Preferential orientation of tracer spheroids in turbulent channel flow

Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number Reτ=100 given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wallnormal direction, which are consistent with earlier observation in low Reynolds number (Reτ=180)wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.

Alignment of inertialess spheroidal particles in flow-structure-dominated regions of turbulent channel flow:shape effect

The alignment of elongated fibers and thin disks is known to be significantly influenced by the presence of fluid coherent structures in near-wall turbulence(Cui et al.2021).However,this earlier study is confined to the spheroids with infinitely large or small aspect ratio,and the shape effect of finite aspect ratio on the alignment is not considered.The current study investigates the shape-dependent alignment of inertialess spheroids in structure-dominated regions of channel flow.With utilizing an ensemble-averaged approach for identifying the structure-dominated regions,we analyze the eigensystem of the linear term matrix in the Jeffery equation,which is governed by both particle shape and local fluid velocity gradients.In contrast to earlier conventional analysis based on local vorticity and strain rate,our findings demonstrate that the eigensystem of the Jeffery equation offers a convenient,effective,and universal framework for predicting the alignment behavior of inertialess spheroids in turbulent flows.By leveraging the eigensystem of the Jeffery equation,we uncover a diverse effect of fluid coherent structures on spheroid alignment with different particle shapes.Furthermore,we provide explanations for both shape-independent alignments observed in vortical-core regions and shape-dependent alignments around near-wall streamwise vortices.

Bifunctional Activated Carbon Ultrathin Fibers:Combining the Removal of VOCs and PM in One Material

Volatile organic compounds(VOCs)and particulate matter(PM)are both frequently present in air as contaminants,posing serious health and environmental hazards.The current filtration of VOCs utilizes entirely different materials compared with PM filtration,adding complexity to air cleaning system.Herein,we design a pitch-based activated carbon ultrathin fibers(PACUFs)for bifunctional air purification.The PACUFs,with fiber diameter of∼1.2μm and specific surface area of 2341 m^(2) g^(−1),provide both high VOCs adsorption capacity(∼706 mg g^(−1))and excellent efficiency of∼97% PM_(0.3) filtration with low pressure drop.In contrast,traditional activated carbon fibers exhibit VOCs adsorption capacity of∼448 mg g^(−1) and PM_(0.3) removal efficiency of only∼36%at an equal area density of∼190 g m^(−2).Theoretical investigations reveal the filtration mechanism of the high-performance bifunctional fibrous PACUFs,considering full advantages of the high surface area,small pore size,and significant micropore volume.

Swimming strategy of settling elongated micro-swimmers by reinforcement learning

Particular types of plankton in aquatic ecosystems can coordinate their motion depending on the local flow environment to reach regions conducive to their growth or reproduction.Investigating their swimming strategies with regard to the local environment is important to obtain in-depth understanding of their behavior in the aquatic environment.In the present research,to examine an impact of the shape and gravity on a swimming strategy,plankton is considered as settling swimming particles of ellipsoidal shape.The Q-learning approach is adopted to obtain swimming strategies for smart particles with a goal of efficiently moving upwards in a two-dimensional steady flow.Strategies obtained from reinforcement learning are compared to those of naive gyrotactic particles that are modeled considering the behavior of realistic plankton.It is found that the elongation of particles improves the performance of upward swimming by facilitating particles’resistance to the perturbation of vortex.In the case when the settling velocity is included,the strategy obtained by reinforcement learning has similar performance to that of the naive gyrotactic one,and they both align swimmers in upward direction.The similarity between the strategy obtained from machine learning and the biological gyrotactic strategy indicates the relationship between the aspherical shape and settling effect of realistic plankton and their gyrotactic feature.

Statistics of Particle Suspensions in Turbulent Channel Flow

Particle dynamics in a turbulent channel flow is considered.The effects of particle concentration and Reynolds number on the particle velocity statistics are investigated.Four different particle response times,τ^(+)=1,5,30 and 100,are examined for three different Reynolds numbers,Re_(∗)=200,360 and 790(based on channel height and friction velocity).The particle concentration evolves with time and statistics obtained during three different sampling periods might be distinctly different.Themean and fluctuating particle velocities are substantially affected both by the particle response time and by the Reynolds number of the flow.

Dispersed multiphase flows:advances in measuring,simulation and modeling

Dispersed multiphase flows are commonly encountered in a wide variety of natural and industrial applications,from sedimentation in rivers and plankton surfacing in oceanic flows to fluidized bed in chemical reactors and pneumatic conveying systems.In these flows,a dispersed phase(con-sisting of particles,droplets and/or bubbles)is transported by a carrier phase.The interaction between the phases re-sults in complex dynamics(e.g.,transport,spatial clustering,and deformation or break-up)that is rich in physics and poses a wide spectrum of theoretical,computational,and experimental challenges.These challenges are at the very core of the research efforts made by the multiphase flow community over the past decades.

A review on gyrotactic swimmers in turbulent flows

Many marine plankton species are motile and perform daily vertical migrations,traveling across water columns over distances of tens of meters.It is intriguing that these tiny and slow swimmers can travel in a certain direction within a turbulent environment.One way to do that is by exploiting gravitaxis,which is a form of taxis characterised by the directional movement of an organism in response to gravity.Many plankton species are able to generate a gravitational torque(e.g.,due to a nonuniform mass distribution)that reorients them upwards.However,the swimming direction is disturbed by the shearing motions and the velocity fluctuations that characterise oceanic turbulence:these can generate a viscous torque that may destabilize the swimmer.The directed locomotion resulting from the combination of gravitational and viscous torques in a flow is termed gyrotaxis,which is known to lead to a non-uniform spatial accumulation of swimmers in patches or layers.These phenomena depend strongly on the non-linear dynamics that originate from the fluid motions,and the study of gyrotactic swimmers in complex flows is attracting growing attention.Numerical simulations of the Navier-Stokes equations coupled with suitable models of gyrotactic swimmers have proven their capability to provide valuable insight into the dynamical and statistical properties of self-propelled organisms.In this paper,we review recent studies and key findings on gyrotactic swimmers in turbulent flows.First,we introduce the most recent results concerning the orientation and vertical migration of gyrotactic swimmers in isotropic turbulence.Second,we discuss the findings on the accumulation of the swimmers.Last,we review recent progresses concerning the behaviour of gyrotactic swimmers in free-surface turbulence.