Momentum Diffusion Research Articles

Heavy quark diffusion in a Polyakov loop plasma

We calculate the transport coefficients, drag and momentum diffusion, of a heavy quark in a thermalized plasma of light quarks in the background of Polyakov loop. Quark thermal mass and the gluon Debye mass are calculated in a non-trivial Polyakov loop background. The constituent quark masses and the Polyakov loop is estimated within a Polyakov loop quark meson (PQM) model. The relevant scattering amplitudes for heavy quark and light partons in the background of Polyakov loop has been estimated within the matrix model. We have also compared the results with the Polyakov loop parameter estimated from lattice QCD simulations. We have studied the temperature and momentum dependence of heavy quark drag and diffusion coefficients. It is observed that the temperature dependence of the drag coefficient is quite weak which may play a key role to understand heavy quark observables at RHIC and LHC energies.

Aircraft Observations of Tropical Cyclone Boundary Layer Turbulence over the South China Sea

Abstract There have been no high-frequency aircraft observations of tropical cyclone (TC) eyewall boundary layer turbulence since two flights into Atlantic hurricanes in the 1980s. We present an analysis of the first TC boundary layer flight observations in the South China Sea by the Hong Kong Observatory comprising four eyewall penetrations. We derive the vertical flux of momentum and vertical momentum diffusivity from observed turbulence parameters. We observe negative (upward) vertical fluxes of tangential momentum near the eyewall consistent with a jet below the flight level near the radius of maximum wind. Our observations of vertical momentum diffusivity support a superlinear relationship between diffusivity and wind speed at the high wind speeds in the inner-core of TCs (power-law exponent of 1.73 ± 0.20) while the few existing boundary layer hurricane observations in the North Atlantic suggest a more linear relationship.

Thermal analysis for hybrid nanofluid past a cylinder exposed to magnetic field

This work has developed mathematical models for thermal transport by treating Al2O3 as nanoparticles of a single type and Al2O3 and Cu as hybrid nanoparticles in a hyperbolic tangent fluid. The solution for the developed mathematical models is computed by FEM in order to compare the thermal performances of the nanofluid and hybrid nanofluid. The convergence, error, and mesh-free analyses are carried out to get physically realistic solutions so that useful information about the underlying thermal physics is extracted. Numerical experiments revealed that the momentum of stretching of the cylinder diffuses faster in a nanofluid than in a hybrid nanofluid. The heat generation rate in the hybrid nanofluid is higher than that in a nanofluid. Simulated results have also revealed that the thermal performance of the hybrid nanofluid is better than that of the nanofluid. Therefore, dispersing hybrid nanoparticles (combination of Cu and Al2O3) in a hyperbolic tangent fluid is recommended for efficient working fluids. Surprisingly, the wall shear stress for the hybrid nanofluid is higher than that of the nanofluid. Numerical data extracted from numerical experiments revealed that the wall heat transfer rate for a hybrid nanofluid is higher than that of the nanofluid. It is also observed that the rate of generation of heat in the hybrid nanofluid is greater than the rate of generation of heat in a nanofluid, which is a drawback of the hybrid nanofluid when it is treated as a coolant. The diffusion of the wall momentum in hybrid nanofluids is less than that in nanofluids. The hybrid nanofluid is a more efficient working fluid because of its high thermal performance when compared with the nanofluid. The intensity of the magnetic field causes a significant reduction in the flow and has a remarkable impact on the momentum boundary layer thickness.

Gyrofluid modeling of magnetosphere in feedback instability

Feedback instability occurs in a coupling system of the magnetosphere and the ionosphere, and is a theoretical model explaining spontaneous development of the quiet aurora. In this study, we extend a model of the magnetosphere in the feedback instability to the gyrofluid model. This extension makes it possible to properly discuss kinetic effects, such as the finite Larmor radius effect, the Landau damping, and the mirror force, on the feedback instability in a framework of a fluid model. The derived model is applied to linear stability analysis of the low-frequency feedback instability in the absence of the mirror force. An analytical expression of the parallel electron momentum diffusion coefficient associated with the electron Landau damping is obtained by considering a Landau-fluid closure model. It is found that high-wavenumber modes are strongly affected by the finite ion Larmor radius effect and the electron Landau damping. In particular, a stabilizing effect of the electron Landau damping is dominant and stronger for high-parallel-wavenumber modes. This result indicates that the lowest-parallel-wavenumber mode primarily grows during the spontaneous development of the quiet aurora.

