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 M18: Aerodynamics: Unsteady IIAerodynamics
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Chair: Amir Danesh-Yazdi, Penn State University, Erie Room: 607 |
Tuesday, November 21, 2017 8:00AM - 8:13AM |
M18.00001: Novel Design for a Wind Tunnel Vertical Gust Generator Zachary Smith, Anya Jones, John Hrynuk Gust response of MAVs is a fundamental problem for flight stability and control of such aircraft. Current knowledge about the gust response of these vehicles is limited and gust interaction often results in damage to vehicles. Studying isolated gust effects on simple airfoil models in a controlled environment is a necessity for the further development of MAV control laws. Gusts have typically been generated by oscillating an airfoil causing the shedding of vortices to propagate through the system. While effective, this method provides only a transient up and downdraft behavior with small changes in angle of attack, not suitable for studying MAV scale gust interactions. To study these interactions, a gust that creates a change in flow angle larger than the static stall angle of typical airfoils was developed. This work was done in a low speed, low turbulence wind tunnel at base operating speed of 1.5 m/s, generating a Reynolds number of 12,000 on a NACA 0012 wing. It describes the fundamental mechanisms of how this gust was generated and the results obtained from the gust generator. The gust, which can alter the flow field in less than 1 second, was characterized using PIV and the interactions with a stationary airfoil at several angles of attack are evaluated. [Preview Abstract] |
Tuesday, November 21, 2017 8:13AM - 8:26AM |
M18.00002: Experimental studies of the rotor flow downwash on the Stability of multi-rotor crafts in descent Marcel Veismann, Christopher Dougherty, Morteza Gharib All rotorcrafts, including helicopters and multicopters, have the inherent problem of entering rotor downwash during vertical descent. As a result, the craft is subject to highly unsteady flow, called vortex ring state (VRS), which leads to a loss of lift and reduced stability. To date, experimental efforts to investigate this phenomenon have been largely limited to analysis of a single, fixed rotor mounted in a horizontal wind tunnel. Our current work aims to understand the interaction of multiple rotors in vertical descent by mounting a multi-rotor craft in a low speed, vertical wind tunnel. Experiments were performed with a fixed and rotationally free mounting; the latter allowing us to better capture the dynamics of a free flying drone. The effect of rotor separation on stability, generated thrust, and rotor wake interaction was characterized using force gauge data and PIV analysis for various descent velocities. The results obtained help us better understand fluid-craft interactions of drones in vertical descent and identify possible sources of instability. [Preview Abstract] |
Tuesday, November 21, 2017 8:26AM - 8:39AM |
M18.00003: Effect of Different Ground Scenarios on Flow Structure of a Rotor At Hover Condition Goktug Kocak, Volkan Nalbantoglu, Mehmet Metin Yavuz The ground effect of a scaled model rotor at hover condition was investigated experimentally in a confined environment. Different ground effect scenarios including full, partial, and inclined conditions, compared to out of ground condition, were characterized qualitatively and quantitatively using laser illuminated smoke visualization and Laser Doppler Anemometry measurements. The results indicate that the presence of the ground affects the flow regime near the blade tip by changing the spatial extent and the path of the vortex core. After the impingement of the wake to the ground, highly unsteady and turbulent wake is observed. Both the mean and the root mean square of the induced velocity increase toward the blade tip. In line with this, the spectral power of the dominant frequency in the velocity fluctuations significantly increases toward the blade tip. All these observations are witnessed in all ground effect conditions tested in the present study. Considering the inclined ground effect in particular, it is observed that the mean induced velocities of the high side (mountain) are higher compared to the velocities of the low side (valley) in contrast to the general trend observed in the present study where the ground effect reduces the induced velocity. [Preview Abstract] |
Tuesday, November 21, 2017 8:39AM - 8:52AM |
M18.00004: Toward the Experimental Characterization of an Unmanned Air System Flow Field John-Michael Velarde, Jacob Connors, Mark Glauser The velocity flow field around a small unmanned air system (sUAS) is investigated in a series of experiments at Syracuse University. Experiments are conducted in the 2'x2' sub-sonic wind tunnel at Syracuse University and the Indoor Flow Lab. The goal of these experiments is to gain a better understanding of the rich, turbulent flow field that a sUAS creates. Comparison to large, multi-rotor manned vehicles is done to gain a better understanding of the flow physics that could be occurring with the sUAS. Regions of investigation include the downwash, above the vehicle, and far downstream. Characterization of the flow is performed using hotwire anemometry. Investigation of several locations around the sUAS show that dominant frequencies exist within the flow field. Analysis of the flow field using power spectral density will be presented as well as looking at which parameters have an effect on these dominant frequencies. [Preview Abstract] |
Tuesday, November 21, 2017 8:52AM - 9:05AM |
M18.00005: Wake instabilities of flow over a spinning circular disk at angle of attack Marcus Lee, Tim Colonius, Beverley McKeon A circular disk spinning about its axis of rotational symmetry is inherently robust to external disturbances due to gyroscopic moments and is therefore a promising configuration for a robust micro air vehicle. However, literature on the wake structures and flow behavior associated with spinning disk aerodynamics remains limited, particularly at angles of attack relevant to flight. We thus use a three-dimensional immersed boundary method for incompressible viscous flows to study the effects of angle of attack, Reynolds number, and tip-speed ratio on spinning disk aerodynamics. We observe a Hopf bifurcation corresponding to a bluff-body wake instability at a critical Reynolds number and/or angle of attack, above which periodic vortex shedding occurs. We then examine how increasing the tip-speed ratio affects the stability and structure of the flow. [Preview Abstract] |
