Article
Mechanics
P. Gualtieri, F. Battista, F. Salvadore, C. M. Casciola
Summary: In this study, two-way momentum-coupled direct numerical simulations were conducted to investigate the effect of particle Stokes number and particle-to-fluid density ratio on turbulence modification in a particle-laden turbulent channel flow. The results showed that particles increase the friction drag, especially for particles with small Stokes numbers. The additional stress caused by the particles results in an increased momentum flux towards the wall, modifying the structure of the buffer and viscous sublayer and increasing velocity fluctuations. This behavior explains the increase in friction drag observed in particle-laden wall-bounded flows.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
T. Zahtila, L. Chan, A. Ooi, J. Philip
Summary: In particle-laden turbulent wall flows, the transport of particles towards solid walls is believed to be controlled by the wall-normal turbulence intensity, which supports particle-eddy interactions modeled by turbophoresis and turbulent diffusion. By directly estimating the turbophoretic and turbulent diffusive coefficients from a generated direct numerical simulation (DNS) database, we can predict the particle concentration as a function of wall-normal distance and time. Our results show that the common gradient diffusion hypothesis may be limited for larger particles with higher Stokes numbers (St(+)), as indicated by a favorable comparison with DNS for smaller St(+) particles. Additionally, we explore the effects of St(+), pipe wall condition, drag and lift forces on particle velocity and statistics, and assess Eulerian-based models for turbophoretic and turbulent diffusive coefficients.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Yan Xia, Zhaosheng Yu, Dingyi Pan, Zhaowu Lin, Yu Guo
Summary: A drag correlation model for laminar particle-laden flows is established based on interfaced-resolved direct numerical simulations. The model takes into account the effects of density ratio and turbulence by correcting the drag correlation and introducing the relative turbulent kinetic energy.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Ryan A. Palmer, Frank T. Smith
Summary: The study describes direct numerical solutions for flow past a body placed in a shear layer adjoining a wall, with particular focus on an inclined flat plate. Increasing local Reynolds number leads to more complex flow characteristics, including enhanced wake responses and increased sensitivity to body orientation.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Ali Rostami, Ri Li, Sina Kheirkhah
Summary: This study investigates the characteristics of droplets, clusters, and voids in sprays injected into a turbulent co-flow. Experimental methods including Mie scattering, interferometric laser imaging, and hotwire anemometry were used. The results show that the mean length scale of the clusters varies linearly with the Stokes number, while the mean void length scale is on the order of the integral length scale. The number density of droplets inside clusters is approximately 7 times larger than that in the voids, and the ratios of droplet number densities in clusters and voids to the total number density are independent of test conditions.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Yi Liu, Yu Guo, Bo Yang, Dingyi Pan, Zhenhua Xia, Zhaosheng Yu, Lian-Ping Wang
Summary: The present study improves the understanding of the sedimentation behavior of two flat disks in a viscous fluid through direct numerical simulation and physical experiments. The results show that the shape of the disks, characterized by dimensionless moment of inertia I* and Reynolds number Re, significantly influences the sedimentation processes. The behavior of the disks transitions from steady falling to periodic swinging as Re increases for flatter disks with smaller I*. Disks with larger I* tend to fall in a drafting-kissing-tumbling mode at low Re and remain separated at high Re. A phase diagram is created based on I* and Re to classify the falling behavior into ten distinctive patterns. The planar or three-dimensional motion of the disks is primarily determined by Re, with turbulent disturbance flows contributing to chaotic three-dimensional rotation. The chance of contact between the two disks is increased when I* and Re are reduced.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Cheng Peng, Lian-Ping Wang
Summary: This study utilizes direct numerical simulations to investigate the changes in mean particle drag caused by turbulent environments and the responsible mechanisms. The study confirms that turbulent environments significantly enhance the particle drag, and the enhancement cannot be explained solely by the nonlinear dependency of drag force on the incoming flow velocity. Two mechanisms of particle-turbulence interactions are identified as responsible for the drag enhancements. General models accounting for turbulence anisotropy are proposed for quantitative drag enhancement predictions. It is also found that the overall particle drag increases with decreasing particle-particle relative gap distance, while the relative enhancement due to turbulence decreases with the particle-particle relative gap distance.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Athanasios Boutsikakis, Pascal Fede, Olivier Simonin
