Momentum Gain Research Articles

Angular momentum gain by electrons under the action of intense structured light

Angular momentum gain by electrons under the action of intense structured light

Footprint of the air-sea momentum transfer saturation observed by ocean wave buoy network in extreme tropical cyclones

Tropical cyclones are one of the most destructive natural phenomena, causing tremendous coastal disasters worldwide. The maximum intensity of tropical cyclones is determined by momentum and heat transfer at the air-sea interface. Momentum transfer corresponds to the momentum loss of tropical cyclones and, consequently, to the underlying ocean's momentum gain causing extreme ocean waves and storm surges. Air-side observations of wind profiles and ocean-side observations of ocean subsurface currents show a slowdown of momentum transfer under high tropical cyclone wind speeds in the previous studies. However, there is still disagreement regarding the slowdown owing to lack of data. Here, we show momentum transfer under high wind speed conditions by observing ocean waves. Although ocean wave observations are highly spatially limited, we deployed a fleet of drifting ocean wave buoys covering the active area of tropical cyclones in the Western North Pacific. The buoy fleet captured extreme waves near the eye of the strongest category 5 tropical cyclone, indicating an ocean wave footprint of the momentum transfer saturation above surface wind speeds of 25 m/s. Our approach from ocean surface wave observation is a unique contribution to determination on air-sea momentum transfer slowdown under extreme wind speeds, which can compensate for the knowledge from conventional air and ocean-side observations. This finding advances tropical cyclones, extreme ocean waves, and storm surge modeling.

Deciphering Controls of Pore‐Pressure Evolution on Sediment Bed Erosion by Debris Flows

AbstractPore‐fluid pressure (PP) plays an important role in bed erosion, but the mechanisms that control PP evolution and the resulting feedbacks on flow dynamics are unclear. Here, we develop a general formulation, allowing quantification of the propensity for PP evolution of saturated and unsaturated bed sediments. We conduct erosion experiments by systematically varying grain composition and water content of beds, for investigating effects of PP evolution on flow erosion. With increasing water content, PP shows a slight rise in deforming beds with drained behavior but significant larger rise in undrained beds. Regardless of bed composition, the erosion rate of beds presents a synchronous change tendency with PP evolution due to the loss in basal friction. PP instigates positive feedback that induces a remarkable gain of flow velocity and momentum on wet beds with undrained behavior. Our results help explain observations of volume growth and long run out of debris flows.

Shell model based description of single-particle and collective excitations in 66Zn

Shell model calculations have been performed in f5/2pg9/2 model space using two different interactions viz., jj44bpn and jun45pn to explore the observed band like structure in 66Zn. Calculated energy values of the excited states are compared with the experimental values and a good agreement is found at low excitation energies. The experimental kinematic moment of inertia and the calculated values using shell model are compared and similar trend is observed. Also, the plotted values of the observed and theoretical alignment suggest very similar gain in the angular momentum of 66Zn. The nature of the variation of calculated energy with spin is very similar to the observation and shows a rotational like behaviour. Observed band crossing phenomenon is also reproduced from the plots of theoretical data points. Thus, these two effective interactions, without any modification, are quite efficient in producing the trend of the observed collectivity in even-even 66Zn nucleus. Configurations and occupation numbers for different states are obtained using the two aforementioned interactions and suggests substantial contribution of 1g9/2 orbital in generating the angular momentum of the high spin states.

Ocean Waves in Large‐Scale Air‐Sea Weather and Climate Systems

AbstractIn spite of massive efforts directed to development of climate models over recent decades, accurate climate simulations and prediction remain a grand challenge. Large sea surface temperature model bias is one of the key indicators of the problem, among others. Qiao and his team suggested the concept of surface wave‐induced turbulence and elaborated the way for such coupling, through turbulent processes at both sides of the air‐ocean interface. These are the wave mixing in the ocean and wave modulation of air‐sea fluxes. This wave‐ocean coupled approach led to essential improvements of performance of ocean circulation and climate models, essentially introducing the next generation of large‐scale air‐sea interaction models. This Commentary reviews three recent JGR publications where the Qiao theory was implemented. The first paper (Huang & Qiao, 2021; https://doi.org/10.1029/2020JC016839) reported that the momentum gain by the ocean could be larger than the wind stress input, due to the ocean waves. The second paper (Chen et al., 2022; https://doi.org/10.1029/2021JC018360) demonstrated that, also due to waves, the drag coefficients in the atmospheric boundary layer have a spatial asymmetry during tropical cyclones. The third paper (Zhao et al., 2022; https://doi.org/10.1029/2022JC019015), based on combination of wave‐coupled effects on both sides of air‐sea interface, demonstrates their impact on simulation of tropical cyclone intensity. All papers are based on unique in situ measurements and their data analysis. The Commentary is further used as an opportunity to outline and review the current state of the small‐large scale coupling topic with respect to ocean, weather and climate modeling.

