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At extreme strain rates, where fast moving dislocations govern plastic deformation, anharmonic phonon scattering imparts a drag force on the dislocations. In this paper, we present calculations of the dislocation drag coefficients of aluminum and copper as functions of temperature and density. We discuss the sensitivity of the drag coefficients to changes in the third-order elastic constants with temperature and density. [ABSTRACT FROM AUTHOR]

The continuous demand and fuel depletion in the automobile industries cause a reduction in fuel consumption, especially in a car which is a classic problem to focus on vehicle body design. The formation of drag force in the car body demands tractive force and significantly affects the engine performance and fuel consumption rate which is not advisable for enhancing aerodynamic efficiency. This paper discusses the methodology to reduce the fuel consumption rate in hatchback cars using a ‘basebleed method’. The hatchback car model with and without basebleed is numerically simulated for the various speed to study the aerodynamic coefficients. The numerical simulation is performed with the k-ε turbulence model for predicting the wake region of both car models with and without basebleed. The numerical study witnessed the hatchback car model with basebleed arrived 6% reduction in the coefficient of drag (CD) compared to without basebleed, which results in a reduction of fuel consumption rate of up to 4.33 %. The research evidence that the stability of the car is not affected while using this basebleed drag reduction method and it is studied from the resultant parameters such as coefficient of lift (CL) and coefficient of side force (CS) and for the varying yaw angle (φ). Further, the research recommends the integration of basebleed at the underbody structure in Hatchback cars to improve the engine fuel consumption without affecting its stability.

The problem of the axisymmetric steady flow of an incompressible couple stress fluid within two concentric spheres is analyzed analytically in this study. In the interior of the solid sphere, translation takes place with a uniform velocity, and in the exterior, translation takes place with a fixed velocity. A study of slip conditions was carried out on the surfaces of the two spheres. In terms of stream functions, modified Bessel functions provide an analytical solution to the problem. As the outer sphere remains stationary, the inner sphere experiences the normalized drag force. A graphic representation of drag is provided based on the slip and spin slip parameters, the couple stress parameter, the separation distance, and the MHD parameter. In addition, the results are compared with earlier results for special cases of translation of one rigid sphere through an unbounded magnetic medium, viscous fluid flow, and Couple stress fluid without MHD between two concentric spheres.

In this study, physical aspects of a ventilated supercavity behind different cavitator geometries such as the hydrodynamic characteristics, distribution of pressure within the cavity, hysteresis phenomenon, and gas leakage mechanism were qualitatively and quantitatively investigated using experimental and numerical methods. For the simulation and tunnel tests, we employed five cavitators, each with different angles (45°, 60°, 90°, 135°, and a 180° cavitator, commonly referred to as a disk cavitator), all sharing the same diameter. The results revealed that the drag force experienced on the cavitator decreased linearly with an increase in the ventilation rate, and a consistent trend was observed for all test cavitator angles. Through experimental measurements, a universal equation has been derived to predict the drag force exerted on a supercavitating vehicle employing a cavitator. In addition, the pressure distribution inside the supercavity was significantly influenced by the angle of the cavitator. The pressure kept almost unchanged in the first half of supercavity; a slight increase in pressure occurred in the remainder of the supercavity. Twin-vortex gas leakage mode was clearly observed. The distance between the two hollow vortices increased significantly, whereas the incline angle of these vortices and the horizontal line changed insignificantly.

The unsteady hydrodynamic drag exerted on an oscillating sphere near a planar wall is addressed experimentally, theoretically and numerically. The experiments are performed by using colloidal-probe atomic force microscopy in thermal noise mode. The resonance frequencies and quality factors are extracted from the measurement of the power spectrum density of the probe oscillation for a broad range of gap distances and Womersley numbers. The shift in the resonance frequency of the colloidal probe as the probe goes close to a solid wall infers the wall-induced variations of the effective mass of the probe. Interestingly, a crossover from a positive to a negative shift is observed as the Womersley number increases. In order to rationalize the results, the confined unsteady Stokes equation is solved numerically using a finite-element method, as well as asymptotic calculations. The in-phase and out-of-phase terms of the hydrodynamic drag acting on the sphere are obtained and agree well with the experimental results. All together, the experimental, theoretical and numerical results show that the hydrodynamic force felt by an immersed sphere oscillating near a wall is highly dependent on the Womersley number. [ABSTRACT FROM AUTHOR]

For a power-law elliptical body, the drag force in a high-speed rarefied gas flow is calculated based on several local models. By solving the variational problem, the exponent in the generatrix for a minimum drag body of various aspect ratio is determined depending on the ellipticity coefficient. [ABSTRACT FROM AUTHOR]

The Thermal Protection System, or TPS, is the layer that shields spacecraft from the extreme aerodynamic heating that occurs during re-entry and launch. We proposed an optimized design of an Integrated Thermal Protection System (ITPS) in this paper. An ITPS provides thermal protection as well as structural load-bearing capability. Thermo-chemical simulations are carried out with the help of the Fully Implicit Ablation and Thermal response program (FIAT) for better understanding of the thermo-chemical behavior of the system under the extreme conditions of a space vehicle. Similarly, a structural analysis is performed to determine the structural integrity of the proposed ITPS design. Both the thermo-chemical and structural analyses show that the proposed corrugated-core sandwich structure is viable in terms of recession, pyrolysis, and structural integrity. [ABSTRACT FROM AUTHOR]

