Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session ZC41: Turbulence: Mixing |
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Chair: Alais Hewes, McGill University Room: 206 |
Tuesday, November 21, 2023 12:50PM - 1:03PM |
ZC41.00001: Geometrical effects on compressible turbulent mixing Eunhye An, Eric Johnsen Compressible turbulent mixing poses challenges to classical turbulent theory due to different energy transfer and dissipation mechanisms. We investigate how shearless compressible turbulent flows mix and decay and how the initial dimensions of interfaces affect the dynamics by conducting high-fidelity numerical simulations. Three different geometries of turbulent flows, which are adjacent to irrotational flow, are considered: plane, cylinder, and sphere. Our results demonstrate that turbulent interfaces propagate as a power-law in time with different exponents, confirmed by dimensional analysis. Additionally, dilatational energy transfer is subject to the dimensions of turbulent flows. This physics is well described by the scaling law. |
Tuesday, November 21, 2023 1:03PM - 1:16PM |
ZC41.00002: Interfacial Dynamics in Mean Shear Free Turbulence Arefe Ghazi Nezami, Blair Johnson The presence of turbulence near a density interface is ubiquitous in oceanic and atmospheric environments, and plays a crucial role in the dynamics of these environments. Turbulence enhances irreversible mixing across density interfaces by increasing the interfacial area through the induction of various instabilities and internal wave generation. Regions of low mean shear turbulence adjacent to a stably stratified sharp density interface have been identified in bay areas (Hodges et al. 2011). Despite its significance, there are still many unknowns about this fundamental flow. In this study, we perform a series of laboratory experiments to investigate the driving mechanisms in the evolution of a density interface due to turbulence present in the ambient. To do so, we generate nearly homogeneous isotropic turbulence with negligible mean shear using a random jet array (RJA) in a two-layered flow. The top layer, continuously stirred by the RJA, is located above the dense quiescent layer. We use particle image velocimetry (PIV) to measure the velocity of the flow and turbulence statistics such as turbulent kinetic energy, integral length scale and dissipation rate. Simultaneously, we incorporate laser induced fluorescence to find the location of the interface as it evolves through time. By modifying the flow characteristics, such as turbulent Reynolds number, and the density stratification, which affects the buoyancy Reynolds number and Richardson number, we are able to identify interfacial dynamics and quantify mixing rate and efficiency in different scenarios. |
Tuesday, November 21, 2023 1:16PM - 1:29PM |
ZC41.00003: Mixing metrics for turbulent multi-scalar flows Alais M Hewes, Laurent Mydlarski The mixing of scalars (i.e., temperature, chemical species concentration, etc.) in turbulent flows plays a vital role in many engineering and scientific fields, examples of which include combustion, environmental pollution dispersion, oceanography, and atmospheric science. In applications involving only a single scalar, mixing may be quantified using mixing metrics such as the unmixedness, a non-dimensional parameter characterizing the scalar variance. (Danckwerts 1952; Dimotakis and Miller 1990). However, approaches to quantify the degree of mixing in flows transporting multiple scalars are less developed. Using both theoretical arguments and experimental data obtained from turbulent coaxial jets in which two scalars are mixed, we investigate the suitability of commonly employed mixing metrics – such as the correlation coefficient, segregation parameter, and unmixedness – in describing multi-scalar mixing. We find that the aforementioned mixing metrics are limited in their ability to fully describe the state of mixing in flows in which more than one scalar is present, necessitating development of a new multi-scalar mixing metric. |
Tuesday, November 21, 2023 1:29PM - 1:42PM |
