Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session M8: Vortex Dynamics and Vortex Flows: Crossflow WakesShear layer Vortexes
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Chair: Neil Cagney, University College London Room: 501 |
Tuesday, November 21, 2017 8:00AM - 8:13AM |
M8.00001: Vortex Interactions from a Finite Span Cylinder with a Laminar Boundary Layer for Varied Parameters Samantha Gildersleeve, Michael Amitay Flow structures around a stationary, wall-mounted, finite-span cylindrical pin were investigated experimentally over a flat plate to explore the effects of varied aspect ratio and pin mean height with respect to the local boundary layer. Nine static pin configurations were tested where the pin's mean height to the local boundary layer thickness were 0.5, 1, and 1.5 for a range of aspect ratios between 0.125 and 1.125. The freestream velocity was fixed at 11 m/s, corresponding to Re$_{\mathrm{D}}$ \textasciitilde 2800, 5600, and 8400, respectively. Three-dimensional flowfields were reconstructed and analyzed from SPIV measurements where data were collected along cross-stream planes in the wake of the pin. This study focuses on three dominant vortical patterns associated with a finite span cylinder: the arch-type vortex horseshoe vortex, and the tip vortices Results indicate that both the aspect ratio and mean height play an important role in the behavior and interactions of these vortex structures which alter the wake characteristics significantly. Understanding the mechanisms by which the vortical structures may be strengthened while reducing adverse local pressure drag are key for developing more efficient means of passive and/or active flow control through finite span cylindrical pins and will be discussed in further detail. [Preview Abstract] |
Tuesday, November 21, 2017 8:13AM - 8:26AM |
M8.00002: Vortex-induced vibrations of a cylinder in planar shear flow Simon Gsell, Remi Bourguet, Marianna Braza Vortex-induced vibrations (VIV) of bluff bodies are common in nature and in engineering applications where flexible or flexibly mounted structures are exposed to wind and ocean currents. VIV have been thoroughly studied through the canonical problem of an elastically mounted, rigid cylinder immersed in uniform flow. However, in the real physical systems where VIV develop, the oncoming flows are usually non-uniform. The present work investigates the impact of a shear of the oncoming current in the cross-flow direction. As a first preliminary step, focus is placed on the fixed cylinder case; the analysis is based on a series of numerical simulations over a wide range of shear rates, at Reynolds number 100. It is found that the shear leads to the cancellation of wake unsteadiness beyond a critical value of the shear rate. Once the rigid cylinder is elastically mounted, free vibrations arise over the entire range of shear rates under study, including beyond the above mentioned critical value. Different flow-structure interaction regimes are uncovered. Some of them exhibit a major deviation from the uniform-flow case, with a profound reconfiguration of the wake patterns and a dramatic amplification of the structural response amplitudes. [Preview Abstract] |
Tuesday, November 21, 2017 8:26AM - 8:39AM |
M8.00003: Experimental study of attached splitter plate effects on the wake of a circular cylinder using finite-time Lyapunov exponents Seth Brooks, Melissa Green Two-component planar particle image velocimetry (PIV) and surface pressure were used to investigate the effects of an attached splitter plate on the formation and shedding of vortices from a circular cylinder. The instantaneous velocity data is phase averaged using the surface pressure. One of the tools used to visualize and characterize the flow is finite-time Lyapunov exponent (FTLE). This is a Lagrangian technique that identifies local separation. Prior literature shows that the addition of an attached splitter plate alters the classic von K\'{a}rm\'{a}n vortex shedding and that splitter plates longer than a certain length suppress the periodic shedding. A separate study proposes that the shedding of a vortex from a circular cylinder is characterized by a hyperbolic saddle leaving the vicinity of the surface and that the shedding time can be identified in real time using a surface pressure. In this study, the effects of splitter plates on the vortex shedding will be investigated where the plate will range in length from $1.5D$ to $5.5D$, where $D$ is the diameter of the cylinder. The FTLE and wake structure results will be compared with those found in previous studies that investigated the wake of bluff bodies with and without splitter plates. [Preview Abstract] |