Revealing the physics of sediment ploughing by geophysical mass flows: Depth-averaged approach

Ploughing is an erosion mechanism where the dam-like front of a geophysical flow shovels sediments along its path to increase its runout. Ploughing is commonly reported in the field. However, the physics of ploughing remain missing in hazard assessments tools owing to the difficulties of simulating the competing effects between erosion and deposition. In this study, 2D non-depth-averaged (non-DA) simulations are used to elucidate the physics of ploughing induced by erosion and deposition. A closure model for erosion and deposition is then developed and implemented into a depth-averaged (DA) framework to facilitate practical simulations for routine hazard assessments. Both the 2D non-DA and DA models are based on the Smoothed Particle Hydrodynamics (SPH) method. The new DA model is evaluated with experimental data and shown to be able to simulate the competing effects of erosion and deposition, which causes flow momentum gain and loss, respectively. The new DA framework is proven to be able to simulate the effects of ploughing, which has been a long-standing limitation of DA models.

SARS-CoV-2 Morphometry Analysis and Prediction of Real Virus Levels Based on Full Recurrent Neural Network Using TEM Images.

The SARS-CoV-2 virus is responsible for the rapid global spread of the COVID-19 disease. As a result, it is critical to understand and collect primary data on the virus, infection epidemiology, and treatment. Despite the speed with which the virus was detected, studies of its cell biology and architecture at the ultrastructural level are still in their infancy. Therefore, we investigated and analyzed the viral morphometry of SARS-CoV-2 to extract important key points of the virus's characteristics. Then, we proposed a prediction model to identify the real virus levels based on the optimization of a full recurrent neural network (RNN) using transmission electron microscopy (TEM) images. Consequently, identification of virus levels depends on the size of the morphometry of the area (width, height, circularity, roundness, aspect ratio, and solidity). The results of our model were an error score of training network performance 3.216 × 10-11 at 639 epoch, regression of -1.6 × 10-9, momentum gain (Mu) 1 × 10-9, and gradient value of 9.6852 × 10-8, which represent a network with a high ability to predict virus levels. The fully automated system enables virologists to take a high-accuracy approach to virus diagnosis, prevention of mutations, and life cycle and improvement of diagnostic reagents and drugs, adding a point of view to the advancement of medical virology.

Tidally induced migration of TESS gas giants orbiting M dwarfs

ABSTRACT According to core-accretion formation models, the conditions under which gas giants will form around M dwarfs are very restrictive. Also, the correlation of the occurrence of these planets with the metallicity of host stars is still unknown due to the intrinsic faintness of M dwarfs in the optical and some intricacies in their spectra. Interestingly, NASA’s Transiting Exoplanet Survey Satellite (TESS) mission has started to create a growing sample of these systems, with 11 observed planets located in close-in orbits: contrary to what is expected for low-mass stars. Tidal interactions with the host star will play a key role in determining the fate of these planets, so by using the measured physical and orbital characteristics of these M-dwarf systems we numerically analyse the exchange of rotational and orbital angular momentum, while constraining the energy dissipation in each system to calculate whether host stars are spun up or spun down, depending on the relationship between the gain and loss of angular momentum by the stellar rotation. We also study the coupled orbital and physical evolution of their gas giant companion and calculate orbital circularization time-scales, as well as the time needed to undergo orbital decay from their current orbital position to the Roche limit. The thorough study of tidal processes occurring over short and long time-scales in star–planet systems like those studied here, can help constrain tidal dissipation rates inside the star and planet, complement tidal theories, and improve estimations of unconstrained properties of exoplanetary systems.

SXP 15.6 – an accreting pulsar close to spin equilibrium?

