given turbulent flow.     

Critical indices for the charged Bose gas

A self-consistent version of the static random phase approximation leads to a quasi-particle energy satisfying ?(k)=Ak^v for small k, where v ≈ 1.8. The critical indices are those of an ideal Bose gas with this spectrum.     

Desynchronization and clustering with pulse stimulations of coupled electrochemical relaxation oscillators

Effects of pulse stimulations on the dynamics of relaxation oscillator populations were experimentally studied in a globally coupled electrochemical system. Similar to smooth oscillations, weakly and moderately relaxational oscillations possess a vulnerable phase, ?_S; pulses applied at ?_S resulted in desynchronization followed by a return to the synchronized state. In contrast to smooth oscillators, weakly and moderately relaxational oscillators exhibited transient and itinerant cluster dynamics, respectively. With strongly relaxational oscillators the pulse applied at a vulnerable phase effected transitions to other cluster configurations without effective desynchronization. Repeated pulse administration resulted in a cluster state that is stable against the perturbation; the cluster configuration is specific to the pulse administered at the vulnerable phase. The pulse-induced transient clusters are interpreted with a phase model that includes first and second harmonics in the interaction function and exhibits saddle type cluster states with strongly stable intra-cluster and weakly unstable inter-cluster modes.     

An implicit high-order spectral difference approach for large eddy simulation

The filtered fluid dynamic equations are discretized in space by a high-order spectral difference (SD) method coupled with large eddy simulation (LES) approach. The subgrid-scale stress tensor is modelled by the wall-adapting local eddy-viscosity model (WALE). We solve the unsteady equations by advancing in time using a second-order backward difference formulae (BDF2) scheme. The nonlinear algebraic system arising from the time discretization is solved with the nonlinear lower–upper symmetric Gauss–Seidel (LU-SGS) algorithm. In order to study the sensitivity of the method, first, the implicit solver is used to compute the two-dimensional (2D) laminar flow around a NACA0012 airfoil at Re = 5 × 10⁵ with zero angle of attack. Afterwards, the accuracy and the reliability of the solver are tested by solving the 2D “turbulent” flow around a square cylinder at Re = 10⁴ and Re =  2.2 × 10⁴. The results show a good agreement with the experimental data and the reference solutions.     

Deprotonation of Transient Guanosyl Cation Radical Catalyzed by Buffer in Aqueous Solution: TR-CIDNP Study

The reaction of deprotonation of the guanosyl cation radical formed in the photoinduced reaction of guanosine monophospate (GMP) with triplet 2,2??-dipyridyl-d₈ is studied in aqueous solution by time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP). In the course of the cyclic photoreaction, spin-polarized products are generated. Their polarization patterns that reflect the properties at the radical stage are analyzed using high-resolution nuclear magnetic resonance. The identification of transient radicals contributing to the polarization kinetics is based on its sensitivity to the degenerate electron exchange reaction of transient radicals with the parent diamagnetic molecules. Degenerate electron exchange is allowed only for the cation radical and manifests itself in the fast decay of the CIDNP signal in time with the rate of decay proportional to the concentration of parent GMP molecules. Because the formation of the neutral transient radical stops the exchange, the deprotonation changes the CIDNP kinetics from a decaying to a growing one. The rate constant of deprotonation, k _d, was obtained from modeling of CIDNP kinetics data with taking into consideration the difference of the CIDNP enhancement factors for neutral and cation guanosyl radicals. The value obtained at pH^* 5 for k _d?=?1?×?10⁶?s^?1 is consistent with the proton dissociation constant of the radical (pK _a?=?3.9). The linear dependence of the deprotonation rate on the buffer concentration is revealed for phosphate, formate, and acetate. Deprotonation is catalyzed by the buffer to a degree that depends on the difference in pK _a value of the buffer and the guanosyl cation radical in full accordance with Eigen??s model.