When inertial particles are dispersed in a turbulent flow at sufficiently high concentrations, the continuous and dispersed phases are two-way coupled. Here, we show via laboratory measurements how, as the suspended particles modify the turbulence, their behaviour is also profoundly changed. In particular, we investigate the spatial distribution and motion of sub-Kolmogorov particles falling in homogeneous air turbulence. We focus on the regime considered in Hassaini & Coletti (J. Fluid Mech., vol. 949, 2022, A30), where the turbulent kinetic energy and dissipation rate were found to increase as the particle volume fraction increases from 10-6 to 5 x 10-5. This leads to strong intensification of the clustering, encompassing a larger fraction of the particles and over a wider range of scales. The settling rate is approximately doubled over the considered range of concentrations, with particles in large clusters falling even faster. The settling enhancement is due in comparable measure to the predominantly downward fluid velocity at the particle location (attributed to the collective drag effect) and to the larger slip velocity between the particles and the fluid. With increasing loading, the particles become less able to respond to the fluid fluctuations, and the random uncorrelated component of their motion grows. Taken together, the results indicate that the concentrated particles possess an effectively higher Stokes number, which is a consequence of the amplified dissipation induced by two-way coupling. The larger relative velocities and accelerations due to the increased fall speed may have far-reaching consequences for the inter-particle collision probability. [ABSTRACT FROM AUTHOR]

This paper examines the analysis of entropy generation in the flow of an MHD Prandtl fluid over a nonlinear stretching sheet. Heat transfer is developed through a convectively heated sheet. The impacts of nonlinear radiation and nonlinear mixed convection are considered. The resulting nonlinear systems are computed for the unique solutions of velocity and temperature profiles. Effects of thermal radiation, the Prandtl number, Prandtl fluid parameters, and the Biot number are discussed. Results for the Nusselt number and skin friction coefficient are analyzed. The impact of the radiation parameter is to improve the rate of heat transport to the flow region. It is stated that temperature distribution increases for greater values of Of. We state that the fluid temperature decreases with the increasing importance of the Prandtl number Pr. Growth in the Prandtl number decreases the rate of thermal diffusion. It shows that the magnitude of drag forces decreases for larger values of Prandtl fluid parameters. Furthermore, curvature and mixed convection parameters boost the flow and heat transfer rate near the cylinder wall. The entropy generation grew up rapidly with larger values of magnetic and Brinkman numbers. The temperature ratio parameter and Prandtl fluid parameters reduce the entropy generation rate. These parameters are also used to control the entropy generation process. [ABSTRACT FROM AUTHOR]

To adhere their silk threads for the construction of webs and to fix the dragline, spiders produce attachment discs of piriform silk. Uniquely, the aquatic spider Argyroneta aquatica spends its entire life cycle underwater. Therefore, it has to glue its attachment discs to substrates underwater. Here we show that Argyroneta aquatica applies its thread anchors within an air layer around the spinnerets maintained by superhydrophobic setae. During spinning, symmetric movements of the spinnerets ensure retaining air in the contact area. The flat structure of the attachment discs is thought to facilitate fast curing of the piriform adhesive cement and improves the resistance against drag forces. Pull-off tests on draglines connected with attachment discs on different hydrophilic substrates point to dragline rupture as the failure mode. The Young´s modulus of the dragline (8.3 GPa) is within the range as in terrestrial spiders. The shown structural and behavioral adaptations can be the model for new artificial underwater gluing devices. Multiple behavioral and functional adaptations enable the unique water living spider Argyroneta aquatica to apply its silk attachment discs for anchoring its dragline within an air layer around its spinnerets under water. [ABSTRACT FROM AUTHOR]

Vertically vibrating a liquid bath may allow a self-propelled wave-particle entity to move on its free surface. The horizontal dynamics of this walking droplet, under the constraint of an external drag force, can be described adequately by an integro-differential trajectory equation. For a sinusoidal wave field, this equation is equivalent to a closed three-dimensional system of nonlinear ODEs. We explicitly define a stability boundary for the system and a quantised criterion for its partial integrability in the meromorphic category. [ABSTRACT FROM AUTHOR]

The presence of particles with a small but finite size, suspended in viscous fluids with low volumetric concentrations, is observed in many applications. The present study focuses on the tridimensional and incompressible lid-driven flow of Newtonian fluids through the application of the immersed boundary method and the Euler–Lagrange approach. These methods are used to numerically predict three-dimensional particle motion by considering nearly neutrally buoyant conditions as well as all relevant elementary processes (drag and lift forces, particle rotation, particle–wall interactions, and coupling between phases). Considering the current stage of the numerical platform, two coupling approaches between phases are considered: one-way and two-way coupling. A single particle is inserted in the cavity after steady-state conditions are achieved. Its three-dimensional motion is obtained from numerical simulations and compared with research data, considering the same conditions, evidently showing that the particle trajectory follows the experimental data until the first collision with a solid surface. After this first contact, there is a deviation between the results, with the two-way coupling results better representing the experimental data than the one-way coupling results. The dimensionless forces' peaks acting on the particles are associated with the relative velocity of the particle near the wall–particle collision position. In terms of magnitude, in general, the drag force has shown greater influence on the particle's motion, followed by the rotation-induced and shear-induced lift forces. Finally, a special application is presented, in which 4225 particles are released into the domain and their dynamic is evaluated throughout dimensionless time, showing similar behavior for both couplings between phases, with variations in local concentrations observed in certain regions. The mean square displacement used to quantify the dispersion evolution of the particles showed that the particulate flow reaches an approximately homogeneous distribution from the moment of dimensionless time tU/S = 130. [ABSTRACT FROM AUTHOR]

