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1.
  • Alghalibi, Dhiya, et al. (författare)
  • Inertial migration of a deformable particle in pipe flow
  • 2019
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 4:10
  • Tidskriftsartikel (refereegranskat)abstract
    • We perform fully Eulerian numerical simulations of an initially spherical hyperelastic particle suspended in a Newtonian pressure-driven flow in a cylindrical straight pipe. We study the full particle migration and deformation for different Reynolds numbers and for various levels of particle elasticity, to disentangle the interplay of inertia and elasticity on the particle focusing. We observe that the particle deforms and undergoes a lateral displacement while traveling downstream through the pipe, finally focusing at the pipe centerline. We note that the migration dynamics and the final equilibrium position are almost independent of the Reynolds number, while they strongly depend on the particle elasticity; in particular, the migration is faster as the elasticity increases (i.e., the particle is more deformable), with the particle reaching the final equilibrium position at the centerline in shorter times. Our simulations show that the results are not affected by the particle initial conditions, position, and velocity. Finally, we explain the particle migration by computing the total force acting on the particle and its different components, viscous and elastic.
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2.
  • Alizad Banaei, Arash, et al. (författare)
  • Inertial settling of flexible fiber suspensions
  • 2020
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 5:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the inertial settling of suspensions of flexible and rigid fibers using an immersed boundary method. The fibers considered are inextensible and slender, with an aspect ratio of 20. For a single Galileo number of Ga = 160, we examine a range of dimensionless bending rigidities 0.1 < gamma < 20 and fiber concentrations 0.5 < nL(3) < 25, with n being the fiber number density and L the fiber length, that spans dilute and semidilute regimes. The settling fibers form streamers, regions where the fibers are packed and settle faster than the average settling velocity of the suspension, for nL(3) > 10. In the low-concentration regions outside the streamers, the fibers either go upward or have low settling velocities. Flexible fibers exhibit higher packing inside the streamers and smaller streamers compared to the streamers formed by the rigid fibers. Due to this higher packing, the flexible fibers settle faster compared to the rigid fibers. The formation of the streamers counterbalances the hindering of the settling velocity at higher concentrations. At higher nL(3), however, the maximum local concentration of fibers relative to a uniform distribution diminishes for both flexible and rigid fibers as the mobility of the fibers becomes limited due to the presence of other fibers in their vicinity. Due to this limited mobility, the deformation of the fibers and their settling orientation become insensitive to nL(3) for nL(3) > 7. In both the dilute and semidilute regimes, flexible fibers are more aligned with the direction perpendicular to gravity compared to rigid fibers.
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3.
  • Almondo, Gino, et al. (författare)
  • Intrinsic viscosity of a suspension of weakly Brownian ellipsoids in shear
  • 2018
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 3:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We analyze the angular dynamics of triaxial ellipsoids in a shear flow subject to weak thermal noise. By numerically integrating an overdamped angular Langevin equation, we find the steady angular probability distribution for a range of triaxial particle shapes. From this distribution we compute the intrinsic viscosity of a dilute suspension of triaxial particles. We determine how the viscosity depends on particle shape in the limit of weak thermal noise. While the deterministic angular dynamics depends very sensitively on particle shape, we find that the shape dependence of the intrinsic viscosity is weaker, in general, and that suspensions of rodlike particles are the most sensitive to breaking of axisymmetry. The intrinsic viscosity of a dilute suspension of triaxial particles is smaller than that of a suspension of axisymmetric particles with the same volume and the same ratio of major to minor axis lengths.
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4.
  • Arratia, Cristóbal, et al. (författare)
  • Absolute/convective secondary instabilities and the role of confinement in free shear layers
  • 2018
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 3:5
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the linear spatiotemporal stability of an infinite row of equal point vortices under symmetric confinement between parallel walls. These rows of vortices serve to model the secondary instability leading to the merging of consecutive (Kelvin-Helmholtz) vortices in free shear layers, allowing us to study how confinement limits the growth of shear layers through vortex pairings. Using a geometric construction akin to a Legendre transform on the dispersion relation, we compute the growth rate of the instability in different reference frames as a function of the frame velocity with respect to the vortices. This approach is verified and complemented with numerical computations of the linear impulse response, fully characterizing the absolute/convective nature of the instability. Similar to results by Healey on the primary instability of parallel tanh profiles [J. Fluid Mech. 623, 241 (2009)], we observe a range of confinement in which absolute instability is promoted. For a parallel shear layer with prescribed confinement and mixing length, the threshold for absolute/convective instability of the secondary pairing instability depends on the separation distance between consecutive vortices, which is physically determined by the wavelength selected by the previous (primary or pairing) instability. In the presence of counterflow and moderate to weak confinement, small (large) wavelength of the vortex row leads to absolute (convective) instability. While absolute secondary instabilities in spatially developing flows have been previously related to an abrupt transition to a complex behavior, this secondary pairing instability regenerates the flow with an increased wavelength, eventually leading to a convectively unstable row of vortices. We argue that since the primary instability remains active for large wavelengths, a spatially developing shear layer can directly saturate on the wavelength of such a convectively unstable row, by-passing the smaller wavelengths of absolute secondary instability. This provides a wavelength selection mechanism, according to which the distance between consecutive vortices should be sufficiently large in comparison with the channel width in order for the row of vortices to persist. We argue that the proposed wavelength selection criteria can serve as a guideline for experimentally obtaining plane shear layers with counterflow, which has remained an experimental challenge.
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5.
  • Atzori, Marco, et al. (författare)
  • High-resolution simulations of a turbulent boundary layer impacting two obstacles in tandem
  • 2023
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 8:6
  • Tidskriftsartikel (refereegranskat)abstract
    • High-fidelity large-eddy simulations of the flow around two rectangular obstacles are carried out at a Reynolds number of 10 000 based on the freestream velocity and the obstacle height. The incoming flow is a developed turbulent boundary layer. Mean-velocity components, turbulence fluctuations, and the terms of the turbulent-kinetic-energy budget are analyzed for three flow regimes: skimming flow, wake interference, and isolated roughness. Three regions are identified where the flow undergoes the most significant changes: the first obstacle's wake, the region in front of the second obstacle, and the region around the second obstacle. In the skimming-flow case, turbulence activity in the cavity between the obstacles is limited and mainly occurs in a small region in front of the second obstacle. In the wake-interference case, there is a strong interaction between the freestream flow that penetrates the cavity and the wake of the first obstacle. This interaction results in more intense turbulent fluctuations between the obstacles. In the isolated-roughness case, the wake of the first obstacle is in good agreement with that of an isolated obstacle. Separation bubbles with strong turbulent fluctuations appear around the second obstacle.
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6.
  • Atzori, Marco, 1992-, et al. (författare)
  • Uniform blowing and suction applied to nonuniform adverse-pressure-gradient wing boundary layers
  • 2021
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:11
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed analysis of the effects of uniform blowing, uniform suction, and body-force damping on the turbulent boundary layer developing around a NACA4412 airfoil at moderate Reynolds number is presented. The flow over the suction and the pressure sides of the airfoil is subjected to a nonuniform adverse pressure gradient and a moderate favorable pressure gradient, respectively. We find that the changes in total skin friction due to blowing and suction are not very sensitive to different pressure-gradient conditions or the Reynolds number. However, when blowing and suction are applied to an adverse-pressure-gradient (APG) boundary layer, their impact on properties such as the boundary-layer thickness, the intensity of the wall-normal convection, and turbulent fluctuations are more pronounced. We employ the Fukagata-Iwamoto-Kasagi decomposition [K. Fukagata et al., Phys. Fluids 14, 73 (2002)] and spectral analysis to study the interaction between intense adverse pressure gradient and these control strategies. We find that the control modifies skin-friction contributions differently in adverse-pressure-gradient and zero-pressure-gradient boundary layers. In particular, the control strategies modify considerably both the streamwisedevelopment and the pressure-gradient contributions, which have high magnitude when a strong adverse pressure gradient is present. Blowing and suction also impact the convection of structures in the wall-normal direction. Overall, our results suggest that it is not possible to simply separate pressure-gradient and control effects, a fact to take into account in future studies on control design in practical applications.
