Momentum Exchange Research Articles

Finite Element Simulation of a Passive Damper System Using Contact Algorithm

Impact dampers are passive devices wherein a moving mass impacts with the primary structure. The vibrations caused due to these impacts are random in nature. During these impacts, exchange of momentum occurs resulting in dissipation of energy. Here, theoretical model of an impact damper system is modelled as MDOF system using Hertzian contact algorithm. Numerical simulation of the above problem was created using ANSYS Finite Element Analysis (FEA) package. Augmented Lagrangian method with Newmark’s time integration scheme is used for simulations. The problem being studied is highly nonlinear in nature with contacts being established during impact. Normal contact force generated due to impact of ball are used to get the transient response of the system.
 Detailed study was carried out with various mass ratio at low frequency and high amplitudes of excitation. Not much studies have been carried out earlier using MDOF systems with contact algorithm. Above modelling and simulation study was helpful to visualise and accurately capture the transient movement of the impact mass within the confined space.

Fluid-Solid Coupling in Kinetic Two-Phase Flow Simulation

Real-life flows exhibit complex and visually appealing behaviors such as bubbling, splashing, glugging and wetting that simulation techniques in graphics have attempted to capture for years. While early approaches were not capable of reproducing multiphase flow phenomena due to their excessive numerical viscosity and low accuracy, kinetic solvers based on the lattice Boltzmann method have recently demonstrated the ability to simulate water-air interaction at high Reynolds numbers in a massively-parallel fashion. However, robust and accurate handling of fluid-solid coupling has remained elusive: be it for CG or CFD solvers, as soon as the motion of immersed objects is too fast or too sudden, pressures near boundaries and interfacial forces exhibit spurious oscillations leading to blowups. Built upon a phase-field and velocity-distribution based lattice-Boltzmann solver for multiphase flows, this paper spells out a series of numerical improvements in momentum exchange, interfacial forces, and two-way coupling to drastically reduce these typical artifacts, thus significantly expanding the types of fluid-solid coupling that we can efficiently simulate. We highlight the numerical benefits of our solver through various challenging simulation results, including comparisons to previous work and real footage.

Quasilinear theory of Brillouin resonances in rotating magnetized plasmas

Both spin and orbital angular momentum can be exchanged between a rotating wave and a rotating magnetized plasma. Through resonances the spin and orbital angular momentum of the wave can be coupled to both the cyclotron rotation and the drift rotation of the particles. It is, however, shown that the Landau and cyclotron resonance conditions which classically describe resonant energy–momentum exchange between waves and particles are no longer valid in a rotating magnetized plasma column. In this case a new resonance condition which involves a resonant matching between the wave frequency, the cyclotron frequency modified by inertial effects and the harmonics of the guiding centre rotation is identified. A new quasilinear equation describing orbital and spin angular momentum exchanges through these new Brillouin resonances is then derived, and used to expose the wave-driven radial current responsible for angular momentum absorption.

Enhancing subsonic ejector performance by incorporating a fluidic oscillator as the primary nozzle: a numerical investigation

Primary flow structures affect ejector performance by impacting momentum transfer to the secondary flow and pressure reduction in the suction chamber. This study investigated the potential benefits of oscillating the primary flow in ejectors to enhance the shear-turbulence mixing and momentum exchange between the primary and secondary flows and improve the entrainment ratio. A vortex-based fluidic oscillator was scaled down and designed to serve as the primary nozzle in a two-dimensional subsonic ejector, embedded between the secondary channels. Prior to installation, optimal values for the nozzle exit position and mixing chamber height were obtained through a parametric study and used in the ejector with the oscillator. To evaluate the impact of the fluidic oscillator on the ejector entrainment ratio, unsteady Reynolds-averaged Navier–Stokes equations were solved using the k–ε standard and k–ω shear-stress transport turbulence models in the Fluent 2022 R2 software. The results indicated that a harmonically oscillating primary flow was generated, increasing the mixing entrainment and momentum transfer while reducing the pressure in the suction and mixing chambers. The oscillator improved the ejector's entrainment ratio by 38.3% compared to the baseline, and 11.6% compared to the parametrically optimized ejector. It also expanded the ejector's performance range.

