Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session Q08: Turbulence: Mixing (3:55pm - 4:40pm CST)Interactive On Demand
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Q08.00001: Enhancing scalar diffusion in grid-generated turbulence Dana Duong, Stavros Tavoularis Once a scalar plume injected from a concentrated source in grid turbulence reaches the ``far field'', its growth rate becomes proportional to the growth rate of the integral length scale of turbulence and so the scalar in an initially thin plume would take a long distance to get mixed with the surrounding fluid. In the present study, we devised means of spreading the plume in the ``near field'', thus reducing the mixing distance. We injected passively heat from an electrically heated ribbon, placed closely behind a perforated plate that was inserted across the flow in a wind tunnel, and then passed the plume through an array of transverse cylinders, located closely downstream of the source. Measurements of velocity and temperature were obtained with hot-wire anemometers, cold-wire thermometers and thermistors. Different array designs were tested and one design was found to produce far field plumes that were twice as wide as in unobstructed grid turbulence. The cost in terms of kinetic energy loss and the disturbance to the turbulent field were also documented. [Preview Abstract] |
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Q08.00002: A peek into the Lagrangian pair dispersion in turbulence at infinite Reynolds number Shiyong Tan, Ashwanth Salibindla, Ashik Ullah Mohammad Masuk, Rui Ni Observing the Richardson-Obukhov cubic scaling law in experiments at large Reynolds number is challenging because it requires tracking particles for a long time before the finite view area starts to play a role. We approached the problem from a different direction by reducing the initial separation between two particles in a water tunnel equipped with a high-concentration particle tracking system. By following particles with almost two and half decades of time scales, we successfully observed the Richardson scaling for the initial separation at about 3$\eta$. The dependence of this scaling exponent versus different initial separation will be discussed, and this dependence provides a new way to study the dispersion dynamics and intermittency at finite and infinite Reynolds number. [Preview Abstract] |
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Q08.00003: Schmidt-number dependence of scalar fluxes and spectra in isotropic turbulence with a mean scalar gradient Tatsuya Yasuda, Toshiyuki Gotoh, Takeshi Watanabe, Izumi Saito We study the Schmidt-number dependence of scalar fluxes and spectra in isotropic turbulence with a uniform mean scalar gradient using the spectral scalar variance equation and Legendre polynomial expansions. For this purpose, we perform direct numerical simulations. In order to sustain statistically stationary isotropic turbulence, we input velocity fluctuations at moderate wavenumbers so that the computational domain of periodic box becomes much larger compared to the velocity integral length scale. The Taylor-microscale Reynolds number being approximately 150, we vary the Schmidt number from unity to 1/4096. Accordingly, the P\'eclet number based on the scalar Taylor microscale varies from 80 to 0.5. When the Schmidt number is very low, the second-order Legendre contribution of shell-summed scalar variance spectral density is almost as significant as the zeroth-order one at any wavenumber. Besides, very large-scale scalar structures emerge which are elongated along the direction of the mean gradient. The formation of such anisotropic structures is due to very rapid scalar diffusion and the mean scalar gradient. [Preview Abstract] |
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Q08.00004: Is turbulent or laminar mixing more efficient in two-dimensional wall-bounded flow? Benjamin Kadoch, Wouter J. T. Bos, Kai Schneider A turbulent flow mixes in general more rapidly a passive scalar than a laminar flow does. From an energetic point of view, for statistically homogeneous or periodic flows, the laminar regime is more efficient. However, the presence of walls may change this picture and it is not {\it a priori} known whether energetically it is desirable to mix a system in turbulent flow or that a laminar flow will succeed a given level of mixedness using less energy. In the present work we will consider a simple two-dimensional flow in a circular container with a circular rod, stirring a passive scalar, considering a broad range of values of the Reynolds and Schmidt numbers. The flows are computed by direct numerical simulation using FLUSI, a Fourier pseudo-spectral code with volume penalization (https://github.com/pseudospectators/FLUSI). We show that for sufficiently large Schmidt number, turbulent flows more efficiently mix a wall-bounded scalar field than a chaotic or laminar flow does. The mixing efficiency is shown to be a function of the P\'eclet number, and a phenomenological explanation yields a scaling law, consistent with the observations (Phys. Rev. E, 101, 043104, 2020). [Preview Abstract] |
