Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session L24: Separated Flows: Vortices and Wakes |
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Chair: Yulia Peet, Arizona State University Room: 232 |
Monday, November 21, 2022 8:00AM - 8:13AM |
L24.00001: Vortex Induced Vibrations of a Double Pendulum Adrian Carleton, Yahya Modarres-Sadeghi We present the dynamics of a double pendulum with a constant cross section undergoing vortex induced vibration (VIV). Angular displacements and forces generated at the pendulum base are reported for a range of flow velocities, and the vortex shedding patterns are analyzed for multiple points of interest throughout the pendulum response. |
Monday, November 21, 2022 8:13AM - 8:26AM |
L24.00002: Vortex formation and flow separation on a scaled low-rise building model Nerion Zekaj, Jeong-ju Kim, Sang Joon Lee, Wei Zhang Damage to low-rise buildings' roofs during hurricane and tornado events seriously threatens community resilience to strong-wind related natural disasters. Peak negative pressures occur due to flow separation and vortex formation on roof edges and corners. Cost-effective flow control strategies to change the aerodynamics of wind around the roof to mitigate such damages are of great importance. We present experimental results of a scaled low-rise building model tested in a boundary layer wind tunnel. We observe flow dynamics and vortex formation through particle image velocimetry (PIV) for various building configurations at Reynolds number of 3800. Moreover, we consider the effects of the incoming wind direction on flow physics at 45° and 90°. These results allow for exploring flow control strategies, for example, fractal patterns in Nature, as a potential method to mitigate strong wind damage. |
Monday, November 21, 2022 8:26AM - 8:39AM |
L24.00003: Experimental investigation of 3-D separation over swept and tapered wings at a moderate Reynolds number Jacob M Neal, Michael Amitay An experimental investigation exploring the effect of sweep angle on tapered and untapered finite span, cantilevered wings was performed using oil flow visualizations and load cell measurements. For all experiments, the wing profile was NACA 0015 and the semi-span aspect ratio was 2. Five planforms were chosen to systematically compare the effects of tapering by sweeping either the leading edge, trailing edge, or both. Two untapered models were tested, one which was unswept and one which was swept back with a sweep angle of 30 degrees, as well as three tapered models, each with a taper ratio of 0.269. The first tapered model had a straight leading edge and a trailing edge swept forward 30 degrees, the second had a straight trailing edge and a leading edge swept aft 30 degrees, and the third had its quarter chord line swept aft 30 degrees. Surface mean topology and aerodynamic forces are presented for a range of angles of attack between 0 degrees to 30 degrees at chord Reynolds number 247,500. |
Monday, November 21, 2022 8:39AM - 8:52AM |
L24.00004: Applicability of Conventional Blockage Corrections to a Fully-Passive Oscillating-Foil Turbine Peter Oshkai, Sierra S Mann, Guy Dumas The influence of flow confinement on the kinematics and hydrodynamic performance of a fully-passive oscillating-foil turbine prototype was investigated experimentally at a Reynolds number of 19,000. The positions of the heave and pitch degrees of freedom were measured using rotary encoders, and quantitative flow patterns were obtained using particle image velocimetry. Quantifying confinement effects is important for comparison between model-scale confined experiments and full-scale unconfined turbine performance. Existing methodologies for correcting blockage effects on turbines are based on applying the conservation laws to flow around an actuator disk, to adjust the confined performance to unconfined performance. The kinematic parameters and energy harvesting performance of the turbine were measured at eight blockage ratios, ranging from 22% to 60%. The turbine performance metrics were then adjusted using two existing blockage correction models, which are based on the actuator disk theory. Both corrections were deemed unacceptable for the case of a fully-passive oscillating-foil turbine. The average corrected power coefficient gave up to 33.8% error compared with the projected unconfined power coefficient from linear extrapolation of the data, and the corrected value for zero confinement was not consistent among the different tested confinement levels. In contrast to the case of a kinematically-constrained turbine, where these corrections were successful, the kinematics of the oscillating foil changed with the changing confinement due to the passive nature of the turbine, making the different operating conditions incomparable. |
Monday, November 21, 2022 8:52AM - 9:05AM |
L24.00005: Critical Points in the wake of bio-inspired undulated cylinders Soundarya Ramasubramanian, Jennifer A Franck, Raul B Cal The unique geometry of seal whiskers consisting of a dual set of undulations along their span has been studied for their significant reduction in drag and vortex-induced vibration (VIV) when compared to smooth elliptical cylinders. These reduced forces and vibrations are intricately linked with the three-dimensional vortex formations downstream. This work investigates the wake structure, including its formation and break-up, by labeling and classifying regions of critical points. Critical points describe the flow topology around their neighborhood and are classified to a first-order approximation based on linear properties of the vector field. The effect on three-dimensional wake patterns and wavelength variation are investigated. The objective of this analysis is to perform correlation techniques and analyze the interrelationship between critical points and other flow properties like vorticity, Q-criterion. |
Monday, November 21, 2022 9:05AM - 9:18AM |