Tuesday, November 21, 2017 8:39AM - 8:52AM |
M8.00004: The Role of the Separation Point in Streamwise VIV of Cylinders of Various Cross-sectional Shapes Stavroula Balabani, Neil Cagney Vortex-Induced Vibration (VIV) is a classic fluid-structure interaction problem and can lead to fatigue damage and catastrophic failure of bodies in cross-flow. VIV acting in the streamwise (flow) direction is relatively poorly understood compared to that acting in the transverse (lift) direction, but can have a very significant effect of the overall response of structures with two or more degrees of freedom (DOFs). We present time-resolved PIV measurements of the wake and structural response of cylinders with a range of cross-sectional shapes, including circular, elliptical, triangular and square. The response of a circular cylinder is characterised by two response branches, in agreement with previous studies. However, it is shown that for geometries with fixed separation points, no significant VIV is observed and vortex-shedding does not lock-in to the vibration frequency. This finding suggests that the fluid excitation caused by the interaction between the cylinder displacement and the shear layers is reliant on the ability of the separation point to vary. It also suggests that control of the separation points may be an effective means of restricting VIV of multi-DOF bodies. [Preview Abstract] |
Tuesday, November 21, 2017 8:52AM - 9:05AM |
M8.00005: Large Eddy Simulation of Vertical Axis Wind Turbine wakes; Part I: from the airfoil performance to the very far wake Philippe Chatelain, Matthieu Duponcheel, Denis-Gabriel Caprace, Yves Marichal, Gregoire Winckelmans A vortex particle-mesh (VPM) method with immersed lifting lines has been developed and validated. Based on the vorticity–velocity formulation of the Navier–Stokes equations, it combines the advantages of a particle method and of a mesh-based approach. The immersed lifting lines handle the creation of vorticity from the blade elements and its early development. Large-eddy simulation (LES) of vertical axis wind turbine (VAWT) flows is performed. The complex wake development is captured in detail and over up to 15 diameters downstream: from the blades to the near-wake coherent vortices and then through the transitional ones to the fully developed turbulent far wake (beyond 10 rotor diameters). The statistics and topology of the mean flow are studied with respect to the VAWT geometry and its operating point. The computational sizes also allow insights into the detailed unsteady vortex dynamics and topological flow features, such as a recirculation region influenced by the tip speed ratio and the rotor geometry. [Preview Abstract] |
Tuesday, November 21, 2017 9:05AM - 9:18AM |
M8.00006: Large Eddy Simulation of Vertical Axis Wind Turbine wakes; Part II: effects of inflow turbulence Matthieu Duponcheel, Philippe Chatelain, Denis-Gabriel Caprace, Gregoire Winckelmans The aerodynamics of Vertical Axis Wind Turbines (VAWTs) is inherently unsteady, which leads to vorticity shedding mechanisms due to both the lift distribution along the blade and its time evolution. Large-scale, fine-resolution Large Eddy Simulations of the flow past Vertical Axis Wind Turbines have been performed using a state-of-the-art Vortex Particle-Mesh (VPM) method combined with immersed lifting lines. Inflow turbulence with a prescribed turbulence intensity (TI) is injected at the inlet of the simulation from a precomputed synthetic turbulence field obtained using the Mann algorithm. The wake of a standard, medium-solidity, H-shaped machine is simulated for several TI levels. The complex wake development is captured in details and over long distances: from the blades to the near wake coherent vortices, then through the transitional ones to the fully developed turbulent far wake. Mean flow and turbulence statistics are computed over more than 10 diameters downstream of the machine. The sensitivity of the wake topology and decay to the TI level is assessed. [Preview Abstract] |
Tuesday, November 21, 2017 9:18AM - 9:31AM |