Anomalous decay of coherence in a dissipative many-body system

Decoherence is ubiquitous in quantum physics, from the conceptual foundations to quantum information processing or quantum technologies, where it is a threat that must be countered. While decoherence has been extensively studied for simple, well-isolated systems such as single atoms or ions, much less is known for many-body systems where inter-particle correlations and interactions can drastically alter the dissipative dynamics. Here we report an experimental study of how spontaneous emission destroys the spatial coherence of a gas of strongly interacting bosons in an optical lattice. Instead of the standard momentum diffusion expected for independent atoms, we observe an anomalous sub-diffusive expansion, associated with a universal slowing down $\propto 1/t^{1/2}$ of the decoherence dynamics. This algebraic decay reflects the emergence of slowly-relaxing many-body states, akin to sub-radiant states of many excited emitters. These results, supported by theoretical predictions, provide an important benchmark in the understanding of open many-body systems.

Pollutant vertical mixing in the nocturnal boundary layer enhanced by density currents and low-level jets: two representative case studies

Turbulent and intermittent characteristics of the nocturnal boundary layer play an important role in determining the transport, diffusion, and storage of momentum, heat, and atmospheric pollutants. Here we use micrometeorological observations and numerical simulations to analyze two coherent turbulent structures—a density current and a low-level jet (LLJ)—that enhance mixing and intermittent turbulence in the nocturnal boundary layer. We analyze in detail their impact on the distribution of simulated pollutant concentrations. The Weather Research and Forecast (WRF) model is coupled with the Community Multiscale Air Quality photochemical model to simulate the dispersion of pollutants for the density-current and LLJ cases. The terms of the pollutant-concentration budget are then analyzed to quantify the contribution of different atmospheric processes on the net $$\hbox {NO}_2$$ and $$\hbox {O}_3$$ concentrations. The results show that the WRF model reproduces successfully the essential characteristics observed in the density-current and LLJ cases. The density-current simulation shows turbulent-mixing enhancement in the frontal zone of the density current, leading to significant vertical pollutant diffusion. Results also indicate that the LLJ is able to transport pollutants from the residual layer to the surface layer due to vertical diffusion. In both cases, the main mechanism responsible for pollutant transport is the horizontal advection.

Turbulent Schmidt Number Measurements Over Three-Dimensional Cubic Arrays

We present turbulent Schmidt number Sc_t estimations above three-dimensional urban canopies, where Sc_t is a property of the flow defined as the ratio of the eddy diffusivity of momentum (K_M) to the eddy diffusivity of mass (D_t). Despite the fact that Sc_t modelling is of great interest, inter alia, for pollutant dispersion simulations conducted via computational fluid dynamics, no universal value is known. Simultaneous measurements of fluid velocity and mass concentration are carried out in a water channel for three staggered arrays of cubical obstacles corresponding to isolated flow, wake-interference, and skimming-flow regimes. A passive tracer is released from a continuous point source located at a height z=1.67H, where H is the obstacle height. The results show an increase of Sc_t with height above the canopy for all three arrays, with values at z=2H (Sc_t = 0.6) about double compared to that at z=H. The observed Sc_t agrees well with that modelled by using a simple formulation for Sc_t based on expressions for K_M and D_t published in previous studies. Comparisons with other Sc_t models found in the literature are also presented and discussed.

Open quantum dynamics induced by light scalar fields

We consider the impact of a weakly coupled environment comprising a light scalar field on the open dynamics of a quantum probe field, resulting in a master equation for the probe field that features corrections to the coherent dynamics, as well as decoherence and momentum diffusion. The light scalar is assumed to couple to matter either through a nonminimal coupling to gravity or, equivalently, through a Higgs portal. Motivated by applications to experiments such as atom interferometry, we assume that the probe field can be initialized, by means of external driving, in a state that is not an eigenstate of the light scalar-field--probe system, and we derive the master equation for single-particle matrix elements of the reduced density operator of a toy model. We comment on the possibilities for experimental detection and the related challenges, and highlight possible pathways for further improvements. This derivation of the master equation requires techniques of non-equilibrium quantum field theory, including the Feynman-Vernon influence functional and thermo field dynamics, used to motivate a method of Lehmann-Symanzik-Zimmermann-like reduction. In order to obtain cutoff-independent results for the probe-field dynamics, we find that it is necessary to use a time-dependent renormalization procedure. Specifically, we show that non-Markovian effects following a quench, namely the violation of time-translational invariance due to finite-time effects, lead to a time-dependent modulation of the usual vacuum counterterms.

