Homogeneity and isotropy in a laboratory turbulent flow

We present a new design for a stirred tank that is forced by two parallel planar arrays of randomly actuated synthetic jets. This arrangement creates turbulence at high Reynolds number with low mean flow. Most importantly, it exhibits a region of 3D homogeneous isotropic turbulence that is significantly larger than the integral lengthscale. These features are essential for enabling laboratory measurements of turbulent suspensions. We use quantitative imaging to confirm isotropy at large, small, and intermediate scales by examining one-- and two--point statistics at the tank center. We then repeat these same measurements to confirm that the values measured at the tank center are constant over a large homogeneous region. In the direction normal to the symmetry plane, our measurements demonstrate that the homogeneous region extends for at least twice the integral length scale $L=9.5$ cm. In the directions parallel to the symmetry plane, the region is at least four times the integral lengthscale, and the extent in this direction is limited only by the size of the tank. Within the homogeneous isotropic region, we measure a turbulent kinetic energy of $6.07 \times 10^{-4}$m$^2$s$^{-2}$, a dissipation rate of $4.65 \times 10^{-5}$m$^2$s$^{-3}$, and a Taylor--scale Reynolds number of $R_\lambda=334$. The tank's large homogeneous region, combined with its high Reynolds number and its very low mean flow, provides the best approximation of homogeneous isotropic turbulence realized in a laboratory flow to date. These characteristics make the stirred tank an optimal facility for studying the fundamental dynamics of turbulence and turbulent suspensions.Comment: 18 pages, 9 figure

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

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 7300–7315, doi:10.1002/2015JC011093.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-volatile and semivolatile 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 ultraviolet time of flight mass spectrometry (GC-VUV-HR-ToFMS). All hydrocarbons with 10–30 carbons were classified by degree of branching, number of cyclic rings, aromaticity, and molecular weight; these hydrocarbons comprise ∼70% of total oil mass. 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 subsurface transport of oil.Funded by Gulf of Mexico Research Initiative2016-05-0

Dr. Price testing groundwater salinity at SRS-6

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QUANTITATIVE VISUALIZATION OF CARBON DIOXIDE GAS TRANSFER AT A TURBULENT FREE SURFACE

Advisor = Edwin A. CowenWe study the aqueous-phase side of the transfer of carbon dioxide gas across an air-water interface. Quantitative imaging techniques are used to directly visualize the physical processes which determine the average gas transfer rate. The interface is a free surface in the absence of mean shear, with turbulence generated on the water side, well away from the free surface, which then transports itself to the free surface. This turbulence is generated far beneath the free surface by an array of upward-pointing synthetic jets which are each driven according to independent random time series. We show that this method of turbulence generation is superior to the traditional grid-stirred tank in that it exhibits weaker mean secondary flows. Using Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) we measure simultaneous concentration and velocity fields, respectively. These are measured in planar fields perpendicular to and intersecting the free surface. From these we calculate turbulent statistics of interest. Namely, the vertical profiles of mean and fluctuating velocity magnitudes, momentum dissipation rate, spatial power spectra for velocity and concentration, and the turbulent mass flux. Examination of the turbulent mass flux field reveals that downward-traveling fluid, which leaves the concentration boundary layer at the surface and enters the bulk, is responsible for the majority of the gas transfer. This is in contrast to the commonly held view that upward-traveling fluid from the bulk dominates gas transfer. The spectrum of the turbulent mass flux field is nearly flat, showing that motions of all sizes in the inertial subrange contribute equally to the mass transfer. This resolves the longstanding question about which size eddies are responsible for gas transfer.This work was supported by: NSF CAREER Grant CTS-0093794, NSF IGERT Site Grant DGE-9870631, and NSF GK-12 Site Grant 0231913. Any opinions, findings, and conclusions or recommendations are those of the author(s) and do not necessarily reflect the views of the National Science Foundation

Sasha Wagner collects surface water samples to assess dissolved organic matter sources and reactivity along the Harney River

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