Momentum Transfer Mechanism Research Articles

Generation of ultrarelativistic vortex leptons with large orbital angular momenta

We put forward a novel method for generating ultrarelativistic vortex positrons and electrons with intrinsic orbital angular momenta (OAM) through nonlinear Breit-Wheeler (NBW) scattering of vortex γ photons. A complete angular momentum-resolved scattering theory has been formulated, introducing angular momenta of laser photons and vortex particles into the conventional NBW scattering framework. We find that vortex positron (electron) can be produced when the outgoing electron (positron) is generated along the collision axis. By unveiling the angular momentum transfer mechanism, we clarify that the OAM of the γ photon and the large angular momenta due to multiple laser photons are entirely transferred to the generated vortex leptons. Furthermore, the cone opening angle and superposition state of the vortex γ photon can be determined via the angular distribution of created pairs in NBW processes. Published by the American Physical Society 2024

Photometric White Dwarf Rotation

We present a census of photometrically detected rotation periods for white dwarf (WD) stars. We analyzed the light curves of 9285 WD stars observed by the Transiting Exoplanet Survey Satellite up to Sector 69. Using Fourier transform analyses and the TESS_localize software, we detected variability periods for 318 WD stars. The 115 high-probability likely single WDs in our sample have a median rotational period of 3.9 hr and a median absolute deviation of 3.5 hr. Our distribution is significantly different from the distribution of the rotational period from asteroseismology, which exhibits a longer median period of 24.2 hr and a median absolute deviation of 12.1 hr. In addition, we reported nonpulsating periods for three known pulsating WDs with rotational periods previously determined by asteroseismology: NGC 1501, TIC 7675859, and G226-29. We also calculated evolutionary models including six angular momentum transfer mechanisms from the literature throughout evolution in an attempt to reproduce our findings. Our models indicate that the temperature–period relation of most observational data is best fitted by models with low metallicity, probably indicating problems with the computations of angular momentum loss during the high-mass-loss phase. Our models also generate internal magnetic fields through the Tayler–Spruit dynamo.

Large eddy simulations of flow over additively manufactured surfaces: Impact of roughness and skewness on turbulent heat transfer

Additive manufacturing creates surfaces with random roughness, impacting heat transfer and pressure loss differently than traditional sand–grain roughness. Further research is needed to understand these effects. We conducted high-fidelity heat transfer simulations over three-dimensional additive manufactured surfaces with varying roughness heights and skewness. Based on an additive manufactured Inconel 939 sample from Siemens Energy, we created six surfaces with different normalized roughness heights, Ra/D=0.001,0.006,0.012,0.015,0.020, and 0.028, and a fixed skewness, sk=0.424. Each surface was also flipped to obtain negatively skewed counterparts (sk=−0.424). Simulations were conducted at a constant Reynolds number of 8000 and with temperature treated as a passive scalar (Prandtl number of 0.71). We analyzed temperature, velocity profiles, and heat fluxes to understand the impact of roughness height and skewness on heat and momentum transfer. The inner-scaled mean temperature profiles are of larger magnitude than the mean velocity profiles both inside and outside the roughness layer. This means, the temperature wall roughness function, ΔΘ+, differs from the momentum wall roughness function, ΔU+. Surfaces with positive and negative skewness yielded different estimates of equivalent sand–grain roughness for the same Ra/D values, suggesting a strong influence of slope and skewness on the relationship between roughness function and equivalent sand–grain roughness. Analysis of the heat and momentum transfer mechanisms indicated an increased effective Prandtl number within the rough surface in which the momentum diffusivity is larger than the corresponding thermal diffusivity due to the combined effects of turbulence and dispersion. Results consistently indicated improved heat transfer with increasing roughness height and positively skewed surfaces performing better beyond a certain roughness threshold than negatively skewed ones.

Real-time dynamics of angular momentum transfer from spin to acoustic chiral phonon in oxide heterostructures.

Chiral phonons have recently been explored as a novel degree of freedom in quantum materials. The angular momentum carried by these quasiparticles is generated by the breaking of chiral degeneracy of phonons, owing to the chiral lattice structure or the rotational motion of ions of the material. In ferromagnets, a mechanism for generating non-equilibrium chiral phonons has been suggested, but their temporal evolution, which obeys Bose-Einstein statistics, remains unclear. Here we report the real-time dynamics of thermalized chiral phonons in an artificial superlattice composed of ferromagnetic metallic SrRuO3 and non-magnetic insulating SrTiO3. Following the photo-induced ultrafast demagnetization in the SrRuO3 layer, we observed the appearance of a magneto-optic signal in the superlattice, which is absent in the SrRuO3 single films. This magneto-optic signal exhibits thermally driven dynamic properties and a clear correlation with the thickness of the non-magnetic SrTiO3 layer, implying that it originates from thermalized chiral phonons. We use numerical calculations considering the magneto-elastic coupling in SrRuO3 to validate our experimental observations and the angular momentum transfer mechanism between the lattice and spin systems in ferromagnetic systems and also to the non-magnetic system.