Heavy quark dynamics in a hot magnetized QCD medium

The heavy quark drag and momentum diffusion have been investigated in a hot magnetized quark-gluon plasma, along the directions parallel and perpendicular to the magnetic field. The analysis is done within the framework of Fokker-Planck dynamics by considering the heavy quark scattering with thermal quarks and gluons at the leading order in the coupling constant. An extended quasiparticle model is adopted to encode the thermal QCD medium interactions in the presence of a magnetic field. Further, the higher Landau level effects on the temperature behaviour of the parallel and perpendicular components of the drag force and diffusion coefficients have studied. It has been observed that both the equation of state and the magnetic field play key roles in the temperature dependence of the heavy quark dynamics.

Dispersion relations in non-relativistic two-dimensional materials from quasinormal modes in H\u01d2rava Gravity

We compute dispersion relations of non-hydrodynamic and hydrodynamic modes in a non-relativistic strongly coupled two-dimensional quantum field theory. This is achieved by numerically computing quasinormal modes (QNMs) of a particular analytically known black brane solution to 3+1-dimensional Hǒrava Gravity. Hǒrava Gravity is distinguished from Einstein Gravity by the presence of a scalar field, termed the khronon, defining a preferred time-foliation. Surprisingly, for this black brane solution, the khronon fluctuation numerically decouples from all others, having its own set of purely imaginary eigenfrequencies, for which we provide an analytic expression. All other Hǒrava Gravity QNMs are expressed analytically in terms of QNMs of Einstein Gravity, in units involving the khronon coupling constants and various horizons. Our numerical computation reproduces the analytically known momentum diffusion mode, and extends the analytic expression for the sound modes to a wide range of khronon coupling values. In the eikonal limit (large momentum limit), the analytically known dispersion of QNM frequencies with the momentum is reproduced by our numerics. We provide a parametrization of all QNM frequencies to fourth order in the momentum. We demonstrate perturbative stability in a wide range of coupling constants and momenta.

Transport coefficients from in-medium quarkonium dynamics

The in medium dynamics of heavy particles are governed by transport coefficients. The heavy quark momentum diffusion coefficient, $\kappa$, is an object of special interest in the literature, but one which has proven notoriously difficult to estimate, despite the fact that it has been computed by weak-coupling methods at next-to-leading order accuracy, and by lattice simulations of the pure SU(3) gauge theory. Another coefficient, $\gamma$, has been recently identified. It can be understood as the dispersive counterpart of $\kappa$. Little is known about $\gamma$. Both $\kappa$ and $\gamma$ are, however, of foremost importance in heavy quarkonium physics as they entirely determine the in and out of equilibrium dynamics of quarkonium in a medium, if the evolution of the density matrix is Markovian, and the motion, quantum Brownian; the medium could be a strongly or weakly coupled plasma. In this paper, using the relation between $\kappa$, $\gamma$ and the quarkonium in medium width and mass shift respectively, we evaluate the two coefficients from existing 2+1 flavor lattice QCD data. The resulting range for $\kappa$ is consistent with earlier determinations, the one for $\gamma$ is the first non-perturbative determination of this quantity.

Strongly coupled N=4 supersymmetric Yang-Mills plasma on the Coulomb branch. II. Transport coefficients and hard probe parameters

We study $\mathcal{N} = 4$ super Yang-Mills theory on the Coulomb branch (cSYM) by using its Type IIB supergravity dual. We compute the transport coefficients, and hard probe parameters of $\mathcal{N} = 4$ cSYM at finite temperature $T$. We use the rotating black 3-brane solution of Type IIB supergravity with a single non-zero rotation parameter $r_{0}$ after analytically continuing $r_{0}\rightarrow -ir_{0}$, and in an ensemble where the Hawking temperature $T$ and a scalar condensate $<\mathcal{O}>\sim r_{0}^4$ are held fixed. We find that the bulk viscosity to entropy density ratio of the large black hole branch decreases with temperature and has a maxima around the critical temperature $T_{c}$ while, for the small black hole branch, it increases with temperature. The other transport coefficients and parameters of hard probes, such as the conductivity, jet quenching parameter, drag force, and momentum diffusion coefficients of the large black hole branch increase with temperature and asymptote to their conformal value while, for the small black hole branch, they decrease with temperature.