ABSTRACT SXP 15.6 is a recently established Be star X-ray binary system in the Small Magellanic Cloud (SMC). Like many such systems, the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. It is shown here that the neutron star in this system is exceptionally close to spin equilibrium averaged over several years, with the angular momentum gain from mass transfer being almost exactly balanced by radiative losses. This makes SXP 15.6 exceptional compared to all other known members of its class in the SMC, all of whom exhibit much higher spin period changes. In this paper, we report on X-ray observations of the brightest known outburst from this system. These observations are supported by contemporaneous optical and radio observations, as well as several years of historical data.

Droplet trampolining on heated surfaces in the transitional boiling regime

Droplet dynamics on a heated surface plays an important role in applications ranging from spray cooling to solidification and pharmaceuticals. Here, we show that in the transitional boiling regime, an unconstrained droplet exhibits trampolining where the droplet bounces to higher heights with each consecutive contact with the substrate. A water droplet of volume 1 μL bounces to a height as high as 6-7 times of its diameter and the height of bounce increases to 18–20 times the diameter as the volume reduces to ∼ 0.1μL. The dynamics of the droplet in the transitional boiling regime results in a non-monotonic variation in the total evaporation time of the droplet with an increase in substrate temperature – an observation that is absent for a constrained droplet. We develop a force-based model to explain the trampolining behavior of the droplet and propose that the gain in momentum of the droplet that results in trampolining is caused by bubble nucleation and escape at the liquid-substrate interface prior to droplet takeoff.

Design and Performance Analysis of NadamSPGD Algorithm for Sensor-Less Adaptive Optics in Coherent FSOC Systems

Sensor-less adaptive optics (SLAO) based on stochastic parallel gradient descent (SPGD) is effective for the compensation of atmospheric turbulence in coherent free-space optical communication (CFSOC) systems. However, SPGD converges slowly and easily falls into local extremes. Therefore, we propose a novel NadamSPGD algorithm for efficient wavefront correction that combines Nesterov-accelerated adaptive moment estimation (Nadam) and SPGD. Specifically, Nesterov’s accelerated gradient momentum (NAG) and adaptive gain coefficients are integrated to conventional SPGD to accelerate its convergence speed and avoid converging to extremum points. Theoretical analysis, numerical simulations and experimental results demonstrate that NadamSPGD can increase the convergence speed by ~50% and significantly improve the robustness of parameters, and thus more efficiently suppress the negative effects of atmospheric turbulence on mixing efficiency (ME) and bit error rate (BER). Our algorithm also presents better dynamic performance under strong turbulence and high Greenwood frequency conditions, and it is more suitable for real-time SLAO systems. This study proves that the NadamSPGD algorithm is suitable for SLAO in the CFSOC system and is a viable substitute for SPGD to improve the quality of optical communications.

Effects of Heat and Momentum Gain Differentiation during Gas Detonation Spraying of FeAl Powder Particles into the Water.

In this paper, dynamic interactions between the FeAl particles and the gaseous detonation stream during supersonic D-gun spraying (DGS) conditions into the water are discussed in detail. Analytical and numerical models for the prediction of momentum and complex heat exchange, that includes radiative effects of heat transfer between the FeAl particle and the D-gun barrel wall and phase transformations due to melting and evaporation of the FeAl phase, are analyzed. Phase transformations identified during the DGS process impose the limit of FeAl grain size, which is required to maintain a solid state of aggregation during a collision with the substrate material. The identification of the characteristic time values for particle acceleration in the supersonic gas detonation flux, their convective heating and heat diffusion enable to assess the aggregation state of FeAl particles sprayed into water under certain DGS conditions.