Summary: The effect of electrostatic forces on the dispersion of small inertial particles transported by turbulent flow has been analyzed using numerical simulations and Lagrangian tracking. The results show that increasing particle charge decreases particle dispersion. It was found that electrostatic forces modify particle agitation by altering the particle entrainment by fluid turbulence, leading to a decrease in particle dispersion.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Pedro Costa, Luca Brandt, Francesco Picano
Summary: The study found that modulation of near-wall turbulence by small inertial particles at different solid mass fractions results in an increase in drag. When the solid mass fraction is small, the impact of particles is weak, and compared to cases with higher volume fractions, fluid Reynolds stresses are attenuated.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Mobin Alipour, Marco De Paoli, Sina Ghaemi, Alfredo Soldati
Summary: In this study, we investigated the dynamics of non-axisymmetric fibers in turbulent channel flow, exploring the impact of fiber curvature on their orientation and rotation rate through statistical analysis of their three-dimensional position, shape, and orientation. Our results provide insights into the behavior of curved fibers in turbulent flow environments and suggest similarities with previous studies on homogeneous isotropic turbulence.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Zhaosheng Yu, Yan Xia, Yu Guo, Jianzhong Lin
Summary: It has been found that the characteristics of particle-laden turbulent flows vary with different Re-p values, and factors such as particle size, density ratio, particle volume fraction, etc., have a significant impact on turbulence intensity.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Soohyeon Kang, Liu Hong, Shyuan Cheng, James L. Best, Leonardo P. Chamorro
Summary: In this study, we experimentally investigated the settling behavior of spherical particles in different quiescent media and at different release frequencies. The results showed that the particle trajectories exhibited preferential in-plane motions in water and ethanol, while they displayed nearly vertical paths in the G60 mixture and oil media. Varying degrees of particle separation resulted in higher terminal velocities and reduced particle drag. The vertical and lateral pair dispersions exhibited ballistic scaling with dependences on the initial separation and the type of medium.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Eric L. Newland, Andrew W. Woods
Summary: This study investigates the behavior of particle-laden turbulent fountains in a uniform horizontal crossflow through experiments. The dynamics of the particle fountains are characterized by the ratio of the crossflow speed to the characteristic fountain speed and the ratio of the Stokes fall speed of particles to the characteristic fountain speed. Three distinct regimes of particle fountain dynamics are identified. In regime I, when the fall speed of particles is much smaller than the characteristic fountain speed, the particles remain well-coupled to the fountain fluid and the flow behaves as a single-phase fountain in the crossflow.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Amy Tinklenberg, Michele Guala, Filippo Coletti
Summary: We conducted an experimental study on the settling of millimeter-sized thin disks in still air. The chosen physical parameters were relevant to the settling of plate crystals in the atmosphere. High-speed imaging was used to reconstruct thousands of trajectories for each disk type, revealing that most disks either fell straight vertically or tumbled while drifting laterally. Two of the three disk sizes exhibited bimodal behavior, with both non-tumbling and tumbling modes occurring with significant probabilities. The data supported a simple scaling of the rotational frequency based on the equilibrium between drag and gravity.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Zhenyu Ouyang, Zhaowu Lin, Jianzhong Lin, Zhaosheng Yu, Nhan Phan-Thien
Summary: In this study, we numerically investigate the hydrodynamics of a spherical swimmer carrying a rigid cargo in a Newtonian fluid. We consider the effects of swimming Reynolds numbers and assembly models on the assembly's locomotion. The results show that the pusher-cargo model swims significantly faster than the other models, and the assemblies with an oblate cargo swim faster than those with a spherical or prolate cargo. Moreover, the pusher-cargo model is more efficient than the other models, and a larger d(s) leads to a smaller carrying hydrodynamic efficiency for the pusher-cargo model, but a greater efficiency for the cargo-pusher model. Additionally, the stability of assembly swimming is influenced by the assembly model and the relative distance between the swimmer and the cargo.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mathematics, Applied
Lucien Vienne, Emmanuel Leveque
Summary: This study introduces a recursive mathematical formulation and numerical schemes for the discrete-velocity Boltzmann equation, as well as a kinetic equation based on flow variables. The numerical simulations show a higher level of numerical dissipation compared to standard schemes, but the scheme can be parametrized to reduce dissipation and improve spectral properties. This modeling can be useful for connecting macroscopic and kinetic representations in hybrid simulations.