This study primarily focuses on conducting, both experimentally and numerically, a modal analysis of a cantilever composite beam. Through extended numerical simulations, we investigate Campbell diagrams, which, depending on the rotation speed of the structure, comprise natural frequencies and their corresponding modal shapes. Our results are categorized into two main aspects: the classical single-mode behavior and an innovative extension involving linearly coupled modal analysis. One key novelty of our research lies in the introduction of an analytical description for coupled mode shapes, which encompass various deformations, including bending, longitudinal deformations, and twisting. The most pronounced activation of dynamic couplings within the linear regime for a 45 ∘ preset angle is observed, though the same is not true of the 0 ∘ and 90 ∘ preset angles, for which these couplings are not visible. In addition to the modal analysis, our secondary goal is to assess the lift, drag forces, and moment characteristics of a rectangular profile in uniform flow. We provide insights into both the static and dynamic aerodynamic responses experienced by the beam within an operational frequency spectrum. This study contributes to a deeper understanding of the dynamics of composite rotating beams and their aerodynamic characteristics. [ABSTRACT FROM AUTHOR]

In this work, a two-fluid phase flow problem involving an axisymmetrical quasi-steady motion of a spherical micropolar droplet translating at a concentric point in a second non-mixable micropolar fluid within a spherical impermeable cavity with a slip surface is analysed under low Reynolds numbers. The two fluid phases that have a microstructure (micropolar fluid) are the case that is being focused on. The Stokes equations are solved inside and outside the droplet for the velocity fields. In addition, based on the concentric position, general solutions in terms of spherical coordinates are obtained. In this case, tangential couple stress and continuity of microrotation are used. For different cases, the normalised drag forces acting on the droplet are represented via graphs for different values of relative viscosity, droplet-to-cavity radii ratio, and the parameter that connects the tangential couple stress with microrotation. The normalised drag force is found to be a monotonically increasing function of the drop-to-cavity radii ratio. It is found that when the droplet-to-cavity radii ratio approaches zero, there is a very strong interaction between the droplet and the cavity. When comparing a solid sphere to a gas bubble, the normalised drag force is larger. Additionally, the results showed that permitting spin and slip at the cavity's interior surface improved the wall correction factor influencing the droplet. The present study is important in the fields of natural, industrial, and biomedical processes such as raindrop formation, liquid–liquid extraction, suspension rheology, sedimentation, coagulation, and the motion of blood cells in an artery or vein. [ABSTRACT FROM AUTHOR]

For turbulent flows in porous media, it is often assumed that the irregularity of the matrix would not affect the macroscopic, double‐averaged (in time and space), parameters of turbulence. Hence, to improve currently used modelling techniques, the ongoing efforts of the community are focused on performing high‐fidelity, scale‐resolving simulations of flows in regular, periodically repeating porous structures. This approach allows for minimizing still large computational cost, however, it assumes that results obtained from those geometries are generalizable to more disordered configurations. We investigate this assumption numerically using large‐eddy simulation and analyse the influence of the irregularity of the porous structure on turbulent characteristics in a fully turbulent flow in a channel half‐filled with porous medium. The flow statistics are examined and compared between four simulations, a reference channel with regular three‐dimensional porous matrix consisting of an array of cubes and three similar geometries created from randomly perturbing the positions of the cubes, with increasing mean value of cube displacement. We study double‐averaged flow properties, the distributions of double‐averaged Reynolds stress tensor, including the anisotropy of macroscopic turbulence and the drag force induced by the flow in the porous region of the channel. The results confirm the idea that the averaged characteristics, including the anisotropy of the stress tensor and the drag force, are not greatly influenced by the moderate perturbation to the geometry of the porous matrix. Additionally, the macroscopic turbulence seems to exhibit a high degree of anisotropy in the porous region and the adjacent boundary layer, suggesting that second‐order closure could be an adequate modelling choice for such flows. [ABSTRACT FROM AUTHOR]

Compressible flow through a branched duct and the motion of a sphere through a high blockage ratio pipe are two important and engaging topics throughout the years. The results of studies on these topics are of practical relevance to many fields such as the gas pipeline technology, air transport systems in gas turbines technology, and tube transportation, etc. However, studies on the motion of a sphere in a branched duct are scarce. Studies of the motion of a sphere in a near-vacuum tube could contribute to the development of a branched Hyperloop system in the future. In this study, we investigated the effect of the tube curvature radius and angle on the aerodynamic characteristics of a sphere during its motion in a branched tube. We examined, quantified, and compared the variation of the drag, side force, and pressure waves for the cases where a sphere enters a branch considering six curvature radii (from 500 to 3000 m), three angles (10°, 15°, and 20°), three speeds (100, 200, and 300m s-1), and two initial pressures (1/1000 and 1 atm) in simulations. The results indicated that the drag and side force vary only in the intersection region (region where the straight tube and branched tube intersect); before and after the intersection region, they are similar. With an increase in the curvature radius, the rate of drag reduction (FD/max(FD)) decreases, while the changes in the angle do not affect variation of drag and side forces. Furthermore, we compared the motion of a sphere in straight and branched directions and found out that the flow in front and behind the sphere was similar for both directions. [ABSTRACT FROM AUTHOR]

Feedforward neural network (FNN) models with strong learning ability and prediction accuracy are crucial for optimization. This paper investigates the effects of the number of training samples and the hidden layers on the accuracy of the FNN model. Meanwhile, under the premise of a high space-fillingness degree, a sample expansion strategy based on the max--min distance criterion is proposed, which ensures that the expanded sample set completely contains the pre-expanded. The strategy can eliminate the interference of sample differences. Furthermore, the multi-objective optimization on the train nose shape is accomplished by minimizing the aerodynamic lift force of the tail car (LT), as well as the aerodynamic drag force of the head (DH) and tail car (DT) using the FNN model. The results indicate that the number of training samples has a greater impact on the prediction error of the FNN model than the number of hidden layers does. Prediction errors decrease as the number of training samples increases and then stabilise, the most accurate one is chosen for nose shape optimization. The DH, DT, and LT all have prediction errors of less than 2%. Compared with the original high-speed train, the DH, DT, and LT of the optimal model are reduced by 5.24%, 3.74%, and 2.61%, respectively. Meanwhile, the correlation analysis reveals that the height of the cab window and the horizontal profile have a significant impact on the aerodynamic characteristics of the high-speed train. [ABSTRACT FROM AUTHOR]

