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 P09: Aerodynamics: Vortex Generators |
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Chair: Robert Breidenthal, University of Washington Room: 213 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P09.00001: Noisy Inflow Advances Jet-Switching in the Wake of a Flapping Airfoil Dipanjan Majumdar, Chandan Bose, Sunetra Sarkar The present work is focused on investigating the effect of noisy input fluctuations on a harmonically plunging elliptic foil in the low Reynolds number regime ($Re = 300$). Simulations are carried out using a discrete forcing Immersed Boundary Method (IBM) based in-house Navier-Stokes solver. The stochastic noisy inlet velocity is modeled as the Ornstein-Uhlenbeck process of chosen correlation length which governs the time-scales present in the velocity spectra. The unsteady flow-field is simulated at different dynamic plunge velocities ($kh$). A thrust producing reverse K\'arm\'an wake is observed at $kh = 1.0$ under steady uniform inflow. As $kh$ increases, at $kh = 1.5$, the reverse K\'arm\'an wake is seen to oscillate alternatively in the upward and downward directions with a small deflection angle, giving the notion of jet switching. However, in the presence of noisy inflow, the organized pattern of the reverse K\'arm\'an wake at $kh = 1.0$ gets distorted as the distances between the vortex cores get altered in a random fashion. At $kh = 1.5$, the onset of jet switching in the trailing wake gets advanced in the presence of incoming gust. Also, the deflection angle and the jet switching frequency are seen to increase compared to the case of the steady uniform inflow. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P09.00002: Unsteady flow structure response to acceleration of non-slender swept wings Han Tu, Matthew Marzanek, David Rival, Melissa Green Previous experiments studying steady translation and axially accelerating translation of a triangular planform wing have shown that at high angles of attack, an axial gust or acceleration can induce flow reattachment. Finite-time Lyapunov exponent (FTLE) analysis reveals spanwise differences of the flow structure around a 45$^{\circ}$ sweep delta wing. Flow reattachment onsets inboard while the flow remains separated from the leading edge further outboard. Under certain axial acceleration magnitudes, a long whip-like nFTLE ridge lifting off the wing surface can be observed. The root of the whip-like structure moves toward the trailing edge and it closely follows a local chordwise pressure peak. It can be inferred that the nFTLE ridges' roots across the span form a line that moves along with the high pressure zone in response to the axial gust. Following this region downstream along the chord is a region of low pressure, consistent with lift being re-established on the wing as the flow reattaches. Vertical accelerations will be studied and combined with axial acceleration cases to obtain further insight of flow structure evolution in highly unsteady environments for non-slender delta wings. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P09.00003: Large Eddy Simulations of controlled aircraft in formation relying on wake sensing. Ignace Ransquin, Denis-Gabriel Caprace, Philippe Chatelain, Jeffrey Eldredge Aircraft formation flight leads to substantial improvements in energetic efficiency even for large separations. Maintaining an energy-saving formation requires the estimation of the preceding aircraft's wake position, and implies a sensing strategy. We propose to leverage the measurements of the six degrees-of-freedom dynamics of the follower aircraft. This study combines the simulation of the aircraft dynamics and the LES of the wakes by means of a Vortex Particle-Mesh method. In a 2-ship formation, the aerodynamics and vorticity sources are modeled using an immersed lifting line approach. The follower aircraft operates an autopilot in the form of a hierarchy of controllers that govern the ailerons, rudder, elevator and thrust in order to achieve wake sensing and tracking. The robustness of the autopilot is verified with the LES of a single aircraft in turbulent flow. Then the influence of a leader's wake on a follower's dynamics is studied through the analysis of the related aerodynamic forces and the resulting wake. Finally, based on an Ensemble Kalman Filter, the position of the leader's wake is estimated and used as the target of the autopilot to maintain the follower in the optimal position. The efficiency of the tracking procedure is analyzed and shows promising results. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P09.00004: LES Exploration of the Near and Far Wake of Wings using a Novel Lifting and Dragging Line Model Denis-Gabriel Caprace, Gregoire Winckelmans, Philippe Chatelain Several wing wakes are investigated by means of a hybrid Vortex Particle-Mesh-based LES flow solver, elected for its ability to capture wake dynamics with minimal spurious dispersion and diffusion. The broad spectrum of scales involved also entails the use of a novel Immersed Lifting and Dragging Line model to shed the vorticity from each wing. Advantageously, this technique waives the need for mollification in the spanwise direction, while remaining compatible with the relaxed CFL condition of vortex methods. Statistics of axial vorticity, velocity deficit, turbulence and circulation distribution are evaluated at different locations over 30 wing spans, in order to shed light on the transition from a coherent near wake to a turbulent vortex system at equilibrium in the far wake. The role of parasitic drag in such transition is also assessed, by comparing configurations with none, moderate and high profile drag. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P09.00005: Effect of Vortex Interaction in Delta Winglet Type Vortex Generators with Slots. Jer-Yo Lee, Giovanni Nino, Robert Breidenthal Vortex generators (VGs) can delay boundary layer separation, generate swirl, and cause flow destabilization in aerodynamic applications. VGs can also be used to enhance convective heat transfer in thermal systems. In the past, different approaches have been brought forward to increase vortex strength by using the interactions between multiple VG arrays or adjusting the VG parameters as its height and angle of attack. In this paper, experiments were performed on delta-shaped VGs in a miniature wind tunnel setup. One set of VGs had a slot across its spanwise direction, while another set of solid VGs was used a baseline for data comparison. The purpose of the novel slot VG design is to exploit the interaction between the primary vortex, induced by the VG at its leading edge, and the secondary vortex generated by the air passing through the slot. Results of different VG configurations such as different VG's angle of attack respect to the air flow and different slot topologies were compared and discussed. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P09.00006: Interaction of the Leading Edge Vortex and Shear Layer Vortices for an Airfoil Undergoing Dynamic Stall Douglas Bohl, Melissa Green 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 angles of \textpm 40\textdegree , \textpm 30\textdegree , and \textpm 20\textdegree at reduced frequencies of $k=$0.2-0.6 and Re$_{\mathrm{c}}=$12000. In all cases a strong leading edge vortex (LEV) is formed with 2-3 weaker shear layer vortices (SLV) also forming along the airfoil surface. In some cases the LEV and the nearest SLV combine. In other cases SLV's combine. And finally in some cases the vortices remain independent of each other. The data suggest the formation a topological saddle forms between the two vortices that correlates with events in which the two vortices combine. The saddle appears below the center of the weaker vortex and moves relative to the vortices as they interact. The vortices are found to combine when the saddle moves above the center of the weaker vortex. The vortices remain distinct when no saddle appears. The data show that the interaction of the LEV and its nearest SLV occurs only for a limited range of the parameter space investigated. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P09.00007: Wake transitions of flexible foils in a viscous uniform flow Min Je Kim, Jae Hwa Lee We perform numerical simulations for two types of rigid and flexible thin foils in a viscous uniform flow to explore the effect of flexibility on wake structures. The thin foils are prescribed by the heaving oscillation motions and the relevant non-dimensional parameters are the chord length based Strouhal number and flapping amplitude. When the dynamical features of the flow wakes are varied with respect to the two parameters, it is possible to make a direct comparison between a rigid and flexible thin foils. The wake transition boundaries of the rigid thin foil are predicted by constant amplitude based Strouhal number lines, consistent with previous studies. However, contrary to the observation from the rigid thin foil, the wake transition boundaries of the flexible thin foil are not predictable by constant amplitude based Strouhal number lines. We find that the sum of the leading and trailing edge circulations plays an important role to determine a wake pattern behind a rigid and flexible foil, and wake transitions are observed beyond critical circulations. -/abstract- This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000537) and the Ministry of Science, ICT {\&} Future Planning (NRF-2017R1A [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P09.00008: The Lift of a Translating Plate with Tip Sweep and Rotation Juhi Chowdhury, Cameron Smith, Matthew Ringuette We conduct towing-tank experiments to study the unsteady lift force produced by a high-angle-of-attack translating wing with a tip panel that rotates inward or outward in the wing plane. The goal is to examine whether combined outboard sweep and rotation can influence the lift for two types of motions: a starting flow and positive/negative 1/2-sine streamwise gusts. This work could provide a method for drones to temporarily boost lift during high-angle-of-attack maneuvers and reduce lift perturbations from streamwise gusts. For the starting flow, our prior flow visualization indicates that early outward panel motion produces a new swept-edge vortex (SEV) and stretches the tip vortex (TV) and trailing-edge vortex outboard, which should enhance lift. For a gust-like forward surge, inward panel motion prior to the gust sheds the panel SEV and TV, and promotes main-wing leading-edge vortex shedding, which should decrease the lift as desired. To test the effect of the modified flow on the force, direct lift measurements are made via a transducer. The influence of varying the main-wing aspect ratio is also examined. The panel actuation timing compared to the starting and gust motions is shown to be important for the lift performance. [Preview Abstract] |
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