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7.
  • Banaei, Arash Alizad, et al. (författare)
  • Numerical study of suspensions of nucleated capsules at finite inertia
  • 2021
  • Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 6:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the rheology of suspensions of capsules with a rigid nucleus at negligible and finite flow inertia by means of numerical simulations. The capsule membrane is modeled as a thin Neo-Hookean hyperelastic material and the nucleus as a rigid particle with radius equal to half the radius of the undeformed spherical capsules. The fluid and solid motion are coupled with an immersed boundary method, validated for both the deformable membrane and the rigid nucleus. We examine the effect of the Reynolds number, capillary number, and volume fraction on the macroscopic properties of the suspensions, comparing with the case of capsules without nuclei. To explain the rheological measurables, we examine the mean capsule deformation, the mean orientation with respect to the flow direction, and the stress budget. The results indicate that the relative viscosity decreases with the capillary number, i.e., increasing deformability, and increases with inertia. The presence of a nucleus always reduces the membrane deformation. Capsules align more in the flow direction at higher capillary numbers and at higher volume fractions, where we also see a significant portion of them oriented with their longer deformed axis in the spanwise direction. When increasing inertia, the alignment with the flow decreases while more capsules orient in the spanwise direction. The first normal stress difference increases with the capillary number and it is always less for the nucleated capsules. Finally, the relative viscosity and the first normal stress difference increase with the capsule volume fraction, an effect more pronounced for the first normal stress difference.
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8.
  • Bempedelis, Nikolaos, et al. (författare)
  • Numerical and experimental investigation into the dynamics of a bubble-free-surface system
  • 2021
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 6:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The aim of the present work is to study the interaction between an oscillating bubble and a free surface. We perform a series of experiments and numerical simulations and attempt to characterize both early and late stages of the interaction. The focus is on providing insight into the mechanisms of bubble-induced atomization. For this reason, we are particularly interested in characterizing the patterns and dynamics of the liquid jets that are formed at the free surface. Observations regarding the evolution of the free surface are presented by measuring the jet's surface area and volume. Finally, based on these quantities, we introduce a metric that may be used to characterize the liquid jetting and predict whether late-time atomization of the interface will occur.
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9.
  • Biancofiore, L., et al. (författare)
  • Streak instability in viscoelastic Couette flow
  • 2017
  • Ingår i: PHYSICAL REVIEW FLUIDS. - : American Physical Society. - 2469-990X. ; 2:4
  • Tidskriftsartikel (refereegranskat)abstract
    • The secondary instability of nonlinear streaks and transition to turbulence in viscoelastic Couette flow are studied using direct numerical simulations. Viscoelasticity is modeled using the FENE-P constitutive equations. Both the polymer concentration beta and Weissenberg number Wi are varied in order to assess their effects on transition at moderate Reynolds number. The base streaks are obtained from nonlinear simulations of the Couette flow response to a streamwise vortex. We select the initial amplitude of the vortex which yields a desired maximum amplitude of the nonlinear streaks during their temporal evolution. The development of streaks in both Newtonian and non-Newtonian flows is primarily due to the action of streamwise vorticity onto the mean shear. In the viscoelastic case, it is also affected by the polymer torque, which opposes the vorticity and becomes more pronounced at large Weissenberg number. Streaks with the same maximum streamwise velocity perturbation can therefore have different total kinetic energy at higher Weissenberg number. At every streak amplitude of interest, harmonic forcing is introduced along the transverse direction to trigger the secondary instability and breakdown to turbulence. We demonstrate that the critical amplitude of the forcing, A(d), increases at large Weissenberg number. The degree of stabilization due to elasticity depends on the initial streak intensity, A(s),(in). For weak streaks the critical amplitude for secondary instability is more sensitive to Wi than for strong ones. This is explained by the existence of two different mechanisms that can trigger transition to turbulence. The perturbation to weak streaks is initially stabilized by the polymer torque which acts to oppose the amplification of wall-normal vorticity and, as a result, delays breakdown to turbulence. The secondary instability of strong streaks, on the other hand, is more immune to this stabilizing influence of the polymer.
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10.
  • Borgnino, M., et al. (författare)
  • Alignment of elongated swimmers in a laminar and turbulent Kolmogorov flow
  • 2022
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 7:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Many aquatic microorganisms are able to swim. In natural environments they typically do so in the presence of flows. In recent years it has been shown that the interplay of swimming and flows can give rise to interesting and biologically relevant phenomena, such as accumulation of microorganisms in specific flow regions and local alignment with the flow properties. Here, we consider a mechanical model for elongated microswimmers in a Kolmogorov flow, a prototypic shear flow, both in steady and in turbulent conditions. By means of direct numerical simulations, supported by analytical calculation in a simplified stochastic setting, we find that the alignment of the swimming direction with the local velocity is a general phenomenon. We also explore how the accumulation of microorganisms, typically observed in steady flows, is modified by the presence of unsteady fluctuations.
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11.
  • Borthakur, Manash Pratim, et al. (författare)
  • Dynamics of a compound droplet under the combined influence of electric field and shear flow
  • 2021
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a numerical investigation on the dynamics of a compound droplet under the combined influence of an applied electric field and shear flow. The paper is carried out by solving the electro-hydrodynamic equations in a two-dimensional framework, and the interface is captured using a volume-of-fluid approach. Both perfect dielectric as well as leaky dielectric fluids are considered. For the case of dielectric fluids, the deformation of both the inner and outer interfaces can be modulated by either variation of the permittivity contrast between the fluids or the applied field strength. The nature of the polarization forces acting at both the interfaces can be either compressive or tensile depending on the magnitude of the permittivity ratio. The investigations for leaky dielectric fluids reveal that the ratio of electrical permittivity and conductivity between the two phases plays a critical role in deciding the magnitude of deformation and orientation of the compound droplet. The variation of charge accumulated at the interfaces modifies the behavior of the Coulombic forces thereby fundamentally altering the droplet deformation and orientation characteristics. Furthermore, it is demonstrated that the electric field can be suitably applied to engender breakup of the compound droplets.
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12.
  • Brandenburg, Axel, et al. (författare)
  • Dynamo effect in decaying helical turbulence
  • 2019
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 4:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity-regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t(-1/2). This is even slower than in the fully helical case, where it decays like t(-2/3). In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t(-2/3) decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.
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13.
  • Brethouwer, Geert (författare)
  • Influence of spanwise rotation and scalar boundary conditions on passive scalar transport in turbulent channel flow
  • 2019
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 4:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Direct numerical simulations of passive scalar transport in turbulent channel flow subject to spanwise rotation are carried out with two different boundary conditions for the scalar. In the first case the scalar transport is driven by an assigned scalar difference at the walls and in the second case by a constant mean streamwise scalar gradient. The Reynolds number Re = U(b)h/nu is fixed at 14 000 and the rotation number Ro = 2 Omega h/U-b is varied from 0 to 0.75, where U-b is the mean bulk velocity, h half the channel gap width, and Omega the rotation rate. This work is a continuation of Brethouwer [J. Fluid Mech. 844, 297 ( 2018)] to further study the influence of rotation and also the influence of scalar boundary conditions on scalar transport in channel flow. Mean scalar profiles and other scalar statistics differ in the two cases with different boundary conditions but are similar in the near-wall region in terms of local wall units. The conclusion of Brethouwer that the Reynolds analogy for scalar-momentum transfer does not apply to rotating channel flow is independent of scalar boundary conditions. Rotation influences the turbulent scalar flux differently than the Reynolds shear stress and strongly reduces the turbulent Prandtl number on the unstable channel side, irrespective of the scalar boundary conditions. Scalar structures are larger than the turbulence structures in rotating channel flow, in contrast to nonrotating channel flow where these are similar.
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14.
