Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G09: Pitching Airfoils |
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Chair: John Hrynuk, US Army Research Lab Room: 213 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G09.00001: On the lift and vortices of an asymmetrically pitching foil Shuji Otomo, Karen Mulleners, Kiran Ramesh, Ignazio Maria Viola Research on unsteady aerodynamics has significantly grown in recent years due to its relevance to bio-inspired flight. We experimentally investigate the influence of kinematics on the unsteady lift force, flow topology, and vortex dynamics on a pitching NACA 0018 foil at a Reynolds number of $3.2 \times 10^4$. We consider the effects of varying pitching frequency and amplitude, as well as amount of asymmetry in angle-of-attack time history. Time-resolved force measurements and particle image velocimetry are performed. We compare the measured lift force with the linear theory of Theodorsen. This analytical model correctly predicts the general trend of the lift force over a pitching period, even for large-angle-of-attack oscillations and non-sinusoidal kinematics. However, when vortical flow structures separate from the foil, the theory overpredicts the lift force. In these conditions, the difference between the linear theory and the measured lift is accounted by a simple model based on the impulse method. Our findings will contribute to the understanding of largely separated flow and to the development of low-order models. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G09.00002: Experimental investigation of the flow and pressure fields around a harmonically pitching airfoil. Jibu Jose, Subhra Shankha Koley, Christopher Williams, Ayush Saraswat, Jin Wang, Joseph Katz Experimental studies of aeroelastic flutter use pitching and heaving foils to characterize the complex flow response to coupled bending and torsion. The current study examines the flow around a harmonically pitching, 50mm chordlength, 189 mm span, modified NACA 0015 hydrofoil oscillating at a frequency of 2.7Hz between 5$^{\mathrm{o}}$ and 20$^{\mathrm{o}}$ incidence. At a freestream velocity of 1 m/s, the Reynolds number is 50,000. The time-resolved stereo-PIV measurements are performed in a refractive index-matched water tunnel, where the refractive index of the acrylic foil is matched with that of the liquid. Data is recorded in the mid span at 1250 frames/s, providing 463 vector maps per cycle. Assuming a 2D flow, the time resolved data is used for calculating the distribution of material acceleration, which is then integrated spatially to determine the pressure distribution. Having the data recorded on both sides simultaneously enables calculation of the lift, pitching moments, and load distribution. Results show that phase lags in the development of the separated region on the suction side create very different flow structures and load distributions during upstroke and downstroke at the same angle both in instantaneous realizations and phase averaged flows. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G09.00003: Wake Properties of an Oscillating Airfoil Undergoing Asymmetric Oscillation Colin Stutz, Douglas Bohl, Melissa Green The flow field around a NACA0012 airfoil undergoing small amplitude, high frequency asymmetric sinusoidal pitching is investigated using Particle Image Velocimetry (PIV). The airfoil is pitched about the quarter chord point with an amplitude of \textpm 4\textdegree at reduced frequencies of $k=$2.6-5.8 for Re$_{\mathrm{c}}=$12000. Pitching symmetries of 50/50, 60/40 and 70/30, where the symmetry is defined as the fraction of the cycle spent in the pitch down and pitch up motion. Wake characteristics (i.e. vortex size, peak vorticity, vortex orientation, vortex convection speed) are investigated by tracking the vortices in the wake over the first chord length of development. Mean thrust/drag are calculated using a control volume approach including fluctuating terms. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G09.00004: Application of FTLE analysis on unsteady flow around pitching airfoils of different amplitude Youwei Liu, Douglas Bohl, Melissa Green The finite-time Lyapunov exponent (FTLE) was used to analyze the unsteady flow around a pitching NACA 0012 airfoil. A total of 15 different cases with fixed $Re_c=12000$, various amplitudes ($\pm20^{\circ}$, $\pm30^{\circ}$, $\pm40^{\circ}$) and reduced frequencies ($k=0.2-0.6$) were investigated. Phase-averaged particle image velocimetry (PIV) was used to obtain 14 windows of the two-component velocity field, which then were stitched together to obtain a window that was approximately 2.37$c$ by 1.65$c$. Due to the temporal and spatial resolution of the data set, FTLE results were able to resolve the structure and interaction of leading edge vortices (LEVs) and trailing edge vortices (TEVs), as well as the influences of amplitude and reduced frequency on the formation, shedding and merging of vortices. For the same amplitude, as frequency increases vortices start shedding later, their trajectories stays closer to airfoil surface, and TEVs become weaker. For the same reduced frequency, as the amplitude decreases primary and secondary LEVs form later. The formation of TEVs is highly dependent the combined influence of frequency and amplitude. With a higher frequency and lower amplitude, the area near the TE is dominated by the main LEV, as it remains attached for longer. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G09.00005: Effect of Chordwise Flexibility on Propulsive Performance of High-Inertia Oscillating Foils Peter Oshkai, Dylan Iverson, Mostafa Rahimpour, Waltfred Lee, Takahiro Kiwata This work studies the effects of chordwise flexibility, inertia and kinematic parameters on propulsive performance of an oscillating foil. Three low-aspect-ratio foils with different flexibilities were undergoing pitch and heave motions in a uniform flow at the Reynolds number of 80000. Forces exerted on the foil were directly measured using a load cell and were used to calculate the thrust and efficiency values. The phase-averaged flow velocity and out-of-plane vorticity in the wake of the foil were obtained using particle image velocimetry. The circulation in the wake was related to the loading on the foil by calculating the moments of vorticity with respect to the pitching axis of the foil. The generated thrust values monotonically increased as a function of the Strouhal number for the considered range of pitching angles. The deformation of the foil resulted in an increased wake width, leading to larger amplitudes of the instantaneous loading on the foil and higher thrust coefficient compared to a reference case of a rigid foil. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G09.00006: Vortex dynamics of a rapidly pitching elliptical airfoil exhibiting dynamic stall at low Reynolds number John Hrynuk, Mingjun Wei The flow features and effects of dynamic stall have been traditionally studied for rotorcraft and in small bio-flyers. In rotorcraft focused dynamic stall studies a sinusoidal pitching motion is commonly used, while a perching maneuver for small vehicles is better represented by a constant pitch-and-hold motion. For pitch-and-hold motions the interaction of Dynamic Stall Vortex (DSV) and Trailing Edge Vortex (TEV) appear to have a significant impact on vortex circulation and convection. To expand the understanding of the interaction of DSV, TEV, and basic vortex shedding phenomenon, an elliptical airfoil shape was pitched about mid-chord with a constant dimensionless pitch rate. Flow fields were captured using PIV at a Reynolds number of Re $=$ 12,000. Results show that depending on the phase angle of the airfoil, the flow is dominated by the DSV, TEV, or traditional bluff body shedding. These regions of phase angle will be delineated and vortex tracking methods will be used to evaluate convection behavior of the trackable structures. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G09.00007: Dynamic stall experiments on various pitching motion profiles of an airfoil at high Reynolds numbers Janik Kiefer, Claudia Brunner, Marcus Hultmark, Martin O. L. Hansen The combination of high Reynolds numbers and unsteady flow conditions depicts a challenge in experimental wind tunnel studies. Unsteady airfoil aerodynamics are commonly described by the reduced frequency k $=$ omega c/2U, where a range of 0 \textless k \textless 0.25 characterizes steady to highly unsteady flow conditions. The Reynolds number scales proportionally with the flow velocity U, whereas the reduced frequency scales inversely proportional. In regular wind tunnels, this leads to unrealistically high pitching frequencies in experimental attempts to achieve high Reynolds numbers simultaneously with high reduced frequencies. Instead, this study takes advantage of a high-pressure flow facility, in which the density of compressed air promotes high Reynolds numbers, while low velocities below 5 m/s allow for low pitching frequencies and large angle amplitudes. A NACA0021 airfoil was equipped with surface pressure sensors to investigate distributed pressures and integrated forces at Reynolds numbers between one and five million. The present study elucidates the differences of various motion profiles on airfoil performance in comparison to the commonly employed sinusoidal pitching motion in dynamic stall conditions. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G09.00008: Dynamic Stall Experiments on a Sinusoidally Pitching Airfoil at High Reynolds Numbers Claudia Brunner, Janik Kiefer, Martin O. L. Hansen, Marcus Hultmark The phenomenon of dynamic stall results in a lift overshoot experienced by an airfoil when its angle of attack is rapidly increased beyond the static stall angle. This overshoot is followed by a sudden drop in the lift force, and large hysteresis as the airfoil returns to its initial angle. Dynamic stall is observed in a variety of applications including helicopters and wind turbines, where it produces rapidly fluctuating loads on the blades. It is also seen in many biological systems, but they often use it advantageously. At low and moderate Reynolds numbers and reduced frequencies, this phenomenon has been extensively investigated, but due to the experimental challenges at high Reynolds numbers, only few studies have been conducted in this regime. In the current study, a NACA 0021 airfoil was oscillated sinusoidally around the static stall angle. A highly pressurized, low-velocity, wind tunnel was used to achieve Reynolds numbers up to 5 $\cdot $ 10$^{\mathrm{6}}$, based on the chord length, and reduced frequencies up to 0.5. Forces and moments on the airfoil, as well as pressure distributions around its surface were recorded. Effects of Reynolds number, reduced frequency, mean angle of attack, and amplitude on the development of dynamic stall will be presented. [Preview Abstract] |
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