Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A16: Flow Control: Passive IControl
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Chair: Humberto Bocanegra Evans, Texas Tech University Room: 603 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A16.00001: Development of a Bio-inspired Microflap Array for Passive Control of Flow Separation Sean Devey, Jackson Morris, Paul Hubner, Amy Lang The shortfin mako shark benefits from its flexible microscopic scales, or denticles; which can passively limit flow separation in water. These denticles can be passively actuated by incipient reversing flow in the lower 5{\%} of the boundary layer, thereby impeding further flow reversal and promoting increased momentum exchange. In air, an array of flow actuated microflaps has the potential to provide similar benefits to man-made systems. Multiple iterations of microflap arrays have been developed and tested in the University of Alabama's Boundary Layer Tunnel. A variety of 3D-printed flaps derived from mako denticle geometries were arranged in rows with freedom to rotate, like mako denticles, to angles up to 50 degrees. Placing the microflap array in separated flow regions allowed for direct observation of the microflap response. Like mako denticles, microflaps with lengths of about 4 mm have been shown to actuate in response to reversing surface flows. This presentation will focus on the development and implementation of passive microflap arrays. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A16.00002: Characterization of Passive Flow-Actuated Microflaps Inspired by Shark Skin for Separation Control Jackson Morris, Sean Devey, Amy Lang, Paul Hubner Thanks to millions of years of natural selection, sharks have evolved into quick apex predators. Previous research has proven shark skin to reduce flow separation, which would result in lower pressure drag. Mako shark skin is made up of microscopic scales on the order of 0.2 mm in size. These scales are hypothesized to be a flow control mechanism, capable of being passively actuated by reversed flow. We believe shark scales are strategically sized to interact with the lower 5 percent of the boundary layer, where reversed flow occurs near the wall. Previous wind tunnel research has shown that it is possible to passively actuate 2D flaps in the lower regions of the boundary layer. This research aims to identify reverse flow conditions that will cause small 3D flaps to actuate. Several sets of microflaps (about 4 mm in length) geometrically similar to shark scales were 3D printed. These microflaps were tested in a low-speed wind tunnel in various reverse flow conditions. Microflaps were observed to be actuated by the reversing flow and flow conditions were characterized using a hot-wire probe. These microflaps have the potential to mimic the mako shark type of flow control in air, passively actuated by reverse flow conditions. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A16.00003: Experimental Study of Unsteady Flow Separation in a Laminar Boundary Layer Andrew Bonacci, Amy Lang, Redha Wahidi, Leonardo Santos Flow separation, caused by an adverse pressure gradient, is a major problem in many applications. Reversing flow near the wall is the first sign of incipient separation and can bristle shark scales which may be linked to a passive, flow actuated separation control mechanism. An investigation of how this backflow forms and how it interacts with shark skin is of interest due to the fact that this could be used as a bioinspired means of initiating flow control. A water tunnel experiment aims to study unsteady separation with a focus on the reversing flow development near the wall within a flat plate laminar boundary layer (Re on order of 10\textasciicircum 5) as an increasing adverse pressure gradient is induced by a rotating cylinder. Unsteady reversing flow development is documented using DPIV. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A16.00004: Low speed streak formation in a separating turbulent boundary layer Leonardo Santos, Amy Lang, Redha Wahidi, Andrew Bonacci Separation control mechanisms present on the skin of the shortfin mako shark may permit higher swimming speeds. The morphology of the scales varies over the entire body, with maximum scale flexibility found on the flank region with an adverse pressure gradient(APG). It is hypothesized that reversing flow close the skin bristles the scales inhibiting further flow reversal and controlling flow separation. Experiments are conducted in water tunnel facility and the flow field of a separating turbulent boundary layer(TBL) is measured using DPIV and Insight V3V. Flow separation is induced by a rotating cylinder which generates a controlled APG over a flat plate (Re $=$ 510000 and 620000). Specifically, the low speed streak(LSS) formation is documented and matches predicted sizing based on viscous length scale calculations. It is surmised that shark scale width corresponds to this LSS sizing for real swimming TBL conditions. However, flow separation control has been demonstrated over real skin specimens under much lower speed conditions which indicates the mechanism is fairly Re independent if multiple scales are bristled as the width of the LSS increases. The formation of reversing flow within the streaks is studied specifically to better understand the process by which this flow initiates scale bristling on shortfin mako skin as a passive, flow actuated separation control mechanism. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A16.00005: Passive Flap Actuation by Reversing Flow in Laminar Boundary Layer Separation Chase Parsons, Amy Lang, Leo Santos, Andrew Bonacci Reducing the flow separation is of great interest in the field of fluid mechanics in order to reduce drag and improve the overall efficiency of aircraft. This project seeks to investigate passive flow control using shark inspired microflaps in laminar boundary layer separation. This study aims to show that whether a flow is laminar or turbulent, laminar and 2D or turbulent and 3D, microflaps actuated by reversing flow is a robust means of controlling flow separation. In order to generate a controlled adverse pressure gradient, a rotating cylinder induces separation at a chosen location on a flat plate boundary layer with Re above 10000. Within this thick boundary layer, digital particle image velocimetry is used to map the flow. This research can be used in the future to better understand the nature of the bristling shark scales and its ability to passively control separation. Results show that microflaps successfully actuated due to backflow and that this altered the formation of flow separation. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A16.00006: Engineered bio-inspired coating for reduction of flow separation Humberto Bocanegra Evans, Ali M. Hamed, Serdar Gorumlu, Ali Doosttalab, Burak Aksak, Leonardo P. Chamorro, Luciano Castillo Flow control using passive strategies has received notable attention in the last decades as a way to increase mixing and reduce skin drag, among others. Here, we present a bio-inspired coating, composed by uniformly distributed pillars with diverging tips, that is able to reduce the recirculation region in highly separated flows. This is demonstrated with laboratory experiments in a refractive index-matching flume at Reynolds number Re$_\theta \approx 1200$. The flow over an expanding channel following a S835 wing section was characterized with the coating and with smooth walls. High-resolution, wall-normal particle image velocimetry show a significant reduction of the reversed flow with the coating, where the region with reverse flow was reduced by $\approx 60\%$. The performance of the micro-scale coating is surprising since the size of the fibers are nearly coincident with the viscous length scale ($k^+ \approx 1$). Additionally, the flow control properties of the surface do not depend on hydrophobicity, giving the coating the capability to work in both air and water media. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A16.00007: Flow around an autonomous underwater vehicle with bio-inspired coating Scott Watkins, Jose Montoya-Segnini, Humberto Bocanegra Evans, Oscar Curet, Serdar Gorumlu, Burak Aksak, Amirkhosro Kazemi, Leonardo Chamorro, Luciano Castillo Flow separation plays a major factor in the form drag of a moving object. In particular, suppressing or reducing flow separation is critical in the energy expenditure of autonomous underwater vehicles. Previous research suggests that bio-inspired micro-fibrillar structures are capable of reducing the boundary layer separation in a turbulent flow. Here, we present laboratory measurements using PIV near the wall and in the wake of two submersible vessel models; one had a coating composed of ordered fibers, and the other had smooth walls. Flow characterization with planar PIV included the presence or absence of a tail fin at multiple angles of attack of the vessels. Preliminary results reveal changes of the flow in the wake of the vessel with coating resulting in lower or similar velocity deficit in the wake compared to the smooth vessel. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A16.00008: Direct Numerical Simulations of Aerofoils with Serrated Trailing-Edge Extensions Muhammad Farrukh Shahab, Mohammad Omidyeganeh, Alfredo Pinelli Owl-feather-inspired technology motivates engineers to develop quieter wings. Direct numerical simulations of NACA-4412 aerofoil with retrofitted flat plate, serrated sawtooth shaped and porous (serrations with filaments) extensions have been performed to study the effects of these modifications on the hydrodynamic characteristics of the turbulent wake and their upstream influence on the interacting boundary layer. A chord based Reynolds number of 100,000 and an angle of attack of 5$^{\circ}$ has been chosen for all simulations, moreover the surface boundary layers are tripped using a a volume forcing method. This contribution will present a detailed statistical analysis of the mean and fluctuating behaviour of the flow and the key differences in the flow topologies will be highlighted. . The preliminary analysis of results identifies a system of counter rotating streamwise vortices for the case of saw-tooth shaped serrations. The presence of the latter is generally considered responsible for an increased parasitic higher frequency noise for serrated aerofoils. To palliate the effect of aforementioned system of streamwise vortices, a filamentous layer occupying the voids of the serrations has been added which is expected to improve the aeroacoustic performance of the system. [Preview Abstract] |
Sunday, November 19, 2017 9:44AM - 9:57AM |
A16.00009: Direct numerical simulation of flow over riblets with the yaw angle Jonghwan Park, Haecheon Choi Direct numerical simulations of turbulent channel flow with riblets with various yaw angles ($\alpha )$ are performed to investigate the effect of the yaw angle on the flow and drag characteristics. The results show that the performance of drag reduction by the riblets is degraded with increasing yaw angle, but drag reduction is still achieved up to the yaw angle of about 20°, which is consistent with previous experimental results. As the yaw angle increases, stronger near-wall vortical structures are generated above riblet surfaces, and the form drag rapidly increases. To estimate the critical yaw angle at which drag reduction no longer occurs, we investigate the relationship between the drag and yaw angle. A scaling analysis is conducted to find the relationship between the form drag and yaw angle, and FIK identity is used to analyze the skin friction drag. We show that the variation of the drag force on riblets is proportional to the square of sin $\alpha $. [Preview Abstract] |
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