  • Brethouwer, Geert (författare)
  • Linear instabilities and recurring bursts of turbulence in rotating channel flow simulations
  • 2016
  • Ingår i: PHYSICAL REVIEW FLUIDS. - : AMER PHYSICAL SOC. - 2469-990X. ; 1:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Intense recurring bursts of turbulence on a long time scale are observed in direct numerical simulations (DNSs) of channel flow subject to rapid spanwise rotation for a range of Reynolds numbers and rotation speeds. A necessary condition for manifestation of cyclic turbulent bursts is that the Reynolds number and rotation speed are sufficiently high. The principal cause of turbulent bursts is a linearly unstable Tollmien-Schlichting-like wave with a wave vector normal to the rotation axis. This exponentially growing wave breaks down through a secondary instability when its amplitude is large causing a burst of turbulence. A new instability develops once turbulence has subsided leading to a continuous and self-sustained cycle of bursts. In several DNSs a recurring instability happens despite the flow being strongly and continuously turbulent in a part of the channel while in other DNSs turbulence is mostly weak between bursts. DNS observations have been compared to linear stability theory using the spatially averaged velocity of the DNS as base flow. Analysis shows that Tollmien-Schlichting waves are linearly unstable if Reynolds number and rotation speed are sufficiently high and bursts are observed. In several cases a good agreement between predicted and observed growth rate and eigenfunction of the instability is found, but in other cases the growth rate is overpredicted by linear stability theory and in some cases a Tollmien-Schlichting instability is predicted but not observed. Further study indicates that when observations and predictions differ turbulence or other modes alter the unstable wave, thereby reducing its ability to extract energy from the mean flow. In none of the analyzed DNSs was a significant nonlinear energy transfer from the unstable mode to other modes noted. Inclusion of an eddy viscosity in linear stability theory did not notably improve correspondence with DNSs.
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15.
  • Cabrera, F., et al. (författare)
  • Experimental validation of fluid inertia models for a cylinder settling in a quiescent flow
  • 2022
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 7:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The precise description of the motion of anisotropic particles in a flow rests on the understanding of the force and torque acting on them. Here, we study experimentally small, very elongated particles settling in a fluid at small Reynolds number. In our experiments, we can, to a very good approximation, relate the rate of rotation of cylindrical tungsten rods, of aspect ratios = 8 and = 16, settling in pure glycerol, to the torque they are experiencing. This allows us to compare the measured torque with expressions obtained either in the slender-rod limit or in the case of spheroids. Both theories predict a simple angle dependence for the torque, which is found to capture very well the experimental results. The slender-rod theory overestimates the results for the two aspect ratios considered, while the expression obtained for a spheroid provides a better approximation for = 16. Comparing our results with those of previous experiments provides further insight on the conditions of validity of the slender-rod theory. The translational dynamics is shown to be in qualitative agreement with the slender-rod and spheroid models, the former one being found to represent better the experimental data.
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16.
  • Canton, Jacopo, et al. (författare)
  • Reynolds number dependence of large-scale friction control in turbulent channel flow
  • 2016
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 1:8
  • Tidskriftsartikel (refereegranskat)abstract
    • The present work investigates the effectiveness of the control strategy introduced by Schoppa and Hussain [Phys. Fluids 10, 1049 (1998)] as a function of Reynolds number (Re). The skin-friction drag reduction method proposed by these authors, consisting of streamwise-invariant, counter-rotating vortices, was analyzed by Canton et al. [Flow, Turbul. Combust. 97, 811 (2016)] in turbulent channel flows for friction Reynolds numbers (Re t) corresponding to the value of the original study (i.e., 104) and 180. For these Re, a slightly modified version of the method proved to be successful and was capable of providing a drag reduction of up to 18%. The present study analyzes the Reynolds number dependence of this drag-reducing strategy by performing two sets of direct numerical simulations (DNS) for Re-tau = 360 and 550. A detailed analysis of the method as a function of the control parameters (amplitude and wavelength) and Re confirms, on the one hand, the effectiveness of the large-scale vortices at low Re and, on the other hand, the decreasing and finally vanishing effectiveness of this method for higher Re. In particular, no drag reduction can be achieved for Re t = 550 for any combination of the parameters controlling the vortices. For low Reynolds numbers, the large-scale vortices are able to affect the near-wall cycle and alter the wall-shear-stress distribution to cause an overall drag reduction effect, in accordance with most control strategies. For higher Re, instead, the present method fails to penetrate the near-wall region and cannot induce the spanwise velocity variation observed in other more established control strategies, which focus on the near-wall cycle. Despite the negative outcome, the present results demonstrate the shortcomings of the control strategy and show that future focus should be on methods that directly target the near-wall region or other suitable alternatives.
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17.
  • Chan, C. , I, et al. (författare)
  • Large-scale and small-scale contribution to the skin friction reduction in a modified turbulent boundary layer by a large-eddy break-up device
  • 2022
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 7:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The role of streamwise length scales (lambda x) in turbulent skin friction generation is investigated using a direct numerical simulation data set of an incompressible zero pressure gradient turbulent boundary layer and the spectral analysis based on the FukagataL73 (2002)]. The total skin friction generation associated with motions scaled with local boundary layer thickness delta of lambda x 3 delta and lambda x 3 delta) contribute to a significant portion of turbulent skin friction. However, it is found that the large-scale ejection and sweep events with streamwise length scales at lambda x 3 delta are equally important. The turbulent skin friction reduction associated with the modification of largeand small-scale quadrant events is studied, using well-resolved simulation data sets of a large-eddy break-up (LEBU) device in a turbulent boundary layer. The results reveal that LEBUs modify both the large- and small-scale ejection and sweep events, yielding a substantial turbulent skin friction reduction.
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18.
  • Chicchiero, Claudio, et al. (författare)
  • Triple-deck analysis of the steady flow over a rotating disk with surface roughness
  • 2021
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of surface roughness on the steady laminar flow induced by a rotating disk submerged by fluid otherwise at rest is investigated here theoretically and numerically. A theory is proposed where a triple-deck analysis is applied leading to a fast evaluation of the steady-flow modification due to the rough surface. The theory assumes that the roughness is much smaller than the boundary-layer height and is characterized by a significantly longer length scale (slender roughness). Only the leading-order correction is developed here, corresponding to a velocity-field correction that is linear with the roughness height. The proposed theory neglects some curvature terms (here partially accounted by means of a stretching of the radial coordinate and of a scaling of the dependent variables). Numerical simulations performed with different roughness geometries (axisymmetric roughness, radial grooves, and localized bumps) have been used to validate the theory. Results indicate that the proposed theory leads to a good quantification of the flow modifications due to surface roughness at a very low computational cost. A demonstration of the capabilities of the theory is finally proposed where the statistical effects on the flow due to a random (but statistically known) roughness distributed on the surface of a rotating disk are characterized.
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19.
  • Chin, R. C., et al. (författare)
  • Backflow events under the effect of secondary flow of Prandtl's first kind
  • 2020
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 5:7
  • Tidskriftsartikel (refereegranskat)abstract
    • A study of the backflow events in the flow through a toroidal pipe at friction Reynolds number Re-tau approximate to 650 is performed and compared with the results in a straight turbulent pipe flow at Re-tau approximate to 500. The statistics and topological properties of the backflow events are analysed and discussed. Conditionally averaged flow fields in the vicinity of the backflow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the azimuthal wall-shear stress when compared to the pipe flow. In the region around the backflow events, critical points are observed. The comparison between the toroidal pipe and its straight counterpart also shows fewer backflow events and critical points in the torus. This is attributed to the secondary flow of Prandtl's first kind present in the toroidal pipe, which is responsible for the convection of momentum from the inner to the outer bend through the core of the pipe, and back from outer bend to the inner bend along the azimuthal direction. These results indicate that backflow events and critical points are genuine features of wall-bounded turbulence, and are not artefacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.
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20.
  • Cifani, P., et al. (författare)
  • Casimir preserving spectrum of two-dimensional turbulence
  • 2022
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 7:8
  • Tidskriftsartikel (refereegranskat)abstract
    • We present predictions of the energy spectrum of forced two-dimensional turbulence obtained by employing a structure-preserving integrator. In particular, we construct a finite-mode approximation of the Navier-Stokes equations on the unit sphere, which, in the limit of vanishing viscosity, preserves the Lie-Poisson structure. As a result, integrated powers of vorticity are conserved in the inviscid limit. We obtain robust evidence for the existence of the double energy cascade, including the formation of the -3 scaling of the inertial range of the direct cascade. We show that this can be achieved at modest resolutions compared to those required by traditional numerical methods.