Color-coded visualization of microbubble clouds interacting with eddies in a spatially developing turbulent boundary layer

Three-dimensional distribution of microbubbles dispersed in a turbulent boundary layer spatially developing along a flat plate is investigated experimentally. The aim of study is to find out visually how microbubbles interact with turbulent eddies to achieve frictional drag reduction on a wall. Setting conditions of the experiments are stream Reynolds number up to 2.3 × 104, microbubble volume fraction inside boundary layer less than 10−5, and microbubble Stokes number smaller than 10−3. Stream-wise vortices in the boundary layer were visualized by flake optics, of which spacing expands with injection of microbubbles. 3-D microbubble distributions were visualized by a color-coded volumetric illumination. The result showed preferentially accumulated microbubble clouds to low-speed streaks close to viscous sublayer and to hair-pin vortices in the buffer layer, leading to reduced diffusivity of microbubbles relative to the turbulent momentum diffusivity.

An experimental study on a coaxial flow with inner swirl: Vortex evolution and flow field mixing attributes

Abstract A 2D particle image velocimetry study of a coaxial flow with inner swirl is presented. An inner swirling jet, produced by tangential injection, interacts with an annular flow generating a recirculating flow field with strong mixing attributes. The characteristics of the cross-plane velocity components of four different test cases are presented (two levels of tangential injection flow rate combined with two levels of annular flow rate) in order to study the mean and turbulent attributes of the swirling vortex. The main features of this complex flow field, which can be considered as the interaction of a typical swirling jet undergoing “vortex breakdown” with an outer annular flow with “backward facing step flow” characteristics, are investigated, focusing on the swirling jet's characteristics. The analysis of the mean and turbulent flow is based on a modified Rossby number, previously proposed by the authors, defined as the ratio of the streamwise velocity jump across the two streams over a typical tangential velocity, which is shown to represent the ratio of the pressure difference due to the streamwise velocity difference and the entrainment of the two flows to that due to the rotation of the swirling vortex. The angular momentum diffusion downstream is evaluated, to assess the mixing between the swirling vortex and the outer flow.

Quantum enhanced estimation of diffusion

Momentum diffusion is a possible mechanism for driving macroscopic quantum systems towards classical behaviour. Experimental tests of this hypothesis rely on a precise estimation of the strength of this diffusion. We show that quantum-mechanical squeezing offers significant improvements, including when measuring position. For instance, with 10dB of mechanical squeezing, experiments would require a tenth of proposed free-fall times. Momentum measurement is better by an additional factor of three, while another quadrature is close to optimal. These have particular implications for the space-based MAQRO proposal -- where it could rule out the spontaneous collapse theory due to Ghirardi, Rimini, and Weber -- as well as terrestrial optomechanical sensing.

Investigation of enhancement of heat transfer in Sutterby nanofluid using Koo–Kleinstreuer and Li (KKL) correlations and Cattaneo–Christov heat flux model

Generalized conservation laws are used to model an enhancement of heat transfer in Sutterby nanofluid over a moving surface subjected to heat flux. The mathematical problems are solved by the finite element method. The code is developed and is validated by comparing the present results with already published benchmarks which are the special case of the present study. The computed solutions are used to study the influence of physical parameters on the momentum diffusion and heat transfer. The obtained behaviors of flow fields are displayed in both numerical and graphical forms Magnetic field slows down the diffusion of the wall momentum and increases the rate of diffusion of heat from the wall into the flow regime. Joule heating phenomenon has a significant influence on the temperature.

