Drag Theory Research Articles

Investigations on the near-wall bubble dynamic behaviors in a diverging channel

This study investigates the movement characteristics and causes of the dramatic deceleration of individual bubbles as they enter a diverging channel near the wall, an important phenomenon for understanding fluid dynamics in the Venturi-type bubble generator. The use of a modified volume of fluid model with a user defined source method based on van der Geld’s drag theory improves the accuracy of bubble velocity predictions. Visualization experiments were conducted to observe air bubble motion in water, focusing on deceleration near the wall, while numerical simulations were employed to complement these observations. The results reveal the identification of forces governing bubble deceleration, such as pressure gradient, drag, added mass, and lateral force (lift and wall lubrication). Pressure gradient and added mass forces of magnitudes of 106 N/m3 were found to dominate the deceleration process, with drag and lift forces contributing to bubble acceleration and lateral motion in low-speed liquid flow, respectively. In addition, simulations revealed the formation of a faster-moving liquid region downstream of the bubble during rapid deceleration, highlighting the critical role of added mass on the bubble dramatic deceleration process.

Variations in gust factor with wind direction and height based on the measurements from a coastal tower during three landfalling typhoons

Using high-frequency onshore wind data from four different heights of a coastal tower, the variations in gust factor with turbulence intensity, height and wind speed were studied under typhoon conditions. The gust factor increases with increasing turbulence intensity and, most often, can be described by a linear relationship with the turbulence intensity. The gust factor decreases with height and is relatively small compared with those presented in the national codes and other studies. A value of 2.5 is acceptable for the peak factor, which is close to the recommended value of the national code in China. The gust factor increases with increasing wind speed and is also affected by the wind direction. The gust factor has a value to that of previously published results when the wind flows roughly perpendicular to the shoreline, and has a smaller value when the wind flows roughly parallel to the shoreline. The phenomenon is caused by the confinement of shoreline on the sea wave development. Sea waves tend to propagate normal to the shoreline because of the refraction effect. As a result, a shorter roughness length exists in the parallel direction to the shoreline. It can be further explained by the weakness in the momentum flux exchange between the air and sea based on the wave form drag theory when the wind flows parallel to the shoreline.

Solute segregation in a moving grain boundary: a phase-field approach

We present a phase-field approach for investigating monolayer and multilayer type solute segregation in a moving Grain boundary (GB). In this model, we introduce an expression for the GB solute interaction potential which allows for easy modification of the shape of the solute segregation profile at the GB. As a consequence, our phase-field simulations capture various segregation profiles in both stationary and migrating GB that agree with Cahn’s solute drag theory. Furthermore, we explore how different segregation profiles evolve at varying GB velocities owing to the inequality of the atomic flux of solute between the front and back faces of the moving GB. At a low-velocity regime, we observe that multilayer segregation results in significantly increased drag force compared to monolayer segregation. At a high-velocity regime, the opposite holds. Our simulation results also provide valuable insights for predicting grain growth in polycrystalline materials in the presence of solute segregation.

Validating continuum theory for Cottrell atmosphere solute drag by molecular dynamics simulations

When a dislocation moves through a field of mobile solute atoms, solutes segregate to the dislocation and form Cottrell atmospheres which exert solute drag forces. Over the last 70 years, continuum theory has been used extensively to estimate these drag forces and their dependence on the dislocation velocity, however few prior works have validated the accuracy of continuum theories. In this work, molecular dynamics (MD) simulations of dislocation motion in face-centered cubic Ni containing interstitial H atoms were performed in order to test the accuracy of continuum theory predictions. Our results demonstrate that continuum theory provides an accurate estimate of the solute drag force for velocities below the critical velocity at which the solute drag force is maximized. Above the critical velocity, continuum theory systematically deviates from MD. Additional analysis reveals that this deviation results from the multi-valued and unstable nature of solute drag under load control, and also from the transition to random solute drag which occurs at high velocities.

Research Progress on Low-Surface-Energy Antifouling Coatings for Ship Hulls: A Review.