Research and Application of the Calculation Method of River Roughness Coefficient with Vegetation

The roughness coefficient is a comprehensive parameter reflecting river resistance, which is widely used in the planning and design of river regulation and flood control projects. In recent years, as the upstream water conservancy and hydro-power projects have been put into operation, the frequency of low flow in the middle and lower reaches has increased, and the frequency of flood flow has decreased. All kinds of vegetation in the river floodplain grow luxuriantly, which causes a change in the river resistance and roughness coefficient. The present study was carried out with theoretical analysis and laboratory tests. A formula for the roughness coefficient calculation was derived based on the momentum equilibrium equation and momentum exchange between the vegetation layer and upper layer. The relationship between the depth-averaged velocity within the vegetation layer and depth-averaged velocity of the whole flow was analyzed. The reliability of the formula was verified by a large amount of previous experimental data. Based on the derived formula, the variation law of the roughness coefficient with vegetation density, vegetation height, and water depth were obtained. For the emerged vegetation flow, the Manning coefficient tended to increase with the increase in the vegetation density and water depth. For the submerged vegetation flow, the Manning coefficient showed a trend of decreasing with the increase in the water depth and increased with the increase in the vegetation height. Finally, the derived formula was applied in the Yueyang reach of the Yangtze River and the Duliujian River. The study can be applied in the fields of water level-flow discharge relationship analysis and the water surface line calculation of vegetated rivers.

Theoretical Analysis for Condensation Heat Transfer Performance Inside Converging-Shaped Microchannel Under Varying-Gravity Conditions

Abstract The fast increasing heat-dissipation requirements under different working conditions such as varying gravity for aerospace industry is drawing more and more attention. Condensation inside microchannel is proved to be a promising technique to tackle this task. To comprehensively and accurately describe the physical phenomenon, a theoretical method considering both momentum exchange caused by vapor condensation and interface temperature drop is developed in this study. Reliability of our theoretical method is verified with both the Comprehensive Shah Correlation and established data. Condensation heat transfer inside converging-shaped microchannel is investigated and the influences of channel size, refrigerant mass flowrate, gravity variation, and converging angle are considered. Converging-shaped microchannel significantly enhances condensation heat transfer, especially for smaller channel with larger refrigerant mass flowrate. Influence of gravity change on condensation performances of vertically configured microchannels both with converging shape and constant cross-sectional area is small.

A contact model based on the coefficient of restitution for simulations of bio-prosthetic heart valves.

A new general contact model is proposed for preventing inter-leaflet penetration of bio-prosthetic heart valves (BHV) at the end of the systole, which has the advantage of applying kinematic constraints directly and creating smooth free edges. At the end of each time step, the impenetrability constraints and momentum exchange between the impacting bodies are applied separately based on the coefficient of restitution. The contact method is implemented in a rotation-free, large deformation, and thin shell finite-element (FE) framework based on loop's subdivision surfaces. A nonlinear, anisotropic material model for a BHV is employed which uses Fung-elastic constitutive laws for in-plane and bending responses, respectively. The contact model is verified and validated against several benchmark problems. For a BHV-specific validation, the computed strains on different regions of a BHV under constant pressure are compared with experimentally measured data. Finally, dynamic simulations of BHV under physiological pressure waveform are performed for symmetrical and asymmetrical fiber orientations incorporating the new contact model and compared with the penalty contact method. The proposed contact model provides the coaptation area of a functioning BHV during the closing phase for both of the fiber orientations. Our results show that fiber orientation affects the dynamic of leaflets during the opening and closing phases. A swirling motion for the BHV with asymmetrical fiber orientation is observed, similar to experimental data. To include the fluid effects, fluid-structure interaction (FSI) simulation of the BHV is performed and compared to the dynamic results.