COMPUTERS & MATHEMATICS WITH APPLICATIONS
(2021)
Article
Engineering, Mechanical
J. Boudet, E. Leveque, H. Touil
Summary: This study presents lattice Boltzmann simulations of corner separation flow in a compressor cascade. The methodology shows good agreement with experimental data and is used to investigate the sensitivity of flow to end-wall boundary-layer thickness. The analysis of wall modeling reveals the influence of curvature and pressure-gradient correction terms on simulation results.
JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME
(2022)
Article
Multidisciplinary Sciences
Dhawal Buaria, Alain Pumir, Eberhard Bodenschatz
Summary: This study analyzed the intense fluctuations of energy dissipation rate in turbulent flows using direct numerical simulations. The study found that the self-amplification of strain and the depletion through vortex stretching and pressure-Hessian contribute to the fluctuations. By analyzing the eigenvalues of strain, it was discovered that the net amplification is solely produced by the third eigenvalue.
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
(2022)
Article
Physics, Fluids & Plasmas
Jingran Qiu, Navid Mousavi, Lihao Zhao, Kristian Gustavsson
Summary: Many plankton species migrate vertically in the ocean using a gyrotactic mechanism. This study proposes an active mechanism for gyrotactic stability that allows alignment with, or against, gravity. The mechanism is minimally affected by turbulence and is more effective than passive mechanisms.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Physics, Fluids & Plasmas
F. Cabrera, M. Z. Sheikh, B. Mehlig, N. Plihon, M. Bourgoin, A. Pumir, A. Naso
Summary: The precise description of the motion of anisotropic particles in a flow depends on the understanding of the force and torque acting on them. In this study, the settling motion of very elongated particles in a fluid was experimentally investigated, and the torque was found to have a simple angle dependence. The experimental results were compared with predictions from slender-rod and spheroid models, with the latter providing a better approximation for particles with an aspect ratio of 16.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Meteorology & Atmospheric Sciences
M. Z. Sheikh, K. Gustavsson, E. Leveque, B. Mehlig, A. Pumir, A. Naso
Summary: Collisions are an important step in the formation of snow aggregates. The collision rate increases with turbulence intensity and shows a more subtle dependence on beta. Increasing beta at low turbulence intensity decreases the collision rate, while increasing it at higher turbulence intensity enhances the collision rate.
JOURNAL OF THE ATMOSPHERIC SCIENCES
(2022)
Article
Physics, Multidisciplinary
A. Gholami, R. Ahmad, A. J. Bae, A. Pumir, E. Bodenschatz
Summary: The beating of cilia and flagella is crucial for various biological functions. This study focuses on the motion of flagella from green algae Chlamydomonas reinhardtii, and accurately describes its movement using principal component analysis. It was found that the flagellar waveform can be described using Fourier modes or as a combination of base-to-tip traveling wave and standing waves. Additionally, the static mode of the flagella is found to be the most sensitive to calcium ion concentration.