Here, we investigate the actuation dynamics of a micro device with different intervening liquids between the actuating components under the influence of Casimir and dissipative hydrodynamic forces. This is enabled via phase space portraits, which demonstrate that by increasing the dielectric response of the intervening layer the autonomous device may not come into stiction due to the decreasing in magnitude Casmir force. Unlike the micro devices that are placed in vacuum with an intervening liquid, the phase portraits show only a spiral trajectory which eventually stops at a rest position due to the strong energy dissipation by the position dependent hydrodynamic drag forces, even when considering sufficiently strong restoring forces. Moreover, it is feasible to expand the area of motion using intervening liquids with lower dynamic viscosity or increasing the slip length of the intervening fluid. Finally, under the influence of an external driven force, which is the realistic case for possible applications, the system can reach stable oscillation at larger separations with an amplitude higher for the liquid that led to lower Casimir and hydrodynamic forces. Hence, the results presented in this study are essential for studying the dynamical behavior of MEMS and their design in liquid environments. [ABSTRACT FROM AUTHOR]

In this study, a new approach based on dielectric barrier discharge (DBD) plasma was used to fabricate super-buoyant dielectric materials for drag-reduction. We used a glass slide as a high-density substrate for buoyancy tests and a model boat made from Polymethyl methacrylate (PMMA) for drag-reduction experiments. In the first step, an atmospheric pressure DBD plasma was used for the deposition of the hydrogenated amorphous carbon (a-C:H) film using argon working gas and toluene precursors. The layer characterizations were performed versus the plasma deposition time by FE-SEM, AFM, FTIR, and Raman analysis. The morphological characterizations showed the formation of some micro and nanostructures on the surface followed by an increase in surface roughness. Moreover, the chemical characterizations suggested the successful formation of a-C:H film. In the next step, the effect of the deposited layer was assessed on the buoyance and drag reduction. Based on the results, the plasma coating produced a super-buoyant glass slide with remarkable load-bearing capacity, high durability, and robustness. In addition, the drag force was reduced noticeably after the plasma treatment. The results of this study promise the use of atmospheric pressure plasma for the fabrication of super-buoyant surfaces for buoyancy applications. [ABSTRACT FROM AUTHOR]

Coastal cities at low-lying elevations face extensive inundation risks when attacked by extreme storm surges or tsunamis. Efficient and accurate modeling of flooding is critical for reducing losses in economic and lives. Therefore, the paradox arises that the computational cost is usually too high for rapid forecasts when considering the complex topography and buildings. The present contribution presents a subgrid-based approach, which combines the subgrid model and a drag force module, to realize efficient modeling of urban flooding at a high resolution. The volume-occupation effects of buildings are incorporated naturally in the subgrid treatment, and the blockage, sheltering, and frictional effects of buildings acting on the flows are considered through a simple, bulk parameterization in the form of drag forces. The model is validated with a laboratory experiment and then applied to an ideal numerical model at a real city scale. The proposed model is compared to three other models using coarse grids, with high-resolution simulation results serving as the benchmark for evaluation. The results demonstrate that the presence of buildings induces noticeable alterations in the distribution of water level and velocity fields within the urban area. The proposed model yields comparable predictions to the benchmark in terms of inundation extent, elevation, and velocity. Furthermore, the model exhibits minor sensitivity to grid size, allowing for a significant reduction in computational time compared to high-resolution simulations. The model achieves a well-balanced combination of high resolution, accuracy, and efficiency for large-scale simulations of coastal urban flooding. [ABSTRACT FROM AUTHOR]

The shape of the tube in submerged floating tunnels (SFTs) plays a critical role in determining their performance and safety in marine environments. In this study, a gradient optimization procedure based on the discrete adjoint method is performed to minimize the drag force under uniform currents action. The free form deformation approach is employed to parameterize the design variables. The physical mechanisms of the optimization process are revealed via unsteady numerical simulations and experimental tests. Moreover, the hydrodynamic performance of the tube with the optimal shapes subjected to a wave–current combination is also evaluated. The results show that the drag coefficient is significantly reduced through optimization by reducing the pressure amplitude in the negative-pressure area. Additionally, the lift oscillation is also suppressed to delay structural fatigue, owing to the strength reduction and increased distance of the wake vortex. The experimental analysis indicates the advantage of the optimal shape in reducing the loads under wave–current actions, accompanied by changes in the frequency distribution of the force and vortex structure. The elliptical shape of the SFT's tube provides significant advantages in drag reduction at high Reynold number. Suggestions on the shape design of the section of SFT are given according to different types of constraints. [ABSTRACT FROM AUTHOR]

