Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E10: Jets, Waves, and Markovianity |
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Chair: Steven Brunton Eurika Kaiser, University of Washington Room: B118-119 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E10.00001: Thin film deposition using rarefied gas jet Dr. Sahadev Pradhan The rarefied gas jet of aluminium is studied at Mach number \textit{Ma }$=$\textit{ (U\textunderscore j / }$\backslash $\textit{sqrt\textbraceleft kb T\textunderscore j / m\textbraceright )}in the range \textit{.01 \textless Ma \textless 2}, and Knudsen number \textit{Kn }$=$\textit{ (1 / (}$\backslash $\textit{sqrt\textbraceleft 2\textbraceright }$\backslash $\textit{pi d\textasciicircum 2 n\textunderscore d H)} in the range \textit{.01 \textless Kn \textless 15}, using two-dimensional (2D) direct simulation Monte Carlo (DSMC) simulations, to understand the flow phenomena and deposition mechanisms in a physical vapor deposition (PVD) process for the development of the highly oriented pure metallic aluminum thin film with uniform thickness and strong adhesion on the surface of the substrate in the form of ionic plasma, so that the substrate can be protected from corrosion and oxidation and thereby enhance the lifetime and safety, and to introduce the desired surface properties for a given application. Here, $H$is the characteristic dimension, \textit{U\textunderscore j}and \textit{T\textunderscore j}are the jet velocity and temperature, \textit{n\textunderscore d}is the number density of the jet, $m$and $d$ are the molecular mass and diameter, and \textit{kb}is the Boltzmann constant. An important finding is that the capture width (cross-section of the gas jet deposited on the substrate) is symmetric around the centerline of the substrate, and decreases with increased Mach number due to an increase in the momentum of the gas molecules. DSMC simulation results reveals that at low Knudsen number \textit{((Kn }$=$\textit{ 0.01);}shorter mean free paths), the atoms experience more collisions, which direct them toward the substrate. However, the atoms also move with lower momentum at low Mach number$,$which allows scattering collisions to rapidly direct the atoms to the substrate. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E10.00002: Turbulence statistics in a negatively buoyant particle plume -- laboratory measurement Ankur Bordoloi, Laura Clark, Gerardo Veliz, Michael Heath, Evan Variano Negatively buoyant plumes of nylon particles are investigated in quiescent salt-water solution using flow visualization and stereoscopic PIV. Particles of the size 2 mm are continuously released through a nozzle from the top inside a water tank using a screw-conveyor based release mechanism. The plume propagates downward due to gravity, and by virtue of interacting particle wakes, becomes turbulent. The two phases are refractive index matched, so that the velocity field in the interstitial fluid can be quantified using PIV. We examine the velocity fields in the fluid phase to characterize turbulence statistics, such as turbulent kinetic energy, Reynolds stresses in the fully developed region of the plume. Further, we develop an image processing method to obtain particle distribution and particle slip inside the plume. In the presentation, we will discuss these results in the light of existing literature for rising plumes of bubbles under similar experimental conditions. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E10.00003: The interaction between atmospheric gravity waves and large-scale flows: an efficient description beyond the non-acceleration paradigm Bruno Ribstein, Gergely Bölöni, Jewgenija Muraschko, Christine Sgoff, Junhong Wei, Ulrich Achatz With the aim of contributing to the improvement of subgrid-scale gravity wave (GW) parameterizations in numerical-weather-prediction and climate models, the comparative relevance in GW drag of direct GW-mean-flow interactions and turbulent wave breakdown are investigated. Of equal interest is how well Wentzel-Kramer-Brillouin (WKB) theory can capture direct wave-mean-flow interactions, that are excluded by applying the steady-state approximation. WKB is implemented in a very efficient Lagrangian ray-tracing approach that considers wave action density in phasespace, thereby avoiding numerical instabilities due to caustics. It is supplemented by a simple wave-breaking scheme based on a static-instability saturation criterion. Idealized test cases of horizontally homogeneous GW packets are considered where wave-resolving Large-Eddy Simulations (LES) provide the reference. In all of theses cases the WKB simulations including direct GW-mean-flow interactions reproduce the LES data, to a good accuracy, already without wave-breaking scheme. The latter provides a next-order correction that is useful for fully capturing the total-energy balance between wave and mean flow. This is not the case when a steady-state WKB implementation is used, as used in present GW parameterizations. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E10.00004: Non-Markov effects in intersecting sprays Mahesh Panchagnula, Dhivyaraja Kumaran, Sri Vallabha Deevi, Arun Tangirala Sprays have been assumed to follow a Markov process. In this study, we revisit that assumption relying on experimental data from intersecting and non-intersecting sprays. A phase Doppler Particle Analyzer (PDPA) is used to measure particle diameter and velocity at various axial locations in the intersection region of two sprays. Measurements of single sprays, with one nozzle turned off alternatively are also obtained at the same locations. This data, treated as an unstructured time series is classified into three bins each for diameter (small, medium, large) and velocity (slow, medium, fast). Conditional probability analysis on this binned data showed a higher static correlation between droplet velocities, while diameter correlation is significantly alleviated (reduced) in intersecting sprays, compared to single sprays. Further analysis using serial correlation measures: auto-correlation function (ACF) and partial auto-correlation function (PACF) shows that the lagged correlations in droplet velocity are enhanced while those in the droplet diameter are significantly debilitated in intersecting sprays. We show that sprays are not necessarily Markov processes and that memory persists, even though curtailed to fewer lags in case of size, and enhanced in case of droplet velocity. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E10.00005: Towards a thorough use of the Mori-Zwanzig formalism for statistical coarse-graining of turbulent flows Ayoub Gouasmi, Eric Parish, Karthik Duraisamy The Mori-Zwanzig formalism provides a mathematically-consistent framework to represent the unresolved physics in coarse-grained simulations. The closure terms that arise can be formally represented as memory or non-Markovian effects. However, the general procedure to compute these memory effects is not tractable in fluid flow problems. Accordingly, existing Mori-Zwanzig closure models only scratch the surface of the framework: they simplify the memory effect by making assumptions that cannot be numerically assessed. We propose a more tractable procedure to approximate memory effects with a good level of accuracy. This approach is demonstrated on the Viscous Burgers Equation and the Kuramoto-Sivanshinsky Equation. Building on these results, we provide perspectives in the development of Mori-Zwanzig-based coarse-grained models for turbulent flows. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E10.00006: A probabilistic approach to modeling and controlling fluid flows Eurika Kaiser, Bernd R. Noack, Andreas Spohn, Louis N. Cattafesta, Marek Morzynski, Guillaume Daviller, Bingni W. Brunton, Steven L. Brunton We extend cluster-based reduced-order modeling (CROM) (Kaiser et al, 2014) to include control inputs in order to determine optimal control laws with respect to a cost function for unsteady flows. The proposed methodology frames high-dimensional, nonlinear dynamics into low- dimensional, probabilistic, linear dynamics which considerably simplifies the optimal control problem while preserving nonlinear actuation mechanisms. The data-driven approach builds upon the unsupervised partitioning of the data into few kinematically similar flow states using a clustering algorithm. The coarse-grained dynamics are then described by a Markov model which is closely related to the approximation of Perron-Frobenius operators. The Markov model can be used as predictor for the ergodic probability distribution for a particular control law approximating the long-term behavior of the system on which basis the optimal control law is determined. Moreover, we combine CROM with a recently developed approach for optimal sparse sensor placement for classification (Brunton et al., 2013) as a critical enabler for in-time control and for the systematic identification of dynamical regimes from few measurements. The approach is applied to a separating flow and a mixing layer exhibiting vortex pairing. [Preview Abstract] |
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