Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L24: Aerodynamics IV |
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Chair: Sunil James, Honeywell Aerospace Room: 319 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L24.00001: On The Flow Physics of Dynamic Stall Inception Dustin Coleman, Flint Thomas, Kyle Heintz, Michael Wicks, Thomas Corke Despite being the focus of many previous investigations, dynamic stall inception is still not fully understood. In this study the flow physics regarding the initiation, growth, and convection of the dynamic stall vortical structure produced during unsteady pitching of a NACA 0015 airfoil are investigated using time-resolved particle imaging velocimetry (TR-PIV) and surface pressure measurements as the primary flow diagnostics. Experiments are conducted at freestream Mach and Reynolds numbers of $M_{\infty} = 0.1$ and Re$_c$ = 2.75e+05, respectively, and over a reduced frequency range of 0.05$-$0.1. The experimental measurements are input to a locally implemented control volume formulation in order to characterize the convection and wall-normal diffusion of spanwise vorticity near the leading edge. In this manner, the near-wall flow physics surrounding dynamic stall vortex (DSV) inception is characterized. Likewise, the evolution of the resultant DSV is characterized in terms of its growth rate, terminal strength, and wall detachment process. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L24.00002: Reduced-order vortex modeling of unsteady non-linear aerodynamics Jeff Eldredge, Darwin Darakananda, Maziar Hemati Non-linear fluid dynamic phenomena are inherent both to flapping wings and to fixed wings during rapid maneuvers. These phenomena, manifested in the interactions of shed vortex structures, are central to the generation of forces and moments. In previous work, we have presented the development and optimization of a low-degree-of-freedom model that captures such phenomena in the motions of point vortices of time-varying strength. Here, we present several extensions of this model toward more complex physics. The model construction is informed from a combination of results from experiments, high-fidelity Navier-Stokes computations, and inviscid vortex sheet simulations. A window-stitching technique is used to develop optimized point vortex models for longer-duration maneuvers. Self-sustained vortex shedding from a wing at large angle of attack is captured with point vortices -- one per shed vortical structure -- using a simple criterion based on the dynamics of the re-attachment point. Finally, the ongoing extension of the model to finite aspect ratio wings is presented. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L24.00003: Physical model of kitesurfing Pawel Zimoch, Adam Paxson, Edward Obropta, Tom Peleg, Sam Parker, A.E. Hosoi Kitesurfing is a popular water sport, similar to windsurfing, utilizing a surfboard-like platform pulled by a large kite operated by the surfer. While the kite generates thrust that propels the surfer across the water, much like a traditional sail, it is also capable of generating vertical forces on the surfer, reducing the hydrodynamic lift generated by the surfboard required to support the surfer's weight. This in turn reduces drag acting on the surfboard, making sailing possible in winds lower than required by other sailing sports. We describe aerodynamic and hydrodynamic models for the forces acting on the kite and the surfboard, and couple them while considering the kite's position in space and the requirement for the kite to support its own weight. We then use these models to quantitatively characterize the significance of the vertical force component generated by the kite on sailing performance (the magnitude of achievable steady-state velocities and the range of headings, relative to the true wind direction, in which sailing is possible), and the degradation in sailing performance with decreasing wind speeds. Finally, we identify the areas of kite and surfboard design whose development could have the greatest impact on improving sailing performance in low wind conditions. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L24.00004: Compressible flow in fluidic oscillators Emilio Graff, Damian Hirsch, Mory Gharib We present qualitative observations on the internal flow characteristics of fluidic oscillator geometries commonly referred to as sweeping jets in active flow control applications. We also discuss the effect of the geometry on the output jet in conditions from startup to supersonic exit velocity. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L24.00005: Lift generation on a flat plate with unsteady motions Xi Xia, Kamran Mohseni The leading edge vortex (LEV) on an airfoil or wing has been considered to be one of the most important sources of lift enhancement according to several previous experimental and theoretical studies. In this work, the unsteady 2D potential flow theory is employed to model the flow field of a flat plate wing undergoing unsteady motions. A multi-vortices model is developed to model both the leading edge and trailing edge vortices (TEVs), which offers improved accuracy compared with using only single vortex at each separation location. The lift prediction is obtained by integrating the unsteady Blasius equation. It is found that the motion of vortices contributes significantly to the overall aerodynamic force on the flat plate. The results of the simulation are then compared with classical numerical, theoretical and experimental data for canonical unsteady flat plat problems. Good agreement with these data is observed. Moreover, these results suggests that the leading edge vortex shedding for small angles of attack should be modeled differently than that for large angles of attack. Finally, the results of vortex motion vs. lift indicate that the lift enhancement during the LEV ``stabilization'' above the wing is a combined effect of both the LEV and TEV motion. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L24.00006: Flow Visualization around a Simplified Two-Wheel Landing Gear Alis Ekmekci, Graham Feltham The flow topology around a simplified two-wheel landing gear model is investigated experimentally by employing the hydrogen bubble flow visualization technique in a recirculating water channel. The landing gear test model consists of two identical wheels, an axle, a main strut and a support strut. The flow Reynolds number based on wheel diameter is 31,500 and wheels with varying geometric details are considered. Flow structures have been identified through analysis of long-time video recordings and linked to the model geometry. In the flow region above the wheels (wing side), the flow in the inter-wheel region either separates prematurely from the inner surfaces of the wheels and forms slant vortices in the near-wake, or remains attached till the aft wheel perimeter. Inclusion of interior wheel wells are found to result in a jet-like ejection as a result of the interaction with the axle and main strut. In the flow region below the wheels (ground side) the near wake contains periodically forming, complex, large-scale structures. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L24.00007: Tip vortex characteristics of rotor in hover Swathi M. Mula, Christopher G. Cameron, Charles E. Tinney, Jayant Sirohi Vortices emanating from the tip of the rotor blades comprise four distinct regions of flow: laminar, transitional, turbulent, and irrotational flows. To investigate the structural instabilities associated with various flow regions within the vortex, the current investigation employs the proper orthogonal decomposition (POD) technique. This technique is applied to blade tip vortices emanated from a reduced-scale, 1.0 m diameter, single-bladed rotor in hover. The rotor is operated at 1500 RPM which corresponds to a $Re_{tip}$ = 218,000 and $M_{tip}$ = 0.23; and at a collective pitch angle of $7.3^{\circ}$. Measurements are undertaken using a two-component PIV system, at various vortex ages. An effort is also made to ensure that there is sufficient resolution within the tip vortex region, to enable the study of local instabilities associated with various flow regions within the vortex. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L24.00008: Perching Dynamics and Development of a Simple Model Michael Puopolo, Jamey Jacob, Ryan Reynolds Aerodynamicists with a vision for bird-like aircraft have been forced to develop new ways of modeling extremely agile flight systems, and in recent years there has been a growing variety of creative approaches that incorporate computer methods, empirical data, and unsteady flow theory. However, there remains a lack of simple and easily transferable models that can be used to predict and control motion of a fixed-wing, perching aircraft in the low Reynolds number flow regime. The authors have developed a simple dynamic model for a perching vehicle with a common fixed wing configuration that uses only input of the system design parameters, in addition to other relevant widely available information, and does not rely on wind tunnel measurements, CFD analysis or other rigorous forms of system identification. The resulting model is presented with a comparison of model simulations to flight data from a perching UAV. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L24.00009: Development of a MEMS shear stress sensor for use in wind tunnel applications Casey Barnard, Jessica Meloy, Mark Sheplak The measurement of mean and fluctuating wall shear-stress in laminar, transitional, and turbulent boundary layers and channel flows has applications both in industry and the scientific community. Currently there is no method for time resolved, direct measurement of wall shear stress at the spatial and temporal scales of turbulent flow structures inside model testing facilities. To address this need, a silicon micromachined differential capacitance shear stress sensor system has been developed. Mean measurements are enabled by custom synchronous modulation/demodulation circuitry, which allows for measurement of both magnitude and phase of incident wall shear stress. Sizes of the largest device features are on the order of relevant viscous length scales, to minimize flow disturbance and provide a hydraulically smooth sensing surface. Static calibration is performed in a flow cell setup, and an acoustic plane wave tube is used for dynamic response data. Normalized sensitivity of 1.34 mV/V/Pa has been observed over a bandwidth of 4.8 kHz, with a minimum detectable signal of 6.5 mPa. Initial results show qualitative agreement with contemporary measurement techniques. The design, fabrication, support electronics, characterization, and preliminary experimental performance of this sensor will be presented. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L24.00010: Flow structure on a rotating wing undergoing deceleration to rest Daniel Tudball Smith, Donald Rockwell, John Sheridan Inspired by the behavior of small biological flyers and micro aerial Vehicles, this study experimentally addresses the flow structure on a low aspect ratio rotating wing at low Reynolds number. The study focuses on a wing decelerating to rest after rotating at constant velocity. The wing was set to a constant 45$^\circ$ angle of attack and, during the initial phase of the motion, accelerated to a constant velocity at its radius of gyration, which resulted in a Reynolds number of 1400 based on the chord length. Stereoscopic PIV was used to construct phase-averaged three-dimensional (volumetric) velocity fields that develop and relax throughout the deceleration and cessation of the wing motion. During gradual deceleration, the flow structure is maintained when normalised by the instantaneous velocity; the distinguishing feature is shedding of a trailing edge vortex that develops due to the deceleration. At higher deceleration rates to rest, the flow structure quickly degrades. Induced flow in the upstream direction along the surface of the wing causes detachment of the previously stable leading edge vortex; simultaneously, a trailing-edge vortex and the reoriented tip vortex form a co-rotating vortex pair, drawing flow downward away from the wing. [Preview Abstract] |
Monday, November 25, 2013 5:45PM - 5:58PM |
L24.00011: Flow Structure on a Wing Due to Unsteady Pitch-Up and Rotation Maneuvers Matthew Bross, Turgut Yilmaz, Donald Rockwell The flow structure along a rectangular (low aspect ratio) wing undergoing pure pitch-up, pitch-up with rotation, and pure rotation is characterized as a function of dimensionless convective time $\tau$ during each maneuver. Quantitative imaging via angular displacement stereo particle image velocimetry was used to determine the three-dimensional velocity field, thereby allowing analysis of the effects of different wing kinematics via representations of Q-criterion, vorticity flux, and velocity and vorticity contours. Despite the difference in wing kinematics, interactions between leading-edge and tip vortices persist across all values of $\tau$. The three-dimensional flow structure involves a symmetric pattern along the wing during pure pitch-up and transforms to a conical leading-edge vortex in conjunction with a tip vortex that extends into the wake for both pitch-up with rotation and pure rotation. This observation suggests that rotational motion has a greater influence than pitching motion in establishing the form and scale of the leading-edge vortex. Finally, sectional images of the flow structure arising from combined pitch-up and rotation were acquired at three different pitch rates relative to a given rate of pure rotation at fixed angle of attack. [Preview Abstract] |
Monday, November 25, 2013 5:58PM - 6:11PM |
L24.00012: The Speed Mach 20 is Quite Impossible in Atmosphere! How to Calculate the Speed Limit When Accelerating an Object in Atmosphere Mwizerwa Pierre Celestin This paper aims to respond to different mysteries which appear in news and blogs around the world about hypersonic flights, Colombia disaster, and hypersonic tests which are going on nowadays. People are really confused and wonder if, when we travel at hypersonic speed, the laws of physics change or if they remain intact. The laws of physics never change. It is we, humans, who have to respect those laws. In this paper, I will try to demonstrate and do all necessary calculations so people may find what is going on. [Preview Abstract] |
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