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Q08.00005: Effect of background turbulence on passive scalar mixing of a heated jet in a turbulent coflow Fanny Legay, Laurent Mydlarski Turbulent jets are commonly encountered flows, which frequently transport a scalar and occur in both natural and man-made settings (e.g. pollutants being dispersed from a smokestack into the atmosphere; injection of reactants into a combustor). In the majority of such contexts, the jet issues into an environment that is turbulent -- a factor that plays a key role in the jet's subsequent evolution, yet one that has been the subject of little study. The objective of this work is to therefore quantify the (longitudinal and radial) velocity and scalar fields of an axisymmetric turbulent jet of heated air emitted into a homogeneous, isotropic turbulent background. Emphasis is placed on simultaneous measurements of the velocity and scalar (viz. temperature) fields, with the aim of both quantifying and further understanding the mixing process. Hot-wire anemometry and cold-wire thermometry are employed to this end. The simultaneous measurements of the jet's velocity and temperature fields are made in background flows of different turbulence intensities, at various downstream positions, for four jet-to-coflow velocity ratios. Statistics of the velocity fields, scalar fields, and combined velocity-scalar statistics (e.g. turbulent scalar fluxes) will be presented and discussed. [Preview Abstract] |
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Q08.00006: The spreading of viruses by airborne aerosols: lessons from a first-passage-time problem for tracers in turbulent flows Dhrubaditya MITRA, Akhilesh Kumar Verma, Akshay Bhatnagar, Rahul Pandit We study the spreading of viruses, such as SARS-CoV-2, by airborne aerosols, via a new first-passage-time problem for Lagrangian tracers that are advected by a turbulent flow: By direct numerical simulations of the three-dimensional incompressible, Navier-Stokes equation, we obtain the time $t_{\rm R}$ at which a tracer, initially at the origin of a sphere of radius $R$, crosses the surface of the sphere for the first time. We obtain the probability distribution function (PDF) and show that it displays two qualitatively different behaviors:(a) for $R\ll L$ where $L$ is the integral scale, the PDF has a power-law tail , with the exponent $\alpha=4$;(b) for $R\gg L$, the tail of the PDF decays exponentially. We develop models that allow us to obtain these asymptotic behaviors analytically. We show how to use the PDF to develop social-distancing guidelines for the mitigation of the spreading of airborne aerosols with viruses such as SARS-CoV-2. [Preview Abstract] |
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Q08.00007: Go with the Flow: How Boundaries of Uniform Concentration Zones are Made Jesse Reijtenbagh, Jerry Westerweel, Willem van de Water Turbulent mixing leaves large distinct regions of poorly mixed scalar: uniform concentration zones. Lagrangian Coherent Structures most probably construct the boundaries of these zones. These structures are formed by maxima of the separation rate of nearby fluid parcels. In our experimental setup, we simultaneously measure the scalar concentration and the velocity field of a turbulent jet in water, using one camera for the velocity field (PIV), and one camera for the concentration (LIF); both move together with the mean flow. This greatly extends the observation time, so that the measurement of long-time separation of fluid elements, both in the future and in the past, becomes accessible and Lagrangian Coherent Structures become much more apparent. [Preview Abstract] |
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Q08.00008: Scaling of decaying turbulence in turbulent/non-turbulent mixing. Eunhye An, Eric Johnsen The free decay of shearless, incompressible, homogeneous, isotropic turbulence (HIT) is a canonical problem in turbulence. In the present study, we evaluate the role of compressibility on turbulent/non-turbulent mixing with no mean shear. For this purpose, we conduct direct numerical simulations (DNS) of freely decaying, shearless, compressible turbulence of different initial turbulent intensities and Mach numbers adjacent to non-turbulent flow. When this inhomogeneity is introduced, kinetic energy is transferred across the interface. The interface propagates at the expected 2/3 power of time predicted by Barenblatt et al. (1987). This result allows us to appropriately scale the decaying turbulent kinetic energy with respect to time. [Preview Abstract] |
