Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L16: Aerodynamics: Unsteady Aerodynamics I |
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Chair: Douglas Bohl, Clarkson University Room: 204 |
Monday, November 23, 2015 4:05PM - 4:18PM |
L16.00001: Experimental Investigation of Dynamic Stall on a NACA0012 Airfoil Undergoing Sinusoidal Pitching Douglas Bohl, Melissa Green In this work, the flow field around a NACA0012 Airfoil undergoing large amplitude sinusoidal pitching is investigated using Particle Image Velocimetry (PIV). The airfoil is pitched symmetrically about the quarter chord point with a peak angle of 20 deg, at reduced frequencies of k$=$0.2-0.6 and Re$_{\mathrm{c}}=$12000. Sixteen different Fields of View are phase averaged and combined to quantify the flow field from 0.75c upstream of the leading edge to 1c downstream of the trailing edge. This provides spatially and temporally resolved data sets that include the downstream evolution of the flow fields. The velocity and vorticity fields, both around the airfoil and downstream of the trailing edge, will be investigated as a function of the reduced frequency to better understand the dynamics (i.e. formation, separation and development) of the leading edge vortex and the resulting downstream flow evolution. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L16.00002: Experimental Investigation of Dynamic Stall on a Finite Span NACA 0012 Wing Wyatt Spellman, Douglas Bohl In this work the velocity and vorticity fields around finite and ``infinite'' span wings with a NACA 0012 profile undergoing constant rate pitching were quantified using Molecular Tagging Velocimetry (MTV). The ``infinite'' span wing was bounded by walls to reduce the effects of the wing tip vortex while the finite span wing was bounded by a wall on one end and unbounded on the other. The wings were pitched from $\alpha =$0 to 55$^{\circ}$ with a constant non-dimensional pitch rate of $\Omega ^{\mathrm{\ast }}=$0.1 at Re$_{\mathrm{c}}=$12000. The Dynamic Stall Vortex (DSV) was identified and tracked using the ``$\Gamma $ criteria.'' The results showed that the formation and trajectory of the DSV for the finite wing case varied with the spanwise location, with the location of the DSV remaining progressively closer to the airfoil surface towards the wingtip. These results were consistent with the ``Omega'' vortex structure previously observed in flow visualization. The DSV was also found to remain closer, and convect away from the airfoil surface slower, at all spanwise measurement planes when compared to the infinite span results. [Preview Abstract] |
(Author Not Attending)
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L16.00003: Analysis of the aerodynamic interaction between two plunging plates in tandem at low Reynolds number for maximum propulsive efficiency Joaquin Ortega-Casanova, Ramon Fernandez-Feria The thrust generated by two heaving plates in tandem is analysed for two particular sets of configurations of interest in forward flight: a plunging leading plate with the trailing plate at rest, and the two plates heaving with the same frequency and amplitude, but varying the phase difference. The thrust efficiency of the leading plate is augmented in relation to a single plate heaving with the same frequency and amplitude in most cases. In the first configuration, we characterize the range of nondimensional heaving frequencies and amplitudes of the leading plate for which the stationary trailing plate contributes positively to the global thrust. The maximum global thrust efficiency, reached for an advance ratio slightly less than unity and a reduced frequency close to 5, is about the same as the maximum efficiency for an isolated plate. But for low frequencies the tandem configuration with the trailing plate at rest is more thrust efficient than the isolated plate. In the second configuration, we find that the maximum thrust efficiency is reached for a phase lag of 180ยบ (counterstroking), particularly for an advance ratio unity and a reduced frequency 4.4, and it is practically the same as in the other configuration and that for a single plate. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L16.00004: Experimental investigation of a large aspect ratio flat plate encountering a steam-wise gust Karen Mulleners, Peter Mancini, Anya Jones While humans are capable of mimicking, and even outperform, the kinematic capabilities of natural flyers, birds and insects are still way ahead of us when it comes to anticipating and dealing with turbulent and gusty flow conditions. To tailor and improve flight control capabilities of low Reynolds number flyers in real weather, we need to bridge this gap of knowledge. As a first step, we experimentally studied the aerodynamic influence of a simplified stream-wise gust on a large aspect ratio flat plate. The experiments were conduction in the $7\times1.5\times1$m$^3$ towing tank at UMD which was equipped with a $4$-axis computer-controlled motion system. The effect of a stream-wise gust was simulated by accelerating or decelerating the wing to a new constant velocity after an initial constant surge. A high-speed camera and light sheet optics were attached to the tow carriage allowing for time-resolved particle image velocimetry along the entire motion in addition to direct force measurements. A proper orthogonal decomposition of the flow field was carried out to study the time scales related to changes induced by the sudden acceleration or deceleration in addition to analyzing the size, position and trajectory of prominent vortices and associated forces during the gust encounter. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L16.00005: Investigation into the Recovery of a Translating Flat Plate Exposed to a Streamwise Acceleration Peter Mancini, Karen Mulleners, Anya Jones This study explores the unsteady aerodynamic response of a wing to streamwise accelerations. A wing was towed through a water tank until reaching steady state ($\>30$ chords), after which the wing accelerated over a prescribed distance to a new constant velocity. Several velocity profiles were investigated, including acceleration and deceleration, as well as various angles of attack ($0^\circ-50^\circ$). Direct force measurements and particle image velocimetry were conducted simultaneously throughout the full length of the motion. This was made possible through the implementation of a unique imaging setup that allows the wing, laser sheet, and camera to move together rigidly down the length of the tank, placing all PIV measurements in the wing-fixed reference frame and allowing for an uninterrupted measurement of the entire wing motion. Lift force results showed that for cases of high leading edge flow separation ($\alpha>20^\circ$), the distance required to reach steady state is drastically lower when recovering from a streamwise acceleration than from accelerating from rest. The concept of vortex formation time was also explored, and via PIV and force results it was confirmed that the formation number consistently lies within the range of 3.6-4.5 for each acceleration profile. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L16.00006: The Direct Numerical Simulation of the Deflected Wake Phenomenon around a Plunging NACA0012 Airfoil at Low Reynolds Numbers Mehmet Sahin, Saliha Banu Yucel, Mehmet Fevzi Unal The deflected wake phenomenon reported by Jones and Platzer (2009) is investigated in detail using direct numerical simulations around a NACA0012 airfoil undergoing harmonic plunging motion. An Arbitrary Lagrangian-Eulerian (ALE) formulation based on an unstructured side-centered finite volume method is utilized in order to solve the incompressible unsteady Navier-Stokes equations. The Reynolds number is chosen to be $252$ and the reduced frequency of plunging motion ($k=2\pi fc/U_{\infty}$) and the plunge amplitude non-dimensionalized with respect to chord are set to $12.3$ and $0.12$, respectively, as in the experimental study of Jones and Platzer (2009). The present numerical simulations reveal a highly persistent transient effect and it takes two orders of magnitude larger duration than the heave period to reach the time-periodic state. In addition, the three-dimensional simulation reveals that the flow field is highly three-dimensional around the leading edge. The calculation reproduces the deflected wake and shows a very good agreement with the experimental wake pattern. The instantaneous vorticity contours, Finite Time Lyapunov Exponent (FTLE) fields and particle traces are presented along with the aerodynamic parameters including the lift and thrust coefficients. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L16.00007: Wing Rock Motion and its Flow Mechanism over a Chined-Body Configuration Yankui Wang, Qian Li, Wei Shi Wing rock motion is one kind of uncommanded oscillation around the body axis over the most of the aircraft at enough high angle of attack and has a strong threat to the flight safety. The purpose of this paper is to investigate the wing rock motion over a typical body-wing configuration with a chined fuselage at fixed angle of attack firstly and four kinds of wing rock motion are revealed based on the flow phenomena, namely non-oscillation, lateral deflection, limit-cycle oscillation and irregular oscillation. Simultaneously, some special relationship between the wing rock motion and the flow over the chined body configuration are discussed. In addition, the evolution of wing rock motion and its corresponding flows when the model undergoes pitching up are also given out. All the experiments have been conducted in a low-speed wind tunnel at a Reynolds number of 1.87*10E5 and angle of attack from 0deg to 65deg. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L16.00008: Unsteady Aerodynamics of ''Roll-Tacking'' in Olympic Class Sailboats Riley Schutt, CHK Williamson When tacking a sailboat (turning a boat through the wind during upwind sailing), racers employ a ``roll-tacking'' technique. During a roll-tack, sailors use body weight movements to roll the boat through extreme angles of heel. This contrasts with a flat-tack, where the boat remains upright throughout the turn. The dynamic heeling motion of a roll-tack causes the sail to vigorously sweep through the air, resulting in large-scale vortex shedding and increased propulsion. In this research, we use a characteristic roll-tack motion derived from on-the-water data. On-the-water data is collected from a full-scale Olympic racing boat sailed by a national champion in the Laser sailboat class. Using this data, we run a series of representative experiments in the laboratory. Two dimensional flexible sail extrusions are built using rapid-prototyping and are tested in a three degree-of-freedom (X, Y, and theta) towing tank. Particle Image Velocimetry and force measurements are used to compare vortex dynamics and propulsive forces generated by roll-tacks versus flat-tacks. An increase in thrust observed during roll-tack tests agrees with on-the-water experiments, which show a racing advantage greater than one boatlength when a roll-tack is performed relative to a flat tack. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L16.00009: Aerodynamics of Unsteady Sailing Kinetics Colin Keil, Riley Schutt, Jennifer Borshoff, Philip Alley, Maximilien de Zegher, CHK Williamson In small sailboats, the bodyweight of the sailor is proportionately large enough to induce significant unsteady motion of the boat and sail. Sailors use a variety of kinetic techniques to create sail dynamics which can provide an increment in thrust, thereby increasing the boatspeed. In this study, we experimentally investigate the unsteady aerodynamics associated with two techniques, ``upwind leech flicking'' and ``downwind S-turns''. We explore the dynamics of an Olympic class Laser sailboat equipped with a GPS, IMU, wind sensor, and camera array, sailed expertly by a member of the US Olympic team. The velocity heading of a sailing boat is oriented at an apparent wind angle to the flow. In contrast to classic flapping propulsion, the heaving of the sail section is not perpendicular to the sail's motion through the air. This leads to heave with components parallel and perpendicular to the incident flow. The characteristic motion is recreated in a towing tank where the vortex structures generated by a representative 2-D sail section are observed using Particle Image Velocimetry and the measurement of thrust and lift forces. Amongst other results, we show that the increase in driving force, generated due to heave, is larger for greater apparent wind angles. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L16.00010: On leading-edge vortex attachment in rotary systems: Incident flow effects Albert Medina, Anya R. Jones The mechanism governing the stable attachment of the leading-edge vortex (LEV) in rotating systems has been believed to be rooting in a balance between the rate vorticity production from the leading-edge shear layer and the convection of vorticity-bearing mass from within the LEV to the surrounding flow field. In such a relation, the accumulation of vorticity within a vortical structure is regulated by convective influences effectively draining the structure of circulatory strength. This work numerically investigates the shear rate-convection balance assertion in low-aspect ratio rectangular flat plates undergoing unidirectional rotation in a steady freestream. The freestream is oriented parallel to the rotational axis and the effect of advance ratio on the resulting flow structures is compared with a rotary plate operating in a quiescent fluid. Depending on advance ratio, the incidence angle of the plate is adjusted to maintain a constant effective attack angle of $\alpha=45^\circ$ based on plate tip speeds. Of interest is the response of the system over a Reynolds number range $Re=\left[ 10^2:10^3\right]$ where axial flow prominence shifts from aft of the leading-edge vortex to within the structure. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L16.00011: Effect of Trailing Edge Shape on the Unsteady Aerodynamics of Reverse Flow Dynamic Stall Andrew Lind, Anya Jones This work considers dynamic stall in reverse flow, where flow travels over an oscillating airfoil from the geometric trailing edge towards the leading edge. An airfoil with a sharp geometric trailing edge causes early formation of a primary dynamic stall vortex since the sharp edge acts as the aerodynamic leading edge in reverse flow. The present work experimentally examines the potential merits of using an airfoil with a blunt geometric trailing edge to delay flow separation and dynamic stall vortex formation while undergoing oscillations in reverse flow. Time-resolved and phase-averaged flow fields and pressure distributions are compared for airfoils with different trailing edge shapes. Specifically, the evolution of unsteady flow features such as primary, secondary, and trailing edge vortices is examined. The influence of these flow features on the unsteady pressure distributions and integrated unsteady airloads provide insight on the torsional loading of rotor blades as they oscillate in reverse flow. The airfoil with a blunt trailing edge delays reverse flow dynamic stall, but this leads to greater downward-acting lift and pitching moment. These results are fundamental to alleviating vibrations of high-speed helicopters, where much of the rotor operates in reverse flow [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L16.00012: Formation and Development of the Dynamic Stall Vortex on a Wing with Leading Edge Tubercles John Hrynuk, Douglas Bohl Humpback whales are unique in that their flippers have leading edge ``bumps'' or tubercles. Past work on airfoils inspired by whale flippers has centered on the static aerodynamic characteristics of these airfoils. The current study uses Molecular Tagging Velocimetry (MTV) to investigate the effects of tubercles on dynamically pitching NACA 0012 airfoils. A baseline (i.e. straight leading edge) wing and one modified with leading edge tubercles are investigated. Tracking of the Dynamic Stall Vortex (DSV) is performed to quantitatively compare the DSV formation location, path, and convective velocity for tubercled and baseline wings. The results show that there is a spanwise variation in the initial formation location and motion of the DSV on the modified wing. Once formed, the DSV aligns into a more uniform spanwise structure. As the pitching motion progresses, the DSV on the modified wing convects away from the airfoil surface later and slower than is observed for the baseline airfoil. The results indicate that the tubercles may delay stall when compared to the baseline airfoil. [Preview Abstract] |
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