Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D15: Flow Control: Shark Skin and Surface Modifications |
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Chair: Amy Lang, The University of Alabama Room: Georgia World Congress Center B302 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D15.00001: Passive Flow-actuated Control of Turbulent Boundary Layer Separation by Shortfin Mako Shark Skin Specimens Leonardo Santos, Amy W Lang, Andrew Bonacci, Jacob Parsons The speedy shortfin mako has flexible scales located on key locations, most specifically the flank region from which specimens were obtained. The scales in this region can reach angles of 50° and their bristling capability by reversing flow has been documented. It is hypothesized that the impedance of reversing flow close to the skin by the scales is the primary mechanism by which flow separation is controlled by this passive, flow-actuated mechanism. Experiments were conducted in a water tunnel facility and the flow field of a separating turbulent boundary layer is measured using DPIV. A controllable adverse pressure gradient (APG) was induced by a rotating cylinder over a flat plate (range of Re = 5.2 xto 8.8 x ). Shark skin specimens (SSS) were placed in two different locations (at the beginning and inside) of the separated region to quantify the separation control capability using backflow coefficient. It is surmised that the shark scales are sized according to the low speed streak (LSS) formation occurring for real swimming turbulent boundary layer conditions. Measurement of the LSS before separation matches the predicted size based on viscous length scale calculations with and without the SSS. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D15.00002: Experimental Study of Unsteady Separation Control in a Laminar Boundary Layer by Shortfin Mako Shark Skin Andrew James Bonacci, Amy W Lang, Leonardo Santos Flow separation, which results in higher drag, is an ongoing issue for flow control innovation. It has been demonstrated that shortifn mako shark scales are capable of being bristled by reversing flow occurring in separated regions resulting in a novel passive, flow actuated separation control mechanism. An investigation of how this backflow forms and interacts with real shark skin specimens is of interest to further understand this mechanism and its potential to control separation. A water tunnel experiment replicates separation with a focus on the reversing flow development near the wall within a laminar boundary layer. Using a rotating cylinder, an unsteady and increasing adverse pressure gradient was induced creating a separated region over a flat plate. Several laminar boundary layers of Re varying from 1.67*10^5 to 2.98*10^5 and approximate thickness of 10 mm were used making them more measurable to DPIV. An unsteady and growing region of separated flow was induced over the plate for both smooth and shortifn mako shark skin surfaces. The two are compared to see how the scales affect the formation, size, and separation point of the reversing flow region. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D15.00003: Actuation of Passive Flaps Modeled after Shark Scales in a Steady Laminar Boundary Layer Separation Bubble Chase Parsons, Amy W Lang, Leonardo Santos, Andrew Bonacci Reducing flow separation is of great interest in the field of fluid mechanics to reduce drag and improve the overall efficiency of aircraft. This project seeks to investigate passive flow control using shortfin mako shark inspired microflaps in a laminar boundary layer separation bubble. Microflaps actuated by reversing flow have the potential to be a robust means of controlling flow separation at various Reynolds numbers. To generate a controlled adverse pressure gradient, a rotating cylinder induces separation at a chosen location within a flat plate laminar boundary layer, ranging from Re = 170,000 to Re = 280,000. Within this thick boundary layer, DPIV is used to measure the flow characteristics. The goal is to better understand the overall mechanism by which shark scales are able to reduce reversing flow with the ultimate aim of fabricating man-made surfaces suitable for real aircraft applications. Results show that microflaps successfully reduced the amount of time-averaged backflow when compared to flat plate cases. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D15.00004: Scale actuation in a 3D printed model of shortfin mako scales in a separating turbulent boundary layer Caleb Stanley, Amy W Lang, Leonardo Santos Passive control of turbulent boundary layer separation is important from an engineering perspective because of the potential for separation to induce a variety of problems including increased pressure drag, loss of lift, and stall events. In this study, the process of actuation for different sized 3D printed micro-flaps, with a design inspired by the geometry of shortfin mako shark scales, was documented. An adverse pressure gradient was generated utilizing a rotating cylinder above a flat plate to induce turbulent boundary layer separation. Quantitative data was acquired using DPIV in a water tunnel and flow characteristics correlating with micro-flap actuation were identified. Analysis reveals a statistical cross-covariance of less than -0.4 between micro-flap angle and the velocity downstream of the micro-flaps for each run. This indicates that a negative downstream velocity correlates with a positive micro-flap angle. The cross-covariance was normalized so that the auto-covariances at zero lag were 1 for both the scale angle and the downstream velocity. Results revealed a tendency for the micro-flaps to remain in either an almost fully actuated (>90%) or fully lowered (~30%) state. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D15.00005: Flow noise reduction using bio-inspired structured surfaces Kaushik Sampath, Charles Rohde, James Wissman, Alec Ikei Turbulent flow over bio-inspired surface features such as shark denticles or riblets has been studied extensively in attempts to reduce drag. Another consequence, noted in aeroacoustics, is a decrease in associated flow noise. Recent advances in 3D printing using polyjet and stereolithography, facilitate accurate fabrication of such complex surface geometries down to 200 μm over large areas (>100 cm2). Streamwise riblets with a characteristic height of 8-20 δν are known to cause a 5-8% reduction in skin-friction drag, while those outside this range result in a drag increase. This study aims to quantify noise reduction by 3D printed riblets of varying properties across this range in water. To maximize the contribution of skin friction to flow noise production, a NACA0012 foil section is towed by a 0.93m long rotating arm at chord Reynolds numbers (Rec) from 0.29 to 3.2×105. A spanwise trip at x/c=0.1 fixes flow transition, ensuring a turbulent boundary layer downstream. The modular foil design, allows for easy replacement of test surfaces, located between x/c=0.15 and 0.7. Wall pressure and noise measurements are performed by pressure sensor and hydrophone arrays respectively, housed internally, to evaluate flow noise reduction in comparison to a smooth wall. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D15.00006: Mitigation of flow separation and debris using a bio-inspired micro-textured coating Jose A Montoya Segnini, Humberto Bocanegra Evans, Jhonathan Carbajal Palacios, Burak Aksak, Leonardo P Chamorro, Luciano Castillo Flow separation in airfoils brings with it an array of negative effects, including reduced lift and increased drag, vibration and noise. Active methods to mitigate flow separation, i.e. synthetic jets and plasma actuators, require extra space and power for sensing and auxiliary systems. Here, we test the functionality of a bio-inspired micro-surface for mitigation of flow separation by carrying out experiments on an S835 airfoil at Re_c ≈ 500,000. The surface consists of an array of mushroom-shaped micro-pillars with a height of approximately 85 µm and a tip diameter of 75 µm. Such coating is applied on the top of the airfoil and the leading edge of the airfoil. Characteristics of the microstructures could also lead to self-cleaning properties and reduce the impact of insect debris at the leading edge of the airfoil. Preliminary data reveal a substantial decrease in the size of the recirculation bubble and up to a 60% decrease in the area of the fluid with reverse flow. Furthermore, viscous losses from turbulence generator appear to be unaffected by the bio-inspired surface coating. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D15.00007: Optimization of two dimensional riblet geometries using the Resolvent analysis. Andrew Chavarin, Mitul Luhar We utilize an extended version of the resolvent formulation proposed by McKeon and Sharma (2010, J. Fluid Mech.) for the optimization of patterned walls for passive turbulence control. Under the resolvent analysis, the Navier-Stokes equations are interpreted as a forcing-response system: the nonlinear convective terms are considered to be a forcing to the linear system, generating a velocity and pressure response. A gain-based decomposition of the forcing-response transfer function---the resolvent operator---yields a set of highly amplified velocity and pressure modes. Previous work has shown that these high-gain modes reproduce statistical and structural features for smooth-wall flows. In the extended formulation, the effect of complex walls is introduced into the governing equations using a volume penalization technique. A gain-based decomposition of this modified system reproduces the deterioration in performance observed in previous direct numerical simulations for channel flow over rectangular riblets of increasing size. Building on these tests, we parametrically study the effect of triangular riblets of varying size and shape to identify optimal geometries. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D15.00008: Scaling of the Flow over a Conformal Vortex Generator with a Laminar Boundary Layer Nicholas A Lucido, Real J KC, Trevor C Wilson, Jamey D Jacob, Aaron S Alexander, Peter Ireland, Buddy Black, Brian R Elbing A novel low-profile flow control device, termed conformal vortex generator (CVG), was recently developed as a flow control device that has been shown to reduce fuel consumption on commercial aircraft. The CVG is a three dimensional backwards facing step with height on the order of the boundary layer thickness. The primary fluid mechanisms responsible for the fuel savings are an open research topic. The focus of the current study is to investigate CVG scaling parameters with a laminar boundary layer inlet condition. A proposed scaling law is presented based on simple geometric and flow parameters. The scaling law is tested at flight scale on a general aviation aircraft (Piper Arrow) using wall shear stress visualization and laboratory scale in a water tunnel with two-dimensional, two component particle image velocimetry. Comparisons between the two scales are consistent with the proposed scaling law. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D15.00009: Computational Investigation of a Low Profile Vortex Generator Trevor C Wilson, Real J KC, Brian R Elbing, Jamey D Jacob, Peter Ireland, Buddy Black, Aaron S Alexander Vortex generators are known to provide several aerodynamic benefits including delayed separation, increased lift, and stabilized shocks by energizing the boundary layer. But, these benefits come at the cost of increased parasitic drag due to the geometry of the traditional vortex generator. The current work studies a low profile vortex generator, termed a conformal vortex generator (CVG), which can successfully energize the boundary layer without the increase in parasitic drag. This study utilizes a commercial computational fluid dynamics (CFD) package (Star-CCM+) to analyze the flow over the CVGs. Due to the low profile of the CVG, quasi-Direct Numerical Simulation (qDNS) was used to capture both large and small scale flow features. Using a flat plate developing boundary layer as inlet boundary condition, the sensitivity of the downstream flow field to the CVG geometry is assessed by varying CVG geometry and flow conditions. These results as well as comparison with water tunnel experimental results will be presented. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D15.00010: Study of Conformal Vortex Generators via Wake Survey Real Jung KC, Nicholas Lucido, Brian R Elbing, Jamey D Jacob, Aaron S Alexander, Buddy Black, Peter Ireland Vortex generators are commonly used in the aircraft industry as a flow control device. A novel low-profile vortex generator, termed a conformal vortex generator (CVG), may provide flow control without the parasitic drag commonly associated with traditional vortex generators. This experimental study varies the CVG configurations on an airfoil model (LA203A) in a wind tunnel and assesses their performance with wake surveys. The configurations were varied by changing the length, width, and the thickness of the CVGs. The test speeds were also varied, thus changing the chord length based Reynolds number. The CVGs were able to generate strong coherent structures that persisted into the far wake region (~5 chord lengths downstream). The strength of those coherent structures increased proportional to Reynolds Number, and the intensity of the coherent structures was sensitive to the CVG geometry. This presentation will compare the coherent structures through velocity profiles and overall drag production from the different CVG configurations as well as compare with canonical configurations (no CVGs and a backward facing step with varying chord-wise locations). |
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