Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A6: Aerodynamics: Control Strategies |
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Chair: James Buchholz, The University of Iowa Room: B114 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A6.00001: Manipulation of Leading-Edge Vortex Evolution by Applied Suction James Buchholz, James Akkala The generation and shedding of vortices from unsteady maneuvering bodies can be characterized within a framework of vorticity transport, accounting for the effects of multiple sources and sinks of vorticity on the overall circulation of the vortex system. On a maneuvering wing, the diffusive flux of secondary vorticity from the surface is a critical contributor to the strength and dynamics of the leading-edge vortex, suggesting that flow control strategies targeting the manipulation of the secondary vorticity flux and the secondary vortex may provide an effective means of manipulating vortex development. Suction has been applied in the vicinity of the secondary vortex during the downstroke of a periodically-plunging flat-plate airfoil, and the flow evolution and aerodynamic loads are compared to the baseline case in which suction is not applied. Observation of the resulting surface pressure distribution and flow evolution suggest that the secondary flux of vorticity and the evolution of the flow field can be altered subject to appropriate position of the suction ports relative to the developing vortex structures, and at a specific temporal window in the development of the vortex. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A6.00002: Performance of active and passive control of an airfoil using CPFD Daniel Asselin, Jay Young, C.H.K. Williamson Birds and fish employ flapping motions of their wings and fins in order to produce thrust and maneuver in flight and underwater. There is considerable interest in designing aerial and submersible systems that mimic these motions for the purposes of surveillance, environmental monitoring, and search and rescue, among other applications. Flapping motions are typically composed of combined pitch and heave and can provide good thrust and efficiency (Read, et al. 2003). In this study, we examine the performance of an airfoil actuated only in the heave direction. Using a cyber-physical fluid dynamics system (Mackowski {\&} Williamson 2011, 2015, 2016), we simulate the presence of a torsion spring to enable the airfoil to undergo a passively controlled pitching motion. The addition of passive pitching combined with active heaving ("Active-Passive" or AP) provides significantly improved thrust and efficiency compared with heaving alone. In many cases, values of thrust and efficiency are comparable to or better than those obtained with two actively controlled degrees of freedom ("Active-Active" or AA). By using carefully-designed passive dynamics in the pitch direction, we can eliminate one of the two actuators, saving cost, complexity, and weight, while maintaining or improving performance. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A6.00003: Feedback Control of an Ahmed Body Flow Exhibiting Symmetry-Breaking Regimes Olga Evstafyeva, Aimee Morgans At motorway speeds two-thirds of usable engine energy of square-back vehicles is spent overcoming the aerodynamic drag. The main source of drag is the bi-stable low pressure wake which forms at the back of the body as the boundary layers separate over the rear edges of the vehicle. Identifying large coherent structures and describing the physics of the wake is, therefore, of great practical importance for understanding the sources of drag and informing drag-reduction strategies. Present work investigates numerically the flow past the Ahmed body - a commonly used test-case for a simplified vehicle geometry, at Reynolds numbers $310 |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A6.00004: Identifying Sources of Lift Production on Rapidly Pitching Trailing Edge Flaps Peter Mancini, Anya Jones, Michael Ol Recent work has delved into the design and quantification of the aerodynamic response of large trailing edge flaps. Ultimately, these flaps would be used as a control mechanism to provide an immediate aerodynamic response to the vehicle, e.g. in the event of a gust encounter. The present work explores the individual sources and contributions of lift in the case of a large, rapidly pitching trailing edge flap. The flap is 50{\%} of the chord length, and thus produces large acceleration and pitch rate terms that dominate the lift production. In the experiment and simulations presented here, the front element remains fixed at a constant angle of attack, while the rear element pitches to a final incidence angle, which in this study ranges from 5 degrees to 40 degrees. Although the front element does not pitch throughout the motion, it is important to consider the time history of the lift distribution on that wing section and assess whether the rapid pitching of the aft element affects the forces experienced on the stationary front element. These results are then used to suggest a simplified method for predicting lift production of a wing with a large trailing flap. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A6.00005: In-Flight Active Wave Cancelation with Delayed-x-LMS Control Algorithm in a Laminar Boundary Layer Bernhard Simon, Nicolo Fabbiane, Timotheus Nemitz, Shervin Bagheri, Dan Henningson, Sven Grundmann This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broad-band Tollmien-Schlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a ``black box'' system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A6.00006: Three dimensional breakdown of an impulsively forced laminar separation bubble Theodoros Michelis, Marios Kotsonis The spatio-temporal behaviour of a short laminar separation bubble is investigated experimentally. The bubble develops on a flat plate driven by an adverse pressure gradient wall at Reynolds number based on displacement thickness at separation of $Re_{\delta^*_s}=975$. The boundary layer is impulsively forced by means of AC dielectric barrier discharge plasma actuator located upstream of the separation point. The full four-dimensional flow development is captured by time resolved tomographic PIV measurements using the multi-pass light amplification technique. Immediately after forcing, a convectively unstable wave packet emerges due to selective amplification of modes which interacts with the reattachment process. The interaction becomes non-linear at the reattachment region, where $\Lambda$ structures typical of laminar separation bubbles are captured before the occurrence of breakdown. The structures and breakdown are characterised in terms of temporal evolution, spanwise coherence and energy budget. The diminishing of $\Lambda$ structures triggers a sharp reduction in size of the separation bubble by interfering with the natural shedding process. As a result, the bubble significantly elongates without shedding undergoing bursting before recovering to its unperturbed state. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A6.00007: Aerodynamic Impact of an Aft-Facing Slat-Step on High Re Airfoils Geoffrey Kibble, Chris Petrin, Jamey Jacob, Brian Elbing, Peter Ireland, Buddy Black Typically, the initial aerodynamic design and subsequent testing and simulation of an aircraft wing assumes an ideal wing surface without imperfections. In reality, however the surface of an in-service aircraft wing rarely matches the surface characteristics of the test wings used during the conceptual design phase and certification process. This disconnect is usually deemed negligible or overlooked entirely. Specifically, many aircraft incorporate a leading edge slat; however, the mating between the slat and the top surface of the wing is not perfectly flush and creates a small aft-facing step behind the slat. In some cases, the slat can create a step as large as one millimeter tall, which is entirely submerged within the boundary layer. This abrupt change in geometry creates a span-wise vortex behind the step and in transonic flow causes a shock to form near the leading edge. This study investigates both experimentally and computationally the implications of an aft-facing slat-step on an aircraft wing and is compared to the ideal wing surface for subsonic and transonic flow conditions. The results of this study are useful for design of flow control modifications for aircraft currently in service and important for improving the next generation of aircraft wings. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A6.00008: Control of a flexible, surface-piercing hydrofoil for high-speed, small-scale applications. Gabriel Bousquet, Michael Triantafyllou, Jean-Jacques Slotine In recent years, hydrofoils have become ubiquitous in the design of high performance surface vehicles such as sailboats. They have proven particularly useful at small scales: while the speed of displacement-hull sailboats of length $L$ is limited by their hull speed $\propto \sqrt{gL}$, due to wave making resistance, such limitations do not apply to hydrofoil crafts and sailboats. Such crafts of length O(1 - 10 m) are capable of reaching speeds in excess of 45 kts, often far faster than the wind. Besides, in the quest for super-maneuverability, actuated hydrofoils enable the efficient generation and control of large forces. With the intent to ultimately enable the design of small-scale, high-speed, and super-maneuverable surface vehicles, we investigate the problem of controlling the lift force generated by a flexible, surface-piercing hydrofoil traveling at high speed through a random wave field. We design a test platform composed of a rudder-like vertical foil, which is actuated in pitch, and instrumented with velocity, force, and immersion sensors. We present a feedback linearization controller, designed to operate over a wide range of velocities and sea states. Validation experiments are carried out on-the-field at speeds ranging from 3 to 10+m/s. [Preview Abstract] |
Sunday, November 20, 2016 9:44AM - 9:57AM |
A6.00009: Tomographic-PIV and TSP for Airfoil Flow-Control Studies Adam Stolt, John Otte, Jesstin Krech, Jordi Estevadeordal Airfoil blades can experience a significant change of angle of attack during operation cycles that can lead to boundary layer separation and dynamic stall. It is unclear how elements distributed at the leading edge would affect the aerodynamic performance, boundary layer separation and transition, and stall behaviors. In the present study, various passive flow control structures, such as distributed dimples and bumps have been investigated and compared to airfoil geometries including the baseline smooth NACA0015 airfoil. Along with standard particle image velocimetry (PIV), a curved-laser sheet PIV, tomographic PIV, and Temperature Sensitive Paint (TSP) techniques have been combined to reveal spanwise flow information in the curved surface of the airfoil. Results show the effects and induced flow patterns of the various elements on boundary layer separation and stall at various angles of attack and compare them with the smooth models. [Preview Abstract] |
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