The adhesion of marine-fouling organisms to ships significantly increases the hull surface resistance and expedites hull material corrosion. This review delves into the marine biofouling mechanism on marine material surfaces, analyzing the fouling organism adhesion process on hull surfaces and common desorption methods. It highlights the crucial role played by surface energy in antifouling and drag reduction on hulls. The paper primarily concentrates on low-surface-energy antifouling coatings, such as organic silicon and organic fluorine, for ship hull antifouling and drag reduction. Furthermore, it explores the antifouling mechanisms of silicon-based and fluorine-based low-surface-energy antifouling coatings, elucidating their respective advantages and limitations in real-world applications. This review also investigates the antifouling effectiveness of bionic microstructures based on the self-cleaning abilities of natural organisms. It provides a thorough analysis of antifouling and drag reduction theories and preparation methods linked to marine organism surface microstructures, while also clarifying the relationship between microstructure surface antifouling and surface hydrophobicity. Furthermore, it reviews the impact of antibacterial agents, especially antibacterial peptides, on fouling organisms' adhesion to substrate surfaces and compares the differing effects of surface structure and substances on ship surface antifouling. The paper outlines the potential applications and future directions for low-surface-energy antifouling coating technology.

Grain growth and segregation in Fe-doped SrTiO3: Experimental evidence for solute drag

In functional ceramics, the impact of dopants on bulk crystals is generally well understood. Their impact on grain boundaries is less well known. The present study investigates the impact of acceptor dopants on grain growth in strontium titanate. Scanning electron microscopy and analytical (scanning) transmission electron microscopy have been used to gain knowledge on Fe segregation behavior, grain sizes, and grain size distributions of SrTiO3. While undoped microstructures show normal grain growth at low temperatures (<1350 °C), doped microstructures evolve bimodally. With increasing acceptor dopant concentration, an increasing population of small grains develops. It is shown that Fe segregates to the grain boundaries due to its negative charge and a positive boundary potential. Thus, the experimental findings seem to be well explained by the theory of solute drag: The diffusion of segregated defects (‘solutes’) at grain boundaries can retard grain boundary migration.

Interlayer Electron-Hole Friction in Tunable Twisted Bilayer Graphene Semimetal.

Charge-neutral conducting systems represent a class of materials with unusual properties governed by electron-hole (e-h) interactions. Depending on the quasiparticle statistics, band structure, and device geometry these semimetallic phases of matter can feature unconventional responses to external fields that often defy simple interpretations in terms of single-particle physics. Here we show that small-angle twisted bilayer graphene (SA TBG) offers a highly tunable system in which to explore interactions-limited electron conduction. By employing a dual-gated device architecture we tune our devices from a nondegenerate charge-neutral Dirac fluid to a compensated two-component e-h Fermi liquid where spatially separated electrons and holes experience strong mutual friction. This friction is revealed through the T^{2} resistivity that accurately follows the e-h drag theory we develop. Our results provide a textbook illustration of a smooth transition between different interaction-limited transport regimes and clarify the conduction mechanisms in charge-neutral SA TBG.

Heterogeneous segregation behavior of Nb at the stepped migrating interface during phase transformation

Not as generally-assumed ‘smooth’, the actual interface should consist of ledge structure with riser and terrace due to crystallographic constraint. However, the combined information of structure and chemistry for such stepped interface has not yet been sufficiently explored. Herein the segregation behaviors of Nb at the migrating ferrite/austenite interface with ledge structure but totally different character are investigated. For the incoherent interface, a typical heterogeneous feature that the Nb atoms segregate at the less mobile terrace rather than at the mobile riser is captured for the first time, while those segregation are absent at the semicoherent interface. By fitting the Nb trans-interface diffusivity, the theoretical calculation based on solute drag theory can well explain the experimental measurements for both types of interfaces. The present results may shed new light on the nanoscaled segregation behaviors of alloying element and stimulate more reliable simulations on the diffusional transformation with ledgewise growth mode.

Microstructure and particle filtration behavior of multimodal filter media made of fibers of different shapes for automotive engine intake application

This article describes microstructure of needle-punched nonwoven filter media comprising polyester fibers of different cross-sectional shapes and reports on their dynamic particle filtration behavior both at clean and clogging stages. The multicomponent fibrous filter media were characterized by their mean fiber shapes which were related to those of the individual components weighted by the corresponding mass fractions. In such media, the fibers of different cross-sectional shapes produced a synergistic effect on the gravimetric filtration efficiency at the clean stage. The fractional filtration efficiency was quite accurately expressed as a sum of fractional filtration efficiencies of individual components weighted by their surface area fractions. The evolution of pressure drop was explained in light of drag theory, and the role of fiber shape in determining the drag coefficient was discussed. Overall, the multimodal fibrous filter media prepared with fibers of different cross-sectional shapes showed a high potential for automotive engine intake application.