NEW JOURNAL OF PHYSICS
(2022)
Article
Physics, Multidisciplinary
Dhawal Buaria, Alain Pumir
Summary: Building on the Navier-Stokes dynamics, this study proposes a simple framework to quantify extreme events and smallest scales of turbulence by considering the vortical structures and the correlation between vorticity and strain rate. The results show excellent agreement with direct numerical simulations of isotropic turbulence, and indicate the shortcomings of current intermittency models. The study emphasizes the importance of the correlation between vorticity and strain in developing an accurate understanding of intermittency in turbulence.
PHYSICAL REVIEW LETTERS
(2022)
Article
Computer Science, Interdisciplinary Applications
Tobias Horstmann, Hatem Touil, Lucien Vienne, Denis Ricot, Emmanuel Leveque
Summary: The lattice Boltzmann method excels in weakly compressible flows due to its efficiency and parallelization, but faces challenges in incompressible flows due to small time-step sizes. Artificially adjusting the speed of sound can increase the time-step, but may lead to discontinuities in mass density and pressure. A trade-off between preserving mass conservation and ensuring pressure continuity can be achieved through a power-law rescaling of mass density.
JOURNAL OF COMPUTATIONAL PHYSICS
(2022)
Article
Physics, Fluids & Plasmas
A. Dubey, K. Gustavsson, G. P. Bewley, B. Mehlig
Summary: This article reveals the complex dependency in droplet collision dynamics through bifurcation analysis, and predicts the strong spatial clustering near certain saddle points.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Mechanics
P-F Yang, J. Fang, L. Fang, A. Pumir, H. Xu
Summary: We derive analytic relations for the second- and third-order moments of the spatial gradient of fluid velocity in compressible turbulence. These relations generalize known relations in incompressible flows and can be approximately applied in a mixing layer. We also discuss the experimental determination of these moments for isotropic compressible turbulence.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Z. Zhang, I. Danaila, E. Leveque, L. Danaila
Summary: The Hall-Vinen-Bekharevich-Khalatnikov (HVBK) model, based on the two-fluid model of Tisza and Landau, is used to investigate quantum turbulence in superfluid helium. This model describes the normal and superfluid components using two coupled Navier-Stokes equations. The transport equations for the third-order moments are derived and reveal the probability of rare and strong fluctuations. Direct numerical simulations of the HVBK flow show the importance of each term in the budget equations and the dominance of transport and pressure-related terms. The flatness of velocity derivatives is studied and found to gradually increase for lower temperatures, indicating the strong locking of the two fluids.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Physics, Fluids & Plasmas
Raffaello Foldes, Emmanuel Leveque, Raffaele Marino, Ermanno Pietropaolo, Alessandro De Rosis, Daniele Telloni, Fabio Feraco
Summary: Simulating plasmas in the Hall-MHD regime is computationally challenging but valuable for studying complex dynamics in astrophysics and fusion machines. In this study, a new code called flame is developed using the kinetic lattice Boltzmann approach to integrate plasma dynamics and solve the induction equation. The code is tested against analytical wave-solutions and compared with the well-established pseudo-spectral code ghost, showing stability and convergence.
JOURNAL OF PLASMA PHYSICS
(2023)
Article
Biology
Junhua Shen, Yan Liu, Meiyu Zhang, Alain Pumir, Liangshan Mu, Baohua Li, Jinshan Xu
Summary: This study demonstrates the effectiveness of analyzing and extracting features from EHG signals to quantify the synchrony and coherence of uterine contractions, showing superiority in predicting the onset of labor.
COMPUTERS IN BIOLOGY AND MEDICINE
(2023)
Article
Chemistry, Physical
Raheel Ahmad, Albert J. Bae, Yu-Jung Su, Samira Goli Pozveh, Eberhard Bodenschatz, Alain Pumir, Azam Gholami
Summary: The researchers built efficient micro-swimmers driven by flagella that consume ATP to propel micron-sized beads. The motion of the beads changes depending on the calcium concentration. This study has important implications for the design of synthetic micro-swimmers and bio-actuated medical micro-robots for targeted drug delivery.