L24.00006: Existence and analysis of anti-symmetric super-coherent states in a wake behind a body of revolution at ReD=5000 Yulia T Peet, Fengrui Zhang In this talk, we investigate the existence of two super-coherent states in a wake behind a body of revolution with a blunt trailing edge, characterized by the opposite direction of a slow azimuthal rotation in the wake in a mean sense. The rotation originates in the recirculation region behind the blunt trailing edge, and manifests itself as a very-low-frequency (VLF) motion with a Strouhal number St~0.001, two orders of magnitude lower than the vortex shedding frequency. The flow also possesses a “cork-screw like” helical structure in the intermediate wake. A targeted perturbation of an instantaneous flow field resulted in a reversal of the orientation of the VLF motion in the near wake, as well as the helix orientation in the intermediate wake. In this talk, we examine the properties of both states using various modal analysis techniques, and present conditionally averaged properties of the flow given the state, as well as unconditionally averaged properties when the two states are combined together. |
Monday, November 21, 2022 9:18AM - 9:31AM |
L24.00007: Laminar flow separation over tapered wings, Part 1: a computational study Jean Ribeiro, Jacob M Neal, Anton Burtsev, Vassilis Theofilis, Michael Amitay, Kunihiko Taira As the first part of our three-way study, we comprehensively characterize the effects of wing taper on laminar flows over NACA 0015 wings at post-stall angles of attack using direct numerical simulations. With a straight leading edge, the wing is tapered by sweeping the trailing edge forward. The flows over such wings generally concentrate unsteadiness towards the wing root. For a straight trailing edge, tapering is analyzed with a backward swept leading edge. Over these wings, vortical structures are advected towards the wing tip. The reduced wing tip chord length of tapered wings constrains the formation of the tip vortex, reducing its strength and streamwise length. We also show how unsteadiness appears over wings with a high leading edge sweep angle. As wings with high sweep angles are tapered, shedding develops near the wing tip. Furthermore, we characterize the effect of tapering on the aerodynamic forces over the wing. We show that tapered wings increase lift with leading-edge sweep, in contrast to untapered wings. The current results provide insights into the effect of wing taper on wake dynamics and support future studies on flow separation over tapered wings at higher Reynolds numbers. |
Monday, November 21, 2022 9:31AM - 9:44AM |
L24.00008: Laminar flow separation over tapered wings, Part 2: an experimental study Brandon Gares, Jacob M Neal, Anton Burtsev, Jean Ribeiro, Vassilis Theofilis, Kunihiko Taira, Michael Amitay As the second part of our three-way study, an experimental investigation on tapered and untapered finite span, cantilevered wings was performed using stereo PIV over the suction surface and in the near wake. For all experiments, the wing profile is NACA 0015, the semi-span aspect ratio is 2, and the chord-based Reynolds number is 600. Five planforms were chosen to systematically compare the effects of tapering by sweeping either the leading edge, trailing edge, or both. Two untapered models were tested, one which was unswept and one which was swept back with a sweep angle of 30 degrees, as well as three tapered models, each with a taper ratio of 0.269. The first tapered model has a straight leading edge and a trailing edge swept forward 30 degrees, the second has a straight trailing edge and a leading edge swept aft 30 degrees, and the third has its quarter chord line swept aft 30 degrees. Surface mean topology, mean vortical structures, and RMS of velocity are strongly influenced by sweep angle. As the leading edge is swept back, flow structures and peak unsteadiness shift towards the tip, and as the trailing edge is swept forward flow structures and unsteadiness is shifted towards the root. |
Monday, November 21, 2022 9:44AM - 9:57AM |
L24.00009: Laminar flow separation over tapered wings, Part 3: A theoretical study Anton Burtsev, Jean Ribeiro, Jacob M Neal, Kunihiko Taira, Michael Amitay, Vassilis Theofilis The final part of this three-way study focuses on linear global stability analysis of flows over tapered wings. Swept and tapered NACA 0015 wings with an aspect ratio of 4 are considered for 200 ≤ Re ≤ 400. Adjoint global modes are computed for these geometries for the first time. Regions of maximum flow receptivity and sensitivity (the wavemaker) are identified for informing flow control. The wavemaker of the leading unstable mode is in the wake inside the laminar separation bubble (LSB). As Re is increased, its spanwise extent shrinks and at Re = 400 the structure changes to two regions associated with top and bottom shear layers of the LSB. As sweep and taper are introduced to the wing, the wavemaker moves in the spanwise direction following the displacement of peak recirculation of the LSB. The regions of flow receptivity are associated with the leading and trailing edges of the wing. Projecting these regions onto the wing surface shows that the peak receptivity to momentum forcing is located near the separation line at the spanwise location of peak recirculation. Present results provide theoretical insights into the instability mechanisms of finite tapered wings and establish a basis for future studies at higher Reynolds numbers. |
Monday, November 21, 2022 9:57AM - 10:10AM |
L24.00010: The Effect of Aspect Ratio and Sweep Angle on 3D Stall Cell Formation on Wings with NACA0015 Airfoil Arash Zargar, Sara Khaleghizadeh, Mehrnoosh Rahbardar, Mahmoud Mani The effects of wing’s aspect ratio and sweep angle are investigated on the characteristics of stall cell phenomenon at different Reynolds numbers and beyond angle of attack (AOA) at which trailing edge flow separation initiates (As). To study the effect of aspect ratio on post-stall flow, three different models with NACA0015 cross-section and AR=3.5, 4.5 and 5.5 were examined experimentally. For the sweep angle effect, four wing models with NACA 0015 airfoils, AR = 4.5 and sweep angles of ?? = 0°, 3°, 6°, and 10° were utilized. The experiments were conducted at different Reynolds numbers varying in the range of Re = 3 × 105 - 6 × 105 based on the wing mid-chord length and free stream velocity (Re = ????0??m/μ). |
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