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 E15: Aerodynamics: Unsteady Airfoil and Wing |
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Chair: Ashok Gopalarathnam, North Carolina State University Room: 203 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E15.00001: Modeling intermittent leading-edge vortex shedding in unsteady airfoil flows with reduced-count discrete vortices Ashok Gopalarathnam, Kiran Ramesh, Arun Vishnu Suresh Babu A discrete-vortex method for unsteady airfoil flows with intermittent leading-edge vortex (LEV) shedding was proposed by Ramesh et al (JFM, 2014). Two novelties were introduced: (i) LEV shedding is initiated using discrete vortices whenever the Leading Edge Suction Parameter (LESP), which is a measure of leading-edge suction, exceeds a critical value, and (ii) the strength of the discrete vortices is determined such that the LESP maintained at the critical value during the shedding process. Although results from this low-order method agree with CFD and experiments, the increasing vortex count with time increases the computational cost. The large number of shed vortices from the TE can be reduced through traditional techniques such as amalgamation and deletion, as they typically convect away from the airfoil and interact only weakly with the airfoil vorticity. The LEV, on the other hand, interacts strongly with the airfoil, and has a large influence on the forces. An approach to reduce the vortex count is desired. Inspired by Wang and Eldredge (TCFD, 2013), we propose a model that has just a single vortex to model an active LEV. The varying strength of this free vortex is determined using our LESP criterion. Results from the method for unsteady airfoil motions are promising. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E15.00002: Assessment of Control Volume Estimation of Thrust for a Sinusoidally Pitching Airfoil at Low Reynolds Number Patrick Hammer, Ahmed Naguib, Manoochehr Koochesfahani The proper estimation of thrust is very important for understanding the aerodynamics of oscillating airfoils at low chord Reynolds number Re. Although direct force measurement is possible, force values at low Re are often small, and separation of the test-model’s inertia forces from the data may not be straightforward. A common alternative is a control-volume (CV) approach, where terms in the integral momentum equation are computed from measured wake velocity profiles. Although it is acceptable to use only the mean streamwise-velocity profile in estimating the streamwise force on stationary airfoils, recent work has highlighted the importance of terms relating the velocity fluctuation and pressure distribution in the wake for unsteady airfoils. The goal of the present work is to capitalize on 2D computational data for a harmonically pitching airfoil at Re in the range 2,000-22,000, where all terms in the momentum-integral equation are accessible, to evaluate the importance of the various terms in the equation and assess the accuracy of the assumptions that are typically made in experiments due to the difficulty in measuring certain terms (such as the wake pressure distribution) by comparing the CV results with the actual computed thrust. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E15.00003: Analytic State Space Model for an Unsteady Finite-Span Wing Jacob Izraelevitz, Qiang Zhu, Michael Triantafyllou Real-time control of unsteady flows, such as force control in flapping wings, requires simple wake models that easily translate into robust control designs. We analytically derive a state-space model for the unsteady trailing vortex system behind a finite aspect-ratio flapping wing. Contrary to prior models, the downwash and lift distributions over the span can be arbitrary, including tip effects. The wake vorticity is assumed to be a fully unsteady distribution, with the exception of quasi-steady (no rollup) geometry. Each discretization along the span has one to four states to represent the local unsteady wake-induced downwash, lift, and circulation. The model supports independently time-varying velocity, heave, and twist along the span. We validate this state-space model through comparison with existing analytic solutions for elliptic wings and an unsteady inviscid panel method. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E15.00004: The Development of the Vorticity Field Downstream of a NACA0012 Airfoil Undergoing Small Amplitude Sinusoidal Oscillation Colin Stutz, Patrick Hammer, Douglas Bohl, Manoochehr Koochesfahani Symmetric small amplitude oscillation of an airfoil produces a semi-infinite array of alternating sign vortices. Under some conditions single vortices of alternating sign are produced for each cycle, whereas under other conditions multiple vortices of each sign are produced. This work investigates a reduced frequency range, k$=$3.5-5.1, for which at low k (3.5-4.5) two vortices of each sign are produced, at higher k (5.0) the two vortices pair, and finally at the highest k (5.1) a single vortex of each sign is produced. The work utilizes highly resolved (spatially and temporally) computations for $\alpha_{\mathrm{max}}=$2$^{\circ}$ and Re$=$12000. For low k's the second vortex has the higher vorticity of the two and the two vortices spread vertically away from each other. As the reduced frequency increases the magnitude of the peak vorticity equalizes between the two structures and they move towards each other as the downsteam distance is increased. At the highest reduced frequencies these two structures merge downstream for form a single structure in the more typical von Karman vortex array. The range of reduced frequencies over which the flow transitions from multi-structure, and spreading in nature, to that of single structures is narrow, k$\approx $4.5 to 5.1. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E15.00005: Evolution and Control of the Leading Edge Vortex on an Unsteady Wing James Akkala, James Buchholz The development of the leading-edge vortex is investigated on a periodically plunging plate within a uniform free stream. Vortex circulation is governed primarily by the strength of the leading edge shear layer, which provides the primary source of circulation, and a substantial opposite-sign contribution due to the pressure-gradient-driven diffusive flux of vorticity from the suction surface of the plate. The latter has been shown to produce a substantial reduction in leading-edge vortex strength, and leads to the development of a secondary vortex whose evolution influences the interaction between the leading edge vortex and the surface, and thus alters the surface pressure gradients. Suction is applied in the vicinity of the secondary vortex in an attempt to regulate the aerodynamic loads in the presence of the leading-edge vortex. The effect on vorticity transport, leading-edge vortex dynamics, and the resulting aerodynamic loads is discussed. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E15.00006: Streamwise Oscillation of Airfoils into Reverse Flow Kenneth Granlund, Anya Jones, Michael Ol An airfoil in freestream is oscillated in streamwise direction to cyclically enter reverse flow. Measured lift is compared to analytical blade element theories. Advance ratio, reduced frequency and angle of attack is varied within those typical for helicopters. Experimental results reveal that lift does not become negative in the flow reversal part, contradicting one theory and supported by another. Flow visualization reveal the leading edge vortex advecting against the freestream to a point in front of the leading edge. [Preview Abstract] |
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