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21.
  • Colabrese, S., et al. (författare)
  • Smart inertial particles
  • 2018
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 3:8
  • Tidskriftsartikel (refereegranskat)abstract
    • We performed a numerical study to train smart inertial particles to target specific flow regions with high vorticity through the use of reinforcement learning algorithms. The particles are able to actively change their size to modify their inertia and density. In short, using local measurements of the flow vorticity, the smart particle explores the interplay between its choices of size and its dynamical behavior in the flow environment. This allows it to accumulate experience and learn approximately optimal strategies of how to modulate its size in order to reach the target high-vorticity regions. We consider flows with different complexities: a two-dimensional stationary Taylor-Green-like configuration, a two-dimensional time-dependent flow, and finally a three-dimensional flow given by the stationary Arnold-Beltrami-Childress (ABC) helical flow. We show that smart particles are able to learn how to reach extremely intense vortical structures in all the tackled cases.
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22.
  • Collins, D., et al. (författare)
  • Lord Kelvin's isotropic helicoid
  • 2021
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 6:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Nearly 150 years ago, Lord Kelvin proposed the isotropic helicoid, a particle with isotropic yet chiral interactions with a fluid so that translation couples to rotation. An implementation of his design fabricated with a three-dimensional printer is found experimentally to have no detectable translation-rotation coupling, although the particle point-group symmetry allows this coupling. We explain these results by demonstrating that in Stokes flow, the chiral coupling of such isotropic helicoids made out of nonchiral vanes is due only to hydrodynamic interactions between these vanes. Therefore it is small. In summary, Kelvin's predicted isotropic helicoid exists, but only as a weak breaking of a symmetry of noninteracting vanes in Stokes flow.
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23.
  • de Martín, Lilian, 1983 (författare)
  • Influence of particle dynamics on the instability for pattern formation in shallow pulsed beds
  • 2018
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 3:12
  • Tidskriftsartikel (refereegranskat)abstract
    • A granular layer can form standing-wave patterns, such as squares, stripes, and hexagons, when it is fluidized with a pulsed gas flow. These patterns resemble the well-known patterns formed in vertically vibrated granular layers, but are governed by different dimensionless numbers. Recent research [de Martin et al., Phys. Rev. Fluids 3, 034303 (2018)] reveals that the onset to pattern formation in shallow pulsed beds can be understood in terms of the dimensionless number Gamma(p) = u(a )/ u(t)(phi) over bar, where u(a), is the amplitude of the gas velocity, u(t) is the terminal velocity of the particles, and (phi) over bar is the average solids volume fraction. In contrast, pattern formation in vertically vibrated granular layers in vacuo is governed by the dimensionless number Gamma(v) = 4 pi(2) f(2) d/g, where f and d are the frequency and displacement of the vibrated plate, respectively, and g is the gravitational acceleration. In addition, the threshold for pattern formation in pulsed beds exhibits a strong dependence with the frequency of the excitation that is not observed in the threshold for pattern formation in vibrated systems. This work explores the origin of these differences by simulating the dynamics of a one-dimensional pulsed array of particles. Simulations reproduce well the experimental stability curves, and reveal that the criterion for instability in shallow pulsed and vibrated systems is actually the same; the layer flight time must be equal to 1/f. In pulsed beds, this criterion is determined by the traveling time of the kinematic wave that forms in each flow pulse. These results provide a theoretical basis to the recent experimental observations and highlights commonalities between the mechanisms behind pattern formation in thin vibrated granular layers and shallow pulsed fluidized beds.
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24.
  • Deshpande, Rahul, et al. (författare)
  • Reynolds-number effects on the outer region of adverse-pressure-gradient turbulent boundary layers
  • 2023
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 8:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the Reynolds-number effects on the outer region of moderate adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) and find that their small-scale (viscous) energy reduces with increasing friction Reynolds number (Reτ). The trend is based on analyzing APG TBL data across 600≲Reτ≲7000 and contrasts with the negligible variation in small viscous-scaled energy noted for canonical wall flows. The data sets considered include those from a well-resolved numerical simulation [Pozuelo, J. Fluid Mech. 939, A34 (2022)0022-112010.1017/jfm.2022.221], which provides access to an APG TBL maintained at near-equilibrium conditions across 1000≲Reτ≲ 2000, with a well-defined flow history, and a new high-Reτ (∼7000) experimental study from the large Melbourne wind tunnel, with its long test section modified to permit development of an APG TBL from a "canonical"upstream condition. The decrease in small-scale energy with Reτ is revealed via decomposing the streamwise normal stresses into small- and large-scale contributions, based on a sharp spectral cutoff. The origin for this trend is traced back to the production of turbulent kinetic energy in an APG TBL, the small-scale contribution to which is also found to decrease with Reτ in the outer region. The conclusion is reaffirmed by investigating attenuation of streamwise normal stresses due to changing spatial resolutions of the numerical grid or hotwire sensors, which reduces with increasing Reτ and is found to be negligible at Reτ∼7000 in this study. The results emphasize that new scaling arguments and spatial-resolution corrections should be tested rigorously across a broad Reτ range, particularly for pressure gradient TBLs.
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25.
  • Dogan, Eda, et al. (författare)
  • Spatial characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of free-stream turbulence
  • 2019
  • Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 4:8
  • Tidskriftsartikel (refereegranskat)abstract
    • Particle image velocimetry (PIV) measurements are performed to examine the structural organization inside a turbulent boundary layer under the influence of free-stream turbulence (FST). In particular, streamwise-wall-normal plane PIV measurements are presented for two cases at two different turbulent intensity levels (about 13% and 8%). The free-stream turbulence is generated using an active grid in a wind tunnel. The statistical information of the flow regarding the wall-normal velocity and Reynolds shear stress are presented. The effect of increasing the turbulence level in the free stream for these flows has been found to have similarities with increasing Reynolds number for high-Reynolds-number canonical flows. Quadrant analysis is performed to determine the contributions of different Reynolds-stress-producing events. In this regard, the distribution of momentum transport events shows some similarity with channel flows, which can be justified by comparison of similar intermittency characteristics of both flows. In addition, the coherent structures found inside the boundary layer have inclined features that are consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organization of the structures inside the boundary layer supports the growing evidence for a universal structure for wall-bounded flows.
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26.
  • Dong, Junhao, et al. (författare)
  • Analog of discontinuous shear thickening flows under confining pressure
  • 2017
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 2:8
  • Tidskriftsartikel (refereegranskat)abstract
    • We use two-dimensional numerical simulations to study dense suspensions of non-Brownian hard particles using the Critical Load Model (CLM) under constant confining pressures. At constant packing fraction this simple model shows shear thickening as the tangential forces get activated upon increased shear stresses. By parameterizing a simple binary system of frictional and nonfrictional particles of different proportions we show that the jamming packing fraction, at which the viscosity diverges, is controlled by the fraction of frictional contacts. The viscosity of dense suspensions can thereby be expressed as a function of the fraction of frictional contacts as well as the packing fraction of solid particles. In addition, we show that there exists a simple relationship between the fraction of frictional contacts and the two control parameters (under confining pressure): the viscous number J and the ratio between the repulsive barrier force and confining pressure. Under confining pressures the viscosity curves are found to depend on the shear protocol, with the possibility of yielding negative dynamic compressibility, an analog to the discontinous shear thickening found at high but constant packing fractions for the same system.
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27.
