Physical Fluid Dynamics Pdf
Laila Berri
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We simulate a spheroidal swimmer through a complex fluid. A spheroidal swimmer model is developed and exerted in a direct-forcing fictitious domain method framework. We find an inertial spheroidal puller with a small swimming intensity swims faster than the counterpart subjected to the Stokes flow regime, a departure from observations of spherical pullers. Within the Giesekus fluid medium, an augmented mobility factor correlates with an increased squirmer velocity, while a larger aspect ratio contributes to neutral squirmer speed enhancement in the presence of fluid inertia. Meanwhile, a slenderer, inertial squirmer with a vigorous swimming intensity expends more energy.
Impacting nanodroplets on pillared surfaces are investigated with molecular dynamics simulations. The difference between the liquid spreading dynamics on flat and pillared surfaces is shown to arise from the intrusion effect of liquid into pillar gaps. The altered dynamics can be understood by modeling the maximum spreading factor βmax. The scaling laws of βmax on flat surfaces can successfully predict βmax on pillared surfaces when the volume term of the bulk droplet is properly accounted for. For a six parameter group of the initial problem we propose a universal phase space which contains six outcome regimes for impacting nanodroplets.
The time evolution of the liquid relative permeability of grain packings initially filled with liquid foam is measured to increase until a constant value is eventually observed. We demonstrate that this evolution is directly related to the coarsening of the liquid foam confined in the pore space. Using the measured bubble size evolution combined with the intrinsic liquid permeability of the bulk foam, we have determined values corresponding to the function that describes the foam confinement effects as a function of the bubble-to-grain size ratio.
Which direction matters more for subgrid-scale (SGS) turbulence? Our analysis reveals that the essence of dynamic SGS models is often condensed in only a few (sometimes, just one) special directions, offering new insights into the success of dynamic large-eddy simulation models.
We address coherent pressure structures in turbulent channel flows through SPOD and resolvent analysis with and without an eddy-viscosity model. The spectral analysis revealed energetic structures in the near-wall region, as well as large-scale and spanwise-coherent structures. Pressure structures are targeted in both SPOD and resolvent analysis by selecting an adequate norm through the quadrature weights and observation operator, respectively. The first SPOD and the leading resolvent modes closely agree and show low-ranking behavior. The analyzed modes comprise quasi-streamwise (for near-wall and large-scale structures) and spanwise vortices with pressure peaking at vortex centers.
Uncharged micron-sized water droplets flying toward each other do not always coalesce due to the cushioning effect of the air between them.For oppositely charged droplets, we discover a regime for which droplets always collide when they move inside the stable manifolds of a saddle point of the relative droplet dynamics. A consequence is that only for small electrical charges does the droplet coalescence rate depend primarily upon the Knudsen number (Kn), the ratio of the mean-free-path of air to the mean droplet radius. For much larger charges, coalescence does not depend upon Kn. Our theory predicts the critical charge at which the transition between the two regimes occurs.
We show that the apparent relaxation time inferred from the exponential thinning regime in viscoelastic pinch-off is not necessarily a material property, as was assumed so far, but depends on the system size for various polymer solutions and filament thinning techniques. It depends on the plate size in Capillary Breakup Extensional Rheometry (CaBER) with both slow and fast plate separation protocols, and on the nozzle size in Dripping-onto-Substrate (DoS), corroborating recent observations with the dripping technique. It is not due to artifacts such as solvent evaporation or polymer degradation and it cannot always be rationalized by finite extensibility effects.
Novel formulations involving a control surface at a distance from the body are developed to compute drift loads on structures composed of an impermeable hull and a perforated surface accurately and efficiently. The developed formulation can not only give all six components of the mean wave drift force and moment, but also determine the drift loads on each individual body of a multi-body system.