Real-time observation of non-equilibrium phonon-electron energy and angular momentum flow in laser-heated nickel.

Identifying the microscopic nature of non-equilibrium energy transfer mechanisms among electronic, spin, and lattice degrees of freedom is central to understanding ultrafast phenomena such as manipulating magnetism on the femtosecond timescale. Here, we use time- and angle-resolved photoemission spectroscopy to go beyond the often-used ensemble-averaged view of non-equilibrium dynamics in terms of quasiparticle temperature evolutions. We show for ferromagnetic Ni that the non-equilibrium electron and spin dynamics display pronounced variations with electron momentum, whereas the magnetic exchange interaction remains isotropic. This highlights the influence of lattice-mediated scattering processes and opens a pathway toward unraveling the still elusive microscopic mechanism of spin-lattice angular momentum transfer.

Heat transfer enhancement with additively manufactured rough surfaces: Insights from large-eddy simulations

To study the impact of additively manufactured (AM) roughness on fluid flow and heat transfer, we performed a series of high-fidelity large-eddy simulations on turbulent heat transfer over a three-dimensional AM rough surface with varying bulk Reynolds number and average roughness height values. We considered rough surfaces created using AM techniques at Siemens based on Nickel Alloy IN939 material with four different mean roughness heights, ks= 1.594, 1.992, 2.630, and 3.984 mm, and the simulations were performed at five bulk Reynolds numbers of 1000, 3000, 6000, 11 700, and 18 000. The temperature was treated as a passive scalar with a Prandtl number of 0.71. To better understand the effect of wall roughness on the momentum and heat transfer mechanism, mean temperature and velocity profiles as well as heat fluxes are presented. The wall-normal Reynolds stress, ⟨ux′ur′⟩, and heat flux, ⟨ur′Θ′⟩, decrease for larger wall roughness heights, Ra, and their respective magnitudes remain very similar for different Ra. A similarity rule for friction factor and heat transfer is used to correlate and interpret the numerical results and compare them with previously existing results, both theoretical and experimental. The assessment of the thermal performance factor illuminates the improvement in heat transfer with the existing surface roughness. By studying the probability density functions of the instantaneous Stanton number, the recirculation zones, which are the result of an adverse pressure gradient, were found to have a profound effect on heat transfer. This is important as it leads to the wall-scaled mean temperature profiles being of larger magnitude than the mean velocity profiles both inside and outside the roughness layer. This means that the temperature wall roughness function, ΔΘ+, differs from the momentum wall roughness function, ΔU+.

Vortex γ photon generation via spin-to-orbital angular momentum transfer in nonlinear Compton scattering

Vortex γ photons with intrinsic orbital angular momenta possess a wealth of applications in various fields—e.g., strong-laser physics, nuclear physics, particle physics, and astrophysics—yet their generation remains unsettled. In this work, we investigate the generation of vortex γ photons via nonlinear Compton scattering of ultrarelativistic electrons in a circularly polarized laser pulse. We develop a quantum electrodynamics scattering theory that explicitly addresses the multiphoton absorption and the angular momentum transfer mechanism. In pulsed laser fields, we unveil the vortex phase structure of the scattering matrix element, discuss how the vortex phase could be transferred to the radiated photon, and derive the radiation rate of the vortex γ photon. We numerically examine the energy spectra and beam characteristics of the radiation, while also investigating the influence of finite laser pulses on the angular momentum and energy distribution of the emitted vortex γ photons. Published by the American Physical Society 2024

Solving the Hydrodynamical System of Equations of Inhomogeneous Fluid Flows with Thermal Diffusion: A Review

The present review analyzes classes of exact solutions for the convection and thermal diffusion equations in the Boussinesq approximation. The exact integration of the Oberbeck–Boussinesq equations for convection and thermal diffusion is more difficult than for the Navier–Stokes equations. It has been shown that the exact integration of the thermal diffusion equations is carried out in the Lin–Sidorov–Aristov class. This class of exact solutions is a generalization of the Ostroumov–Birikh family of exact solutions. The use of the class of exact solutions by Lin–Sidorov–Aristov makes it possible to take into account not only the inhomogeneity of the pressure field, the temperature field and the concentration field, but also the inhomogeneous velocity field. The present review shows that there is a class of exact solutions for describing the flows of incompressible fluids, taking into account the Soret and Dufour cross effects. Accurate solutions are important for modeling and simulating natural, technical and technological processes. They make it possible to find new physical mechanisms of momentum transfer for the design of new types of equipment.

