Javier Urzay

The elastohydrodynamic force on a sphere near a soft wall

The influence of soft boundaries on the forces experienced by a small sphere undergoing slow translation and rotation near a wall is investigated using asymptotic and numerical methods. The clearance between the sphere and the wall is assumed to be small, so that the lubrication approximation holds in the gap. The forces induced by boundary deformation break the symmetry of the Stokes equations, leading to irreversibility of the motion of the sphere and yielding a nonzero lift force. A general formulation, applicable to any constitutive equation of the wall, is presented, and an asymptotic analysis for slightly soft boundaries is developed and applied to a thin compressible elastic layer coating a rigid surface. Expressions are derived for the elastohydrodynamic lift exerted on a sphere moving parallel to the wall, which include the influence of the sphere rotation in the direction of its motion. The results extend and correct previous work, and are different from those for the elastohydrodynamic motion o...

Multi-scale statistics of turbulence motorized by active matter

A number of micro-scale biological flows are characterized by spatio-temporal chaos. These include dense suspensions of swimming bacteria, microtubule bundles driven by motor proteins, and dividing and migrating confluent layers of cells. A characteristic common to all of these systems is that they are laden with active matter, which transforms free energy in the fluid into kinetic energy. Because of collective effects, the active matter induces multi-scale flow motions that bear strong visual resemblance to turbulence. In this study, multi-scale statistical tools are employed to analyze direct numerical simulations (DNS) of periodic two- (2D) and three-dimensional (3D) active flows and compare them to classic turbulent flows. Statistical descriptions of the flows and their variations with activity levels are provided in physical and spectral spaces. A scale-dependent intermittency analysis is performed using wavelets. The results demonstrate fundamental differences between active and high-Reynolds number turbulence; for instance, the intermittency is smaller and less energetic in active flows, and the work of the active stress is spectrally exerted near the integral scales and dissipated mostly locally by viscosity, with convection playing a minor role in momentum transport across scales.

Asymptotic theory of the elastohydrodynamic adhesion and gliding motion of a solid particle over soft and sticky substrates at low Reynolds numbers

This analysis makes use of asymptotic analyses and numerical methods to address, in the limit of small Reynolds and ionic Peclet numbers and small clearances, the canonical problem of the forces exerted on a small solid spherical particle undergoing slow translation and rotation in an incompressible fluid moving parallel to an elastic substrate, subject to electric double-layer and van der Waals intermolecular forces, as a representative example of particle gliding and the idealized swimming dynamics of more complex bodies near soft and sticky surfaces in a physiological solvent. The competition of the hydrodynamic, intermolecular and surface-deformation effects induces a lift force, and drag-force and drift-force perturbations, which do not scale linearly with the velocities, and produce a non-additivity of the intermolecular effects by reducing the intensity of the repulsive forces and by increasing the intensity of the attractive forces. The lift force enhances a reversible elastohydrodynamic adhesion regime in both ionized and deionized solvents, in which lateral motion and lift-off from the substrate can occur. An irreversible elastohydrodynamic adhesion regime, produced by elastic instabilities in the form of surface bifurcations in the substrate, is found to exist for both positive and negative lift forces and is enhanced by small gliding velocities and large substrate compliances, for which critical thresholds are calculated for both ionized and deionized solvents. Elastohydrodynamic corrections are derived for the critical coagulation concentration of electrolyte predicted by the Derjaguin― Landau―Verwey―Overbeek (DLVO) standard theory of colloid stabilization. The corrected DLVO critical concentration is unable to describe the adhesion process when the substrate is sufficiently compliant or when the solvent is deionized. These effects may have consequences on the lateral motility and adhesion of small particles and swimming micro-organisms to soft and sticky substrates, in which the reversible or irreversible character of the adhesion process may be influenced not only by the solvent ionic strength, as described by the DLVO theory, but also by the motion kinematics and the substrate mechanical properties.

Constant-energetics physical-space forcing methods for improved convergence to homogeneous-isotropic turbulence with application to particle-laden flows

This study investigates control-based forcing methods for incompressible homogeneous-isotropic turbulence forced linearly in physical space which result in constant turbulent kinetic energy, constant turbulent dissipation (also constant enstrophy), or a combination of the two based on a least-squares error minimization. The methods consist of proportional controllers embedded in the forcing coefficients. During the transient, the controllers adjust the forcing coefficients such that the controlled quantity achieves very early a minimal relative error with respect to its target stationary value. Comparisons of these forcing methods are made with the non-controlled approaches of Rosales and Meneveau [“Linear forcing in numerical simulations of isotropic turbulence: Physical space implementations and convergence properties,” Phys. Fluids 17, 095106 (2005)] and Carroll and Blanquart [“A proposed modification to Lundgren’s physical space velocity forcing method for isotropic turbulence,” Phys. Fluids 25, 105114 (2013)], using direct numerical simulations (DNS) and large-eddy simulations(LES). The results indicate that the proposed constant-energetics forcing methods shorten the transient period from a user-defined artificial flow field to Navier-Stokes turbulence while maintaining steadier statistics. Additionally, the proposed method of constant kinetic-energy forcing behaves more robustly in coarse LES when initial conditions are employed that favor the occurrence of subgrid-scale backscatter, whereas the other approaches fail to provide physical turbulent flow fields. For illustration, the proposed forcing methods are applied to dilute particle-laden homogeneous-isotropic turbulent flows; the results serve to highlight the influences of the forcing strategies on the disperse-phase statistics.

