Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q29: Turbulent Mixing II |
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Chair: Emmanuel Villermaux, Aix-Marseille University Room: Georgia World Congress Center B401 |
Tuesday, November 20, 2018 12:50PM - 1:03PM |
Q29.00001: Helicity in turbulent channel flow with Lagrangian computations Quoc Nguyen, Dimitrios V Papavassiliou Coherent 3D flow structures are important for turbulent transport. Utilizing helicity and normalized helicity is an effective way to identify and differentiate between primary and secondary vortices, and to trace vortex-core streamlines [1]. Defined as the dot-product of velocity and vorticity [2], high values of helicity reflect high speed and vorticity when the angle between them is small. In this study, flow in a turbulent channel is simulated by DNS, followed by Lagrangian Scalar Tracking of passive scalar markers [3,4]. These are released from instantaneous line sources at different distances from the channel wall. The channel flow DNS is for friction Reynolds number of 300, and the passive scalars have a Schmidt number ranging from 0.7 to 2,400. By calculating the helicity and normalized helicity along the marker trajectories, the correlation between vortex flow structures with particle dispersion can be explored and the flow structures that are important for scalar turbulent transport can be identified. References 1. Levy Y, Degani D, Seginer A, AIAA J. 28(8), 1347, 1990 2. Moffat HK, JFM, 35(1), 117, 1969 3. Nguyen Q, Papavassiliou DV, Phys. Fluids, 28(12), 125103, 2016 4. Nguyen Q, Feher S, Papavassiliou DV, Fluids 2(3), 46, 2017 |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q29.00002: A novel forcing technique to reproduce scalar mixing of turbulent jets in a 3D periodic box Kyupaeck Jeff Rah, Guillaume Blanquart For most numerical simulations of passive scalars in 3D periodic boxes, a forcing technique is required to sustain the scalar energy. However, the existing forcing techniques are not based on physics, but rather arbitrary numerical methods that sustain the scalar energy. In this current investigation, we aim to develop a forcing technique to simulate a realistic turbulent mixing process inside a 3D periodic box. The target flow is an axisymmetric jet with passive scalars on its centerline. The velocity and scalar information of turbulent jets on the centerline is applied to the scalar transport equation via a Reynolds decomposition. The result is a combination of three terms in the scalar equation: a mean scalar gradient term, a linear forcing term, and a velocity-scalar non-linear term. These new forcing terms are derived to replicate the scalar mixing properties of jets in a triply periodic DNS. A set of DNS has been performed with the new forcing terms, and various turbulent parameters, such as the scalar variance and velocity-scalar correlation, are compared against experiments. |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q29.00003: Deep Learning of PDF Turbulence Closure Maziar Raissi, Hessam Babaee, Peyman Givi A new data-driven method is presented for learning the PDF turbulence closure using deep learning. The method is based on a recently developed physics-informed deep learning model and relies on the physics as expressed by partial differential equations. We solve the single-point PDF equation in homogeneous turbulence using deep neural networks to describe the classical binary scalar mixing problem. In this setting, the neural network learns the conditional expected statistics via observing the PDF data. The performance of this data-driven strategy is appraised against the exact solution where the PDF is given by the amplitude mapping closure (AMC) of Kriachnan. |
Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q29.00004: Numerical study of turbulent channel flow perturbed by spanwise topographic heterogeneity: amplitude and frequency modulation within low- and high-momentum pathways Ankit Awasthi, William Anderson We studied the effects of spanwise heterogeneous, topographically-driven secondary flows on inner-outer interaction in turbulent channel flow. This topography induces domain-scale, counter-rotating vortices. The vortices are anchored to the topography such that prominent upwelling and downwelling occurs above the low and high roughness, respectively. We have quantified the extent to which such secondary flows disrupt the distribution of spectral density across constituent wavelengths throughout the depth of the flow, which has direct implications for amplitude and frequency modulation. The distinct outer peak associated with large-scale motions -- the “modulators” -- is preserved within the upwelling zone, but vanishes in the downwelling zone. Within the downwelling zones, structures are steeper and shorter. Single- and two-point correlations for amplitude and frequency modulation demonstrate insensitivity to resolution across cases. We also show a pronounced crossover between the single- and two-point correlations, a product of modulation quantification based upon Parseval's theorem (i.e., spectral density, not the wavelength at which energy resides, defines the strength of modulation). |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q29.00005: On Shapes and Forms: Population balance dynamics of corrugated stirred fronts Emmanuel Villermaux We introduce a unified framework to discuss the emergence of corrugations on material interfaces transported by random media. Relating the shape of these interfaces to the stirring field giving birth to it, we formalize a population balance dynamics for the $r-$elements (segments of length $r$) needed to cover the interface contour in the course of its deformation. As long as corrugations grow kinematically, shapes change continuously, their fractal dimension $d_f(r,t)$ is a non-monotonous function of the scale $r$, and increases in time $t$ with no bounds. Interface creation and destruction balance however in self-propagating fronts like flames, and in fronts smearing by molecular diffusion, through a mixing induced overlap mechanism, leading to a stationary shape. These findings, which help reexamining old observations in a new perspective, also reconcile kinetics with geometry. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q29.00006: Flow Topology and Alignment Analysis of Passive Scalar Mixing in Shock Turbulence Interaction Xiangyu Gao, Ivan Bermejo-Moreno, Johan Larsson, Lin Fu, Sanjiva K Lele |
Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q29.00007: Abstract Withdrawn Direct Numerical simulation was performed for Spatial plane mixing layer. The emergence of highly coherent 2D structure in Plane mixing layer as observed in the transverse vorticity contour and its further transition into 3D structures is presented. Vorticity is seen to be constant in the eye of coherent vorticity structure for a plane mixing layer. The nature of instability of these coherent structures is being studied . |
Tuesday, November 20, 2018 2:21PM - 2:34PM |
Q29.00008: Initialization of decaying turbulence to study turbulent/non-turbulent interfaces Eunhye An, Eric Johnsen We present a new method for initializing decaying homogeneous, isotropic turbulence to bypass transients produced in a number of other approaches. We propose to re-scale the turbulence in Fourier space after a number of eddy turnover times to achieve a desired spectrum and intensity. We demonstrate the viability of this approach using the canonical problem of the decay of homogeneous, isotropic turbulence, and further use this approach to study mixing at interfaces separating fields of different intensities. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q29.00009: The formation and mixing of a reverse density stratification. Jason Olsthoorn, Edmund W. Tedford, Gregory A Lawrence Northern lakes are typically ice-covered during the winter months. Under the ice, these lakes can also become `reverse stratified' due to the temperature of maximum density of lake water being above its freezing point. Field studies of a brackish lake in Northern Alberta have recorded temperature profiles within the water column over several years and have demonstrated the formation of such a stratification. In addition, these measurements identify several interesting structural features to the density stratification for which we do not yet have an explanation. We have developed a theoretical framework to estimate the heat fluxes for in this regime.
This presentation will discuss an idealize fluid-mechanics problem for convection near the temperature of maximum density of lake water, using direct numerical simulations. We will demonstrate how a nonlinear equation of state modifies the transport and mixing of an unsteady density stratification. We then related these results back to laboratory and field measurements, with a particular focus on the distribution of the density field. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q29.00010: The influence of propagation angle on vortex ring-induced mixing Benjamin M Jackson, Stuart B Dalziel The propagation angle of a vortex ring travelling towards the interface in a two-layer density stratification has a considerable influence on the mixing induced. In the limit of a shallow angle, the vortex ring produces an interfacial wave in its wake when sufficiently close to the interface. The flow remains largely laminar and the ring acts minimally to mix the stratification. Conversely, at low Ri, a vertically downward propagating vortex ring will cause significant volumes of lower layer fluid to be transported into the upper layer during the turbulent energy cascade, leading to considerable mixing. Here, we present new experimental results investigating the mixing induced by the periodic generation of vortex rings fired at an angle in a two-layer stratification. We demonstrate how deviating from the axisymmetric, vertically propagating case impacts the evolution of the density field and the long-term mixing regime. |
Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q29.00011: Differential diffusion and active-scalar turbulence at high Schmidt numbers Pui-Kuen Yeung, Matthew P Clay, K. Ravikumar Recent progress inn GPU-optimized algorithm development (Clay {\em et al.} Comput. Phys. Commun., 2018) has provided a new capability to study turbulent mixing at high Schmidt numbers with demanding resolution requirements. Here we consider both passive and active scalars. Differential diffusion of passive scalars is studied in Schmidt number regimes comparable to those of temperature and salinity (7 and 700) in the ocean. The correlation between fluctuating gradients of the different scalars agrees well with a semi-empirical formula motivated by past simulation data at more modest Schmidt numbers. For active scalars our primary interest is in modifications of the turbulence structure due to one or two scalars with uniform mean gradients in the vertical, with their dynamical effects representable by Boussinesq-type terms in the equations of motion. An active scalar of higher Schmidt number is seen to have a stronger effect on small-scale statistics, such as oscillating mean-square vorticity components in a horizontal plane, although some form of saturation of these effects at sufficiently high Schmidt number is possible. The largest simulations to be presented will be at $8192^3$ grid resolution. |
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