Temperature dependence of the static quark diffusion coefficient

The propagation of a low momentum heavy quark in a deconfined quark-gluon plasma can be understood in terms of a Langevin description. The momentum exchange with the plasma in thermal equilibrium can then be parametrized in terms of a single heavy quark momentum diffusion coefficient κ, which needs to be determined nonperturbatively. In this work, we study the temperature dependence of κ for a static quark in a gluonic plasma, with a particular emphasis on the temperature range of interest for heavy ion collision experiments.

Tunneling induced swapping of orbital angular momentum in a quantum dot molecule

In this paper, we have examined the effectiveness exchange of optical vorticity via three-wave mixing (TWM) technique in a four-level quantum dot (QD) molecule by means of the electron tunneling effect. Our analytical analysis demonstrates that the TWM procedure can result in the production of a new weak signal beam that may be absorbed or amplified within the QD molecule. We have taken into account the electron tunneling as well as the relative phase of the applied lights to assess the absorption and dispersion characteristics of the newly generated light. We have discovered that the slow light propagation and signal amplification can be achieved. Our results show that the exchange of the orbital angular momentum of light can transfer from coupling optical vortex light to the new generated light in high efficiency.

Theory of porous media with the advection term and mass exchange between phases

Earlier modeling approaches in the field of mixtures assumed the solid phase of the mixture was the predominant constituent and simplified the constitutive laws for the fluid phase. In the field of mixtures, it has been argued that methods such as the theory of poroelasticity are incapable of accurately capturing mass exchange between phases and stress and strain fields. Mass exchange between phases and accurate stress and strain variations of each phase are indispensable to many fields of study, such as bone remodeling, fault movement in rocks, and shock waves in soils. This study presents an alternative method for capturing the stress and strain fields of mixtures, focusing on the advection term in the balance of linear momentum and mass exchange for each phase. In addition, this extended theory has the potential for measuring the shear stress field of a viscous fluid component, considering complex boundary conditions, and modeling chemical reactions between mixture phases. The capability of the model has been evaluated by analyzing 2D and 3D examples, including a realistic bone remodeling scenario. Calculations have shown a maximum difference of 20% in the displacement fields. Thus, results clearly demonstrate the importance of considering the advection term and mass exchange between phases, as its application causes significant variations in the displacement and stress fields of the mixture. The developed model of this study can be considered as a general theory, which yields the classic poroelastic model with certain manipulations.

Head-on collision of large-scale high density plasmas jets: A first-principle kinetic simulation approach

In the double-cone ignition (DCI) inertial confinement fusion (ICF) scheme, head-on collision of high density plasma jets is one of the most distinguished feature when compared with the traditional central ignition and fast ignition of ICF. However, the application of traditional hydrodynamic simulation methods becomes limited, due to serious plasma penetrations, mixing, and kinetic physics that might occur in the collision process. To overcome such limitations, we propose a new simulation method for large-scale high density plasmas. This method takes advantages of modern particle-in-cell simulation techniques and binary Monte Carlo collisions, including both long-range collective electromagnetic fields and short-range particle–particle interactions. Especially, in this method, the restrictions of simulation grid size and time step, which usually appear in a fully kinetic description, are eliminated. In addition, collisional coupling and state-dependent coefficients, which are usually approximately used with different forms in fluid descriptions, are also removed in this method. Energy and momentum exchanges among particles and species, such as thermal conductions and frictions, are modeled by “first principles” kinetic approaches. The correctness and robustness of the new simulation method are verified, by comparing with fully kinetic simulations at small scales and purely hydrodynamic simulations at large scale. Following the conceptual design of the DCI scheme, the colliding process of two plasma jets with initial density of 100 g/cc, initial thermal temperature of 65 eV, and counter-propagating velocity at 300 km/s is investigated using this new simulation method. Quantitative values, including density increment, increased plasma temperature, confinement time at stagnation, and conversion efficiency from the colliding kinetic energy to thermal energy, are obtained with a density increment of about three times, plasma temperature of 400 eV, confinement time at stagnation of 50 ps, and conversion efficiency of 85%. These values agree with the recent experimental measurements at a reasonable range.

