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 D3: Vortex Dynamics: Applications on the Living Environment |
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Chair: Douglas Hart, Massachusetts Institute of Technology Room: B110-111 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D3.00001: PIV Analysis of Wake Induced by Real Harbor Seal Whiskers Joseph Bunjevac, Aidan Rinehart, Justin Flaherty, Wei Zhang Harbor Seals are able to accurately detect minute disturbances in the ambient flow using their whiskers, which is attributed to the exceptional capability of the whiskers to suppress vortex-induced vibrations in the wake. To explore potential applications for designing smart devices, such as high-sensitivity underwater flow sensors and drag reduction components, research has studied the role of key parameters of the whisker morphology on wake structure. Due to the inherent variation in size and angle of incidence along the length of whiskers, it is not well understood how a real seal whisker changes wake structure, in particular the vortex shedding behavior. This work aims to understand the flow around a single real seal whisker using Particle Image Velocimetry at low Reynolds numbers (i.e. a few hundred) in a water channel. Variations in flow structure are inspected between several different real whiskers and whisker models. The results will provide insights of the effects of the natural geometry of the harbor seal whiskers on wake flow compared to idealized whisker-like models. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D3.00002: How do seal whiskers suppress vortex shedding Aidan Rinehart, Justin Flaherty, Joseph Bunjavick, Vikram Shyam, Wei Zhang Certain seal whiskers possess a unique geometry that significantly reduces the vortex-induced vibration; which has attracted great attention to understand how the unique shape re-organizes the wake structure and its potential for passive flow control. The shape of the whiskers can be described as an elliptical cross-section that is lofted along the length of the whisker. Along the entire length of the whisker the ellipse varies in major and minor axis as well as angle of incidence with respect to the axis of the whisker. Of particular interest in this study is to identify what effect the angle of incidence has on the flow structure around the whisker, which has been overlooked in the past. The study will analyze the wake structure behind various scaled-up whisker models using particle image velocimitry (PIV). These whisker models share common geometry dimensions except for the angle of incidence. Flow conditions are created in a water channel and a wind tunnel, covering a wide range of Reynolds number (a few hundreds to thousands), similar to the ambient flow environment of seals and to the targeted aero-propulsion applications. This study will help address knowledge gaps in understanding of how certain geometry features of seal whiskers influence the wake and establish best practices for its application as effective passive flow control strategy. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D3.00003: The effect of bottom friction on tidal dipolar vortices and the associated transport Matias Duran-Matute, Leon Kamp, GertJan van Heijst Tidal dipolar vortices can be formed in a semi-enclosed basin as the tides flow in and out through an inlet. If they are strong enough to overcome the opposing tidal currents, these vortices can travel away from the inlet due to their self-propelling mechanism, and hence, act as an efficient transport agent for suspended material. We present results of two-dimensional numerical simulations of the flow through an idealized tidal inlet, with either a linear or a nonlinear parameterization of the bottom friction. We then quantify the effect of the bottom friction on the propagation of the dipolar vortex and on its ability as a transport agent by computing the flushing and residence times of passive particles. Bottom friction is detrimental to the ability of tidal dipolar vortices to propagate and hinders transport away from the inlet. The magnitude of this effect is related to the relative duration of the tidal period as compared to the typical decay time scale of the vortex dipole. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D3.00004: Turbulent coherent-structure dynamics in a natural surface storage zone: Mechanisms of mass and momentum transport in rivers Cristian Escauriaza, Jorge Sandoval, Emmanuel Mignot, Luca Mao Turbulent flows developed in surface storage zones (SSZ) in rivers control many physical and biogeochemical processes of contaminants in the water. These regions are characterized by low velocities and long residence times, which favor particle deposition, nutrient uptake, and flow interactions with reactive sediments. The dynamics of the flow in SSZ is driven by a shear layer that induces multiple vortical structures with a wide range of temporal and spatial scales. In this work we study the flow in a lateral SSZ of the Lluta River, a high-altitude Andean stream (~4,000 masl), with a Re=45,800. We describe the large-scale turbulent coherent structures using field measurements and 3D numerical simulations. We measure the bed topography, instantaneous 3D velocities at selected points, the mean 2D free-surface velocity field, and arsenic concentration in the sediment. Numerical simulations of the flow are also performed using a DES turbulence model. We focus on the mass and momentum transport processes, analyzing the statistics of mass exchange and residence times in the SSZ. With this information we provide new insights on the flow and transport processes between the main channel and the recirculating region in natural conditions. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D3.00005: PIV measurements and flow characteristics downstream of mangrove root models. Amirkhosro Kazemi, Oscar Curet Mangrove forests attracted attentions as a solution to protect coastal areas exposed to sea-level rising, frequent storms, and tsunamis. Mangrove forests found in tide-dominated flow regions are characterized by their massive and complex root systems, which play a prominent role in the structure of tidal flow currents. To understand the role of mangrove roots in flow structure, we modeled mangrove roots with rigid and flexible arrays of cylinders with different spacing between them as well as different configurations. In this work, we investigate the fluid dynamics downstream of the models using a 2-D time-resolved particle image velocimetry (PIV) and flow visualization. We carried out experiments for four different Reynolds number based on cylinder diameters ranges from 2200 to 12000. We present time-averaged and time-resolved flow parameters including velocity distribution, vorticity, streamline, Reynolds shear stress and turbulent kinetic energy. The results show that the flow structure has different vortex shedding downstream of the cylinders due to interactions of shear layers separating from cylinders surface. The spectral analysis of the measured velocity data is also performed to obtain Strouhal number of the unsteady flow in the cylinder wake. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D3.00006: Vortex wake of tip loaded rotors at low Reynolds numbers Omer Savas, Onur Bilgi The effect of tip tabs on the flow characteristics of a three bladed rotor is investigated using strain gauge thrust measurements, flow visualization and particle image velocimetry at chord Reynolds numbers of $0.4-2.9\times10^5$. The tab angles of attack of $0, \ \mp3^{\circ}\ \& \ \mp5^{\circ}$ with respect to the rotation of the rotor are used to vary the tip loading. The rotor wakes and thrust characteristics at positive angles of attack, when the tip loading is outward, are qualitatively similar to those with no-tabs. In contrast, when the tip loading is inward at zero and negative angles of attack, the vortex wake is radically altered; the thrust nearly vanishes, even reverses with increasing inward loading. The key factors influencing the behavior of the wake are the vortex system off the tabs and their associated {\it downwash}, which is inward for the outward tab loading and causes increased volume and momentum flux and outward for the inward tab loading and causes expansion of the wake and nearly complete loss of thrust. At negative angles of attack, the flow fields exhibit a quasi-steady bound ring vortex system around at the edge of the rotor disk and the flow direction on the pressure side of the rotor disk reverses: it flows toward the rotor disk. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D3.00007: Numerical study of the effects of rotating forced downdraft in reproducing tornado-like vortices. Jinwei Zhu, Shuyang Cao, Tetsuro Tamura Appropriate physical modeling of a tornado-like vortex is a prerequisite to studying near-surface tornado structure and tornado-induced wind loads on structures. Ward-type tornado simulator modeled tornado-like flow by mounting guide vanes around the test area to provide angular momentum to converging flow. Iowa State University, USA modified the Ward-type simulator by locating guide vanes at a high position to allow vertical circulation of flow that creates a rotating forced downdraft in the process of generating a tornado. However, the characteristics of the generated vortices have not been sufficiently investigated till now. In this study, large-eddy simulations were conducted to compare the dynamic vortex structure generated with/without the effect of rotating forced downdraft. The results were also compared with other CFD and experimental results. Particular attention was devoted to the behavior of vortex wander of generated tornado-like vortices. The present study shows that the vortex center wanders more significantly when the rotating forced downdraft is introduced into the flow. The rotating forced downdraft is advantageous for modeling the rear flank downdraft phenomenon of a real tornado. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D3.00008: From salps to robots: estimating thrust in propulsive pulsed jets using wake kinematics Athanasios Athanassiadis, Douglas Hart Both animals and robots can achieve high maneuverability underwater by using pulsed jets for propulsion. However, in cases where multiple jets are required, it remains unclear how jet placement and timing will affect propulsive performance. In recent experiments,\footnote{A.G. Athanassiadis, and D.P Hart. Phys. Rev. Fluids, 1(3), 034501. (2016).} we demonstrate how vortex interactions reduce thrust production for simultaneously pulsed jets. Our results rely on force estimates using high-speed laser fluorescence imaging of the jet wakes. By combining measurements of wake kinematics with analytical models, we are able to estimate force production from just the fluorescence videos. In this talk, I will discuss the force estimation technique, and how this approach helped to reveal design strategies that would \textit{benefit} from the wake interactions. [Preview Abstract] |
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