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 A18: Aerodynamics: Heaving & PitchingAerodynamics
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Chair: Azar Eslam-Panah, Pennsylvania State University Room: 607 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A18.00001: Onset of orbital motion in a perturbed trailing vortex Gregory Fishman, Donald Rockwell The unsteady flow structure of a trailing vortex from a wing undergoing small amplitude, heaving motion is investigated using stereo particle image velocimetry. The effect of Strouhal number on the onset and development of orbital motion of the trailing vortex is examined through space-time representations of axial and azimuthal vorticity, axial velocity deficit and swirl ratio. The response of the vortex can be categorized according to the Strouhal number. At the lowest value, unidirectional excursions of the vortex occur, with insignificant orbital motion over a streamwise extent of approximately one hundred radii of the vortex. At a moderate value of Strouhal number, unidirectional motion of the vortex evolves into orbital motion with increasing streamwise distance. At the highest Strouhal number, pronounced orbital motion of the vortex occurs at the trailing edge of the wing and is amplified in the streamwise direction. Changes in curvature along the vortex are closely linked to significant variations of axial and azimuthal vorticity, axial velocity deficit and swirl ratio for all three regimes of vortex response. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A18.00002: Flow Measurements of a Plunging Wing in Unsteady Environment. Jesse Wengel, Rungun Nathan, Bo Cheng, Azar Eslam-Panah Despite the great progress in their design and control, Unmanned Aerial Vehicles (UAVs) are tremendously troubled while flying in turbulent environments, which are common in the lower atmospheric boundary layer (ABL). A nominally 2D plunging wing was developed and tested in the presence of unsteady wake to investigate the effect of the flow disturbances on vorticity fields. The experiments were conducted in a water channel facility with test section width of 0.76 m, and a water depth of 0.6 m. The unsteady wake in the form of von K\'{a}rm\'{a}n Vortex Street was generated by a cylinder located upstream of the plunging wing. The plunge amplitude and frequency of the oscillation were adjusted to bracket the range of Strouhal numbers relevant to the biological locomotion (0.25\textless St\textless 0.35). Free-stream velocity is held constant at 0.13 m/s for all the cases, producing a chord-based Reynolds number of 10,000. These cases were selected because there is a significant lack of knowledge describing the topology of the flow field in presence of upstream vortical structures. First, the dye flow visualization technique was used to qualitatively observe the wake behind the cylinder, mainly to position the wing with respect to the upstream vortical structure. Second, time-resolved Particle Image Velocimetry (PIV) was employed to quantitatively study the effect of unsteady wake on the flow measurements of the plunging wing. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A18.00003: An Experimental Investigation of an Airfoil Traversing Across a Shear Flow Borhan A. Hamedani, Ahmed Naguib, Manoochehr Koochesfahani While the aerodynamics of an airfoil in a uniform approach flow is well understood, less attention has been paid to airfoils in non-uniform flows. An aircraft encounters such flow, for example, during landing through the air wake of an aircraft carrier. The present work is focused on investigating the fundamental aerodynamics of airfoils in such an environment using canonical flow experiments. To generate a shear approach flow, a shaped honeycomb block is employed in a wind tunnel setup. Direct force measurements are performed on a NACA 0012 airfoil, with an aspect ratio of 1.8, as the airfoil traverses steadily across the shear region. Measurements are conducted at a chord Reynolds number \textit{Re}$_{c}\approx $ 75k, based on the mean approach stream velocity at the center of the shear zone, for a range of airfoil traverse velocities and angles of attack (0 -- 12 degree). The results are compared to those obtained for the same airfoil when placed statically at different points along the traverse path inside the shear zone. The comparison enables examination of the applicability of quasi-steady analysis in computing the forces on the moving airfoil. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A18.00004: Wake vortex properties and thrust production of a harmonically-pitching flexible airfoil at low Reynolds number David Olson, Ahmed Naguib, Manoochehr Koochesfahani Many of the natural flyers have deformable wing structures and exhibit complex kinematics in order to produce lift and thrust. Replicating all of these conditions in the laboratory (or in simulations) is extremely difficult, and drawing explicit connections to basic unsteady aerodynamics models and theories is even more complicated. Therefore, simplified wing structure and kinematics are typically used to facilitate drawing out these connections. In this work, measurements are conducted using a rigid and a chordwise-flexible NACA 0009 airfoils when harmonically pitched about the quarter chord point. Molecular tagging velocimetry is used to characterize the wake and estimate the thrust based on the momentum integral equation as function of the reduced frequency and the pitching amplitude. The results obtained using the two different airfoils are compared in order to examine the influence of structural flexibility. Consistent with the literature, chordwise flexibility is found to enhance thrust production and the circulation of the vortices shed into the wake, for a certain range of frequencies and amplitudes. Additional characterizations are undertaken of the wake vortex structure and its scaling. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A18.00005: Reynolds number effect on airfoil wake structures under pitching and heaving motion. Kyung Chun Kim, Hamidreza Karbasian Detached Eddy Simulation (DES) and particle image velocimetry (PIV) measurements were performed to investigate the wake flow characteristics of an airfoil under pitching and heaving motion. A NACA0012 airfoil was selected for the numerical simulation and experiments were carried out in a wind tunnel and a water tunnel at Reynolds number of 15,000 and 90,000, respectively. The airfoil oscillated around an axis located ¼ distance from the leading edge chord. Two different angles of attack, 20° and 30°, were selected with ±10° maximum amplitude of oscillation. In order to extract the coherent flow structures from time-resolved PIV data, proper orthogonal decomposition (POD) analysis was performed on 1,000 instantaneous realisations for each condition using the method of snapshots. Vorticity contour and velocity profiles for both PIV and DES results are in good agreement for pitching and heaving motion. At high Reynolds number, 3D stream-wise vortices appeared after generating span-wise vortices. The higher maximum angle of attack allows the leading edge vortex to grow stronger and that the angle of attack appears to be more important in influencing the growth of the leading edge vortex structure than the reduced frequency. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A18.00006: Pitch and heave dynamics of an elastically-mounted cyber-physical hydrofoil Yunxing Su, Kyohei Onoue, Michael Miller, Kenneth Breuer The energy harvesting performance of an elastically-mounted hydrofoil (chord, $c$, span, $s$) subject to a prescribed pitching motion is studied using a cyber-physical force-feedback control system. We vary the mass, $m$, the frequency of the pitching motion, $\omega$, the parameters of the elastic support (stiffness, $k$ and damping, $b$) and the Reynolds number, $Re$. The extracted energy is obtained from measured heave force and velocity, $F \dot{y}$. The ratio between the pitching frequency and the natural frequency of the system, $\omega/\sqrt{k/m}$, and the damping coefficient, $b/(0.5\rho U s c)$, are found to play a major role. In particular, the maximum power output is achieved at a frequency ratio of 1, which corresponds to an optimal phase difference of $90^{\circ}$ between the driven pitch and passive heave motions. At the resonance condition, the damping coefficient defines the heaving amplitude, $H$, and thus the width of the wake and the Strouhal number, $St = fH/U$. The power coefficient, $C_p = |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A18.00007: Passive pitching of splitters in the trailing edge of elliptic cylinders Liu Hong, Yaqing Jin, Haotian Qiu, Leonardo P. Chamorro The pitching of hinged splitters in the trailing edge of elliptic cylinders was experimentally studied at various angle of attack AoA of the cylinder, Reynolds numbers, splitter length, aspect ratio AR of the cylinder and freestream turbulence levels. High-resolution telemetry and hotwire anemometry were used to characterize and gain insight on the dynamics of splitters and wake flow. Results show that the motions of the splitters contain various dominating modes, fp and fv, which are induced by the mean flow and wake dynamics. High turbulence dampens the coherence of the regular vortex shedding leading to negligible fv. For sufficiently long splitter, namely twice of the semi-major axis of the cylinder, dual vortex shedding mode exists close to the leading and trailing edges of the splitter. In general, the splitters oscillate around an equilibrium position nearly parallel to the mean direction of the flow; however, a skewed equilibrium is also possible with a strong recirculation region of low AR and high AoA. Flow measurements indicate that although the splitter pitching exhibits two dominant vortex shedding modes in various configurations, only the higher frequency is transmitted to the wake. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A18.00008: Energy extraction from a semi-passive flapping-foil turbine with active heave and passive pitch Matthieu Boudreau, Guy Dumas, Kevin Gunther Due to the inherent complexity of the mechanisms needed to prescribe the heaving and the pitching motions of optimal flapping-foil turbines, several research groups are now investigating the potential of using unconstrained passive motions. The amplitude, the phase and the frequency of such free motions are thus the result of the interaction of the blade with the flow and its elastic supports, namely springs and dampers. In parallel with our current study on fully-passive flapping-foil turbines (Boudreau et al., APS 2016), we investigate in this work the possibility of using a semi-passive turbine. Unlike previous semi-passive turbines studied in the literature, we propose a turbine with a passive pitching motion and an active heaving motion constrained to be a sine wave with desired amplitude and frequency. As most of the energy extracted by flapping-foil turbines comes from the heaving motion, it is natural to connect an electric generator to this degree of freedom, thereby allowing one to constrain this motion. It is found that large-amplitude pitching motions leading to a considerable energy extraction can arise under different circumstances and mechanisms, either forced by the heaving motion or driven by an instability of the pitching motion itself. [Preview Abstract] |
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