Don't be a Drag, Just be a Priest: The Clothing and Identity of the Galli of Cybele in the Roman Republic and Empire

ABSTRACTThis paper explores the clothing and representation of the galli priests of Cybele in late Republican and early Imperial Rome. The galli were male‐bodied, but practiced self‐castration and wore traditionally feminine clothing and makeup, which the article argues placed them outside any expected gender binary and allowed them to inhabit a non‐binary identity. The article applies contemporary drag theory and the relationship between clothing and identity, particularly in respect of (assumed) incongruity, to explore the ways in which the galli's identity is visible in the Roman world. In the final section, a case study is made of the way the galli are represented in Lucian of Samosata's On the Syrian Goddess.

The universal steady lift and drag theory and the physical origin of lift

Since the birth of modern aerodynamics, various theories on lift and drag have been developed and validated extensively in aeronautical applications. However, the far-field force theory had long remained at low-speed incompressible flow. Based on the analytical solutions of the linearized Navier-Stokes equations in the steady far field, the authors and their collaborators extended the classic Kutta-Joukowski lift theorem to both two- and three-dimensional viscous and compressible flows, and thus filled the long-standing gap in theoretical aerodynamics. Why can the simple formulas based on linearized approximation still be accurately valid for highly nonlinear complex flows? This issue of great interest involves the methodological characteristics and physical mechanism behind the unified force theory and is the first task of this article. Moreover, there has been an abnormal phenomenon regarding the physical origin of lift that, despite the already mature and fully verified rigorous lift theory, various different hypotheses still keep surfacing frequently in various nonscientific publications and media. This indicates that the issue is really complicated and has not been thoroughly clarified in textbooks, monographs, and classrooms around the world. Now, the universality and high conciseness of the unified theory enable one to reach a clear answer to this issue by rigorous logical arguments in the most direct way. This is the second task of this article.

A delay in sampling information from temporally autocorrelated visual stimuli

Much of our world changes smoothly in time, yet the allocation of attention is typically studied with sudden changes – transients. A sizeable lag in selecting feature information is seen when stimuli change smoothly. Yet this lag is not seen with temporally uncorrelated rapid serial visual presentation (RSVP) stimuli. This suggests that temporal autocorrelation of a feature paradoxically increases the latency at which information is sampled. To test this, participants are asked to report the color of a disk when a cue was presented. There is an increase in selection latency when the disk’s color changed smoothly compared to randomly. This increase is due to the smooth color change presented after the cue rather than extrapolated predictions based on the color changes presented before. These results support an attentional drag theory, whereby attentional engagement is prolonged when features change smoothly. A computational model provides insights into the potential underlying neural mechanisms.

Photon drag of superconducting fluctuations in two-dimensional systems

The theory of photon drag of superconducting fluctuations in the two-dimensional electron gas is developed. It is shown that the frequency dependence of the induced current is qualitatively similar to the case of photon drag of conventional two-dimensional degenerate electron gas. With the decreasing temperature the magnitude of the effect increases dramatically and the current of superconducting fluctuations carries an additional power of reduced temperature in comparison with the Aslamazov-Larkin contribution. The magnitude of the developed effect is expected sufficient to be visible against the conventional photocurrent background.

Dislocation drag from phonon wind in an isotropic crystal at large velocities

ABSTRACTThe anharmonic interaction and scattering of phonons by a moving dislocation, the photon wind, imparts a drag force on the dislocation. In early studies, the drag coefficient B was computed and experimentally determined only for dislocation velocities v much less than transverse sound speed, . In this paper, we derive analytic expressions for the velocity dependence of B up to in terms of the third-order continuum elastic constants of an isotropic crystal, in the continuum Debye approximation. In so doing we point out that the most general form of the third order elastic potential for such a crystal and the dislocation-phonon interaction requires two additional elastic constants involving asymmetric local rotational strains, which have been neglected previously. We compute the velocity dependence of the transverse phonon wind contribution to B in the range 1–90% for Al, Cu, Fe, and Nb in the isotropic Debye approximation. The drag coefficient for transverse phonons scattering from screw dislocations is finite as , whereas B is divergent for transverse phonons scattering from edge dislocations in the same limit. This divergence indicates the breakdown of the Debye approximation and sensitivity of the drag coefficient at very high velocities to the microscopic crystalline lattice cutoff. We compare our results to experimental results wherever possible and identify ways to validate and further improve the theory of dislocation drag at high velocities with realistic phonon dispersion relations, inclusion of lattice cutoff effects, MD simulation data, and more accurate experimental measurements.