The Momentum Transfer Mechanism of a Landslide Intruding a Body of Water

Landslide-generated waves occur as a result of the intrusion of landslides such as mud flows and debris flows into bodies of water such as lakes and reservoirs. The objective of this study was to determine how the momentum is transferred from the sliding mass to the body of water on the basis of theoretical analysis and physical model experiments. Considering the viscoplastic idealization of natural landslides, the theoretical model was established based on the momentum and mass conservation of a two-phase flow in a control volume. To close the theoretical equations, slide thickness and velocity passing through the left boundary of the control volume were estimated by lubrication theory, and the interaction forces between the slide phase and water phase, including hydrostatic force and drag force, were given by semiempirical equations fitted with experimental data obtained using the particle image velocimetry (PIV) technique. The near-field velocity fields of both the sliding mass and the body of water, as well as the air–water–slide interfaces, were determined from the experiments. The theoretical model was validated by comparing the theoretical and experimental data of the slide thickness and slide velocity, as well as the momentum variations of the two phases in the control volume.

Linear and nonlinear frequency-domain modelling of oscillatory flow over submerged canopies

An analytical and experimental study of flow velocities within submerged canopies of rigid cylinders under oscillatory flows is presented, providing insights into the momentum transfer mechanisms between the different flow harmonics. The experimental dataset covers an unprecedented wide range of flow amplitudes with in-canopy velocity reductions ranging between 0.2 and 0.8 of the free stream velocity (from inertia- to drag-dominated in-canopy flow). Results from the analytical model with nonlinear drag compare favourably to the experimental data. Having application of theories for free surface waves over canopies in mind, the effects of linearization of the drag are analysed by comparing sinusoidal and nonlinear model predictions. Finally, a unified prediction formula for in-canopy velocities for sinusoidal, velocity-skewed, and velocity-asymmetric free stream velocities is presented. The formula depends on two non-dimensional parameters related to inertia and drag forces, and the unified formula allows for easy assessment of the maximum in-canopy velocity.

Nonlinearity-Induced Optical Torque.

Optically induced mechanical torque driving rotation of small objects requires the presence of absorption or breaking cylindrical symmetry of a scatterer. A spherical nonabsorbing particle cannot rotate due to the conservation of the angular momentum of light upon scattering. Here, we suggest a novel physical mechanism for the angular momentum transfer to nonabsorbing particles via nonlinear light scattering. The breaking of symmetry occurs at the microscopic level manifested in nonlinear negative optical torque due to the excitation of resonant states at the harmonic frequency with higher projection of angular momentum. The proposed physical mechanism can be verified with resonant dielectric nanostructures, and we suggest some specific realizations.

Localized shear as a quasi-plastic mechanism of momentum transfer in liquids

Two types of dispersion relations (DRs) in condensed media and associated momentum transfer mechanisms are discussed: gapless DRs corresponding to acoustic modes and DRs with energy (or frequency) gap. Studying the viscoelastic effects in condensed media based on the generalization of the Maxwell-Frenkel approach, the emphasis was placed on the gap states in the momentum transfer mechanisms (GMS), when the k-gap was accompanied by qualitative changes in the DRs type. In this paper, the gap states in the mechanisms of momentum transfer in liquids are associated with the effects of shear elasticity and the formation of collective modes of quasi-plastic shears under conditions of the established type of critical phenomena in the ensembles of shear defects — structural-scaling transitions. It has been found that the formation of collective shear modes, having the character of self-similar solutions of the autosoliton type, accounts for the qualitative change in the dispersion properties corresponding to the quasi-plastic momentum transfer mechanisms operating in the characteristic range of loading times. The results of comparison with the data of original experiments confirm the initiation of quasi-plastic mechanisms of momentum transfer in liquids during the formation of collective shear modes and corresponding change in dissipative properties.