Impact of temperature dependent diffusion coefficients on heat and mass transport in viscoelastic liquid using generalized Fourier theory

This article considers the behavior of diffusion coefficients (varying with respect to the fluid temperature) by using Cattaneo–Christov (generalized Fourier) models based on the first law of thermodynamics and conservation of diffusion of specie. Mathematical models are developed and the finite element method is used for computational purposes. The computer programme is tested and validated. An excellent agreement between published results is observed. The results obtained from numerical experiments are displayed graphically and the numerical values of momentum, temperature and concentration gradients versus emerging physical parameters are tabulated. The elasticity of the fluid is responsible for restoration of the deformation of viscoelastic liquid for the cases of constant viscosity and temperature dependent viscosity. The rate of diffusion of wall momentum slows down as the elasticity of the fluid (through the Deborah number) is increased. The qualitative and quantitative behavior of constant diffusion coefficients are noted. The diffusion of momentum in viscoelastic liquid is compared to the viscoelastic liquid of diffusion coefficients. As an increase in the ratio of momentum diffusion to thermal diffusion reduces the rate of diffusion of momentum of wall into the viscoelastic liquid, it is noted that wall temperature diffuses into viscoelastic liquid of constant viscosity faster than the diffusion of wall temperature into viscoelastic liquid of temperature dependent diffusion coefficients. It is also observed that concentration of specie in viscoelastic liquid of constant viscosity is less than the concentration in liquid of temperature dependent viscosity.

Full O(a) improvement in electrostatic QCD

EQCD is a 3D bosonic theory containing SU(3) and an adjoint scalar, which efficiently describes the infrared, nonperturbative sector of hot QCD and which is highly amenable to lattice study. We improve the matching between lattice and continuum EQCD by determining the final unknown coefficient in the O(a) matching, an additive scalar mass renormalization. We do this numerically by using the symmetry-breaking phase transition point of the theory as a line of constant physics. This prepares the ground for a precision study of the transverse momentum diffusion coefficient C(qperp) within this theory. As a byproduct, we provide an updated version of the EQCD phase diagram.

Direct Numerical Simulation of a buoyant triple jet at low-Prandtl number

Mixing of buoyant streams is a phenomenon of relevance in many practical cases like pollutant emission in the atmosphere, discharges from marine outfalls and cooling of fuel rods in nuclear reactors to name a few. A canonical configuration for this class of flows consists in three buoyant jets at different temperatures vertically entering a pool from the bottom. This work reports a Direct Numerical Simulation study performed on the triple jet configuration. The Reynolds number based on the average jet centerline velocity and jet width is set to Re=5000 and mixed convection regime is established at a Richardson number, Ri=0.25. In order to represent flows occurring inside liquid metal fast reactors, the selected Prandtl number is Pr=0.031.Statistics computed show that in the first stages of development, the three jets undergo a strong interaction. In that same region the shedding of large-scale vortices is originated accompanied by low-frequency undulations. Further from the inlet, the three jets are observed to coalesce in a single, isothermal stream. The analysis of momentum fluxes clarifies the mutual entrainment mechanism behind coalescence, which is commonly known as Coandă effect. At distances larger than ten times the jet width the self-similar characteristics of single and isothermal planar jets are recovered. The flow configuration presented includes several peculiar features, namely buoyancy effects at low Prandtl number, interaction between jets and the presence of multiple shear layers. This leads to an irregular behaviour of the turbulent diffusivity of momentum and heat as well as the misalignment between the temperature gradient and turbulent heat flux. Therefore the flow can be considered very complex and might constitute a demanding test bench for the development and validation of turbulence models.

Classical model for diffusion and thermalization of heavy quarks in a hot medium: memory and out-of-equilibrium effects * * The work of M. R. and S. K. D. is supported by the National Science Foundation of China (11805087, 11875153) and by the Fundamental Research Funds for the Central Universities (862946)

We consider a simple model for the diffusion of heavy quarks in a hot bath, modeling the latter by an ensemble of oscillators distributed according to either a thermal distribution or to an out-of-equilibrium distribution with a saturation scale. In this model it is easy to introduce memory effects by changing the distribution of oscillators: we model them by introducing a Gaussian distribution, , which can be deformed continuously from a function, giving a Markov dissipation, to a broad kernel with memory. Deriving the equation of motion of the heavy quark in the bath, we remark how dissipation comes out naturally as an effect of the back-reaction of the oscillators on the bath. Moreover, the exact solution of this equation allows to define the thermalization time as the time necessary to remove any memory of the initial conditions. We find that the broadening of the dissipative kernel, while keeping the coupling fixed, lowers the thermalization time. We also derive the fluctuation-dissipation theorem for the bath, and use it to estimate the kinematic regime in which momentum diffusion of the heavy quark dominates over drift. We find that diffusion is more important as long as is small, where and denote the initial energy of the heavy quark and the average energy of the bath, respectively.