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 G3: Vortex Dynamics: Mechanisms and Plates |
Hide Abstracts |
Chair: Melissa Green, Syracuse University Room: B110-111 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G3.00001: Identification and tracking of hairpin vortex auto-generation in turbulent wall-bounded flow Yangzi Huang, Melissa Green Hairpin vortices have been widely accepted as component structures of turbulent boundary layers. Their properties (size, vorticity, energy) and dynamic phenomena (origin, growth, breakdown) have been shown to correlate to the complex, multi-scaled turbulent motions observed in both experiments and simulations. As established in the literature, the passage of a hairpin vortex creates a wall-normal ejection of fluid, which encounters the high-speed freestream resulting in near-wall shear and increased drag. A previously generated simulation of an isolated hairpin vortex is used to study the auto-generation of a secondary vortex structure. Eulerian methods such as the Q criterion and Γ2 function, as well as Lagrangian methods are used to visualize the three-dimensional hairpin vortices and the auto-generation process. The circulation development and wall-normal location of both primary and secondary hairpin heads are studied to determine if there is a correlation between the strength and height of the primary hairpin vortex with the secondary hairpin vortex auto-generation. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G3.00002: Vortex Clusters and Their Time Evolution in High- Reynolds-Number Turbulence Takashi Ishihara, Atsuya Uno, Koji Morishita, Mitsuo Yokokawa, Yukio Kaneda Time series data (with a time interval of $4\tau_\eta$) obtained by high-resolution direct numerical simulations (DNSs) of forced incompressible turbulence in a periodic box, with a maximum of $12288^3$ grid points and Taylor micro-scale Reynolds numbers $R_\lambda$ up to $2300$, are used to study the vortex dynamics in high Reynolds number ($Re$) turbulent flows. Here $\tau_\eta$ is the Kolmogorov time scale. A visualization method to handle such large-scale data was developed for this study. In the high $Re$ turbulence generated by the DNS, we observed the dynamics of tube-like vortex clusters of various sizes, which are constructed by strong micro vortices. For example, we observed the generation of the tube-like clusters of various sizes and the processes of their merging and breakdown. We also observed layer-like vortex clusters of the order of the integral length scale forming shear layers in the high $Re$ turbulence. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G3.00003: Vortex reconnection in the K-type transitional channel flow Yaomin Zhao, Yue Yang, Shiyi Chen Vortex reconnection, as the topological change of vortex lines or surfaces, is a critical process in transitional flows, but is challenging to accurately characterize in shear flows. We apply the vortex-surface field (VSF), whose isosurface is the vortex surface consisting of vortex lines, to study vortex reconnection in the K-type temporal transition in channel flow. Based on the VSF, both qualitative visualization and quantitative analysis are used to investigate the reconnection between the hairpin-like vortical structures evolving from the opposite channel halves. The incipient vortex reconnection is characterized by the vanishing minimum distance between a pair of vortex surfaces and the reduction of vorticity flux through the region enclosed by the VSF isolines on the spanwise symmetric plane. In addition, we find that the surge of the wall friction coefficient begins at the identified reconnection time, which is discussed with the induced velocity during reconnection and the Biot-Sarvart law. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G3.00004: Thermodynamic effects during vortex reconnection Jean-Pierre Hickey Thermodynamic and compressibility effects during vortex reconnection are studied using fully-resolved, high-order direct numerical simulations. The bridging and reconnection of coherent structures is believed to play a key role in the understanding of the turbulent energy cascade as it provides a mechanism for the energy transfer from the large scale down to the dissipation scale. Previous studies have focused on the incompressible cases to understand the hydrodynamics of reconnection process. Here, the focus lies on the thermodynamic effects, more specifically, the temperature and density changes and concomitant thermophysical variations resulting from the large pressure gradients at the time of reconnection. The initial setup replicates the anti-parallel vortex configuration proposed by Melander and Hussain (CTR Summer program, 1988). The simulations are conducted with a third-order in time, sixth-order compact finite difference (in space) schemes for the solution of the fully compressible, Navier--Stokes equations at Reynolds numbers (circulation/viscosity) from 1000 to 5000. The results will highlight the importance of the thermodynamic effects during the reconnection process and the dependence on the local Mach number of the flow. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G3.00005: Vortex breakdown in simple pipe bends Jesse Ault, Sangwoo Shin, Howard Stone Pipe bends and elbows are one of the most common fluid mechanics elements that exists. However, despite their ubiquity and the extensive amount of research related to these common, simple geometries, unexpected complexities still remain. We show that for a range of geometries and flow conditions, these simple flows experience unexpected fluid dynamical bifurcations resembling the bubble-type vortex breakdown phenomenon. Specifically, we show with simulations and experiments that recirculation zones develop within the bends under certain conditions. As a consequence, fluid and particles can remain trapped within these structures for unexpectedly-long time scales. We also present simple techniques to mitigate this recirculation effect which can potentially have impact across industries ranging from biomedical and chemical processing to food and health sciences. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G3.00006: Vortex-breakdown and wall-separation states in swirling flows in a straight pipe Yuxin Zhang, Zvi Rusak, Shixiao Wang The appearance of vortex-breakdown and wall-separation states in various incoming swirling flows to a straight circular pipe is investigated. Fixed-in-time profiles of the axial and circumferential velocities and of the azimuthal vorticity are prescribed at the pipe inlet. A parallel flow state is set at the pipe outlet. Following the theory of Wang \& Rusak (1997), the outlet state of the steady flow problem is determined for a long pipe by solutions of the columnar (axially-independent) Squire-Long equation. For each of the incoming flows studied, these solutions include the base columnar flow state, a decelerated flow along the centerline, an accelerated flow along the centerline, a vortex-breakdown state and a wall-separation state. These theoretical predictions are numerically realized by flow simulations based on the unsteady flow equations. The simulations shed light on the base flow stability and the dynamics of initial perturbations to the various states. The present study extends all the six bifurcation diagrams of solutions studied in Leclaire \& Sipp (2010), who stopped the development of branches of steady states once breakdown and wall-separation states first appear. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G3.00007: Characterization of Vortex Development on a Pitching Plate Kevin Wabick, James Buchholz The formation and evolution of leading-edge vortices (LEVs) is ubiquitous on a broad range of aerodynamic structures and natural fliers, and can have a significant impact on aerodynamic loads. The formation of LEVs is considered on a pitching flat plate at a chord-based Reynolds number of 10$^4$ with varying dimensionless pitch rate through characterization of the sources and sinks of vorticity that contribute to their growth and evolution. The effect of pitch rate is examined, and the flow field evolution and measured fluxes of vorticity are compared to prior measurements on a purely plunging plate, to isolate the effects of rotation on the factors contributing to vortex strength and development. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G3.00008: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 9:44AM - 9:57AM |
G3.00009: Evaluating vortex sheet models for separated flow past a flat plate Monika Nitsche, Ling Xu Numerical studies of separated flows using the full governing equations are computationally expensive. In practice, low order point vortex or vortex sheet models are often used instead. These models are based on simple algorithms to satisfy the Kutta condition at sharp edges. Here, we use highly resolved direct numerical simulations of flow past a finite flat plate to evaluate vortex sheet models for separation. We obtain values for the shed circulation, vortex trajectory and vortex sizes as a function of time and Reynolds number, for accelerated flow past a flat plate at an angle to the incoming flow. We then compare the viscous results with results from a vortex sheet model and determine the extent to which the model reproduces the flow. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G3.00010: Vortex dynamics and surface pressure fluctuations on a normal flat plate Arman Hemmati, David H. Wood, Robert J. Martinuzzi, Simon W. Ferrari, Yaoping Hu The effect of vortex formation and interactions on surface pressure fluctuations is examined in the wake of a normal flat plate by analyzing Direct Numerical Simulations at Re$=$1200. A novel local maximum score-based 3D method is used to track vortex development in the region close to the plate where the major contributions to the surface pressure are generated. Three distinct vortex shedding regimes are identified by changes in the lift and drag fluctuations. The instances of maximum drag coincide with impingement of newly formed vortices on the plate. This results in large and concentrated areas of rotational and strain contributions to generation of pressure fluctuations. Streamwise vortex straining and chordwise stretching are correlated with the large ratios of streamwise to chordwise normal stresses and regions of significant rotational contribution to the pressure. In contrast at the minimum drag, the vorticity field close to the plate is disorganized, and vortex roll-up occurs farther downstream. This leads to a uniform distribution of pressure. [Preview Abstract] |
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