A Brief Introduction to Beam Position Monitors for Charged Particle Accelerators

While charged particle accelerators have their origin in the research related to fundamental and high-energy physics, they expanded their applications to many other fields of applied research in physics, material science, biology, chemistry, and medical science, including the treatment of patients, to name some. However, in all types of accelerators assembles of charged particles, e.g., electrons, protons, ions, sometimes their antimatter partners positrons or p-bars (anti-protons) are accelerated to a desired energy, typically in an evacuated, metallic tube, called beam pipe. The acceleration is performed by providing an electric field of high gradient to the charged particle assemble in the direction of motion. In most cases this is performed by resonant radio-frequency (RF) cavities, which are fed by a high-power RF source. However, new acceleration schemes using high power lasers, plasma wakefields, etc. are also studied these days. A guide-field along the accelerator, typically provided by sets of different types of magnets, ensures the charged particles stay within the transverse boundaries of the beam pipe, near its center, travelling on the wanted trajectory. Accelerating elementary particles, like electrons, or quasi-elementary particles, like protons, to very high energies (GeV range) results their velocity $v$ to be close to speed-of-light $c$ , typically expressed as relative velocity $\beta=v/c\approx 1$ , and any further acceleration manifests in a gain of momentum $p=\gamma m_{0}v$ , with $\gamma=1/\sqrt{1-\beta^{2}}$ being the Lorentz factor and $m_{0}$ being the mass of the particle at rest, 0.511 MeV and 938.26 MeV for electron and proton, respectively. The use of resonant RF cavities for the acceleration, causes the particles to form bunches, which fill a longitudinal range of typically a few millimeters up to some meters, depending on $f_{RF}, \gamma$ and other factors. This means, the large number of charged particles in the accelerator are not uniformly distributed along the beam-line but appear in bunches of typical $N=10^{8}-10^{11}$ particles per bunch, and the minimum space between the center of those bunches is defined by the RF bucket length, given by the wavelength $\lambda_{RF}=v/f_{RF}$ of the RF system.

Study of Momentum Diffusion with the Effect of Adiabatic Focusing

Abstract The momentum diffusion of charged energetic particles is an important mechanism of the transport process in astrophysics, the physics of fusion devices, and laboratory plasmas. In addition to the momentum diffusion term for a uniform field, we obtain an additional momentum diffusion term due to the focusing effect of the large-scale magnetic field. After evaluating the coefficient of the additional momentum diffusion term, we find that it is determined by the sign of the focusing characteristic length and the cross helicity of the turbulent magnetic field. Furthermore, by deriving the mean momentum change rate contributed from the additional momentum diffusion term, we identify that the focused field provides an additional momentum loss or gain process.

Axial momentum gains of ions and electrons in magnetic nozzle acceleration

Fully kinetic simulations of magnetic nozzle acceleration were conducted to investigate the axial momentum gains of ions and electrons with electrostatic and Lorentz forces. The axial momentum gains per ion and electron are directly calculated from the kinetics of charged particles, indicating that electrons in the magnetic nozzle obtain the net axial momentum by the Lorentz force, even though they are decelerated by the electrostatic force. Whereas ions are also accelerated by the electrostatic force, the axial momentum gain of electrons increases significantly with increasing magnetic field strength and becomes dominant in the magnetic nozzle. In addition, it is clearly shown that the axial momentum gain of electrons is due to the electron momentum conversion from the radial to the axial direction, resulting in a significant increase in the thrust and exhaust velocity.

Effects of Navier slip on film condensation heat transfer over upward facing horizontal flat surfaces with free edges

Transport phenomena involving condensate liquids generated from the phase-change heat transfer in microchannels and in engineered superhydrophobic surfaces require consideration of slip effects. Herein, the laminar film condensation over upward facing flat slabs and circular disks of finite sizes with free edges in the presence of wall slip effects is investigated. By considering the Navier slip model and extending the classical Nusselt analysis, the mass, momentum, and energy of the liquid film in two-dimensional/axisymmetric coordinates are solved for the film thickness and the heat transfer rate in non-dimensional form. Numerical solution yields the local structure of the condensate film profile and the Nusselt number for different values of slip coefficient. The results reveals that the condensate film on horizontal surfaces becomes thinner and the overall heat transfer rate is enhanced with an increase in the slip coefficient. In particular, an analysis of the results indicates a power law dependence of the Nusselt number on the non-dimensional slip coefficient with an exponent close to 0.5. Significant enhancement in phase change heat transfer follow from the modification of the local velocity profiles within the condensate film, resulting from the additional momentum gain near the wall surfaces due to increases in slip effects.