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Q08.00009: On the scalar-dissipation rate in a temporal jet flow with variable viscosity and mass diffusivity Luminita Danaila, Yacine Brahami, Emilien Varea, Michael Gauding The counterpart of Taylor's predictions for turbulent mixing stands on the assumption that the mean dissipation rate of the scalar variance is independent of the local value of molecular properties (mass diffusivity, or thermal conductivity). Whilst different forms of Taylor's postulate were assessed for decaying homogeneous isotropic turbulence, its validity for decaying shear flows with variable viscosity and mass diffusivity has never been investigated. We use DNS of a jet, that evolves into a different fluid, R times more viscous and diffusive. The ratio R varies between 0.25 and 4. Of specific interest is the dependence of scalar dissipation and the norm of the scalar gradient, on the ratio R. The results are as follows. 1) In the jet core, scalar gradients adapt to diffusivity variations, rendering scalar dissipation independent of these fluctuations, and thus locally validating the classical paradigm. 2) When statistics are conditioned on the distance to the so called Turbulent/Non-Turbulent Interface, we find an intense and persistent effect of the diffusivity variations on the conditional scalar dissipation and on the scalar gradient. Therefore, classical mixing paradigm is not tenable at the T/NT interface, as as diffusivity gradients are maintained during mixing. [Preview Abstract] |
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Q08.00010: Buoyancy-driven homogeneous turbulence with large density fluctuations Denis Aslangil, Daniel Livescu, Arindam Banerjee We explore the dynamics of buoyancy-driven homogeneous variable-density turbulence (HVDT) by using high-resolution (2048$^{\mathrm{3}})$ direct numerical simulations (DNS) within a triply periodic domain. Initially, large regions of two pure miscible fluids with different densities are randomly distributed and start to move in opposite directions upon application of an acceleration field. These motions enhance the stirring and turbulence generation due to the different buoyancy forces in the flow domain. Thus, the available potential energy (PE) is converted into kinetic energy (KE). Simultaneously, the fluids are molecularly mixing, which reduces the PE. At some point, the turbulence dissipation starts to overcome the turbulence generation, which leads to a decay in KE. In this talk, we summarize our recent findings on the large density contrast effects on the highly non-linear evolution of the HVDT. Briefly, increasing the density contrast between the mixing fluids causes a significant divergence between the turbulence structure of the classical single fluid turbulence and turbulence with large density fluctuations as the lighter fluid regions become turbulent faster and reach higher turbulence intensities than the heavier fluid regions due to their smaller inertia. [Preview Abstract] |
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Q08.00011: Investigating the structure of turbulence in disparate viscosity mixing in a co-axial jet mixer Gokul Pathikonda, Michael Cameron Ahmad, Mustafa Usta, Irfan Khan, Cyrus Aidun, Devesh Ranjan When active turbulent mixing is coupled with mixing-limited complex reactions (for eg., consecutive-competitive reactions), changes in turbulent structure significantly affects the reaction products and yield. The turbulent mixing between two liquids of disparate viscosities is investigated experimentally using a simultaneous PIV and PLIF diagnostics in a co-flowing jet configuration for different viscosity ratios ($m$). The simultaneous measurements of velocity and mixture-fraction (and thus the viscosity) fields enable studying the coupled mixing dynamics between the velocity and scalar fluctuations. A skewed distribution of turbulent kinetic energy towards the low viscosity fluid is identified, which can significantly affect the effective local reaction stoichiometry of a hypothetical reaction. Conditional estimates of turbulent kinetic energy and scalar dissipation with respect to local viscosity are studied for similar trends and the effect of viscosity ratio on the same. Finally, the local stress-strain eigenvector alignments are studied to identify changes in local turbulent structure with increasing viscosity disparity. [Preview Abstract] |
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Q08.00012: Non-reacting and reacting experimental investigation of disparate viscosity turbulent mixing in a coaxial jet mixer to investigate mixing-limited consecutive competitive reaction systems Michael Cameron Ahmad, Gokul Pathikonda, Mustafa Usta, Irfan Khan, Cyrus Aidun, Devesh Ranjan Industrial chemical processes often involve mixing limited reactions between fluids of disparate viscosities. The viscosity gradients within the reacting fluids lead to different mixing processes which significantly alter yield. The jet in co-flow served as a canonical case to perform this variable viscosity mixing study. The mixing physics for the disparate viscosity jet in coflow were captured with simultaneous high resolution PIV and PLIF. The resultant yield for the same flows was observed with a novel inline spectroscopic technique. The technique measured the products of reaction between 1-Napthol and Diazotized Sulfanilic acid. The concentrations of mono- (primary) and di-azo (secondary) reaction products were averaged diametrically visible-light absorption spectroscopy at various distances from the injector. Significant structural mixing differences were noted for cases with disparate viscosity. Increases in the viscosity disparity led to decreased selectivity of the desired product. [Preview Abstract] |
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