The present study is focused on the problems of gas–solid two-phase flow transport in the film cooling hole that cause film flow obstruction and cooling failure. To study the unsteady development process of the deposition layer in the film hole, a simulation method combining computational fluid dynamics and the discrete element method was used, and a film hole flow model was established. The effect of gas phase and solid phase characteristics on clogging and deposition in the film hole was studied. The following conclusions are drawn: The inlet/outlet pressure ratio is inversely proportional to the clogging degree of the film hole. The inlet/outlet pressure determines the deposition behavior by affecting the initial momentum and drag force of particles. In the Stokes number range of 1.58–14.26, the deposition in the film hole first increases and then decreases. There is a Stokes number with the most severe clogging. The Stokes number determines the deposition pattern by affecting the relative magnitudes of the drag force and interaction forces of particles. The particle surface energy is positively correlated with film hole clogging. The particle surface energy determines the stability of the deposition layer by influencing the strength of the force chain network. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of gas−solid fluidized beds with different particle densities using a coupled computational fluid dynamics−discrete element method. The accuracy of the numerical method is experimentally verified, and the characterization parameters of the gas−solid fluidized bed are evaluated, including velocity distribution, bubble equivalent diameter, dead zone angle, particle force, bed pressure, and mixing index. The effects of five particle densities on the gas−solid fluidized bed were analyzed in detail while contact and drag models were fixed. The predicted results show that an increase in particle density will reduce the distribution of translational and rotational velocities, which will have an inhibitory effect on the bubbles in the bed, with a positive correlation with the angle of the dead zone. It is explained that the main forces on particles during fluidization are contact force, drag force, and pressure gradient force, and the normal contact force is two orders of magnitude larger than the drag force and the pressure gradient force. The phenomenon that the normal contact force is much larger than the tangential contact force is explained. The increase in particle density has an increasing effect on the bed pressure, takes more time to reach a good degree of mixing, and reduces the mixing performance. The study of the effect of particle density on gas−solid fluidized beds can provide theoretical guidance for the structural design as well as theoretical development of subsequent fluidized beds. [ABSTRACT FROM AUTHOR]

Bubbling liquefaction of dense particles is one of the most common forms of industrial fluidization in gas–solid flow systems. Computational fluid dynamics and the discrete element method are important tools for studying dense gas–solid flows. In these methods, the momentum transfer between phases relies on a drag model, so a reasonable choice of drag model is crucial for accurately predicting the hydrodynamic behavior of dense gas–solid flows. This paper investigates the effect of different drag models on the flow behavior prediction of dense gas–solid flow for the "Small-Scale Challenge Problem-I" published by the National Energy Technology Laboratory in 2013. The gas–solid fluidization characteristics, such as instantaneous particle flow processes, particle velocity vector distributions, changes in the fluidized bed height, and average gas phase pressure drops, were compared for different drag models. A detailed validation analysis of each dominant drag model was carried out in conjunction with the experimental data. The results show that the drag model significantly affects the numerically predicted results of particles' hydrodynamic behavior, especially in terms of the bed height variation and the remixing behavior of particles. These research results are expected to improve the predictive accuracy of gas–solid flow hydrodynamic behavior and provide guidance for designing and optimizing fluidized beds, which has theoretical and practical significance. [ABSTRACT FROM AUTHOR]

Industrial operations often involve multiphase flows, where the motion of bubbles plays a crucial role in determining hydrodynamic properties. In the context of modern practices, advanced computational solvers are increasingly employed to simulate the interactions and trajectories of bubbles within complex flows. This study stands out in its unique examination of the computational software COMSOL's capability to accurately simulate the upward motion of a single bubble within quiescent water, akin to a liquid-like environment. The novelty lies in the comprehensive coverage of various container diameters (ranging from 10 to 80 mm) and heights (from 25 to 300 mm) relative to the air bubble diameter (ranging from 0.5 to 8 mm). Through meticulous comparisons between nine empirical formulas and numerically projected bubble ascent through a water column, a remarkable level of agreement emerges, underscoring the precision and consistency of the simulations. These simulations unveil intriguing findings, shedding light on the intricate interplay of forces governing bubble behavior. Notably, variations in drag forces induce changes in bubble shapes as a function of diameter, while the ascent of bubbles is accompanied by distinctive vortices, resulting in fascinating asymmetry. Furthermore, vorticity concentrates within the bubble, particularly in lighter fluid regions characterized by reduced pressure. The study also unveils how larger aspect ratios minimize flow drag, consequently boosting ascent velocities, and demonstrates the influence of container diameter on the rising velocity. Gravitational forces are found to reduce ascent velocities at greater column heights, while the rate of air bubble rise escalates with its size. This meticulous exploration of bubble dynamics in multiphase flows yields invaluable insights for diverse industrial applications, ultimately enhancing our understanding of this complex phenomenon. The strong alignment observed between empirical formulations and numerical simulations within the COMSOL framework underscores the utility of such computational tools for the study and design of multiphase flows' intricate dynamics. [ABSTRACT FROM AUTHOR]