Generation of tunable terahertz pulse carrying orbital angular momentum in non-uniform vortex plasma channel

An amplitude modulated Laguerre-Gaussian (LG) laser beam carrying orbital angular momentum (OAM) is considered for terahertz radiation using non-uniform plasma vortex channel under relativistic conditions. The suggested approach involves the exchange of angular momentum between a non-uniform vortex plasma and a laser beam. During the interaction of non-uniform vortex plasma with photons, the laser angular momentum changes and twisted terahertz radiation is generated. The resultant THz radiation is shown to be substantially dependent on the beam width parameter and non-uniformity in the plasma channel. The amplitude of THz increases with beam focusing and the orbital angular momentum parameter. Due to higher amplitude of THz this study is useful in THz applications like, detection of counterfeit drugs, explosive material, analyze and identify defects in materials, medical imaging and communication.

Influence of submergence ratio on flow and drag forces generated by a long rectangular array of rigid cylinders at the sidewall of an open channel

This paper discusses how the submergence ratio, defined as the ratio between the flow depth, D, and the height, h, of the solid rigid cylinders forming the array affects flow and turbulence structure inside and around a rectangular array of cylinders placed adjacent to one of the channel sidewalls. As the array becomes submerged, a vertical shear layer develops in between the top face of the array and the free surface, which strongly increases flow three-dimensionality and modifies how the momentum exchange between the array and the surrounding open water regions occurs with respect to the case of an emerged array where only a horizontal shear layer forms as part of the incoming flow approaching the array is deflected laterally. Eddy-resolving simulations are conducted for several values of the solid volume fraction, ϕ, and of the submergence ratio, 1.0 ≤ D/h ≤ 4.0. Similar to the limiting case of an emerged array (D/h = 1.0), the width- and depth-averaged streamwise velocity inside the array reaches a constant value after an initial adjustment region in the submerged-array cases. For D/h ≥ 1.33, the mean normal velocities through the top and side faces of the array do not become equal to zero downstream of the initial adjustment region. The flow inside the array reaches an equilibrium regime where the local flux of fluid leaving the array through its side face is balanced by the local flux of fluid entering the array through its top face. This regime is observed until close to the end of the array. For D/h ≥ 2.0, the horizontal shear layer vortices do not generate successive regions of high and low streamwise velocity and bed friction velocity inside the array, as is observed in the emerged cases. With respect to the emerged case, the size of the shear layer vortices and the shear layer width increase for low submergence ratios before decreasing rapidly for D/h ≥ 1.33. Significant three-dimensional effects are present inside the array and the horizontal shear layer for cases with both emerged and submerged arrays. In the ϕ = 0.08 cases, strong upwelling and downwelling motions are observed inside the array for D/h ≥ 1.33, while the circulation of the streamwise-oriented cell of secondary flow forming close to the lateral face of the array peaks when the submergence ratio is close to 1.33. For constant ϕ, the total streamwise drag force normalized with the height and width of the array increases with increasing submergence ratio. As the array submergence increases, the cylinders near the front of the array contribute less to the total force acting on the cylinders forming the array. For constant D/h, the total streamwise drag force acting on the array increases with ϕ for the submerged and emerged cases.