  • Dubey, Anshuman, et al. (författare)
  • Bifurcations in droplet collisions
  • 2022
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 7:6
  • Tidskriftsartikel (refereegranskat)abstract
    • Saffman and Turner [P. G. Saffman and J. S. Turner, J. Fluid Mech. 1, 16 (1956)] argued that the collision rate for droplets in turbulence increases as the turbulent strain rate increases. However, the numerical simulations of Dhanasekaran et al. [J. Dhanasekaran et al., J. Fluid Mech. 910, A10 (2021)] in a steady straining flow showed that the Saffman-Turner model is oversimplified because it neglects droplet-droplet interactions. These result in a complex dependence of the collision rate on the strain rate and on the differential settling speed. Here we show that this dependence is explained by a sequence of bifurcations in the collision dynamics. We compute the bifurcation diagram when strain is aligned with gravity and show that it yields important insights into the collision dynamics. First, the steady-state collision rate remains nonzero in the limit Kn -> 0, contrary to the common assumption that the collision rate tends to zero in this limit (Kn is a nondimensional measure of the mean free path of air). Second, the nonmonotonic dependence of the collision rate on the differential settling speed is explained by a grazing bifurcation. Third, the bifurcation analysis explains why so-called closed trajectories appear and disappear. Fourth, our analysis predicts strong spatial clustering near certain saddle points, where the effects of strain and differential settling cancel.
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28.
  • Einarsson, Jonas, et al. (författare)
  • Spherical particle sedimenting in weakly viscoelastic shear flow
  • 2017
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 2:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider the dynamics of a small spherical particle driven through an unbounded viscoelastic shear flow by an external force. We give analytical solutions to both the mobility problem (the velocity of a forced particle) and the resistance problem (the force on a fixed particle), valid to second order in the dimensionless Deborah and Weissenberg numbers, which represent the elastic relaxation time of the fluid relative to the rate of translation and the imposed shear rate. We find a shear-induced lift at O(Wi), a modified drag at O(De(2)) and O(Wi(2)), and a second lift that is orthogonal to the first, at O(Wi(2)). The relative importance of these effects depends strongly on the orientation of the forcing relative to the shear. We discuss how these forces affect the terminal settling velocity in an inclined shear flow. We also describe a basis set of symmetric Cartesian tensors and demonstrate how they enable general tensorial perturbation calculations such as the present theory. In particular, this scheme allows us to write down a solution to the inhomogeneous Stokes equations, required by the perturbation expansion, by a sequence of algebraic manipulations well suited to computer implementation.
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29.
  • Fan, Yitong, et al. (författare)
  • Decomposition of the mean friction drag in adverse-pressure-gradient turbulent boundary layers
  • 2020
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 5:11
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, we exploit the Renard-Deck identity [J. Fluid Mech. 790, 339 (2016)] to decompose the mean friction drag in adverse-pressure-gradient turbulent boundary layers (APG-TBLs) into three components, associated with viscous dissipation, turbulence kinetic energy production, and spatial growth of the flow, respectively. We consider adverse-pressure-gradient turbulent boundary layers developing on flat plates and airfoils, with friction Reynolds numbers in the range 200 < Re-tau < 2000, and with Rotta-Clauser pressure-gradient parameters (beta) ranging from 0 to 50. The effects of Reynolds number, adverse pressure gradient, and the pressure-gradient history on the contributing components are individually investigated, and special attention is paid to the comparisons with zero-pressure-gradient turbulent boundary layers (ZPG-TBLs). Our results indicate that the inner peaks of the dissipation and production terms are located at y(+) approximate to 6 and y(+) approximate to 16.5, respectively, and their outer peaks scale with the 99% boundary-layer thickness (delta(99)), i.e., y/delta(99) approximate to 0.7 and 0.53, respectively. These results are independent of the friction Reynolds number, the magnitude of beta, and its development history. Moreover, the spatial-growth component is negative in the investigated APG-TBLs, and its magnitude increases with beta.
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30.
  • Feneuil, Blandine, et al. (författare)
  • Experimental and numerical investigation of bubble migration in shear flow : Deformability-driven chaining and repulsion
  • 2023
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 8:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the interaction-induced migration of bubbles in shear flow and observe that bubbles suspended in elastoviscoplastic emulsions organize into chains aligned in the flow direction, similarly to particles in viscoelastic fluids. To investigate the driving mechanism, we perform experiments and simulations on bubble pairs, using suspending fluids with different rheological properties. First, we notice that, for all fluids, the interaction type depends on the relative position of the bubbles. If they are aligned in the vorticity direction, then they repel, if not, then they attract each other. The simulations show a similar behavior in Newtonian fluids as in viscoelastic and elastoviscoplastic fluids, as long as the capillary number is sufficiently large. This shows that the interaction-related migration of the bubbles is strongly affected by the bubble deformation. We suggest that the cause of migration is the interaction between the heterogeneous pressure fields around the deformed bubbles, due to capillary pressure.
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31.
  • Fistler, Marco, 1989, et al. (författare)
  • Turbulence modulation in particle-laden stationary homogeneous isotropic turbulence using one-dimensional turbulence
  • 2020
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 5:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Turbulence modulation in particle-laden stationary homogeneous isotropic turbulence is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, ODT is extended in two ways. First, a forcing scheme that maintains statistical stationarity is introduced. Unlike direct numerical simulation (DNS) of forced turbulence, the ODT framework accommodates forcing that is not directly coupled to the momentum equation. For given forcing the ODT energy dissipation rate is therefore the same in particle-laden cases as in the corresponding single-phase reference case. Second, previously implemented one-way-coupled particle phenomenology is extended to two-way coupling using the general ODT methodology for flow modulation through interaction with any specified energy and momentum sources and sinks. As in a DNS comparison case for Re-lambda = 70, turbulence modulation is diagnosed primarily on the basis of the fluid-phase kinetic-energy spectrum. Because ODT involves subprocesses with straightforward physical interpretations, the ODT mechanisms of particle-induced turbulence modulation are clearly identified and they are plausibly relevant to particleladen Navier-Stokes turbulence. ODT results for the ratio of particle-phase and fluid-phase kinetic energies as a function of particle Stokes number and mass loading are reported for the purpose of testing these predictions in the future when these quantities are evaluated experimentally or using DNS.
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32.
  • Fistler, Marco, 1989, et al. (författare)
  • Turbulence modulation in particle-laden stationary homogeneous shear turbulence using one-dimensional turbulence
  • 2020
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 5:12
  • Tidskriftsartikel (refereegranskat)abstract
    • Turbulence modulation in particle-laden stationary homogeneous shear turbulence (HST) is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, an ODT formulation previously used to study turbulence modulation in forced homogeneous isotropic turbulence (HIT) is extended, so that the model emulates the anisotropic character of HST and, potentially, anisotropic turbulence in general. This is done by limiting the kinetic-energy redistribution during an eddy event to an exchange involving two velocity components, where the three possible choices of the omitted component define three eddy types whose relative likelihoods control the anisotropy. Comparisons of ODT and direct-numerical-simulation results with reference to signatures of turbulence modulation are the basis of a broader ODT parameter study that is reported. Owing to the reduced dimensionality of ODT, it is found that the fidelity of the model for single-phase HST does not extend to particle effects on flow anisotropy, but for quantities averaged over components, parametric trends are captured. The consistent approach to case comparisons that was introduced in the HIT study to evaluate sensitivities to particle-phase parameters in a given flow configuration is extended here to a cross-comparison of HST and HIT model results, and its efficacy is again confirmed. The results provide an overall characterization of the potential for ODT to support the incorporation of particle-induced turbulence modulation into subgrid-scale closures of large-eddy simulations.
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33.
  • Forooghi, Pourya, et al. (författare)
  • Direct numerical simulation of flow over dissimilar, randomly distributed roughness elements : A systematic study on the effect of surface morphology on turbulence
  • 2018
  • Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 3:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Direct numerical simulations are used to investigate turbulent flow in rough channels, in which topographical parameters of the rough wall are systematically varied at a fixed friction Reynolds number of 500, based on a mean channel half-height h and friction velocity. The utilized roughness generation approach allows independent variation of moments of the surface height probability distribution function [thus root-mean-square (rms) surface height, skewness, and kurtosis], surface mean slope, and standard deviation of the roughness peak sizes. Particular attention is paid to the effect of the parameter Delta defined as the normalized height difference between the highest and lowest roughness peaks. This parameter is used to understand the trends of the investigated flow variables with departure from the idealized case where all roughness elements have the same height (Delta = 0). All calculations are done in the fully rough regime and for surfaces with high slope (effective slope equal to 0.6-0.9). The rms roughness height is fixed for all cases at 0.045h and the skewness and kurtosis of the surface height probability density function vary in the ranges -0.33 to 0.67 and 1.9 to 2.6, respectively. The goal of the paper is twofold: first, to investigate the possible effect of topographical parameters on the mean turbulent flow, Reynolds, and dispersive stresses particularly in the vicinity of the roughness crest, and second, to investigate the possibility of using the wall-normal turbulence intensity as a physical parameter for parametrization of the flow. Such a possibility, already suggested for regular roughness in the literature, is here extended to irregular roughness.
