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 A3: Vortex Dynamics: Cylinders |
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Chair: Yahya Modarress-Sadeghi, University of Massachusetts, Amherst Room: B110-111 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A3.00001: The effect of boundary conditions on VIV of a fully submerged flexible cylinder Mahdiar Edraki, Banafsheh Seyed-Aghazadeh, Yahya Modarres-Sadeghi A series of experiments was conducted in a re-circulating water tunnel, in which Vortex-Induced Vibration (VIV) of a fully submerged, tension-dominated cylinder with different boundary conditions was studied. While in most previous studies, either the cylinder was not fully submerged in flow or the boundary conditions for the cylinder were different at the two ends, in the current study the cylinder is fully submerged and the boundary conditions are carefully controlled. The cylinder was held fixed at both ends and was placed perpendicular to the uniform incoming flow direction. Different symmetric and asymmetric boundary conditions for the cylinder, i.e., clamped-clamped, simply supported, and clamped-hinged were tested. Continuous response of the cylinder in both the crossflow and inline directions were reconstructed from limited number of measurement points based on modal expansion theorem modified using Modal Assurance Criterion (MAC). Amplitudes and frequencies of oscillations were studied in the reduced velocity range of U*$=$ 5.5-32.5 and the Reynolds number range of Re $=$ 200-1220. Modes up to four were excited in the crossflow direction for a cylinder with a length of L$=$0.3 m and an aspect ratio of 73. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A3.00002: Boundary layers and forces on a cylinder in vortex-induced vibrations Ravi Chaithanya Mysa, Sumana Inaganti, Vinh-Tan Nguyen, Chang Wei Kang The flow, load on the cylinder and its motion are coupled to each other in a vortex-induced vibration (VIV). The boundary layer on the cylinder is affected by the shedding of vortices and motion of the cylinder. The motion of the cylinder will be in certain phase with respect to shedding vortices depending on the reduced velocity. Based on their phase difference, the cylinder motion dynamics can affect boundary layers differently. This, in turn changes force acting on the cylinder and subsequently its motions. It is believed that boundary layer and its movements on the cylinder play an important role in explaining the responses in vortex induced vibrations. Numerical experiments have been performed on the vortex induced vibrations of the cylinder in laminar flow environment for different mass ratios to better understand its complex coupled physics. The boundary layer on the cylinder is calculated at different locations on the cylinder during its VIV responses. The variation of the boundary layer is characterized in pre-lock-in, lock-in and post-lock-in regions with respect to the pressure and viscous forces generated on the cylinder. The connection between the forces generated, shed vortices and the motion will be presented with the help of boundary layer variation and flow contours. Their respective correlations will be established while the dependence of pattern of shedding vortices on the phase of the cylinder motion will be discussed. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A3.00003: Symmetry breaking in flow-induced vibration of rotating prisms with non-circular cross-sections Banafsheh Seyed-Aghazadeh, Yahya Modarres-Sadeghi Flow-induced vibration of a flexibly-mounted triangular prism, free to oscillate in the crossflow direction with imposed rotation about its axis, was studied experimentally. Depending on the angle of attack, non-rotating prisms with triangular cross-sections could experience both Vortex-Induced Vibration (VIV) and galloping. In particular, the objective of this study was to investigate how the imposed rotation could affect the galloping instability for such prisms. The rotation rate, $\alpha $, defined as the ratio of the surface velocity and the free stream velocity, was varied in the range of $\alpha =$0-2.6. The amplitudes and frequencies of oscillations were measured in a Reynolds number range of Re$=$420-2100 and reduced velocity of Vr$=$2.8-14. The oscillation was found to be limited to a range of reduced velocities, and the lock-in range became narrower at higher rotation rates where the oscillation ceased beyond the rotation rate around $\alpha =$2.4. This rotation rate at which the oscillation ceased and the decrease in the width of the lock-in range was very similar to those observed previously for VIV of a rotating circular cylinder. The tests were repeated for a square-cross-section prism and similarly, the oscillation was observed to cease at the same range of rotation rate around $\alpha =$2.4. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A3.00004: Optical Tracking Measurement on Vortex Induced Vibration of Flexible Riser with Short-Length Buoyance Module Dixia Fan, Honglin Du, Michael Triantafyllou We address experimentally the vortex induced vibrations (VIV) of long flexible cylinders. We employ optical tracking, using an array of high speed cameras. Compared to strain gauges and accelerometers, this non-intrusive approach, allows direct measurement of the flexible cylinder displacement with far denser spatial distribution. The measurements reveal essential features of flexible cylinder VIV, including complex geometries such as cylinders containing short-length buoyancy modules, with module to cylinder diameter ratio of 1:3.2 and module to bare cylinder length ratio of 1:1. The experiments are conducted with aspect ratio of 170 and 3 different coverage ratios, of 100{\%}, 50{\%} and 20{\%}. The measurements demonstrate bi-frequency response due to excitation from both buoyancy module and bare cylinder, at low Strouhal number, down to values of 0.08, and the generation of traveling wave patterns. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A3.00005: Multiple Long-Time Solutions for Intermediate Reynolds Number Flow past a Circular Cylinder with a Nonlinear Inertial and Dissipative Attachment Antoine B. E. Blanchard, Lawrence A. Bergman, Alexander F. Vakakis, Arne J. Pearlstein We consider two-dimensional flow past a linearly-sprung cylinder allowed to undergo rectilinear motion normal to the mean flow, with an attached ``nonlinear energy sink'' consisting of a mass allowed to rotate about the cylinder axis, and whose rotational motion is linearly damped by a viscous damper. For \textit{Re} \textless 50, where the flow is expected to be two-dimensional, we use different inlet transients to identify multiple long-time solutions, and to study how they depend on \textit{Re} and a dimensionless spring constant. For fixed values of the ratio of cylinder density to fluid density, dimensionless damping coefficient, and ratio of the rotating mass to the total mass, we find that different inlet transients lead to different long-time solutions, including solutions that are steady and symmetric (with a motionless cylinder), time-periodic, quasi-periodic, and chaotic. The results show that over a wide range of the parameters, the steady symmetric motionless-cylinder solution is locally, but not globally, stable. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A3.00006: Relating surface pressure to Lagrangian wake topology around a circular cylinder in cross flow Matthew Rockwood, Melissa Green The tracks of Lagrangian saddles, identified as non-parallel intersections of positive and negative-time finite-time Lyapunov exponent (FTLE) ridges, have been shown to indicate the timing of von Karman vortex shedding in the wake of bluff bodies. The saddles are difficult to track in real-time, however, since future flow field data is needed for the computation of the FTLE fields. In order to detect the topological changes without direct access to the FTLE, the saddle dynamics are correlated to measurable surface quantities on a circular cylinder in cross flow. The Lagrangian saddle found upstream of a forming and subsequently shedding vortex has been shown to accelerate away from the cylinder surface as the vortex sheds. In previous numerical results at $Re=150$, this acceleration coincides with the peak in lift force over the cylinder, and also with a minimum in the static pressure at a location slightly upstream of the mean separation location. In the current work, this result is compared with experimental data at $Re=O(10,000)$. Successful validation would provide a strategy for locating sensitive regions on the cylinder surface where vortex shedding could be detected using simple pressure transducers. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A3.00007: Two-dimensional wakes of a variable diameter cylinder Wenchao Yang, Mark Stremler It is well known that periodic variations in the position of a circular cylinder can produce a variety of complex vortex wake patterns. We will discuss what we believe is the first investigation of the wake patterns produced by a stationary circular cylinder undergoing periodic variations in the cylinder diameter. In our experiments, cylinder variations are produced by oscillating a cone perpendicularly through a flowing soap film. The wake flow generates thickness variations in the thin soap film, allowing direct observation of wake patterns through visualization of interference fringes. We consider diameter variations ranging from 0.1 to 0.5 times the mean diameter, with the Reynolds number varying from 50 to 150. The frequency of the diameter's variation influences the wake patterns. When the variation frequency is negligible compared to the vortex shedding frequency, the wake is a quasi-steady representation of fixed cylinder shedding. We will discuss wake pattern bifurcations that occur as the variation frequency becomes comparable to the vortex shedding frequency. Comparisons will be made with the wake patterns generated by a constant-diameter circular cylinder forced to oscillate transverse to the free stream. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A3.00008: Wake structure of an oscillating cylinder in a flowing soap film Mark Stremler, Wenchao Yang When a circular cylinder oscillates with respect to a uniform background flow, a variety of wake patterns can be observed in which multiple vortices are generated during each shedding cycle. Thorough investigations of the possible wake patterns behind a cylinder undergoing forced oscillations have been conducted by C.H.K. Williamson using two-dimensional characterization of a three-dimensional flow. Attempts to reproduce the structural bifurcations using two-dimensional computational models have been only moderately successful. A flowing soap film, an experimental system with quasi-two-dimensional flow, provides an alternative method for investigating the role of system dimensionality in the structure and dynamics of complex vortex wakes. Wake patterns are observed directly through interference fringes caused by thickness variations in the soap film. Such systems have been used for decades to visualize wake structure, but they have not previously been used to conduct an analog of Williamson's work. We will discuss the results of an ongoing parametric study of the wake structure produced by a circular cylinder undergoing forced oscillations transverse to the background flow in an inclined soap film system. [Preview Abstract] |
Sunday, November 20, 2016 9:44AM - 9:57AM |
A3.00009: Investigation of Vortical Flow Patterns in the Near Field of a Dynamic Low-Aspect-Ratio Cylinder Samantha Gildersleeve, Michael Amitay The flowfield and associated flow structures of a low-aspect-ratio cylindrical pin were investigated experimentally in the near-field as the pin underwent wall-normal periodic oscillations. Under dynamic conditions, the pin is driven at the natural wake shedding frequency with an amplitude of 33{\%} of its mean height. Additionally, a static pin was also tested at various mean heights of 0.5, 1.0, and 1.5 times the local boundary layer thickness to explore the effect of the mean height on the flowfield. Three-dimensional flowfields were reconstructed and analyzed from SPIV measurements where data were collected along streamwise planes for several spanwise locations under static and dynamic conditions. The study focuses on the incoming boundary layer as it interacts with the pin, as well as two main vortical formations: the arch-type vortex and the horseshoe vortex. Under dynamic conditions, the upstream boundary layer is thinner, relative to the baseline, and the downwash in the wake increases, resulting in a reduced wake deficit. These results indicate enhanced strength of the aforementioned vortical flow patterns under dynamic conditions. The flow structures in the near-field of the static/dynamic cylinder will be discussed in further detail. [Preview Abstract] |
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