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 E7: Separation Flows in Aero and HydroDynamics |
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Chair: Anya Jones, University of Maryland Room: B115 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E7.00001: Adaptation of the Leishman-Beddoes Dynamic Stall Model for Reverse Flow Andrew Lind, Anya Jones The Leishman-Beddoes dynamic stall model has long been used for the prediction of unsteady airloads acting on rotorcraft and wind turbines. However, little work has been completed that attempts to model the unsteady airloads experienced by a blade in the reverse flow region of a high advance ratio rotor. The present work describes modifications to the Leishman-Beddoes model and evaluates its suitability for the prediction of unsteady airloads for a sinusoidally oscillating NACA 0012 in reverse flow. Specifically, the ability of the model to capture early dynamic stall vortex formation (due to the sharp aerodynamic leading edge) and delayed reattachment is assessed. Results from the modified Leishman-Beddoes model are compared to measured unsteady pressure distributions for reduced frequencies up to 0.511 and a maximum pitch angle of 25 degrees. The model is also evaluated against numerical simulations of reverse flow dynamic stall where complete pressures distributions (and thus unsteady airloads) are available. This work is foundational for the development of more complex low-order models of the reverse flow region of a high advance ratio rotor where the time-varying local freestream and spanwise flow are also expected to play an important role. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E7.00002: Experimental Investigation of a Yawed Airfoil in Reverse Flow Dynamic Stall Luke Smith, Dr. Andrew Lind, Dr. Anya Jones When a rotating blade enters high advance ratio flight, a significant portion of the blade is subject to reverse flow, where flow travels from the blade`s geometric trailing edge to the geometric leading edge. The purpose of this work is to determine the influence of spanwise flow on a blade undergoing dynamic stall in reverse flow. Without spanwise flow, an oscillating sharp trailing edge airfoil in reverse flow experiences separation about its sharp aerodynamic leading edge, leading to the formation of a dynamic stall vortex at low angles of attack. With spanwise flow, an airfoil experiences a delay in lift stall, possibly due to the convection of a vortex along the freestream. This work characterizes the three-dimensional flow field of an oscillating airfoil at static yaw angles in reverse flow. Time-resolved velocity fields and chordwise pressure distributions are presented for several span locations, reduced frequencies, and Reynolds numbers. The unsteady velocity fields allow for the identification of dynamic stall vortex locations, and the unsteady pressure distributions allow for the analysis of spanwise variation in aerodynamic forces. By comparing the yawed and un-yawed cases, this work illustrates the relative importance of spanwise flow in reverse flow dynamic stall. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E7.00003: Formation mechanisms of rapid pressure recovery around a laminar separation bubble Donghwi Lee, Taku Nonomura, Akira Oyama, Kozo Fujii Large-eddy simulations around $5\%$ thickness flat plate are conducted at $Re_{c}=5,000, 8,000$ and $20,000$ and formation mechanisms of rapid pressure recovery in the surface pressure distribution around laminar separation bubbles are analyzed. Three analyses are applied to investigate the mechanisms of rapid pressure recovery. First, by using the Reynolds averaged streamwise pressure gradient equation, it is confirmed that the "overall Reynolds stress diffusion (ORSD)" is an important factor for inducing rapid pressure recovery. Second, we decompose the ORSD into the "normal Reynolds stress diffusion in the streamwise direction" and the "tangential Reynolds stress diffusion (TRSD) in the wall-normal direction". We show that the TRSD in the wall-normal direction, which corresponds to the momentum transfer in the same direction, is the main contributor to the ORSD. Third, the TRSD in the wall-normal direction is decomposed into two- and three-dimensional components. The results indicate that the rapid pressure recovery is strongly affected by the presence of Reynolds stress rather than by the type of physical phenomena that creates the Reynolds stress. In other words, the three-dimensional turbulent structures are not a necessary condition for the rapid pressure recovery. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E7.00004: Experimental Study of Unsteady Separation in a Laminar Boundary Layer Andrew Bonacci, Amy Lang, Redha Wahidi, Leo Santos Separation, caused by an adverse pressure gradient, can be a major problem to aircraft. Reversing flow occurs in separated regions and an investigation of how this backflow forms is of interest due to the fact that this could be used as a means of initiating flow control. Specifically, backflow can bristle shark scales which may be linked to a passive, flow actuated separation control mechanism. An experiment was conducted in a water tunnel to replicate separation, with a focus on the reversing flow development near the wall within a laminar boundary layer. Using a rotating cylinder, an adverse pressure gradient was induced creating a separated region over a flat plate. In this experiment the boundary layer grows to sizes great enough that the scale of the flow is increased, making it more measurable to DPIV. In the future, this research can be utilized to better understand flow control mechanisms such as those enabled by shark skin. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E7.00005: The actuation of microflaps inspired by shark scales deeply embedded in a boundary layer Jackson Morris, Amy Lang, Paul Hubner Thanks to millions of years of natural selection, sharks have evolved to become quick apex predators. Shark skin is made up of microscopic scales on the order of 0.2 mm in size. This array of scales is hypothesized to be a flow control mechanism where individual scales are capable of being passively actuated by reversed flow in water due to their preferential orientation to attached flow. Previous research has proven shark skin to reduce flow separation in water, which would result in lower pressure drag. We believe shark scales are strategically sized to interact with the lower 5 percent of the boundary layer, where reversed flow occurs close to the wall. To test the capability of micro-flaps to be actuated in air various sets of flaps, inspired by shark scale geometry, were rapidly prototyped. These microflaps were tested in a low-speed wind tunnel at various flow speeds and boundary layer thicknesses. Boundary layer flow conditions were measured using a hot-wire probe and microflap actuation was observed. Microflap actuation in airflow would mean that this bio-inspired separation control mechanism found on shark skin has potential application for aircraft. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E7.00006: Reversing flow development in a separating turbulent boundary layer Leonardo Santos, Amy Lang, Redha Wahidi, Andrew Bonacci Fast swimming sharks have micro-structures on their skin consisting of bristling scales. These scales are hypothesized to~bristle~in response to backflow~generated from the separated turbulent boundary layer (TBL) in regions of adverse pressure gradient (APG) on the shark body. Vortices are trapped in the~cavities~between the scales, which induce momentum exchange between the higher momentum fluid in the outer flow and that in the separated region. This momentum exchange causes reattachment of the separated TBL, causing the scales to return to the unbristled~location, and the cycle continues. The rows of scales have widths that are~comparable~to the spanwise length scale of the intermittent backflow patches~that appear in the region of incipient detachment of TBLs. In this~experimental~investigation, correlations between the shark scale's width and the spanwise size of the low backflow streaks are examined, as well as details of the incipient detachment region. The~experiments~are conducted in a water tunnel facility and the flow field is measured using PIV. Turbulent boundary layers are subjected to an APG via a rotating cylinder. Separated TBLs are investigated on a flat plate. [Preview Abstract] |
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