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34.
  • Fouxon, Itzhak, et al. (författare)
  • Theory of hydrodynamic interaction of two spheres in wall-bounded shear flow
  • 2020
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 5:5
  • Tidskriftsartikel (refereegranskat)abstract
    • The seminal Batchelor-Green's (BG) theory on the hydrodynamic interaction of two spherical particles of radii a suspended in a viscous shear flow assumes unbounded fluid. In the present paper we study how a rigid plane wall modifies this interaction. Using an integral equation for the surface traction we derive the expression for the particles' relative velocity as a sum of the BG's velocity and the term due to the presence of a wall at finite distance, z(0). Our calculation is not the perturbation theory of the BG solution, so the contribution due to the wall is not necessarily small. We indeed demonstrate that the presence of the wall is a singular perturbation, i.e., its effect cannot be neglected even at large distances. The distance at which the wall significantly alters the particles interaction scales as z(0)(3/5). The phase portrait of the particles' relative motion is different from the BG theory, where there are two singly connected regions of open and closed trajectories both of infinite volume. For finite z(0), besides the BG's domains of open and closed trajectories, there is a domain of closed (dancing) and open (swapping) trajectories that do not materialize in an unbounded shear flow. The width of this region grows as 1/z(0) at smaller separations from the wall. Along the swapping trajectories, which have been previously observed numerically, the incoming particle is turning back after the encounter with the reference particle, rather than passing it by, as the BG theory anticipates. The region of dancing trajectories has infinite volume and is separated from a BG-type domain of closed trajectories that becomes compact due to presence of the wall. We found a one-parameter family of equilibrium states that were previously overlooked, whereas the pair of spheres flows as a whole without changing its configuration. These states are marginally stable and their perturbation yields a two-parameter family of the dancing trajectories, whereas the test particle is orbiting around a fixed point in a frame comoving with the reference particle. We suggest that the phase portrait obtained at z(0) >> a is topologically stable and can be extended down to rather small z(0) of several particle diameters. We confirm this hypothesis by direct numerical simulations of the Navier-Stokes equations with z(0) = 5a. Qualitatively the distant wall is the third body that changes the global topology of the phase portrait of two-particle interaction.
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35.
  • Fries, Johan, et al. (författare)
  • Angular dynamics of small crystals in viscous flow
  • 2017
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 2:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The angular dynamics of a very small ellipsoidal particle in a viscous flow decouples from its translational dynamics and the particle angular velocity is given by Jeffery's theory. It is known that cuboid particles share these properties. In the literature a special case is most frequently discussed, namely that of axisymmetric particles with a continuous rotation symmetry. Here we compute the angular dynamics of crystals that possess a discrete rotation symmetry and certain mirror symmetries but do not have a continuous rotation symmetry. We give examples of such particles that nevertheless obey Jeffery's theory. However, there are other examples where the angular dynamics is determined by a more general equation of motion.
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36.
  • Ge, Zhouyang, et al. (författare)
  • Effective slip over partially filled microcavities and its possible failure
  • 2018
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 3:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by the emerging applications of liquid-infused surfaces (LIS), we study the drag reduction and robustness of transverse flows over two-dimensional microcavities partially filled with an oily lubricant. Using separate simulations at different scales, characteristic contact line velocities at the fluid-solid intersection are first extracted from nanoscale phase field simulations and then applied to micronscale two-phase flows, thus introducing a multiscale numerical framework to model the interface displacement and deformation within the cavities. As we explore the various effects of the lubncant-toouter-fluid viscosity ratio A2/A0 th(mu)over tilde( )c(mu)over tilde(1), thary number Ca, the static contact angle A> and t theta(s), filling fraction of the cavity <5, we f delta d that the effective slip is most sensitive to the parameter S. The effects of A2/A1 an(mu)over tilde( )A(mu)over tilde(a )re ge theta(s)erally intertwined but weakened if <5 < 1. delta M 1er, for an initial filling fraction S = 0.94 delta our results show that the effective slip is nearly independent of the capillary number when it is small. Further increasing Ca to about O.OIA1/A20.01(mu)over tilde(1)/(mu)over tilde(2)ntify a possible failure mode, associated with lubricants draining from the LIS, for A2/A1 A (mu)over tilde(2)1(mu)over tilde(1)V less than or similar to y viscous lubricants (e.g., A2/A1 > (mu)over tilde()),(mu)over tilde(h)owever, are immune to such failure due to their generally larger contact line velocity.
  •  
37.
  • Ge, Zhouyang, et al. (författare)
  • Effective slip over partially filled microcavities and its possible failure
  • 2018
  • Ingår i: Physical Review Fluids. - : APS. - 2469-990X. ; 3:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by the emerging applications of liquid-infused surfaces (LIS), we study the drag reduction and robustness of transverse flows over two-dimensional microcavities partially filled with an oily lubricant. Using separate simulations at different scales, characteristic contact line velocities at the fluid-solid intersection are first extracted from nanoscale phase field simulations and then applied to micronscale two-phase flows, thus introducing a multiscale numerical framework to model the interface displacement and deformation within the cavities. As we explore the various effects of the lubricant-to-outer-fluid viscosity ratio ˜?2/˜?1, the capillary number Ca, the static contact angle ??, and the filling fraction of the cavity ?, we find that the effective slip is most sensitive to the parameter ?. The effects of ˜?2/˜?1 and ?? are generally intertwined but weakened if ?<1. Moreover, for an initial filling fraction ?=0.94, our results show that the effective slip is nearly independent of the capillary number when it is small. Further increasing Ca to about 0.01⁢˜?1/˜?2, we identify a possible failure mode, associated with lubricants draining from the LIS, for ˜?2/˜?1≲0.1. Very viscous lubricants (e.g., ˜?2/˜?1>1), however, are immune to such failure due to their generally larger contact line velocity.
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38.
  • Ge, Zhouyang, et al. (författare)
  • Irreversibility and rate dependence in sheared adhesive suspensions
  • 2021
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:10
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent experiments report that slowly sheared noncolloidal particle suspensions unexpectedly exhibit rate(omega)-dependent complex viscosities in oscillatory shear, despite a constant relative viscosity in steady shear. Using a minimal hydrodynamic model, we show that van der Waals attraction gives rise to this behavior. At volume fractions phi = 20-50%, the complex viscosities in both experiments and simulations display power-law reductions in shear, with a phi-dependent exponent maximum at phi = 40%, resulting from the interplay between hydrodynamic, collision, and adhesive interactions. Furthermore, this rate dependence is accompanied by diverging particle diffusivities and pronounced cluster formations after repeated oscillations.  Previous studies established that suspensions transition from reversible absorbing states to irreversible diffusing states when the oscillation amplitude exceeds a ϕ-dependent critical value γc0,ϕ. Here, we show that a second transition to irreversibility occurs below an ω-dependent critical amplitude, γc0,ω≤γc0,ϕ, in the presence of weak attractions.
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39.