The role of sea spray in air-sea fluxes during Typhoon Molave: a study based on drifting buoy observations

The exchange of heat and momentum between the ocean and the atmosphere greatly affects the growth of typhoons. Utilizing the meteorological and oceanic variables observed by a Drifting Air-sea Interface Buoy (DrIB) during Typhoon Molave, a new air-sea turbulent fluxes product (referred to as DrIB product) is developed with the consideration of the thermal and dynamic effects of sea spray in the Coupled Ocean Atmosphere Response Experiment algorithm. The performances of two reanalysis products, ERA5 and MERRA2, under typhoon conditions are evaluated by comparing them to the DrIB observations. In particular, the air-sea turbulent fluxes during Typhoon Molave are systematically studied. The averaged heat (momentum) flux of the DrIB product is ~200% (~30%) higher than the reanalysis. However, the reanalysis products have higher latent heat than the DrIB product, because the reanalysis products have lower wind speed, smaller air-sea temperature difference, and drier atmosphere. The sea spray-induced mean heat (momentum) flux increase is ~1% (8%) in normal weather and is ~5% (17%) at the during-typhoon stage. Sea spray amplifies the dominance of wind speed on heat fluxes and weakens the contribution of air-sea temperature and humidity differences to heat fluxes. Sea spray starts to obviously contribute to the heat fluxes at a 10-m wind speed of ~10 m/s, and it non-linearly accelerates the air-sea heat exchange at a 10-m wind speed of ~20 m/s. When the 10-m wind speed is less than 20 m/s, the basic momentum flux (without sea spray effects) at the air-sea interface is roughly one or two orders of magnitude higher than the sea spray-induced momentum flux. Including the sea spray effects, the maximum momentum flux can even double at the 10-m wind speed of ~30 m/s.

Granular media deformation and fluid flow as overlapping, concurrent, coupled multilayered depth-averaged framework

A novel coupled depth-averaged formulation for the combination of multilayered fluid and monolayer granular flows as a set of interpenetrating continua has been developed in this study. Granular and fluid phases are coupled through an interaction force pair comprised of fluid stresses, and drag forces. Fluid phase is discretized in the vertical direction through a set N of virtual layer interfaces to obtain a variation of flow properties along depth. A novel algorithm for mass and momentum exchange between fluid layers for flow through and over porous media has been proposed. Eigenstructure analysis of the coupled system shows that the system may be discretized in N+1 separate yet coupled Riemann problems. An improved version of well-known HLL method is developed for solution of the model (Finite Volume Method) in a regular grid. The resistive source–sink terms are embedded implicitly within the interfacial flux. Based on the choices of state variables and layer-interface porosity, four variants of the coupled system are developed. A set of benchmark experimental test-cases and a representative test involving both rigid and deformable granular media has been simulated using the four variants to establish the robustness of the model and the coupling procedure.

A fluid–solid momentum exchange method for the prediction of hydroelastic responses of flexible water entry problems

The paper presents a Fluid Structure Interaction (FSI) method for hydroelastic water entry. The method assumes that the momentum exchange between the fluid and solid body can be used for the calculation of pressure, deformation and stresses arising during impact. The flexible fluid–structure interactions of flat plates entering water are solved using a computational code that employs the finite volume method to discretise both fluid and solid equations. This provides a better matching of momentum on the fluid–solid interface. The momentum arising in the solid body that emerges after the impact is defined as the momentum exchange, and is shown to increase linearly under the increase of non-dimensional impact speed. The ratio of the maximum pressure arising in an elastic body entering water to that of a rigid body is termed relative pressure and is shown to decrease linearly as a function of momentum exchange. The latter verifies the main hypothesis of this paper, namely that ‘the pressure acting on an elastic body can be predicted using an unsophisticated equation that uses the momentum exchange.’ The deformation and stresses arising in elastic plates entering water are demonstrated to be functions of momentum exchange and can be found using simple equations formulated via parametrisation of data. It is concluded that subject to further validation, the method could be extended for the prediction of hydroelastic response of other sections/bodies entering water.