A viscous fluid in contact with a solid channel that has a preferential affinity with respect to a second fluid embedded in the channel leads to a spontaneous imbibition process. Due to the increasing friction of the invading phase, the invading fluid scales diffusively in time. What happens when a third liquid is a lubricant coating the channel? We show that when the lubricant viscosity is decreased, dissipation switches from being localized in the bulk of the invading phase, to the lubricant layers. This leads to a new crossover, below which diffusive dynamics are not observed. Our results open up the possibility of using this mechanism in SLIPS and LIS to control capillary flows.
Ion selective concentration shocks have been shown to develop in electrochemical cells with homogeneous porous media of low surface charge. Here we demonstrate through a set of simulations that heterogeneity in the porous structure can lead to substantial differences in separation performance. Both variation in surface charge and characteristic pore size, result in vortical flow in the depleted area, affecting multiple metrics such as energy consumption, water recovery, and desalination. These effects can also be observed for hierarchical media and may be exploited in future designs of porous materials in electrochemical applications beyond shock electrodialysis
Inspired by industrial processes in oil and gas well plugging and abandonment (P&A) operations, this study investigates the injection of heavy, thick fluids into lighter fluids. By experimenting and analyzing flow behaviors using dimensionless numbers such as Reynolds number, Froude number, inclination angle, Bingham number, and viscosity ratio, we identified different flow regimes such as stable and unstable slumping, separation, and mixing. These findings offer valuable insights for improving fluid flow analysis in applications like 3D printing and other industrial processes.
The follower force model is a fundamental model for active filaments, commonly utilized to model microtubule-motor protein complexes and collections of cilia. In this work we perform a thorough analysis of this model, employing techniques from computational dynamical systems, adapted from high Reynolds number fluid dynamics, to map out the bifurcations in the system and classify emergent states. This approach allows us to bridge the gap between 2D and 3D analyses, in particular establishing the initial buckling as a double Hopf bifurcation. Additionally, we identify the existence of a quasiperiodic solution at the second bifurcation, and categorize the dynamics at higher values of forcing.
Cohesive forces occur at the particle scale and have effects up to the macroscopic scale. Using the canonical configuration of a column of grains collapsing under its own weight in air, this paper reports that a bulk description framework of cohesive effects can account for the macroscopic observations. Experiments are reported with two different cohesion sources, capillary bridges or a polymer coating, for the collapse of rectangular and cylindrical granular columns. The bulk framework is shown to capture the effects of cohesion on the final deposit for both sources of cohesion.
We study numerically and analytically the effect of Navier slip on the orientational dynamics and effective shear viscosity of a semi-dilute suspension of two-dimensional particles with either circular or elongated (plate-like) shape, interacting only via hydrodynamic and contact forces. We show that at dilute concentrations slip causes the elongated particles to align in the flow direction, whilst for large concentrations tumbling of the particles occurs due to particle-particle interactions. We show this change in orientational microstructure directly impacts the effective viscosity of the suspension: a minimum in the effective viscosity occurs at a threshold concentration.
The natural frequency of a fluid overlying on a wavy wall in general reduces. This reduction is observed by a shift in the minimum of the Faraday threshold, i.e., in the parametric acceleration versus frequency plot.
Laboratory experiments of ocean gravity currents show laminar transport supplemented by spontaneous and intermittent cascading of dense water. Statistical analysis reveals self organized criticality of the downward transport. Cascading intrusions are a major contributor of turbulence and vorticity in the ocean interior.
After the presentation, Allum and co-author Marek Stastna (University of Waterloo) will be available to answer attendee questions in a live Q&A session moderated by Phys. Rev. Fluids Board Member, Pascale Garaud (UC Santa Cruz).
APS has selected 156 Outstanding Referees for 2024 who have demonstrated exceptional work in the assessment of manuscripts published in the Physical Review journals. A full list of the Outstanding Referees is available online.
Physical Review Fluids publishes a collection of papers associated with the 2022 Gallery of Fluid Motion. These award winning works were presented at the annual meeting of the APS Division of Fluid Dynamics.