  • Gowda, V. Krishne, et al. (författare)
  • Formation of colloidal threads in geometrically varying flow-focusing channels
  • 2021
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:11
  • Tidskriftsartikel (refereegranskat)abstract
    • Threads of colloidal dispersions can be formed in microfluidic channel systems and are often used for analytical purposes or to assemble macroscopic structures from colloidal particles. Here, we report a combined experimental and numerical study of thread formation in channel systems with varying geometry. In the reference flow-focusing configuration, the sheath flows impinge the core flow orthogonally while in four other channel configurations, the sheath flows impinge the core flow at different confluence angles, which are both positive and negative with respect to the reference sheath direction. Tomographic measurements of the thread development are made using optical coherence tomography (OCT) and are compared to numerically simulated 3D data. The numerical simulations performed with an immiscible fluid solver show good agreement with the experiments in terms of 3D thread shapes, wetted region morphologies, and velocity fields provided an ultralow interfacial tension is applied between the low viscosity (solvent) sheath flows and the high viscosity (dispersion) core flow. Such an ultralow interfacial tension is motivated by the so-called Korteweg stresses induced as a result of the concentration gradient between two miscible fluids in nonequilibrium state. These stresses mimic the effect of interfacial tension, and are often modeled as an effective interfacial tension (EIT), an approach chosen in the present work as well. The value of interfacial tension applied in the simulations was determined through an optimization procedure and compares well with a value deduced from a scaling analysis utilizing the downstream development of experimentally determined thread shape. The experimental and numerical results show that for channel configurations with modest deviations from orthogonal sheath flows, the effect on the thread is similar regardless of whether the sheath flows are co- or counterflowing the core flow. In fact, for these cases, the effect of co- and counterflowing sheath flows can be reproduced with orthogonal sheath flows, if the sheath channel width is increased. However, for channel configurations with larger deviations from orthogonal sheath flows, the effects on the thread are direction dependent. The one-to-one comparison and analysis of numerical and experimental results bring useful insights to understand the behavior of miscible systems involving high-viscosity contrast fluids. These key results provide the foundation to tune the flow-focusing for specific applications, for example in tailoring the assembly of nanostruc-tured materials.
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40.
  • Guo (郭佳诚), Jiacheng, et al. (författare)
  • Vortex dynamics and fin-fin interactions resulting in performance enhancement in fish-like propulsion
  • 2023
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 8:7
  • Tidskriftsartikel (refereegranskat)abstract
    • The leading-edge vortex (LEV) formation on the caudal fin (CF) has been identified as playing a key role in efficient lift-based thrust production of fish-like propulsion. The enhancement of the CF LEV through its interaction with vortices formed upstream due to a median fin with a distinct shape is the focus of this paper. High-speed, high-fidelity videos and particle imaging velocimetry (PIV) were obtained from rainbow trout during steady forward swimming to visualize the undulatory kinematics and two-dimensional flow behavior. Body kinematics are quantified using a traveling-wave formulation that is used to prescribe the motion of a high-fidelity three-dimensional surface model of the fish body for a computational fluid dynamics (CFD) study. The pressure field of the CFD result is compared and validated with the PIV result from the experiment. Using CFD, the vortex forming and shedding behaviors of the anal fin (AF) and their capturing and interaction with the trunk (TK) and the CF are visualized and examined. Coherent AF-bound LEVs are found to form periodically, leading to thrust production of the AF. The vortices subsequently shed from the AF are found to help stabilize and reinforce the LEV formation on the CF by aiding LEV initiation at stroke reversal and enhancing LEV during a tail stroke, which leads to enhancement of lift-based thrust production. The CF is found to shed vortex tubes (VTs) that create backward-facing jets, and the ventral-side VT and the associated backward jets are both strengthened by vortices shed by the AF. An additional benefit of the AF is found to be reduction of body drag by reducing the lateral crossflow that leads to loss of beneficial pressure gradient across the body. Through varying AF-CF spacing and AF height, we find that CF thrust enhancement and TK drag reduction due to the AF are both affected by the position and size of the AF. The position and area of the AF that led to the most hydrodynamic benefit are found to be the original, anatomically accurate position and size. In this paper, we demonstrate the important effect of vortex interaction among propulsive surfaces in fish-like propulsion.
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41.
  • Hejazi, B., et al. (författare)
  • Emergent scar lines in chaotic advection of passive directors
  • 2017
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 2:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We examine the spatial field of orientations of slender fibers that are advected by a two-dimensional fluid flow. The orientation field of these passive directors are important in a wide range of industrial and geophysical flows. We introduce emergent scar lines as the dominant coherent structures in the orientation field of passive directors in chaotic flows. Previous work has identified the existence of scar lines where the orientation rotates by p over short distances, but the lines that were identified disappeared as time progressed. As a result, earlier work focused on topological singularities in the orientation field, which we find to play a negligible role at long times. We use the standard map as a simple time-periodic two-dimensional flow that produces Lagrangian chaos. This class of flows produces persistent patterns in passive scalar advection and we find that a different kind of persistent pattern develops in the passive director orientation field. We identify the mechanism by which emergent scar lines grow to dominate these patterns at long times in complex flows. Emergent scar lines form where the recent stretching of the fluid element is perpendicular to earlier stretching. Thus these scar lines can be labeled by their age, defined as the time since their stretching reached a maximum.
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42.
  • Hu, Ruifeng, et al. (författare)
  • General attached eddies : Scaling laws and cascade self-similarity
  • 2023
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 8:4
  • Tidskriftsartikel (refereegranskat)abstract
    • The attached-eddy model (AEM) is one of the most successful coherent-structure-based phenomenological models in wall turbulence. In the classical AEM, the probability density of eddy pe is assumed to follow an inverse law with the eddy size he, i.e., pe oc h-1 e , to satisfy a constant Reynolds-shear-stress distribution in the inertial layer of canonical wall-bounded turbulent flows. In this paper, we first extend the AEM to general attached eddies with pe oc h-" e , where " is an arbitrary positive real number. Scaling laws for velocity covariance (Reynolds stress) by general attached eddies are derived. Preliminary evidence for the validity of the model is provided from adverse-pressure-gradient turbulent boundary layer and turbulent wing boundary layer flows. Second, considering that the eddy cascade self-similarity is manifested by generalized power laws for probability density pe, population density Me, area coverage Ce, and volume fraction Ve of eddies, i.e., pe oc h-"e, Me oc h-ae , Ce oc h-?e , and Ve oc h-zeta e, we directly connect the exponents with the fractal dimension De of the general attached eddies in a simple and clear way. The present paper highlights that the scaling laws of velocity covariance in the inertial layer of wall-bounded turbulent flows can be directly linked to the characteristics of the cascade self-similarity of the general attached eddies. We believe that the scaling laws derived here and the generalized power-law relationships are useful for a deeper understanding of the connection between coherent structures and turbulence statistics.
  •  
43.
  • Ishida, Takahiro, et al. (författare)
  • Laminar-turbulent patterns with rough walls
  • 2017
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 2:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Oblique large-scale laminar-turbulent patterns are found near the onset of turbulence in subcritical planar shear flows. Their robustness to the introduction of wall roughness is investigated numerically in plane Couette flow as a function of the Reynolds number and the roughness height. The effect of roughness is considered on either one or two walls and is modeled numerically using a parametric model suggested recently [Busse and Sandham, J. Fluid Mech. 712, 169 (2012)]. In the case of two rough walls, the patterns are robust for a mean roughness height up to 10% of the wall gap, but the flow shows larger turbulent fractions with increasing roughness height. In the case of one rough wall only, the trend is similar, but the onset Reynolds number decreases faster with increasing roughness height. Roughness height levels above 15% of the wall gap give rise to new coherent structures, including turbulent bands with nonoblique interfaces. The energetic efficiency of the various regimes is investigated by monitoring the friction factor versus the friction Reynolds number. The mechanisms allowing for streamwise localization of the stripe patterns are discussed, with or without roughness, in the light of various low-order models.
  •  
44.
