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 A35: Turbulence: General I |
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Chair: Edward Smith, Imperial College London Room: Oregon Ballroom 204 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A35.00001: A Molecular Dynamics Simulation of the Turbulent Couette Minimal Flow Unit Edward Smith What happens to turbulent motions below the Kolmogorov length scale? In order to explore this question, a 300 million molecule Molecular Dynamics (MD) simulation is presented for the minimal Couette channel in which turbulence can be sustained. The regeneration cycle and turbulent statistics show excellent agreement to continuum based computational fluid dynamics (CFD) at Re=400. As MD requires only Newton’s laws and a form of inter-molecular potential, it captures a much greater range of phenomena without requiring the assumptions of Newton's law of viscosity, thermodynamic equilibrium, fluid isotropy or the limitation of grid resolution. The fundamental nature of MD means it is uniquely placed to explore the nature of turbulent transport. A number of unique insights from MD are presented, including energy budgets, sub-grid turbulent energy spectra, probability density functions, Lagrangian statistics and fluid wall interactions. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A35.00002: Response of a turbulent von Karman swirling flow to anisotropy at the molecular scale Tim Gruenberg, Thomas Roesgen We ask if and how the large-scale structure of a turbulent von Karman swirling flow depends on anisotropies introduced at the smallest scales. We generate such anisotropy at the viscous scale in a paramagnetic colloid whose rheology is modified by an external, uniform magnetic field. We report measurements in a high Reynolds number turbulence experiment ($R_{\lambda}$ = 120). Ultrasound velocimetry provides records of tracer particle velocity. Distinct changes in the velocity statistics can be observed extending from the dissipative scales up to the mean flow topology. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A35.00003: Fundamental Entrainment Observations (VSL, etc.) for a SSSL John Foss, Kyle Bade, Douglas Neal, Richard Prevost Fundamental observations of the entrainment process on the low speed side of a high $Re$ self-preserving single stream shear layer have been made using PIV realizations. The $Re$ value was: $U_0 \theta_{mid}/\nu$ = 6.75*$10^4$, where $\theta_{mid}$ = 13.7 cm is the momentum thickness at the mid-location ($x/\theta(0)$ = 390) of the observations. The VSL (Viscous Super Layer), 15-20 $\eta_K$ thick, is bounded by a well-defined border where the non-vortical/vortical transition occurs. The Kolmogorov microscale ($\eta_K$) was determined from the mean-square vorticity adjacent to the VSL. A threshold level to define the border ($\omega_z \theta_{mid}/U_0$ = 0.221) was selected by examination of the data. Quantitative measures of the entrainment process have been obtained, including: $i$) the convoluted length of the border ($L_b$) made non-dimensional with respect to the length ($L_m$) of the temporally averaged flow field ($L_b/L_m$ = 2.8) and $ii$) $\langle v_b^2 \rangle / v_e^2$ = 17, as a measure of the sink-effect at the border. $v_b$ is the measured velocity at the border; $v_e$ is the well-established entrainment velocity far from the active shear layer whose value: $v_e / U_0$ = 0.035, corresponds to the growth of the self-preserving SSSL (d$\theta /$d$x$). [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A35.00004: Modeling the stochastic dynamics of moving turbulent spots over a slender cone at Mach 5 during laminar-turbulent transition Brian Robbins, Rich Field, Mircea Grigoriu, Ryan Jamison, Mikhail Mesh, Katya Casper, Lawrence DeChant During reentry, a hypersonic vehicle undergoes a period in which the flow about the vehicle transitions from laminar to turbulent flow. During this transitional phase, the flow is characterized by intermittent formations of localized turbulent behavior. These localized regions of turbulence are born at the onset of transition and grow as they move to the aft end of the flight vehicle. Throughout laminar-turbulent transition, the moving turbulent spots cause pressure fluctuations on the outer surface of the vehicle, which leads to the random vibration of the structure and its internal components. In light of this, it is of great interest to study the dynamical response of a flight vehicle undergoing transitional flow so that aircraft can be better designed to prevent structural failure. In this talk, we present a statistical model that calculates the birth, evolution, and pressure field of turbulent spots over a generic slender cone structure. We then illustrate that the model appropriately quantifies intermittency behavior and pressure loading by comparing the intermittency and root-mean-square pressure fluctuations produced by the model with theory and experiment. Finally, we present results pertaining to the structural response of a housing panel on the slender cone. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A35.00005: Investigation of RANS Model Deficiencies for Flow and Heat Transfer Simulations in a Pin-Fin Array Zengrong Hao, Catherine Gorle Quantifying turbulence model-form uncertainties of Reynolds-averaged Navier-Stokes (RANS) models is a challenging topic, largely because model inaccuracies can vary greatly across flow regions. The objective of the present study is to gain knowledge on where and how RANS models violate reality in representative heat exchanger geometries, such that a UQ method for use in optimization studies can be developed. To achieve this objective we performed a large-eddy simulation (LES) of the flow and heat transfer for a pin-fin array, and analyzed where RANS fails to predict the key features. The LES results are validated against experimental data available from Ames et al. (J. Turbo., 2005) and Ames and Dvorak (J. Turbo., 2006). The RANS simulations showed significant deviations from the LES for mean velocity profiles downstream of certain pins and for the Nusselt number distribution on the fins. A detailed comparison of the turbulent quantities illustrates a general underestimation of the Reynolds stresses and turbulent heat fluxes in near-fin regions, and incorrect trends in some pin-wake regions near the channel center plane. Based on this analysis, we draw conclusions that will support the development of a turbulence model UQ method for heat exchangers. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A35.00006: ABSTRACT WITHDRAWN |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A35.00007: Analysis of non-stationary turbulent flows using Multivariate EMD and Matching Pursuits Arvind Mohan, Lionel Agostini, Datta Gaitonde, MIguel Visbal Time-series analysis of highly transient non-stationary turbulent flow is challenging. Traditional Fourier based techniques are generally difficult to apply because of the highly aperiodic nature of the data. Another significant obstacle is assimilating multivariate data, such as multiple variables at a location or from different sources in a flow-field. Such an analysis has the potential to identify sensitive events common among these sources. In this work, we explore two techniques to address these challenges - Multivariate Empirical Mode Decomposition and Matching Pursuits, on deep dynamic stall of a plunging airfoil in a mixed laminar-transitional-turbulent regime. Although primarily used for neuroscience applications, we use them in fluid mechanics and highlight their significant potential to overcome limitations of more traditional techniques. Application of these methods highlight different stages in the development of stall. A first stage shows development of 2-D boundary layer oscillations at frequencies similar to those associated with trailing edge vortices. Subsequently, new instabilities arise due to imminent separation. The separation bubble itself is characterized by relatively higher frequency content, and further analysis indicates its 3-D collapse. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A35.00008: Investigation of coherent structures in a superheated jet using decomposition methods Avick Sinha, Shivasubramanian Gopalakrishnan, Sridhar Balasubramanian A superheated turbulent jet, commonly encountered in many engineering flows, is complex two phase mixture of liquid and vapor. The superposition of temporally and spatially evolving coherent vortical motions, known as coherent structures(CS), govern the dynamics of such a jet. Both POD and DMD are employed to analyze such vortical motions. PIV data is used in conjunction with the decomposition methods to analyze the CS in the flow. The experiments were conducted using water emanating into a tank containing homogeneous fluid at ambient condition. Three inlet pressure were employed in the study, all at a fixed inlet temperature. 90\% of the total kinetic energy in the mean flow is contained within the first five modes. The scatterplot for any two POD coefficients predominantly showed a circular distribution, representing a strong connection between the two modes. We speculate that the velocity and vorticity contours of spatial POD basis functions show presence of K-H instability in the flow. From DMD, eigenvalues away from the origin is observed for all the cases indicating the presence of a non-oscillatory structure. Spatial structures are also obtained from DMD. [Preview Abstract] |
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