Evan A. Variano

Shape-dependence of particle rotation in isotropic turbulence

We consider the rotation of neutrally buoyant axisymmetric particles suspended in isotropic turbulence. Using laboratory experiments as well as numerical and analytical calculations, we explore how particle rotation depends upon particle shape. We find that shape strongly affects orientational trajectories, but that it has negligible effect on the variance of the particle angular velocity. Previous work has shown that shape significantly affects the variance of the tumbling rate of axisymmetric particles. It follows that shape affects the spinning rate in a way that is, on average, complementary to the shape-dependence of the tumbling rate. We confirm this relationship using direct numerical simulations, showing how tumbling rate and spinning rate variances show complementary trends for rod-shaped and disk-shaped particles. We also consider a random but non-turbulent flow. This allows us to explore which of the features observed for rotation in turbulent flow are due to the effects of particle alignment i...

Refractive-index-matched hydrogel materials for measuring flow-structure interactions

In imaging-based studies of flow around solid objects, it is useful to have materials that are refractive-index-matched to the surrounding fluid. However, materials currently in use are usually rigid and matched to liquids that are either expensive or highly viscous. This does not allow for measurements at high Reynolds number, nor accurate modeling of flexible structures. This work explores the use of two hydrogels (agarose and polyacrylamide) as refractive-index-matched models in water. These hydrogels are inexpensive, can be cast into desired shapes, and have flexibility that can be tuned to match biological materials. The use of water as the fluid phase allows this method to be implemented immediately in many experimental facilities and permits investigation of high-Reynolds-number phenomena. We explain fabrication methods and present a summary of the physical and optical properties of both gels, and then show measurements demonstrating the use of hydrogel models in quantitative imaging.

The contribution of an overlooked transport process to a wetland's methane emissions

Wetland methane transport processes affect what portion of methane produced in wetlands reaches the atmosphere. We model what has been perceived to be the least important of these transport processes: hydrodynamic transport of methane through wetland surface water and show that its contribution to total methane emissions from a temperate freshwater marsh is surprisingly large. In our 1 year study, hydrodynamic transport comprised more than half of nighttime methane fluxes and was driven primarily by water column thermal convection occurring overnight as the water surface cooled. Overall, hydrodynamic transport was responsible for 32% of annual methane emissions. Many methane models have overlooked this process, but our results show that wetland methane fluxes cannot always be accurately described using only other transport processes (plant-mediated transport and ebullition). Modifying models to include hydrodynamic transport and the mechanisms that drive it, particularly convection, could help improve predictions of future wetland methane emissions.

Slip velocity of large neutrally buoyant particles in turbulent flows

We discuss possible definitions for a stochastic slip velocity that describes the relative motion between large particles and a turbulent flow. This definition is necessary because the slip velocity used in the standard drag model fails when particle size falls within the inertial subrange of ambient turbulence. We propose two definitions, selected in part due to their simplicity: they do not require filtration of the fluid phase velocity field, nor do they require the construction of conditional averages on particle locations. A key benefit of this simplicity is that the stochastic slip velocity proposed here can be calculated equally well for laboratory, field and numerical experiments. The stochastic slip velocity allows the definition of a Reynolds number that should indicate whether large particles in turbulent flow behave (a) as passive tracers; (b) as a linear filter of the velocity field; or (c) as a nonlinear filter to the velocity field. We calculate the value of stochastic slip for ellipsoidal and spherical particles (the size of the Taylor microscale) measured in laboratory homogeneous isotropic turbulence. The resulting Reynolds number is significantly higher than 1 for both particle shapes, and velocity statistics show that particle motion is a complex nonlinear function of the fluid velocity. We further investigate the nonlinear relationship by comparing the probability distribution of fluctuating velocities for particle and fluid phases.

Rotation of Nonspherical Particles in Turbulent Channel Flow.

The effects of particle inertia, particle shape, and fluid shear on particle rotation are examined using direct numerical simulation of turbulent channel flow. Particles at the channel center (nearly isotropic turbulence) and near the wall (highly sheared flow) show different rotation patterns and surprisingly different effects of particle inertia. Oblate particles at the center tend to rotate orthogonally to their symmetry axes, whereas prolate particles rotate around their symmetry axes. This trend is weakened by increasing inertia so that highly inertial oblate spheroids rotate nearly isotropically about their principle axes at the channel center. Near the walls, inertia does not move the rotation of spheroids towards isotropy but, rather, reverses the trend, causing oblate spheroids to rotate strongly about their symmetry axes and prolate spheroids to rotate normal to their symmetry axes. The observed phenomena are mostly ascribed to preferential orientations of the spheroids.