M8.00007: Turbulent wake vortex system at equilibrium, and its interaction with a ground at $Re_\Gamma = 2.0\, 10^5$ Gregoire Winckelmans, Olivier Thiry, Matthieu Duponcheel, Laurent Bricteux, Ivan De Visscher A turbulent two-vortex system (T-2VS) at statistical equilibrium is first obtained. This is done by putting two vortices with analytical circulation distribution in a weak homogeneous isotropic turbulence field (obtained by LES), and then further running the LES. The system goes unstable and a turbulent equilibrium is obtained after some transient. This is believed to be representative of the state reached by the wake behind a real aircraft, when it is fully rolled up. The T-2VS is also characterized (vortex circulation profile, turbulence, etc.), which provides useful information to support further modeling and theory. Next, that T-2VS wake is put in near ground proximity, and a wall-resolved LES at $Re_\Gamma= \Gamma/ \nu = 2.0\, 10^5$ is carried out: this is ten times larger than the published works on wake vortices interacting with a ground. As the T-2VS comes down, it goes in ground effects (IGE), and then strongly interacts with the ground and rebounds. The physics of the interaction with the ground are analyzed (also relatively to previous works where the 2VS was made of 2D analytical vortices with white noise added), and diagnostics are obtained: vortex rebound and trajectory, vortex global circulation decay, state of the vortices long after rebound (also an equilibrium). [Preview Abstract] |
Tuesday, November 21, 2017 9:31AM - 9:44AM |
M8.00008: On the dynamics in the vicinity of a shear layer of a recirculation bubble past a canopy patch Wing Lai, Dan Troolin, Jin-Tae Kim, Yaqing jin, Hong Liu, Leonardo Chamorro The 3D dynamics of the flow around the shear layer above the recirculation bubble past a canopy patch was explored with time-resolved volumetric PIV in a refractive-index-matching channel. The tunnel was operated in a free surface mode with a flow depth 0.4 m. The canopy had a width 0.12 m and a spanned the cross section of the flume; it consisted of rectangular elements with square cross section of side 6.4 mm side and 37.5 mm height, placed in a staggered layout. The measurements were made using a volumetric system, assembled with three high-speed 4MP cameras. The region of interest (50\textasciicircum 3 mm3) was illuminated with a high-speed laser. Results revealed distinctive patterns characterized by periods of low, developing and intense mixing. These three types of motions modulated the coherence, and distribution of the energetic vortices. Each type is defined in terms of the spatial distribution of the stagnation points. In the low mixing, those were mostly located in a plane; conversely, the points were spatially distributed in the high mixing state. Velocity spectra revealed intense turbulence energy around the shear layer at a frequency consistent with a Strouhal number St$=$0.2, with the canopy height as length scale. [Preview Abstract] |
Tuesday, November 21, 2017 9:44AM - 9:57AM |
M8.00009: Volumetric PIV behind mangrove-type root models Amirkhosro Kazemi, Keith Van de Riet, Oscar M. Curet Mangrove trees form dense networks of prop roots in coastal intertidal zones. The interaction of mangroves with the tidal flow is fundamental in estuaries and shoreline by providing water filtration, protection against erosion and habitat for aquatic animals. In this work, we modeled the mangrove prop roots with a cluster of rigid circular cylinders (patch) to investigate its hydrodynamics. We conducted 2-D PIV and V3V in the near- and far-wake in the recirculating water channel. Two models were considered: (1) a rigid patch, and (2) a flexible patch modeled as rigid cylinders with a flexible hinge. We found that Strouhal number changes with porosity while the patch diameter is constant. Based on the wake signature, we defined an effective diameter length scale. The volumetric flow measurements revealed a regular shedding forming von K\'{a}rm\'{a}n vortices for the rigid patch while the flexible patch produced a less uniform wake where vortices were substantially distorted. We compare the wake structure between that 2-D PIV and V3V. This analysis of the hydrodynamics of mangrove-root like models can also be extended to understand other complex flows including bio-inspired coastal infrastructures, damping-wave systems, and energy harvesting devices. [Preview Abstract] |
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