Strong Electronic Winds Blowing under Liquid Flows on Carbon Surfaces

Solid-liquid interfaces display a wealth of emerging phenomena at nanometer scales, which are at the root of their technological applications. While the interfacial structure and chemistry have been intensively explored, the potential coupling between liquid flows and the solid’s electronic degrees of freedom has been broadly overlooked up till now. Despite several reports of electronic currents induced by liquids flowing in various carbon nanostructures, the mechanisms at stake remain controversial. Here, we unveil the molecular mechanisms of interfacial liquid-electron coupling by investigating flow-induced current generation at the nanoscale. We use a tuning fork atomic force microscope to deposit and displace a micrometric liquid droplet on a multilayer graphene sample, and observe an electronic current induced by the droplet displacement. The measured current is several orders of magnitude larger than previously reported for water on carbon, and further boosted by the presence of surface wrinkles on the carbon surface. Our results point to a peculiar momentum transfer mechanism between the fluid molecules and graphene charge carriers, mediated mainly by the solid’s phonon excitations. These findings open new avenues for active control of nanoscale liquid flows through the solid walls’ electronic degrees of freedom.Received 28 June 2022Revised 27 October 2022Accepted 12 January 2023DOI:https://doi.org/10.1103/PhysRevX.13.011020Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNanofluidicsPhysical SystemsGrapheneCondensed Matter, Materials & Applied Physics

Flyrock in surface mining - Limitations of current predictive models and a better alterative through modelling the aerodynamics of flyrock trajectory

Historical approaches to the problem of flyrock based on correlation studies and regression analysis, including artificial neural networks and similar techniques, are inherently incapable of addressing two core issues - root causes of flyrock and projection velocity. A further shortcoming of correlation techniques is that they give no information on the influence of rock size and shape on the flight distance. The scaled depth of burial model for crater blasting in the collar zone and bench face does not specifically address the question of flyrock velocity. A third approach, based on flight trajectory calculations, often neglects the very significant effects of air resistance on the trajectory. Some trajectory models incorporate air resistance but use an implausible fragment velocity model that cannot propel sizeable rocks to distances much beyond 150 m. Nonetheless, trajectory calculation incorporating the effects of air drag affords the most promising approach to the prediction of flyrock range. A unique and insightful feature of the proposed realistic flight modelling is that it collapses all suspected causes of flyrock, many of which are not well understood, to just a single parameter - the launch velocity. This indicates that the root causes of flyrock lie in the mechanisms of momentum transfer to broken rock and suggests new avenues of study.

On the role of inertia in channel flows of finite-size neutrally buoyant particles

We consider suspensions of finite-size neutrally buoyant rigid spherical particles in channel flow and investigate the relevance of different momentum transfer mechanisms and the relation between the local particle dynamics and the bulk flow properties in the highly inertial regime. Interface-resolved simulations are performed in the range of Reynolds numbers $3000 \leq Re \leq 15\ 000$ and solid volume fractions $0 \leq \phi \leq 0.3$ . The Lagrangian particle statistics show that pair interactions are highly inhomogeneous and dependent on the distance from the wall: in their vicinity, the underlying mean shear drives the pair interactions, while a high degree of isotropy, dictated by more frequent collisions, characterizes the core region. Analysis of the momentum balance reveals that while the particle-induced stresses govern the dynamics in dense conditions, $\phi =0.3$ , and moderate Reynolds numbers, $Re <10\ 000$ , the turbulent stresses take over at higher Reynolds numbers. This behaviour is associated with a reduced particle migration toward the channel core, which decreases the importance of the particle-induced stress and increases the turbulent activity. Our results indicate that Reynolds stresses and the associated velocity fluctuations, characteristics of near-wall turbulence, prevail at high inertia over the resistance to deformation presented by the particles for volume fractions lower than 30 %.

Transfer of Orbital Angular Momentum to Freely Levitated High-Density Objects in Airborne Acoustic Vortices

Vortex-field acoustic levitation is an application of acoustic vortices that allows the manipulation of a wide range of materials in various media. However, to date, its levitation capability in air is limited to relatively low-density objects. Here, by optimizing an array-tube setup, we levitate iridium ($22.56\phantom{\rule{0.2em}{0ex}}\mathrm{g}/{\mathrm{cm}}^{3}$) in a first-order acoustic vortex in air. This technique makes it possible to observe the orbital angular momentum transfer carried by acoustic vortices to freely levitated high-density objects, of which the highest rotation speed reaches up to 6500 revolutions per second and increases with a decrease in the object size and an increase in the source amplitude. It is revealed that the thermal-viscous boundary layer dissipation is the dominant effect and leads directly to the rapid rotation of high-density objects. This work can expand the application scope of acoustic vortices, especially in the fields of containerless processing of various materials and rapid rotation of objects, and help to understand the mechanisms of orbital angular momentum transfer to matter.