Regimes of Spray Vaporization and Combustion in Counterflow Configurations

This article addresses the problem of spray vaporization and combustion in axisymmetric opposed-jet configurations involving a stream of hot air counterflowing against a stream of nitrogen carrying a spray of fuel droplets. The Reynolds numbers of the jets are assumed to be large, so that mixing of the two streams is restricted to a thin mixing layer that separates the counterflowing streams. The evolution of the droplets in their feed stream from the injection location is seen to depend fundamentally on the value of the droplet Stokes number, St, defined as the ratio of the droplet acceleration time to the mixing-layer strain time close to the stagnation point. Two different regimes of spray vaporization and combustion can be identified depending on the value of St. For values of St below a critical value, equal to 1/4 for dilute sprays with small values of the spray liquid mass-loading ratio, the droplets decelerate to approach the gas stagnation plane with a vanishing axial velocity. In this case, the droplets located initially near the axis reach the mixing layer, where they can vaporize due to the heat received from the hot air, producing fuel vapor that can burn with the oxygen in a diffusion flame located on the air side of the mixing layer. The character of the spray combustion is different for values of St of order unity, because the droplets cross the stagnation plane and move into the opposing air stream, reaching distances that are much larger than the mixing-layer thickness before they turn around. The vaporization of these crossing droplets, and also the combustion of the fuel vapor generated by them, occur in the hot air stream, without significant effects of molecular diffusion, generating a vaporization-assisted nonpremixed flame that stands on the air side outside the mixing layer. Separate formulations will be given below for these two regimes of combustion, with attention restricted to the near-stagnation-point region, where the solution is self-similar and all variables are only dependent on the distance to the stagnation plane. The resulting formulations display a reduced number of controlling parameters that effectively embody dependences of the structure of the spray flame on spray dilution, droplet inertia, and fuel preferential diffusion. Sample solutions are given for the limiting cases of pure vaporization and of infinitely fast chemistry, with the latter limit formulated in terms of chemistry-free coupling functions that allow for general nonunity Lewis numbers of the fuel vapor.

Aerodynamic generation of electric fields in turbulence laden with charged inertial particles

Self-induced electricity, including lightning, is often observed in dusty atmospheres. However, the physical mechanisms leading to this phenomenon remain elusive as they are remarkably challenging to determine due to the high complexity of the multi-phase turbulent flows involved. Using a fast multi-pole method in direct numerical simulations of homogeneous turbulence laden with hundreds of millions of inertial particles, here we show that mesoscopic electric fields can be aerodynamically created in bi-disperse suspensions of oppositely charged particles. The generation mechanism is self-regulating and relies on turbulence preferentially concentrating particles of one sign in clouds while dispersing the others more uniformly. The resulting electric field varies over much larger length scales than both the mean inter-particle spacing and the size of the smallest eddies. Scaling analyses suggest that low ambient pressures, such as those prevailing in the atmosphere of Mars, increase the dynamical relevance of this aerodynamic mechanism for electrical breakdown.How lightning occurs in dusty atmospheres remains largely unknown because of the complexity of the turbulent flows involved. Di Renzo and Urzay reveal a flow-driven mechanism of charge separation by simulating turbulence laden with hundreds of millions of electrically charged inertial particles.

Computational hydrodynamics and optical performance of inductively-coupled plasma adaptive lenses

This study addresses the optical performance of a plasma adaptive lens for aero-optical applications by using both axisymmetric and three-dimensional numerical simulations. Plasma adaptive lenses are based on the effects of free electrons on the phase velocity of incident light, which, in theory, can be used as a phase-conjugation mechanism. A closed cylindrical chamber filled with Argon plasma is used as a model lens into which a beam of light is launched. The plasma is sustained by applying a radio-frequency electric current through a coil that envelops the chamber. Four different operating conditions, ranging from low to high powers and induction frequencies, are employed in the simulations. The numerical simulations reveal complex hydrodynamic phenomena related to buoyant and electromagnetic laminar transport, which generate, respectively, large recirculating cells and wall-normal compression stresses in the form of local stagnation-point flows. In the axisymmetric simulations, the plasma motion is co...

Publisher Correction: Aerodynamic generation of electric fields in turbulence laden with charged inertial particles

The original version of this Article contained an error in the last sentence of the second paragraph of the ‘Atmospheric rarefaction effects’ section of the Results, which incorrectly read ‘The other one emulates the rarefied, CO2-rich Martian atmosphere (μ♂ = 1.3 × 10−5 N s m−2) at 6.9 mbar and 210 K, which gives ρ♂ = 1.6 × 10−12 kg m−3.’ The correct version states ‘ρ♂ = 1.6 × 10−2 kg m−3’ in place of ‘ρ♂ = 1.6 × 10−12 kg m−3’. This has been corrected in both the PDF and HTML versions of the Article.