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 H09: Aerodynamics: General |
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Chair: John Farnsworth, University of Colorado Boulder Room: 213 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H09.00001: Enhancement of gust using jet at trailing edge of airfoil: A novel technique Dipendra Gupta, Jaywant H. Arakeri Sudden and sharp change in flow velocity, termed as a gust, is an important parameter to study flight performance, especially of micro-air vehicles (MAVs) and aircraft. There have been several techniques to create gusts in a wind tunnel including using pitching foils at the entrance of the test section. One conventional method is using two airfoils with a certain spacing to create a gust. The model aircraft to be tested is placed downstream between the pitching foils. The main limitation with this technique is the low gust intensity (Ig) that can be achieved. We propose a new method to enhance Ig using jet at the trailing edge of the pitching foils. Numerical simulation shows the gust intensity, using the proposed method, to increase by 2-7 times compared to that by conventional techniques for a particular Re, jet velocity and the range of reduced frequencies considered. Moreover, the spacing of foils ensures shear-free smooth flow in the near region surrounding the model, unlike some existing methods for gust enhancement. This technique provides a simple, economical and controlled way to study gust response of MAVs and aircraft in wind tunnels. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H09.00002: The effects of freestream turbulence on the pressure distributions and lift characteristics of airfoil Leon Li, R. Jason Hearst Airfoils employed in engineering applications are often subjected to significant flow variations which can have adverse impacts on their aerodynamic performance and structural stability, for example a wind turbine operating in the atmospheric boundary layer or an airplane on final approach. The impact of these incoming (or freestream) variations is presently not well understood. To gain a better understanding, one must explore a wide parameter space of freestream parameters. This study uses 9 different passive grid configurations to alter the freestream turbulence (FST) in a wind tunnel in order to measure the impact of FST on the pressure distribution and lift characteristics of a NREL S826 reference airfoil. The FST intensity ranges from 0.5{\%} to 5.4{\%}, and the chord-normalized integral length scale varies between 0.07 and 0.24. The chord Reynolds number (Re) was varied between 200-400k. Preliminary results show that an increase in turbulence intensity increases both the maximum lift coefficient and the lift slope. The latter observation contrasts with some results reported in the literature with a smaller FST parameter space and different Re range. The impact on the pressure distribution and separation points will also be examined. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H09.00003: Collaborative Experiments and Simulations of an Unsteady Free-Jet Wind Tunnel for the Study of Gust Interactions John Farnsworth, Kenneth Jansen, Dasha Gloutak, Mark Blanco An unsteady wind tunnel facility was recently developed at the University of Colorado by incorporating a set of counter-rotating louver vanes at the inlet to the low-speed, open-return wind tunnel. The facility was designed such that the wind tunnel can be reconfigured for testing in both a standard closed test-section configuration and an open test-section or free jet configuration. In both configurations, the wind tunnel can operate with maximum test section velocities up to 30 m/s and an unsteady peak to peak amplitude on the order of 50\% of the maximum. High fidelity computational fluid dynamics simulations of the unsteady wind tunnel facility were performed and are validated against experimental measurements. Operating the facility in the closed configuration produces a velocity perturbation that propagates through the test-section nearly instantaneously and thus can be assumed to be purely unsteady. In contrast, the open test-section or free jet configuration produces a velocity perturbation that propagates near the mean convective speed of the flow through the excitation of vortical structures in the surrounding shear layer. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H09.00004: Computational Fluid Dynamic Simulations of a Finite NACA 0015 Wing in an Unsteady Flow Mark Blanco, Dasha Gloutak, John Farnsworth, Kenneth Jansen Computational fluid dynamic simulations were performed for a finite span NACA 0015 rectangular wing section subjected to an unsteady surging flow in a simulated, open test-section or free jet wind tunnel. The full wing and wind tunnel facility was completely modeled and simulated for accurate comparison to collaborative experimental investigations being performed in parallel. At moderate to high angles of attack the flow over the wing section is observed to undergo periodic, three-dimensional separation and reattachment as the surging flow decelerates and then accelerates, respectively. A detailed analysis of the three-dimensional flow field behavior is discussed focusing on the time varying response of the flow field, surface pressure distribution, and wall shear stress. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H09.00005: Experimental Measurements of a Finite NACA 0015 Wing in an Unsteady Flow as Compared to Theory Dasha Gloutak, Emanuele Costantino, Mark Blanco, Kenneth Jansen, John Farnsworth Force, moment and velocity measurements of a semi-span, NACA 0015 rectangular wing subjected to unsteady flow are compared to classical surging airfoil theory. Unsteady streamwise flow, generated by a louver system at the wind tunnel inlet, consisted of maximum velocity amplitudes of 40\% at frequencies up to 3Hz, with mean chord Reynolds Number below 150,000. These sinusoidal velocity gusts were imposed on the wing in the closed test section and free jet wind tunnel configurations. In the former, velocity changes occurred instantaneously over the entire chord length of the wing, and in the latter the velocity changes propagated at the gust's convective speed. Quasi-steady and unsteady lift coefficient responses were compared to Isaacs' unsteady airfoil theory, which details the lift behavior of an infinite airfoil at constant angle of attack with a variable streamwise velocity. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H09.00006: Boundary Layer Characterization of a NACA-0012 Airfoil Plunging in Uniform-Shear Flow using 1c-MTV Mitchell Albrecht, Ahmed Naguib, Manoochehr Koochesfahani Non-uniform approach flows can occur in environments through which aircraft navigate, such as the air wake of an aircraft carrier or aircraft formations. However, few studies have investigated the effects of viscous shear approach flow on airfoils. Our previous work shows that, in a Galilean reference frame, the lift coefficient on a NACA-0012 airfoil plunging in uniform-shear flow is greater than that of a stationary airfoil under the same flow conditions at near-stall positive angles of attack. To elaborate on this deviation from quasi-steady conditions, the current work examines the difference in lift behavior by characterizing the boundary layer around the plunging airfoil in uniform-shear flow. Single-component molecular tagging velocimetry is used to measure the streamwise velocity component of the flow on the suction side of the airfoil surface. Various boundary layer characteristics will be presented for the plunging airfoil as it traverses across the shear layer, and compared with the stationary airfoil at the same cross-stream position, flow conditions, and effective angles of attack. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H09.00007: Finite Wing Hydrodynamic Forces during Water-to-Air Interface Transition Warren Weisler, Rajmohan Waghela, Dr. Kenneth Granlund, Dr. Matthew Bryant Wings are used for numerous applications in both air and water and their lift generation in either domain is well understood. However, the lift generated by a wing when it is transitioning out of water and into air has not been quantified to date. This experimental study aims to examine the lift generated by a wing as it is translated through the water-to-air interface. Studies on egress velocities of 0.2 - 0.75 m/s were conducted to examine the effect of velocity on lift generation over a range of angles of attack from 0- 10 degrees. To examine the effects of starting depth, the translation of the wing was started at depths from 9.5c, 7c, 5c, and 3c. The experimental campaign utilized a rectangular NACA 0015 wing with a chord of 10 cm and an aspect ratio of 4. It was observed that the profile of lift generation during transition was dependent on velocity. At the ``slower'' velocities there is a spike in lift when the wing leading edge nears the water surface, sometimes more than double the steady values. As the speed increases, the spike in lift disappears and lift decreases before the leading edge even reaches the surface. The results from the angle of attack testing show that the transition profile appears to scale with angle of attack. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H09.00008: Three Dimensional Simulation of Dynamic Stall on a Dynamically Pitching Airfoil at Re $=$ 12,000 Harry Werner IV, Douglas Bohl, Brian Helenbrook, Chunlei Liang Dynamic stall (DS) is a flow separation phenomenon affecting airfoils that experience dynamic changes in Angle of Attack (AOA) beyond the airfoil's static stall angle. Passive control of DS could extend the lifespan and efficiency of rotors, wind turbine blades, and turbo machinery, improve the performance and stability of micro air vehicles, and increase the performance and maneuverability of fixed wing aircraft during high AOA maneuvers. The development of effective methodology for passive control of dynamic stall requires in-depth understanding of the fundamental flow physics governing DS phenomena. This work examines the fundamental flow phenomena of dynamic stall through Large Eddy Simulation (LES) of a dynamically pitching NACA 0012 airfoil at low Reynolds Number (Re $=$ 12,000) and a constant nondimensional pitch rate of $\Omega $*$=$0.1. The simulation is a pitch and hold scenario, whereby the airfoil is rotated from 0 to 50\textdegree AOA at a non-dimensional pitch rate of 0.1. The airfoil is held static at 50\textdegree AOA for an additional 10 chord times to observe vortex formation and convection. Results are compared to experimental measurements. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H09.00009: Unsteady spectra of velocity field for a canonical dynamic stall process Rohit Gupta, Sabrina Henne, Karen Mulleners, Phillip Ansell Dynamic stall is a multiscale phenomenon that is commonly associated with airfoils undergoing rapid maneuvers. The knowledge of the dominant modes of instability in the flow field is critical for the design of effective actuation strategies for the control of dynamic stall. The objective of the present investigation was to extract the time-dependent spectral content of the velocity field about a NACA 0012 airfoil model subjected to a linear pitch-up maneuver in a water tunnel at a chord-based Reynolds number of 10$^{\mathrm{4}}$. This objective was achieved through a novel strategy involving a combination of empirical mode decomposition and the Riesz transformations. The velocity field was acquired using high-fidelity, time-resolved particle image velocimetry. The acquired velocity measurements were processed through the empirical mode decomposition algorithm to obtain a set of oscillatory modes, known as the intrinsic mode functions, and an underlying trend, termed as the residue. The dominant amplitudes and frequencies were extracted through a direct application of the Riesz transform. The resulting velocity field spectra were then analyzed at several critical stages of the dynamic stall process. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H09.00010: Evaluation of the unsteady aerodynamic forces of an oscillating airfoil undergoing dynamic stall using impulse theory Firas Siala, James Liburdy The unsteady forces generated by a heaving and pitching airfoil are evaluated from velocity fields using the finite-domain vortex impulse theory. Time-resolved velocity fields are obtained experimentally using two-component particle image velocimetry measurements at reduced frequencies of $k = fc/U$ = 0.06–0.16 (where f is oscillation frequency, $c$ is chord length and $U$ is free stream velocity) with heaving and pitching amplitudes fixed at $h_0$ = 0.6$c$ and $\theta_0$=75$^{\circ}$∘, respectively. The concept of moment-arm dilemma associated with the impulse equation is revisited to shed-light on its physical impact on the calculated forces. It is shown that by selecting an objectively defined origin of the moment-arm, the finite-domain impulse force formulation can be greatly reduced to two terms that have a clear physical interpretation: (i) the time rate of change of the impulse of vortical structures within the control volume and (ii) the Kutta-Joukowski force that indirectly captures the contributions of vortical structures outside of the control volume. Furthermore, it is shown that for the reduced-form of the impulse equation to be valid, a critical distance of 0.85$c$ or greater from the airfoil trailing edge to the downstream control volume boundary is required. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H09.00011: Investigation of compressibility effects on a plunging airfoil under dynamic stall conditions Renato Fuzaro Miotto, Datta Gaitonde, William Roberto Wolf, Miguel Visbal Large-eddy simulations are performed to study the compressibility effects on an airfoil under deep dynamic stall condition. In the present work, an SD7003 airfoil in plunging motion is considered at a chord Reynolds number of Re = 60,000 and freestream Mach numbers M = 0.1 and 0.4. These conditions aim to meet the current renewed interest in low- and moderate-Reynolds number unsteady aerodynamics, which finds applications in the design of small unmanned air vehicles and micro air vehicles. The current numerical methodology has already been validated for M = 0.1 and is herein extended for other flow regimes. In addition, modal decomposition techniques are also employed to analyze the complex off-surface flow structures associated with the dynamic stall, and to assess its onset mechanism for different compressible scenarios. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H09.00012: Blunt nose airfoils for improved stall performance and transonic aerodynamics Yuxin Zhang, Matthew Kraljic, Zvi Rusak We numerically study the aerodynamic performance of blunter than classical airfoils in terms of both the delay of critical Mach number for transonic shock waves to higher values and the increase of stall angle of attack at low subsonic speeds. The approach is motivated by the optimal critical airfoils developed by Schwendeman et al. (ZAMP 1993) and the recent results on the delay of stall by Kraljic \& Rusak (PRFluids 2019). The nose of the airfoils is given by a canonic shape $y=(ax)^{1/a}$ where $a \ge 2$. The effect of changing the airfoil’s nose parameter $a$, thickness ratio $\delta$, and location of maximum thickness $x_t/c$ from leading edge on the wave drag and stall angle of attack are determined. Increasing $a$ and decreasing $x_t$ at a fixed $\delta$ help in deriving airfoil shapes with better performance at subsonic and transonic speeds for application in wings of transport jet airplanes, rotors of helicopters, and rotating turbines. [Preview Abstract] |
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