Impact Of Porous-Media Topology On Turbulent Fluid Flow: Time-Resolved PIV Measurements

An investigation of the interactions between a turbulent duct flow above a 3D porous medium with a periodic topology is reported. Both spatially and temporally resolved point measurements of velocity in the free flow and at the interface of the porous medium using the Particle Image Velocimetry (PIV) technique are presented. A Schwarz Primitive triply periodic minimal surface (TPMS) with a frequency of f = Π/20 printed from polyamide PA12 with a porosity of ϕ = 0.92 was investigated as a porous structure. The periodicity of the model results in two different topologies, hill- and valley-alignment, each of which represent a measurement plane. These different alignments show a strong influence on the turbulent flow above the porous me dium. In addition to the time-averaged velocity field for the two alignments, velocity profiles as well as turbulent kinetic energy profiles at selected positions in flow direction are also shown. The influence of the turbulent channel flow on the porous model is shown by a contour plot of the time-resolved velocity, where this represents the turbulent fluctuation at a point in t ime. For the time-resolved plot, the same influence of the different topology to the respective alignments is shown. With these time-resolved measurements, transient flow processes could be resolved, such as the detachment and attachment of the flow to the porous surface, which are also shown in this paper. These measurements provide an insight into the mechanism of energy, mass and momentum transfer at the interface between the free stream and the porous medium. The results presented are useful for further understanding of turbulent flows on flush mounted complex 3D structures and the influence of the periodic surface on the free flow.

Conservation equations in the relativistic and non-relativistic fluids with the three mechanisms for energy-momentum transferring

In this paper we obtain the conservation equations of momentum-energy of a relativistic fluid in the flat space-time. Three mechanisms for energy and momentum transferring are considered, which are: perfect fluid, stress viscosity and heat transfer mechanisms. The effect of magnetic field is ignored in this paper. Also, the connection of relativistic and non-relativistic equations are seen in this paper, so, the non-relativistic equations can be modified with the relativistic influences.The method of deriving energy and momentum conservation equations and connection of relativistic and non-relativistic conservation equations of this paper, is available to use for a variety of energy and momentum transfer mechanisms, also, applicable to all metrics.

Kinetic modeling of fractal aggregate mobility

Although the mobility or transport parameters, such as lift drag and pitching moments for regular-shaped particulates, are widely studied, the mobility of irregular fractal-like aggregates generated by the aggregation of monomers is not well understood. These particulates which are ubiquitous in nature, and industries have very different transport mechanisms as compared to their spherical counterpart. A high-fidelity direct simulation Monte Carlo (DSMC) study of two fractal aggregates of different shapes or dimensions is undertaken in the slip and transitional gas regime to understand the underlying mechanism of gas-particle momentum transfer that manifests as the orientation-averaged mobility parameters of the particulates. The study specifically focuses on the viscous contribution of these parameters and develops a non-linear correlation for drag and lift parameters p and q obtained from DSMC by normalizing the axial and lateral forces. The drag parameter p predicted a monotonic increase in fractal particulate drag with respect to a spherical monomer while the lift parameter q shows an initial increasing trend but a decreasing tendency toward the high Mach number or high compressibility regime. The approximate model that captures the compressibility and rarefaction effects of the fractal mobility is used to study the evolution of these particulates in a canonical Rankine vortex to illustrate the wide disparity in the trajectories of the fractal aggregate vs a spherical geometry approximation generally found in the literature.

Reynolds number dependence of turbulent heat transfer over irregular rough surfaces

To study the scaling of turbulent heat transfer over a rough surface, we performed a series of direct numerical simulations on turbulent heat transfer over a three-dimensional irregular rough surface with varying the friction Reynolds numbers and relative roughness values. We considered rough surfaces with three different relative roughness values of 1/1.9, 1/4.3, and 1/9.0, and the simulations were performed at three friction Reynolds numbers of 115, 250, and 550. The temperature was treated as a passive scalar with a Prandtl number of unity. Regarding the scaling of the Reynolds analogy factor, which is defined as the ratio of the doubled Stanton number to the skin friction coefficient, a correlation function with the skin friction coefficient, equivalent roughness, and Prandtl number provides an accurate account of the effects of relative roughness, roughness Reynolds number, and friction Reynolds number. For scaling the turbulent momentum and energy fluxes, we introduced the decomposition of the turbulent fluxes into the smooth wall profiles at matched friction Reynolds numbers and their deviatoric components. The baseline smooth wall profile was found to account for the effect of the friction Reynolds number, while the deviatoric component incorporated the effect of the roughness Reynolds number. The dispersion fluxes, namely, the dispersive covariance and dispersion heat flux, were dominantly affected by the roughness Reynolds number rather than the friction Reynolds number. To obtain a better understanding of the effect of wall roughness on the momentum and heat transfer mechanisms, we analyzed the spatial and time-averaged momentum and energy equations and discussed the physical mechanisms that caused a decrease in the mean velocity and temperature from smooth wall profiles.