The axisymmetric motion of a solid spherical particle embedded in a hydrogel medium in the presence of a planar wall surface is investigated semi-analytically. The hydrogel medium is modeled as a porous medium saturated with a microstructure fluid of micropolar type. The no-slip velocity and no-slip spin boundary conditions are considered at both the particle surface and the plane wall surface. The sixth-order differential equation describing the stream function of the micropolar fluid flow through the voids of the porous medium is constructed under the assumption of low Reynolds numbers. The general solution of the equation satisfied by the stream function in the porous region is obtained from the superposition of basic solutions in both cylindrical and spherical coordinates. To satisfy first the boundary condition at the planar surface, we apply the Fourier–Bessel transforms and then at the surface of the particle by a boundary collocation technique. The collocation scheme for the normalized drag force acting on the particle is calculated with good convergence for various values of the relevant parameter. Our results are in good agreement with the available data in the literature. The findings of the present investigation demonstrate that the presence of the planar surface, micropolarity, and permeability parameters has significant effects on the drag force. This study is motivated by its potential application on micro- and ultra-filtration. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of a morphing airfoil is numerically and experimentally investigated. The morphing airfoil is designed based on macro fiber composites, capable of trailing edge flapping during 10–90 Hz with a maximum amplitude of 0.55 mm. A numerical model with flexible deformation walls based on the experiment is established to precisely restore the actual dynamic morphing instead of segmental deformation to explore the transient aerodynamic performance of high-frequency flapping. The drag coefficient is reduced by 2.07% at the flapping frequency (ff) of 37.5 Hz compared with the rigid airfoil, while the drag coefficient and the lift coefficient increase by 4.8% and 5.8% for ff at 600 Hz. The vortex is broken up by flapping, and the corresponding position has been forwarded to the tail. Dynamic mode decomposition shows that the wing's flapping dominates the second mode and the high-frequency vortex has changed to low-frequency. The energy of higher modes is transferred to lower-order modes that the first mode's power has risen sharply from 49.29% of the rigid airfoil to 91.83%. In the wind tunnel experiment, the lift and drag forces are increased by 1.88% and 0.77% at the flapping frequency of 40 Hz, respectively. Furthermore, the lift force frequency is locked by flapping and changes from 124.9 Hz of the rigid airfoil to the flapping frequency, consistent with the computational fluid dynamics results. The research has provided a solution to reduce the drag force and increase the lift force of the aircraft by the trailing edge flapping. [ABSTRACT FROM AUTHOR]

In this investigation, two-dimensional flow past square cylinders with slip boundary have been studied with the lattice Boltzmann method. Three modes, which are a single cylinder, an oblique cylinder, and side-by-side cylinders, are investigated with Reynolds numbers from 25 to 200 while relative slip length ranges from 0 to 0.1. It can be concluded that both the flow state and the slip boundary have a great effect on the drag reduction rate. For a single square cylinder, drag forces decrease with larger slip length while the Strouhal number is almost constant. For an oblique cylinder, the slip length also has effects on the stability of the flow except the drag and lift forces. Vortex separation delays with a slip wall of oblique cylinder. For side-by-side cylinders, the jet between two cylinders is not conducive to the drag reduction rate of the slip boundary. Moreover, the application of slip boundary may also lead to additional drag force as vortex separation intensifies, which is extremely different from simple channel flows. Studies have shown that the slip boundary does not always reduce the drag in some complex flow fields. It can be concluded that the drag reduction effect of slip boundaries is more effective in uniform flow. [ABSTRACT FROM AUTHOR]

In this paper, the flow and particle trajectories, induced by standing surface acoustic waves (SSAWs) in a poly-dimethylsiloxane microchannel, are investigated by establishing a two-dimensional cross-sectional model with the finite element method and improved boundary conditions. Extensive parametric studies are conducted regarding the channel height, ranging from 0.2 to 4.0 times the spacing of the repetitive vertical interference pattern, to investigate its influences on the flow field and microparticle aggregation. The first-order flow field is found to be related to the channel height, exhibiting a periodic spatial distribution and oscillatory variation in its amplitude as the height changes. We theoretically analyze the propagation mechanism of the acoustic waves in the vertical direction and thus determine the periodicity of the wave interference pattern. Furthermore, we find that the speed of the particle aggregation is a function of the channel height, so the channel height can be optimized to maximize the strength of the first-order flow field and thus minimize the time of particle aggregation. The optimum heights can reduce the aggregation time by up to 76%. In addition, the acoustophoretic motions of microparticles exhibit a spatially dependent pattern when the channel height becomes larger than a quarter of the wavelength of the SAW, which can be explained by the change in the ratio between the radiation force and the streaming drag force from position to position. Our findings provide guidelines to the design and optimization of SSAW-based acoustofluidic devices. [ABSTRACT FROM AUTHOR]

The forces arising on the ship's rudderat different angles of attack in an unbounded flow are investigated. The components of the resulting force on the rudder are represented in terms of the rudder lift and drag forces, as well as in terms of the normal and tangential forces on the rudder. The well-known mathematical models of hydrodynamic rudder coefficients are analyzed, and their disadvantages are found. New mathematical models of hydrodynamic coefficients have been obtained, in particular, the coefficients of rudder lift and drag, which take into account the aspect ratio of the rudder, its relative thickness and can be applied to any angle of attack of the flow on the rudder. On specific examples for rudders of the NACA series, the adequacy of the proposed models and their consistency with known experimental studies are illustrated. It is shown how the rudder lift and drag change, as well as the components of the resulting force for the maximum possible range of changes in the local drift angle and the rudder angle. [ABSTRACT FROM AUTHOR]

In recent years, underwater robotics has become very important because it can be applied to various fields such as underwater exploration, underwater inspection, marine industry, and environmental monitoring. Fin plays an essential role in the movement of underwater robots, providing operation, control, and efficient propulsion. This research aims to design and develop a unique robotic fin for underwater robots to improve their handling and propulsion efficiency. The goal is to improve the power density and propulsion efficiency of underwater survey robots. The study is based on a comprehensive analysis of experience and a performance evaluation. Five types of tail fin models were used in the study. The experimental results showed that the performance of the fin design can be compared with existing configurations under different conditions. The best design parameters will be determined by analyzing the experimental results. The results of this study will contribute to underwater robotics by providing a concept of the principles of fin design and its impact on the performance of robotics. [ABSTRACT FROM AUTHOR]

In this paper, we developed a mathematical model to simulate virus transport through a viscous background flow driven by the natural pumping mechanism. Two types of respiratory pathogens viruses (SARS-Cov-2 and Influenza-A) are considered in this model. The Eulerian–Lagrangian approach is adopted to examine the virus spread in axial and transverse directions. The Basset–Boussinesq–Oseen equation is considered to study the effects of gravity, virtual mass, Basset force, and drag forces on the viruses transport velocity. The results indicate that forces acting on the spherical and non-spherical particles during the motion play a significant role in the transmission process of the viruses. It is observed that high viscosity is responsible for slowing the virus transport dynamics. Small sizes of viruses are found to be highly dangerous and propagate rapidly through the blood vessels. Furthermore, the present mathematical model can help to better understand the viruses spread dynamics in a blood flow. [ABSTRACT FROM AUTHOR]