Tuesday, November 21, 2017 9:05AM - 9:18AM |
M18.00006: Shape matters: improved flight in tapered auto-rotating wings Yucen Liu, Lionel Vincent, Eva Kanso Many plants use gravity and wind to disperse their seeds. The shape of seed pods influence their aerodynamics. For example, Liana seeds form aerodynamic gliders and Sycamore trees release airborne “helicopters.” Here, we use carefully-controlled experiments and high-speed photography to examine dispersion by tumbling (auto-rotation) and we focus on the effect of geometry on flight characteristics. We consider four families of shapes: rectangular, elliptic, tapered, and sharp-tip wings, and we vary the span-to-chord ratio. We find that tapered wings exhibit extended flight time and range, that is, better performance. A quasi-steady two-dimensional model is used to highlight the mechanisms by which shape affects flight performance. These findings could have significant implications on linking seedpod designs to seed dispersion patterns as well as on optimizing wing design in active flight problems. [Preview Abstract] |
Tuesday, November 21, 2017 9:18AM - 9:31AM |
M18.00007: Bistability of flight states for heavy falling plates Edwin Lau, Wei-Xi Huang Interactions of falling flat plates in two-dimensional flows is presented through direct numerical simulation and immersed boundary method. The transition from steady falling to tumbling flight for heavy plates is presented. At steep angles of release, the plates undergo a period of amplitude increasing fluttering motion before developing to tumble. For the same fluid-solid system of Reynolds number Re and moment of inertia I*, shallow angles of release develop to a state of steady falling after a period of diminishing fluttering amplitude. Simulations further construct a mapping of this bistable region. Relationships among Re, I*, and the critical angles of release separating the two flight states are also provided. The inclusion of this finding on the mapping of flight states suggests fluttering motion as a transitional state before the onset of tumble. [Preview Abstract] |
Tuesday, November 21, 2017 9:31AM - 9:44AM |
M18.00008: Fluidic Energy Harvester Optimization in Grid Turbulence Amir Danesh-Yazdi, Niell Elvin, Yiannis Andreopoulos Even though it is omnipresent in nature, there has not been a great deal of research in the literature involving turbulence as an energy source for piezoelectric fluidic harvesters. In the present work, a grid-generated turbulence forcing function model which we derived previously is employed in the single degree-of-freedom electromechanical equations to find the power output and tip displacement of piezoelectric cantilever beams. Additionally, we utilize simplified, deterministic models of the turbulence forcing function to obtain closed-form expressions for the power output. These theoretical models are studied using experiments that involve separately placing a hot-wire anemometer probe and a short PVDF beam in flows where turbulence is generated by means of passive and semi-passive grids. From a parametric study on the deterministic models, we show that the white noise forcing function best mimics the experimental data. Furthermore, our parametric study of the response spectrum of a generic fluidic harvester in grid-generated turbulent flow shows that optimum power output is attained for beams placed closer to the grid with a low natural frequency and damping ratio and a large electromechanical coupling coefficient. [Preview Abstract] |
Tuesday, November 21, 2017 9:44AM - 9:57AM |
M18.00009: A Galloping Energy Harvester with Attached Flow Petr Denissenko, Igor Khovanov, Sam Tucker-Harvey Aeroelastic energy harvesters are a promising technology for the operation of wireless sensors and microelectromechanical systems, as well as providing the possibility of harvesting wind energy in applications were conventional wind turbines are ineffective, such as in highly turbulent flows, or unreliable, such as in harsh environmental conditions. The development of aeroelastic energy harvesters to date has focused on the flutter of airfoils, the galloping of prismatic structures, and the vortex induced vibrations. We present a novel type of galloping energy harvester with the flow becoming attached when the oscillation amplitude is high enough. With the flow attached, the harvester blade acts closer to an aerofoil than a bluff body, which results in a higher efficiency. The dynamics of a prototype device has been characterised experimentally with the use of a motion tracking system. The flow structure in the vicinity of the device has been studied using smoke visualisation and PIV measurements. A lumped parameter mathematical model has been developed and related to the experimental results. [Preview Abstract] |
Tuesday, November 21, 2017 9:57AM - 10:10AM |
M18.00010: Velocity-intermittency structure for wake flow of the pitched single wind turbine under different inflow conditions Ryan Crist, Raul Bayoan Cal, Naseem Ali, Stanislav Rockel, Joachim Peinke, Michael Hoelling The velocity-intermittency quadrant method is used to characterize the flow structure of the wake flow in the boundary layer of a wind turbine array. Multifractal framework presents the intermittency as a pointwise Hölder exponent. A 3x3 wind turbine array tested experimentally provided a velocity signal at a 21x9 downstream location, measured via hot-wire anemometry. The results show a negative correlation between the velocity and the intermittency at the hub height and bottom tip, whereas the top tip regions show a positive correlation. Sweep and ejection based on the velocity and intermittency are dominant downstream from the rotor. The pointwise results reflect large-scale organization of the flow and velocity-intermittency events corresponding to a foreshortened recirculation region near the hub height and the bottom tip. [Preview Abstract] |
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