  • Izbassarov, Daulet, et al. (författare)
  • Dynamics of an elastoviscoplastic droplet in a Newtonian medium under shear flow
  • 2020
  • Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 5:11
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamics of a single elastoviscoplastic drop immersed in plane shear flow of a Newtonian fluid is studied by three-dimensional direct numerical simulations using a finite-difference and level-set method combined with the Saramito model for the elastoviscoplastic fluid. This model gives rise to a yield stress behavior, where the unyielded state of the material is described as a Kelvin-Voigt viscoelastic solid and the yielded state as a viscoelastic Oldroyd-B fluid. Yielding of an initially solid drop of Carbopol is simulated under successively increasing shear rates. We proceed to examine the roles of nondimensional parameters on the yielding process; in particular, the Bingham number, the capillary number, the Weissenberg number, and the ratio of solvent and total drop viscosity are varied. We find that all of these parameters, and not only the Bingham number, have a significant influence on the drop dynamics. Numerical simulations predict that the volume of the unyielded region inside the droplet increases with the Bingham number and the Weissenberg number, while it decreases with the capillary number at low Weissenberg and Bingham numbers. A new regime map is obtained for the prediction of the yielded, unyielded, and partly yielded modes as a function of the Bingham and Weissenberg numbers. The drop deformation is studied and explained by examining the stresses in the vicinity of the drop interface. The deformation has a complex dependence on the Bingham andWeissenberg numbers. At low Bingham numbers, the droplet deformation shows a nonmonotonic behavior with an increasing drop viscoelasticity. In contrast, at moderate and high Bingham numbers, droplet deformation always increases with drop viscoelasticity. Moreover, it is found that the deformation increases with the capillary number and with the solvent to total drop viscosity ratio. A simple ordinary differential equation model is developed to explain the various behaviours observed numerically. The presented results are in contrast with the heuristic idea that viscoelasticity in the dispersed phase always inhibits deformation.
  •  
45.
  • Izbassarov, Daulet, et al. (författare)
  • Polymer drag reduction in surfactant-contaminated turbulent bubbly channel flows
  • 2021
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:10
  • Tidskriftsartikel (refereegranskat)abstract
    • Polymer additives are commonly utilized to manipulate bubbly flows in various applications. Here we investigate the effects of clean and contaminated bubbles driven upward (upflow) in Newtonian and viscoelastic turbulent channel flows. Interface-resolved direct numerical simulations are performed to examine sole and combined effects of soluble surfactant and viscoelasticity using an efficient three-dimensional finite-difference-front-tracking method. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is accomplished by a nonlinear equation of state that relates the surface tension to the surfactant concentration. For Newtonian turbulent bubbly flows, the effects of Triton X-100 and 1-pentanol surfactant are examined. It is observed that the sorption kinetics highly affect the dynamics of bubbly flow. A minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters restoring the single-phase behavior while even two orders of magnitude more 1-pentanol surfactant is not adequate to prevent the formation of layers. For viscoelastic turbulent flows, it is found that the viscoelasticity promotes formation of the bubble wall-layers and thus the polymer drag reduction is completely lost for the surfactant-free bubbly flows, while the addition of small amount of surfactant (Triton X-100) in this system restores the polymer drag reduction resulting in 25% drag reduction for the Wi = 4 case.
  •  
46.
  • Jiang, F., et al. (författare)
  • Inertial torque on a small spheroid in a stationary uniform flow
  • 2021
  • Ingår i: Physical Review Fluids. - 2469-990X. ; 6:2
  • Tidskriftsartikel (refereegranskat)abstract
    • How anisotropic particles rotate and orient in a flow depends on the hydrodynamic torque they experience. Here we compute the torque acting on a small spheroid in a uniform flow by numerically solving the Navier-Stokes equations. Particle shape is varied from oblate (aspect ratio lambda = 1/6) to prolate (lambda = 6), and we consider low and moderate particle Reynolds numbers (Re <= 50). We demonstrate that the angular dependence of the torque, predicted theoretically for small particle Reynolds numbers, remains qualitatively correct for Reynolds numbers up to Re similar to 10. The amplitude of the torque, however, is smaller than the theoretical prediction, the more so as Re increases. For Re larger than 10, the flow past oblate spheroids acquires a more complicated structure, resulting in systematic deviations from the theoretical predictions. Overall, our numerical results provide a justification of recent theories for the orientation statistics of ice crystals settling in a turbulent flow.
  •  
47.
  • Johansson, Petter, et al. (författare)
  • Electrowetting diminishes contact line friction in molecular wetting
  • 2020
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 5:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We use large-scale molecular dynamics to study the dynamics at the three-phase contact line in electrowetting of water and electrolytes on no-slip substrates. Under the applied electrostatic potential the line friction at the contact line is diminished. The effect is consistent for droplets of different sizes as well as for both pure water and electrolyte solution droplets. We analyze the electric field at the contact line to show how it assists ions and dipolar molecules to advance the contact line. Without an electric field, the interaction between a substrate and a liquid has a very short range, mostly affecting the bottom, immobilized layer of liquid molecules which leads to high friction since mobile molecules are not pulled towards the surface. In electrowetting, the electric field attracts charged and polar molecules over a longer range, which diminishes the friction.
  •  
48.
  • Johansson, Petter, et al. (författare)
  • Molecular origin of contact line friction in dynamic wetting
  • 2018
  • Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 3:7
  • Tidskriftsartikel (refereegranskat)abstract
    • A hydrophilic liquid, such as water, forms hydrogen bonds with a hydrophilic substrate. The strength and locality of the hydrogen bonding interactions prohibit slip of the liquid over the substrate. The question then arises how the contact line can advance during wetting. Using large-scale molecular dynamics simulations we show that the contact line advances by single molecules moving ahead of the contact line through two distinct processes: either moving over or displacing other liquid molecules. In both processes friction occurs at the molecular scale. We measure the energy dissipation at the contact line and show that it is of the same magnitude as the dissipation in the bulk of a droplet. The friction increases significantly as the contact angle decreases, which suggests suggests thermal activation plays a role. We provide a simple model that is consistent with the observations.
  •  
49.
  • Kamble, Chetna, et al. (författare)
  • Closure modeling in near-wall region of steep resolution variation for partially averaged Navier-Stokes simulations
  • 2022
  • Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 7:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We seek to develop a closure model to enable scale-resolving simulation (SRS) of a turbulent flow to optimally switchover from a Reynolds-averaged Navier-Stokes (RANS) calculation at the wall to a specified degree of resolution in the wake or free-stream region. The closure model is derived by (i) using the physical principle that the total energy of resolved and unresolved scales should be conserved in the switchover region and (ii) establishing consistency with equilibrium boundary layer scaling of the partially resolved field. The model development is performed in the context of a partially averaged Navier-Stokes (PANS) scale-resolving method by quantifying and modeling the commutation terms resulting from varying resolutions in the wall-normal direction. The resulting wall-modeled PANS (WM-PANS) is used to compute the turbulent channel flow in the Re,. range 180 - 8000. The influence of the RANS-SRS switchover location on the computed flow field is examined. It is then demonstrated that the mean flow is reproduced with reasonable accuracy at modest computational effort without discernible log-layer mismatch even at the highest Reynolds number considered. While the Reynolds stresses are also recovered accurately over most of the flow domain, a noticeable computational transition from RANS to unsteady SRS flow behavior is observed and the underlying physics is examined. Irrespective of the location of computational transition, the unsteady features of the flow away from the wall are well captured. It is demonstrated that the proposed closure is able to inject the appropriate amount of resolved turbulence without the need for artificially generated synthetic turbulence. Overall, WM-PANS presents an accurate and computationally viable option for scale-resolving computations of near-wall high-Reynolds number flows.
  •  
50.
  • Kato, Kentaro, et al. (författare)
  • Boundary-layer transition over a rotating broad cone
  • 2019
  • Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 4:7
  • Tidskriftsartikel (refereegranskat)abstract
    • The route to turbulence in the boundary layer on a rotating broad cone is investigated using hot-wire anemometry measuring the azimuthal velocity. The stationary fundamental mode is triggered by 24 deterministic small roughness elements distributed evenly at a specific distance from the cone apex. The stationary vortices, having a wave number of 24, correspond to the fundamental mode and these are initially the dominant disturbance-energy carrying structures. This mode is found to saturate and is followed by rapid growth of the nonstationary primary mode as well as the stationary and nonstationary first harmonics, leading to transition to turbulence. The amplitudes of these are plotted in a way to highlight the continued growth after saturation of the fundamental stationary mode.
  •  
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