ZC41.00004: Statistics at small scales of a passive scalar field with a uniform mean scalar gradient in isotropic turbulence Yukio Kaneda, Katsunori Yoshimatsu This talk presents a theory of the statistics at small scales of a scalar field convected passively by statistically homogeneous and isotropic incompressible turbulent flow under a uniform mean scalar gradient. The theory is based on an extension of the idea of linear response theory of turbulence reviewed by Y. Kaneda [J. Stat. Mech. (2020) 034006]. In order to examine the theory, we performed direct numerical simulation (DNS) of a passive scalar field under periodic boundary conditions for the velocity field u_{i} (i=1,2,3) and the fluctuating part θ of the scalar field. The velocity field is forced at low wavenumbers so that the total kinetic energy per unit mass is kept time-independent. The DNS is based on an alias-free spectral method and a fourth-order Runge-Kutta method. In the main DNS, the Schmidt number is unity and the Taylor-microscale Reynolds number is approximately 260. Particular attention is paid to the effects of the mean scalar gradient on the anisotropy of the mixed velocity-scalar structure function <δu_{i }(r)δθ(r) δθ(r) > in the inertial-convective range, where δf(r)≡f(x+r)-f(x). Results from the theory are consistent with those of the DNS. |
Tuesday, November 21, 2023 1:42PM - 1:55PM |
ZC41.00005: Energetics of turbulent mixing driven by the Faraday instability in rotating miscible fluids. Narinder Singh, Anikesh Pal Direct numerical simulations (DNS) are performed to investigate the influence of rotation on the turbulent mixing driven by Faraday instability in two miscible fluids of small contrasting density subjected to periodic vertical vibrations. We quantify the irreversible mixing which depicts the conversion of the available potential energy (APE) to the background potential energy (BPE) through irreversible mixing rate (Μ). We demonstrate that at lower forcing amplitudes, the turbulent kinetic energy (t.k.e.) increases with an increase in the Coriolis frequency f till (f/ω)^{2}<0.25, where ω is the forcing frequency, during the sub-harmonic instability phase. This enhancement of t.k.e. is attributed to the excitement of more unstable modes. The irreversible mixing sustains for an extended period with increasing (f/ω)^{2 }till 0.25 owing to the prolonged sub-harmonic instability phase and eventually ceases with instability saturation. When (f/ω)^{2}>0.25, the Coriolis force significantly delays the onset of the sub-harmonic instabilities. The strong rotational effects result in lower turbulence because the bulk of the APE expends to BPE, decreasing APE that converts back to t.k.e. reservoir for (f/ω)^{2}>0.25. Therefore, in the subsequent oscillation, the t.k.e. available to contribute to the external energy input from periodic forcing is small. Since the instability never saturates for (f/ω)^{2}>0.25, conversion of APE to BPE via Μ continues, and we find prolonged irreversible mixing. At higher forcing amplitudes, the instability delaying effect of rotation is negligible, and the turbulence is less intense and short-lived. Therefore, the irreversible mixing phenomenon also ends quickly for (f/ω)^{2}<0.25. However, when (f/ω)^{2}>0.25, a continuous irreversible mixing is observed. We also examine the mixing efficiency in terms of Μ and find that the mixing is efficient at lower forcing amplitudes and rotation rates of (f/ω)^{2}>0.25 because the major portion of APE expends to BPE. |
Tuesday, November 21, 2023 1:55PM - 2:08PM |
ZC41.00006: Mixing and laminarization characteristics of coaxial jets with disparate viscosity Mustafa Usta, Gokul Pathikonda, Irfan Khan, Devesh Ranjan, Cyrus K Aidun Understanding the complex mixing phenomena in co-axial turbulent jets with fluids of disparate viscosity is crucial for numerous industrial applications, like reactive mixing. In this study, we perform highly-resolved three-dimensional Large-eddy simulations (LES) to investigate the mixing characteristics. Velocity and scalar fields are validated against in-house PIV and PLIF measurements, respectively. Since viscosity has a direct effect on flow regime, two cases, TTT (turbulent-turbulent-turbulent) and TLL (turbulent-laminar-laminar), are considered to represent different flow conditions. We investigate evolving interfacial dynamics, turbulent-non-turbulent interface phenomena, momentum equation budget, mixing behavior, and laminarization with increasing viscosity. The interfacial wave evolution shows distinct behavior: m1 (low viscosity) experiences rapid homogenization, while m40 (high viscosity) exhibits "mushroom"-like structures transitioning to folded-segregated patterns. The magnitude and orientation of variable viscosity terms in RANS equations are quantified using LES data, revealing their significance and gradient diffusion hypothesis failure. This research provides valuable insights into mixing physics and RANS limitations in hybrid flow conditions. |