SOLPS-ITER predictive simulations of the impact of ion-molecule elastic collisions on strongly detached MAST-U Super-X divertor conditions

The role of ion-molecule () elastic collisions in strongly detached divertor conditions has been studied in the MAST-U Super-X configuration using SOLPS-ITER. Two strongly detached steady state solutions were compared, one obtained through a main-ion fuelling scan and the other through a nitrogen seeding scan at fixed fuelling rate. A significant difference in the electron–ion recombination (EIR) levels was observed; significant EIR in strongly detached conditions in the fuelling scan and negligible EIR throughout the seeding scan. This is partly because the fuelling scan achieves electron temperatures () as low as 0.2 eV near the divertor target, compared to 0.8 eV in the seeding scan (EIR increases strongly below eV), and partly due to higher divertor plasma densities achieved in fuelling scan. Features of the strongly detached seeded cases, i.e. higher temperatures and negligible EIR, are recovered in the fuelling scan by turning off elastic collisions. Analysis suggests that dissipation mechanisms like line radiation and charge exchange (important for detachment initiation) become weak when falls below 1 eV, and that elastic collisions are necessary for further heat dissipation and access to strongly recombining conditions in the fuelling scan. In the seeding scan, heat dissipation through elastic collisions is weak. This could be because our nitrogen seeding simulations do not include interactions between nitrogen ions and neutrals, and the strongly detached cases contain high levels of in the divertor. As a result, the acts like a reservoir of energy and momentum which appears to weaken the impact of elastic collisions on the divertor plasma energy and momentum balance, making it more difficult to access recombining conditions. This suggests that some of the differences between seeding and fuelling scans could be because energy and momentum exchange between impurities and neutrals is not sufficiently captured in our simulations.

Contrasting Responses of Surface Heat Fluxes to Tropical Deforestation

AbstractDeforestation alters the exchange of heat, moisture, and momentum between the Earth's surface and the atmosphere, which can significantly affect the surface energy balance and water budget. However, changes in surface heat fluxes in response to deforestation are diverse among multi‐model simulations. This study explores factors that might cause different changes in surface heat fluxes due to tropical deforestation with NCAR Community Land Model. The results show that the changes in surface heat fluxes are related to the mean‐state flux partitioning conditions over the deforested areas. Deforestation tends to decrease both surface heat fluxes under conditions with smaller evaporative fractions (EFs), and tends to decrease the latent heat fluxes and increase the sensible heat fluxes under conditions with greater EFs. A similar relation can be found in the Land Use Model Intercomparison Project, which indicates that the varying simulated flux partitioning conditions might contribute to the diverse changes in surface heat fluxes among models.

Four-way coupled modelling of swirling particle-laden flow in Methane-central coaxial jets

A novelty particle subgrid scale model is established to model the effect of gas flow on particle motion behaviors and a four-way coupling method to describe the particle-particle collisions by granular temperature equation is also developed. Menthane-central jets of particle-laden swirling flow in coaxial chamber are modeled by a multiphase second-order moment model and numerically predicted by the large-eddy simulation. Simulation results of two-phase hydrodynamics are in good agreement with experimental measurement. Evolution of coherent structure, recirculation region, periodic vortices formation, breakup, coalescence and pair, stronger mixing are captured and analyzed successfully. Compared to gas-annular flow, it is benefit for the stabilization of flame due to the strongly radial spread of particle dispersions and mixing momentum exchanges. The core-annular flow structures of particle concentration are observed, and instantaneous particle velocity are complied with the normal distribution. At shear layers of core-jetting regions of X/R=0.56, correlations of axial-tangential gas and particle fluctuation velocities are approximately 3.0 and 6.0 times larger than those of far downstream flows.

Significant phonon drag effect in wide band gap GaN and AlN

Understanding the electrical and thermal transport properties of group-III nitride semiconductors is crucial for their electronic and thermoelectric applications. This study investigates the impact of the phonon drag effect, which refers to the momentum exchange between nonequilibrium phonons and electrons, on electronic transport in GaN and AlN. Results show that, even at room temperature, phonon drag significantly enhances mobility and the Seebeck coefficient. The study delves into the specific mechanisms for this.