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 D6: Aerodynamics: Rotation and Passive Control Strategies |
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Chair: Tim Colonius, California Institute of Technology Room: B114 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D6.00001: On the rotation and pitching of flat plates. Yaqing Jin, Sheng Ji, Leonardo P. Chamorro Wind tunnel experiments were performed to characterize the flow-induced rotation and pitching of various flat plates as a function of the thickness ratio, the location of the axis of rotation and turbulence levels. High-resolution telemetry, laser tachometer, and hotwire were used to get time series of the plates motions and the signature of the wake flow at a specific location. Results show that a minor axis offset can induce high-order modes in the plate rotation under low turbulence due to torque unbalance. The spectral decomposition of the flow velocity in the plate wake reveals the existence of a dominating high-frequency mode that corresponds to a static-like vortex shedding occurring at the maximum plate pitch, where the characteristic length scale is the projected width at maximum pitch. The plate thickness ratio shows inverse relation with the angular velocity. A simple model is derived to explain the linear relation between pitching frequency and wind speed. The spectra of the plate rotation show nonlinear relation with the incoming turbulence, and the dominating role of the generated vortices in the plate motions. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D6.00002: An Experimental Study on the aerodynamic and aeroacoustic performances of Maple-Seed-Inspired UAV Propellers Hui Hu, zhe Ning Due to the auto-rotating trait of maple seeds during falling down process, flow characteristics of rotating maple seeds have been studied by many researchers in recent years. In the present study, an experimental investigation was performed to explore maple-seed-inspired UAV propellers for improved aerodynamic and aeroacoustic performances. Inspired by the auto-rotating trait of maple seeds, the shape of a maple seed is leveraged for the planform design of UAV propellers. The aerodynamic and aeroacoustic performances of the maple-seed-inspired propellers are examined in great details, in comparison with a commercially available UAV propeller purchased on the market (i.e., a baseline propeller). During the experiments, in addition to measuring the aerodynamic forces generated by the maple-seed-inspired propellers and the baseline propeller, a high-resolution Particle Image Velocimetry (PIV) system was used to quantify the unsteady flow structures in the wakes of the propellers. The aeroacoustic characteristics of the propellers are also evaluated by leveraging an anechoic chamber available at the Aerospace Engineering Department of Iowa State University. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D6.00003: Dynamic flow reattachment on a rotating blade undergoing dynamic stall Vrishank Raghav, Narayanan Komerath A 2-bladed rigid rotor undergoing retreating blade dynamic stall in a low-speed wind tunnel was used to study the 3-dimensional flow reattachment at the end of the dynamic stall cycle. Phase-locked stereoscopic Particle Image Velocimetry was used to capture the velocity field during reattachment. Continuing from prior studies on the inception and progression of 3-D rotating dynamic stall for this test case, phase-resolved, ensemble-averaged results are presented for different values of rotor advance ratio at varying spanwise stations along the blade. The results show the nominal reattachment getting delayed in rotor azimuth with higher advance ratio. At low advance ratio reattachment starts at the leading-edge and progresses towards the trailing-edge with vortex shedding transporting excess vorticity away from the leading-edge. At higher advance ratio, vortex shedding is not observed, instead the vortical structure shrinks in size while the flow close to the trailing-edge appears to reattach. At the higher advance ratio conditions, spanwise vorticity transport appears to be the mechanism to transport excess vorticity away from the leading-edge. The possible causes for a switch in mechanism of vorticity transport are also presented. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D6.00004: Lift-optimal aspect ratio of a revolving wing at low Reynolds number Thierry Jardin, Tim Colonius Lentink \& Dickinson (2009) showed that rotational acceleration stabilized the leading-edge vortex on revolving, low-aspect-ratio wings, and hypothesized that a Rossby number of around 3, which is achieved during each half-stroke for a variety of hovering insects, seeds, and birds, represents a convergent high-lift solution across a range of scales in nature. Subsequent work has verified that, in particular, the Coriolis acceleration is responsible for LEV stabilization. Implicit in these results is that there exists an optimal aspect ratio for wings revolving about their root, because it is otherwise unclear why, apart from possible physiological reasons, the convergent solution would not occur for an even lower Rossby number. We perform direct numerical simulations of the flow past revolving wings where we vary the aspect ratio and Rossby numbers independently by displacing the wing root from the axis of rotation. We show that the optimal lift coefficient represents a compromise between competing trends where the coefficient of lift increases monotonically with aspect ratio, holding Rossby number constant, but decreases monotonically with Rossby number, when holding aspect ratio constant. For wings revolving about their root, this favors wings of aspect ratio between 3 and 4. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D6.00005: The effects of leading edge roughness on dynamic stall John Hrynuk Dynamic stall is a fundamental flow phenomenon that is commonly observed for insect flight and rotorcraft. Under certain conditions a leading edge vortex forms generating large but temporary lift forces. Historically, computations studying dynamic stall on airfoil shapes have struggled to predict this vortex formation time and separation point. Reduced order models and CFD have performed well when experiments have been performed to develop separation models, but this has limited the development of robust design tools. The current study looks at the effect of leading edge surface roughness on the formation of the Dynamic Stall Vortex (DSV). Roughness elements were applied to the leading edge of a NACA 0012 airfoil and PIV data of the vortex formation process was recorded. Measurements were taken at a Reynolds number of Re $=$ 12,000 and baseline smooth NACA 0012 data was also recorded for comparison. Surface roughness elements, below the typical scale modeled by CFD, are shown to change DSV formation angle and location. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D6.00006: Turbulent separation delay via tuned wall-impedance on a NACA 4412 airfoil in pre-stalled conditions. Julien Bodart, Grigory Shelekhov, Carlo Scalo, Laurent Joly We have performed large-eddy simulations of turbulent separation control via imposed wall-impedance on a NACA-4412 airfoil in near-stalled conditions (Mach, $M_\infty = 0.3$, and chord-Reynolds numbers, $Re_c = 1.5\times 10^6$ and angle of attack, $\alpha = 14^{\circ{}}$), inspired by the experimental setup of Coles \& Woodcock (1979). We impose complex impedance boundary conditions (IBCs) using the implementation developed by Scalo, Bodart and Lele, \emph{Phys. Fluids} {\bf 27}, 035107 (2015), representing an array of sub-surface-mounted tunable Helmholtz cavities with resonant frequency, $f_\textrm{res}$, covered by a porous sheet with permeability inversely proportional to the impedance resistance. Generation of spanwise-oriented Kelvin-Helmholtz (KH) rollers of size $l_{KH,0} \simeq U_\infty / f_\textrm{res}$is observed in areas of sustained mean shear, which are convectively amplified along the shear-layer and reenergizing the separated flow via vortical-induced mixing and entrainment of irrotational fluid. Their characteristic initial size $l_{KH,0}$ is determined by the periodic wall-transpiration pattern induced, in turn, by acoustic resonance in the cavities. Several resonant frequencies and impedance have been tested, bracketing optimal conditions for control. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D6.00007: MOVED TO G35.006 |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D6.00008: Aerodynamic performance of an airfoil with a prescribed wall protuberance at low Reynolds numbers. Carlos Duque-Daza, Cristian Mejia, Diego Camacho, Duncan Lockerby Numerical simulations of flow around a modified NACA0012 airfoil, featuring a small surface perturbation on the upper wall, were performed at two low Reynolds numbers. The aerodynamic performance was examined under conditions of incompressible steady state flow. Simulations at different angles of attack (AOA) were performed: 0, 6, 9.25 and 12 degrees for Re$=$5000, and 6, 9.25 and 12 for Re$=$50000. The effect of the wall-perturbation was assessed in terms of changes of drag and lift coefficients, and alterations of the upper wall turbulent boundary layer. Examination of mean velocity profiles reveals that the wall perturbation promotes boundary-layer separation near the leading edge and increase of the skin friction drag. An arguably improvement of the effectiveness, i.e. ratio of lift to drag, was observed for the modified profile for Re $=$ 5000, especially at AOA of 6 degrees. This effect seems to be caused by a double effect: boundary layer separation approaching the leading edge and an increase of the lift coefficient caused by the larger pressure drop on the upper surface. The effect of the perturbation was always negative for the airfoil operating at Re$=$50000, independently of AOA. [Preview Abstract] |
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