Tuesday, November 21, 2023 2:08PM - 2:21PM |
ZC41.00007: The dispersion and deformation of molecular patterns written in turbulent air Willem Van De Water, Nico J Dam, Enrico Calzavarini Molecular tagging is used to study the dispersion and deformation of patterns written in turbulent air. The writing is done by fusing O_{2} and N_{2} molecules into NO in the focus of a strong ultraviolet laser beam. By crossing several of these laser beams, patterns that have both small and large scales can be painted. The patterns are visualized a while later by inducing fluorescence of the NO molecules with a second UV laser and registering the image. |
Tuesday, November 21, 2023 2:21PM - 2:34PM |
ZC41.00008: On the Taylor-Prandtl controversy Emmanuel Villermaux, Lucas Rotily In 1932, G. I. Taylor wrote a fascinating article suggesting that, in two dimensional turbulent flows at least, it is not the momentum of the eddies which is conserved from one step of their random walk to the other (the so-called Reynolds-Prandtl analogy), but their vorticity, and that therefore the conservation equations for the velocity u and the concentration of a passive scalar c must be different. Taylor's `vorticity transport' theory thus predicts that, in a 2D wake or a jet, the c-profile across the jet (scaled by its maximal value) is exactly twice as large as the axial u-profile (i. e. u(r)=c(r)^{2}). |
Tuesday, November 21, 2023 2:34PM - 2:47PM |
ZC41.00009: Kinetic dissipation and creation of atomic mix Erik L Vold, Jan Velechovsky, Susan Kurien In computational fluid dynamics (CFD), distinguishing the atomically mixed components in a multi-fluid mixture is essential for accurate predictions of species reactivities and transport coefficients dependent upon sub-grid physical scales. In this work, the Kolmogorov scales for velocity in turbulent mixing are used to construct a model for the evolution of the atomically mixed component where fluid instabilities cascade and dissipate by kinetic processes. At the smallest distinguishable hydrodynamic wavelength, equal to the Kolmogorov scale, a velocity is defined as a classical kinetic diffusivity coefficient over the Kolmogorov length scale. This scale sets the maximum momentum diffusivity length in a complex hydrodynamic or turbulent field where the continuous eddy shearing limits that maximum value of the diffusive gradient scale. This kinetic diffusive velocity relative to a turbulent velocity is shown to scale with a turbulent Reynolds number to the power of -1/4, consistent with the Kolmogorov scaling for velocities, while the mass to momentum diffusivities are related through the Schmidt number and the Batchelor scale. The resulting mass flux associated with the diffusive transport term creates atomic mix, with a specific volume, mass, and molar fraction evolving in time, which can be distinguished from pure unmixed components in the flow, and also distinguished from numerical mixing at fluid interfaces. Example results for the case of a plasma fluid near ICF (Inertial Confinement Fusion) conditions are examined and compared in a CFD resolved flow and in the BHR (Besnard-Harlow-Rauenzahn) turbulent mix model. |
Tuesday, November 21, 2023 2:47PM - 3:00PM |
ZC41.00010: Challenges in turbulence modeling and simulation of free-surface flows Lubna Abdelaal Arafa Hassan Margha, Björn Windén, Sharath S Girimaji Free-surface wave flows are of much importance in ocean engineering applications pertaining to ships, coastal barrier systems, offshore platforms, wave energy converters, and fixed/floating offshore wind turbines. Numerical Wave Tanks (NWTs) and Numerical Tow Tanks (NuTTs) which are used for engineering design depend on adequately accurate numerical simulations of turbulence effects in these free-surface flows. The steep gradients of density and velocity at the air-water interface pose several challenges to turbulence modeling and simulation. In this work, we examine the importance of (i) the realizability characteristics of the RANS (Reynolds-Averaged Navier-Stokes) turbulence model, and (ii) the initial state of kinetic energy and eddy viscosity at the interface. Unrealizable turbulence models and inappropriate initial viscosity levels can lead to significant errors. The physics underlying the extreme sensitivity to these model/simulation features at the air-water interface is explicated. The utility of scale-resolving simulations (SRS) is also investigated. |
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