Segregation of Sn on migrating interfaces of ferrite recrystallisation: quantification through APT measurements and comparison with the solute drag theory

Solute atoms play an important role in grain boundary migration phenomena, which are critical for understanding the microstructural evolution in metals. Here we investigate the segregation of Sn at migrating boundaries during recrystallisation in a ferritic Fe-Si-Sn alloy using atom probe tomography. Experimental evidence demonstrated a solute depletion zone ahead of the moving interface. The behaviour of Sn was rationalized with the Cahn solute drag theory. A difference in segregation between two investigated migrating interfaces was observed and could be explained by a variation in either binding energy, intrinsic velocity of each interface or in driving force.

A delay in sampling information from temporally autocorrelated visual stimuli

Much of our world changes smoothly in time, yet the allocation of attention is typically studied with sudden changes - transients. A sizeable lag in selecting feature information is seen when stimuli change smoothly. Yet this lag is not seen with temporally uncorrelated rapid serial visual presentation (RSVP) stimuli. This suggests that temporal autocorrelation of a feature paradoxically increases the latency at which information is sampled. To test this, participants are asked to report the color of a disk when a cue was presented. There is an increase in selection latency when the disk's color changed smoothly compared to randomly. This increase is due to the smooth color change presented after the cue rather than extrapolated predictions based on the color changes presented before. These results support an attentional drag theory, whereby attentional engagement is prolonged when features change smoothly. A computational model provides insights into the potential underlying neural mechanisms.

Wave drag on asymmetric bodies

An asymmetric body with a sharp leading edge and a rounded trailing edge produces a smaller wave disturbance moving forwards than backwards, and this is reflected in the wave drag coefficient. This experimental fact is not captured by Michell’s theory for wave drag (Michell Lond. Edinb. Dubl. Phil. Mag. J. Sci., vol. 45 (272), 1898, pp. 106–123). In this study, we use a tow-tank experiment to investigate the effects of asymmetry on wave drag, and show that these effects can be replicated by modifying Michell’s theory to include the growth of a symmetry-breaking boundary layer. We show that asymmetry can have either a positive or a negative effect on drag, depending on the depth of motion and the Froude number.

Phonon wind and drag of excitons in monolayer semiconductors

We study theoretically the non-equilibrium exciton transport in monolayer transition metal dichalcogenides. We consider the situation where excitons interact with non-equilibrium phonons, e.g., under the conditions of localized excitation where a ``hot spot'' in formed. We develop the theory of the exciton drag by the phonons and analyze in detail the regimes of diffusive propagation of phonons and ballistic propagation of phonons where the phonon wind is formed. We demonstrate that a halo-like spatial distribution of excitons akin observed in [Phys. Rev. Lett. 120, 207401 (2018)] can be formed as a result of the exciton drag by non-equilibrium phonons or Seebeck effect.

Effect of Solute Nb on Grain Growth in Fe-30 Pct Mn Steel

Niobium is known to segregate strongly to grain boundaries in steel. The strong interaction of Nb with grain boundaries results in an important reduction of the grain boundary mobility and strong retardation of recrystallization and grain growth. In this study, the effect of Nb on the mobility of grain boundaries in a second-generation Fe-30 pct Mn TWIP steel is investigated. Grain growth kinetics was measured in a series of Nb-containing Fe-30 pct Mn model alloys. An estimate of the grain boundary mobility was obtained for various temperatures and niobium contents. It was found that Nb slows down the mobility of random high-angle grain boundary segments that are not twin related. The effect of solute Nb on grain growth, in the presence of annealing twins, was modeled using Cahn’s theory of solute drag coupled with a recently formulated twin-inhibited grain growth model. The results are shown to be in very good agreement with the experimentally determined growth kinetics.

Magneto-Coulomb Drag and Hall Drag in Double-Layer Dirac Systems.

We develop a theory of Coulomb drag due to momentum transfer between graphene layers in a strong magnetic field. The theory is intended to apply in systems with disorder that is weak compared to Landau level separation, so that Landau level mixing is weak but strong compared to correlation energies within a single Landau level, so that fractional quantum Hall physics is not relevant. We find that, in contrast to the zero-field limit, the longitudinal magneto-Coulomb drag is finite and, in fact, attains a maximum at the simultaneous charge neutrality point (CNP) of both layers. Our theory also predicts a sizable Hall drag resistivity at densities away from the CNP.