Stellar hardening of massive black hole binaries: the impact of the host rotation

ABSTRACT Massive black hole binaries (MBHBs) with masses of ∼104 to $\sim 10^{10} \, \mathrm{M}_{\odot {}}$ are one of the main targets for currently operating and forthcoming space-borne gravitational wave observatories. In this paper, we explore the effect of the stellar host rotation on the bound binary hardening efficiency, driven by three-body stellar interactions. As seen in previous studies, we find that the centre of mass (CoM) of a prograde MBHB embedded in a rotating environment starts moving on a nearly circular orbit about the centre of the system shortly after the MBHB binding. In our runs, the oscillation radius is ≈ 0.25 (≈ 0.1) times the binary influence radius for equal mass MBHBs (MBHBs with mass ratio 1:4). Conversely, retrograde binaries remain anchored about the centre of the host. The binary shrinking rate is twice as fast when the binary CoM exhibits a net orbital motion, owing to a more efficient loss cone repopulation even in our spherical stellar systems. We develop a model that captures the CoM oscillations of prograde binaries; we argue that the CoM angular momentum gain per time unit scales with the internal binary angular momentum, so that most of the displacement is induced by stellar interactions occurring around the time of MBHB binding, while the subsequent angular momentum enhancement gets eventually quashed by the effect of dynamical friction. The effect of the background rotation on the MBHB evolution may be relevant for LISA sources, that are expected to form in significantly rotating stellar systems.

Angular momentum redistribution in laser-induced demagnetization

The ultimate fate of the spin moment in ultrafast demagnetization is to appear as a gain in momentum of the lattice. However, the route by which it occurs remains to be conclusively demonstrated. Employing state-of-the-art time-dependent density functional theory we show that spin-orbit coupling at femtosecond timescales drives a transfer of spin angular momentum into orbital angular momentum, which via the Coulomb term transfers from this orbital angular momentum instantaneously to the lattice. The rate of change of angular and spin momenta have, respectively, predominantly extrinsic (the pulse duration) and intrinsic (spin-orbit coupling) timescales. This facilitates the design of pulses to clearly disentangle the physics of angular momentum redistribution. While experiments predominantly use bulk elemental Ni, we propose Co in thin film geometry with an experimentally realizable time resolution for ultrashort pulses will allow clear observation of the ultrafast transfer of momentum via orbital degrees of freedom to the lattice.

Boundary-Layer Control by Means of Pulsed Jets at Different Inclination Angles

Finite amounts of compressed air are impulsively emitted through a rectangular outlet at an axial velocity of . Their interaction with a steady crossflow with a zero-pressure-gradient turbulent boundary layer is studied on the basis of phase-locked particle image velocimetry measurements for a parameter space that is spanned by the jet inclination angle defined with respect to the surface tangent downstream of the outlet and the ratio between the axial jet velocity and the crossflow velocity . Two types of vortex structures are observed. For , , jets penetrate into the crossflow, and distinct, asymmetrical vortex rings are produced. For , as well as for , , starting jets attach to the wall as leading vortex half-rings are formed. The latter type is better suited to energize the boundary layer as low-momentum fluid is shifted away from the surface and high-momentum fluid from the freestream is entrained into the jet wake. In terms of the overall gain in streamwise momentum, the impulse provided due to overpressure during the rapid jet initiation is of major importance in the present study, and its exploitation may enable a significant enhancement in future flow control applications.

The effects of laser polarization and wavelength on injection dynamics of a laser wakefield accelerator

Here, we investigate the effects of laser polarization and wavelength on electron injection dynamics in a laser wakefield accelerator. During the ionization process, electrons gain residual momentum and kinetic energy via above threshold ionization, which has a strong dependence on laser polarization. A circularly polarized laser pulse results in a much higher residual momentum and kinetic energy gain for the ionized electrons compared with the linearly polarized case. This residual momentum results in particle injection because of the sensitivity of particle trapping to the initial conditions and enhanced the total injected beam charge in both experiments and particle-in-cell simulations. Due to the strong correlation of above threshold ionization with laser wavelength, in this work we extended the investigation to long wavelength (up to 20 μm) drive pulses using particle-in-cell simulations. Owing to the gain in kinetic energy, it may be expected that the charge trapped would consistently increase for circular polarization with increasing laser wavelength, but this was not observed. Instead, there are oscillations with wavelength in the relative trapped charge between linear and circular polarization cases, which arise because of ionization and heating effects on the plasma. Our studies highlight the complex interplay between several different physical effects, including injection regimes—above threshold ionization assisted injection, wave-breaking injection by carrier-envelope-phase effects and ionization injection—ionization gradient induced laser pulse evolution, and thermal modifications to the wake structure that need considering when extrapolating laser wakefield acceleration to different wavelength regimes.