Modeling comprehensive chemical composition of weathered oil following a marine spill to predict ozone and potential secondary aerosol formation and constrain transport pathways

Releases of hydrocarbons from oil spills have large environmental impacts in both the ocean and atmosphere. Oil evaporation is not simply a mechanism of mass loss from the ocean, as it also causes production of atmospheric pollutants. Monitoring atmospheric emissions from oil spills must include a broad range of volatile organic compounds (VOC), including intermediate- and semi-volatile compounds (IVOC, SVOC), which cause secondary organic aerosol (SOA) and ozone production. The Deepwater Horizon (DWH) disaster in the northern Gulf of Mexico during Spring/Summer of 2010 presented a unique opportunity to observe SOA production due to an oil spill. To better understand these observations, we conducted measurements and modeled oil evaporation utilizing unprecedented comprehensive composition measurements, achieved by gas chromatography with vacuum ultra-violet time of flight mass spectrometry (GC-VUV-HR-ToFMS) . All hydrocarbons with 10 to 30 carbons were classified by degree of branching, number of cyclic rings, aromaticity, and molecular weight; these hydrocarbons comprise ∼50% of total oil mass.[Worton et al.] Such detailed and comprehensive characterization of DWH oil allowed bottom-up estimates of oil evaporation kinetics. We developed an evaporative model, using solely our composition measurements and thermodynamic data, that is in excellent agreement with published mass evaporation rates and our wind tunnel measurements. Using this model we determine surface slick samples are composed of oil with a distribution of evaporative ages and identify and characterize probable sub-surface transport of oil. This article is protected by copyright. All rights reserved.

Rotations of small, inertialess triaxial ellipsoids in isotropic turbulence

The statistics of rotational motion of small, inertialess triaxial ellipsoids are computed along Lagrangian trajectories extracted from direct numerical simulations of homogeneous isotropic turbulence. The particle angular velocity and its components along the three principal axes of the particle are considered, expanding on the results presented by \citet{ChevillardMeneveau13}. The variance of the particle angular velocity, referred to as the particle enstrophy, is found to increase for particles with elongated shapes. This trend is explained by considering the contributions of vorticity and strain-rate to particle rotation. It is found that the majority of particle enstrophy is due to fluid vorticity. Strain-rate-induced rotations, which are sensitive to shape, are mostly cancelled by strain-vorticity interactions. The remainder of the strain-rate-induced rotations are responsible for weak variations in particle enstrophy. For particles of all shapes, the majority of the enstrophy is in rotations about the longest axis, which is due to alignment between the longest axis and fluid vorticity. The integral timescale for particle angular velocities about each axis reveals that rotations are most persistent about the longest axis, but that a full revolution is rare.

Collision of oil droplets with marine aggregates: Effect of droplet size

Interactions between oil droplets and marine particle aggregates, such as marine snow, may affect the behavior of oil spills. Marine snow is known to scavenge fine particles from the water column, and has the potential to scavenge oil droplets in the same manner. To determine the degree to which such a process is important in the evolution of oil spills, we quantify the collision of oil droplets and marine aggregates using existing collision rate equations. Results show that interaction of drops and aggregates can substantially influence the drop size distribution, but like all such processes this result is sensitive to the local concentration of oil and aggregates. The analysis also shows that as the size distribution of oil droplets shifts toward larger droplets, a greater fraction of the total oil volume collides with marine aggregates. This result is robust to a variety of different assumptions in the collision model. Results also show that there is not always a dominant collision mechanism. For example, when droplets and aggregates are both close to 10 μm in radius, shear and differential settling contribute nearly equally to the collision rate. This overlap suggests that further research on the interaction of shear and differential settling could be useful.

Air-water gas exchange by waving vegetation stems

Exchange between wetland surface water and the atmosphere is driven by a variety of motions, ranging from rainfall impact to thermal convection and animal locomotion. Here, we examine the effect of wind-driven vegetation movement. Wind causes the stems of emergent vegetation to wave back and forth, stirring the water column and facilitating air-water exchange. To understand the magnitude of this effect, a gas transfer velocity (k600-value) was measured via laboratory experiments. Vegetation-waving was studied in isolation by mechanically forcing a model canopy to oscillate at a range of frequencies and amplitudes matching those found in the field. The results show that stirring due to vegetation-waving produces k600-values from 0.55 cm/hr to 1.60 cm/hr. The dependence of k600 on waving amplitude and frequency are evident from the laboratory data. These results indicate that vegetation-waving has a non-negligible effect on gas transport; thus it can contribute to a mechanistic understanding of the fluxes underpinning biogeochemical processes.