We present trajectory simulation-based modeling to capture the interactions between ions and charged grains in dusty or complex plasmas. Our study is motivated by the need for a self-consistent and experimentally validated approach for accurately calculating the ion drag force and grain charge that determine grain collective behavior in plasmas. We implement Langevin dynamics in a computationally efficient predictor–corrector approach to capture multiscale ion and grain dynamics. Predictions of grain velocity, grain charge, and ion drag force are compared with prior measurements to assess our approach. The comparisons reveal excellent agreement to within ± 20 % between predicted and measured grain velocities [Yaroshenko et al., Phys. Plasmas 12, 093503 (2005) and Khrapak et al., Europhys. Lett. 97, 35001 (2012)] for 0.64 , 1.25 μ m grains at ∼ 20 − 500 Pa. Comparisons with the measured grain charge [Khrapak et al., Phys. Rev. E 72, 016406 (2005)] under similar conditions reveal agreement to within ∼ 20 % as well. Measurements of the ion drag force [Hirt et al., Phys. Plasmas 11, 5690 (2004); IEEE Trans. Plasma Sci. 32, 582 (2004)] are used to assess the viability of the presented approach to calculate the ion drag force experienced by grains exposed to ion beams of well-defined energy. Excellent agreement between calculations and measurements is obtained for beam energies >10 eV, and the overprediction below 10 eV is attributed to the neglect of charge exchange collisions in our modeling. Along with critical assessments of our approach, suggestions for future experimental design to probe charging of and momentum transfer onto grains that capture the effect of space charge concentration and external fields are outlined. [ABSTRACT FROM AUTHOR]

Bridge inspections are critical for maintaining structural integrity and must be inspected on a regular basis to ensure their dependability. Aerial vehicles provide a safer, costefficient, and time-saving method for inspection. However, UAVS (Unmanned Aerial Vehicles) are limited from flying near the bridge structure due to their exposed propellers. Using shelled UAVs addressed the collision problem while giving a good visual condition and increasing motion effectiveness. In this study, a shelled UAV system that structural inspectors can use to perform a close visual inspection of the bridge was developed. The newly developed shelled UAV features a passive rotating shell with a two-axis gimbal. This shelled UAV is waterproof, modular, capable of water takeoff and landing, and integrated with a crack detection system. A functional shelled UAV was fabricated, and test flights were successfully conducted. The results of the computational simulations and actual flight tests showed that the shelled UAV is overall safe and effective in terms of its strength-to-weight consideration, drag force, and stability performance. Moreover, crack detection systems and software applications were developed. A curated dataset was also produced for the purpose of training the crack detection system. The U-Net architecture was used as the segmentation model trained on the dataset. The trained model was effective and could predict and segment cracks in the gathered dataset images. The functionality of the Data Acquisition App, Damage Detection App, and Inspection Details App was tested and verified. [ABSTRACT FROM AUTHOR]

Natural or human-induced intra-annual variation of river discharge alters estuarine hydrological regimes and further affects habitat conditions for saltmarsh vegetation, particularly at the river mouth bar. In this study, numerical experiments were performed in Delft3D to simulate the evolution of a schematized river mouth bar under prototypical unsteady river discharge scenarios. The simulated hydrodynamic and morphodynamic changes were used to drive a vegetation dynamics model developed based on Spartina alterniflora in real time to model the resultant vegetation responses throughout the plant life history. Our results show that the imposed seasonal high flow can create more potential suitable habitat for the vegetation expansion and at the same time, cause marsh erosion through flood-induced drag force and substrate erosion. The overall effect of the trade-off between expansion and erosion depends on the timing, magnitude and duration of the high flow as well as its carried sediment concentration, leading to three vegetation response regimes, namely, minimal impact with small flood, erosion with big flood and low sediment supply, and expansion with big flood and high sediment supply. Besides, the timing of the high flow determines whether the vegetation has enough time to occupy the newly created subaerial area after the high flow and thereby affects the overall saltmarsh extent. The proposed vegetation response regimes are verified in principle in real cases such as Yellow River Estuary, Wax Lake Delta and Yangtze River Estuary. Our findings can help inform water diversion projects in river deltas to restore coastal wetlands in terms of suitable sediment supply and timing, etc. [ABSTRACT FROM AUTHOR]

This paper focused on the investigation of the blades for a carousel rotor of a wind turbine with a vertical axis. Cross sections of the single coherent (SC) and the dual coherent (DC) blades were compared in terms of the aerodynamic forces and aerodynamic torque generated during rotor operation for various wind attack angles. The design of the DC blade is novelty proposed by the authors. The main objective of the study was to determine the influence of the blade cross-section on the propelling torque of a wind turbine with three blades, which is an important parameter for rotor starting. First, experimental studies were carried out in a wind tunnel for real-size blade models. A CFD analysis of the airflow around the blades was then conducted. The obtained results were used to evaluate the suitability of applying the studied blade types in the design of the carousel wind rotor. The assessment compared the drag force and the lift force as well as aerodynamic torque as a function of a wind attack angle. It was concluded that the rotor with three DC blades involved mainly the drag force in contrast to the rotor with three SC blades that also involved the lift force to a greater extent. Despite the rotor with DC blades obtained greater values of the drag forces on the blades, the rotor with SC blades obtained a greater starting torque. [ABSTRACT FROM AUTHOR]

This study purposes to estimate the spatial distribution of vegetation removal on a gravel bed and braided river, called the Tenryu in Central Japan. Aerial photograph and flood flow simulation are used to observe the vegetation change. Frequent changes of river width, bed level, bar areas indicate that it is an active river geomorphologically. High magnitudes of flood flow have direct impacts on bar area changes and vegetation dynamics. High discharge and less vegetation in the bar area produced extensive land erosion in the year 1990. High annual flood flow promotes vegetation removal and low flood flow in the preceding year accelerates vegetation invasion. Numerical simulation of flood flow can estimate the spatial conditions of vegetation wash out or vegetation remaining with minor misjudgment. Inclusion of sediment transport equation in the simulation may reduce the misjudgment during computation. [ABSTRACT FROM AUTHOR]

Electrocoalescence has long been known for the separation of a water‐in‐oil emulsion. An associated challenge with electrocoalescers is the undesired noncoalescence and consequently chain formation of aqueous phase droplets. This leads to low separation efficiency and damage of electrical equipment. Recently Hasib et al. proposed an electric field modulated scheme that showed significant improvement in dehydration of water‐in‐oil emulsions. They investigated the range of modulation parameters when the scheme is most effective. The fundamental process in electrostatic dehydration of an emulsion is the interaction between a pair of water droplets. In the present study, two suspended aqueous drops in insulated oil experiments are compared for their behavior under unmodulated and modulated electric fields. Further, a model is developed and the experimental behavior under unmodulated and modulated electric fields is compared with numerical solutions. The model predicts the experimental observations accurately by balancing electrostatic, electrophoresis, dipolar, and resisting viscous drag forces, ignoring the end (bridge) effect during the contact. The study shows that the increase in the rate of dehydration of a water‐in‐oil emulsion under modulated electric fields with an increase in duty ratio and its near independence on the modulation time period can be explained by the two‐drop studies. However, several other processes such as multidrop interactions as well as scavenging of fine droplets by charged droplets created as intermediates in the interaction of two droplets cannot be explained by the two‐drop studies. [ABSTRACT FROM AUTHOR]

The rainstorms that initiate over mountains, propagate to, and suddenly intensify in nearby plains pose great challenges for weather forecasters and the public. Being in a mountain‐plain transition zone, the city of Beijing is prone to rainfall extremes induced by weakly forced mountain‐to‐plain storms. Despite efforts to understand their possible mechanisms, it remains unclear whether and how these mountain‐to‐plain precipitation extremes are affected by urbanization in Beijing. Here we use a high‐resolution weather prediction model to hindcast a weakly forced mountain‐to‐plain extreme rainfall event impacting Beijing on 23 June 2011. We incorporate spatially varying urban canopy parameters into the model to assess their effect on rainfall simulation and find no significant improvement in model performance. By comparing simulations under current and pre‐urbanization scenarios, we find that urbanization intensifies the regional average rainfall within Beijing's Sixth Ring Road by ∼23%, with an increase in peak hourly rainfall rate reaching 27 mm hr−1. The higher surface roughness and enhanced vertical motions in urban areas facilitate the transport of warm and humid air from lower to mid‐upper layers, fueling the development and maintenance of convective storms. The dragging effect of urban canopy leads to more moisture accumulation and slower‐moving convections, resulting in more rainfall falling in urban areas. Our study underscores the key role of local urbanization in shaping the mountain‐to‐plain rainstorms in Beijing, which may help improve the forecast capacity for these weather extremes and related urban resilience planning. Plain Language Summary: This study aims to investigate whether and how urbanization affects mountain‐to‐plain rainstorms in Beijing, which pose considerable challenges for weather forecasters and urban residents. A high‐resolution weather forecast model is employed to simulate a typical weakly forced mountain‐to‐plain extreme precipitation event that hit Beijing on 23 June 2011. Incorporating spatially varying urban parameters into the model has a limited effect on the model's ability to reproduce this rainfall case. The findings reveal that urbanization increases rainfall totals in urban areas by nearly 23%, because of the higher urban land surface roughness and drag forces that enhance vertical motions and lower‐level water vapor convergence. Moreover, the urban canopy of buildings and roads acts as a barrier, causing more moisture to accumulate and retard the advancement of rainstorms, resulting in more rainfall in urban areas. This understanding could potentially enhance the capability of weather forecasting and disaster planning for these extreme rainfall events, thereby contributing to the strategy development of urban resilience. Key Points: Gridded urban canopy parameters have a limited effect on model's skill in simulating a typical mountain‐to‐plain rainfall event in BeijingUrbanization‐induced rainfall increase is linked to urban surface dynamics under a weak urban heat islandThe high urban surface roughness and drag force of urban canopy collectively contribute to the precipitation intensification in urban areas [ABSTRACT FROM AUTHOR]

This study utilized 3D Large-eddy simulation to investigate the crosswind drag force on a square cylinder subjected to transverse wind fluctuation. Two distinct methods were employed to generate the fluctuation: a prescribed sine function at the inlet boundary and an upwind barrier. The frequency was normalized in the same Strouhal number form. The transverse wind fluctuation with a normalized frequency above 0.05 tends to excite the square cylinder transversely with the same frequency band. The frequency effect also exists on the square cylinder located downwind an obstacle half the square cylinder's size. However, an obstacle 2.5 times the size of the square cylinder generates a cross-wind fluctuation with the normalized frequency of 0.04, which cannot excite the square cylinder transversely. The frequency effect from the upwind barrier significantly dampens with the distance and disappears at 8–10 times the square cylinder size. [ABSTRACT FROM AUTHOR]