Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session R25: Flow Control VIII: Surface Modulation, Interface Speed and Other Effects |
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Chair: Beverley J. McKeon, California Institute of Technology Room: 320 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R25.00001: The effect of mako sharkskin on laminar flow separation Michael Bradshaw, Amy Lang, Philip Motta, Maria Habegger, Robert Hueter Many animals possess effective performance enhancing mechanisms, such as the denticles found on the skin of the shortfin mako shark (\textit{Isurus oxyrinchus)}. The shortfin mako, one of the fastest sharks on the planet, is covered by small, tooth-like scales that vary in flexibility over the body. Previous biological findings have shown that the scales increase in flexibility from the leading to trailing edge over the pectoral fin as well as on various sections of the body. It is believed that the scale bristling may provide a mechanism for flow separation control that leads to decreased drag and increased maneuverability. This study involved testing a left pectoral fin of a shortfin mako shark as well as a cylinder with a sharkskin specimen applied circumferentially in a water tunnel facility under static, laminar conditions. Digital Particle Image Velocimetry (DPIV) was used to characterize the flow over the surfaces. Various Reynolds numbers were tested for both configurations, as well as several AOAs for the pectoral fin. The flow over the fin and cylinder were compared to a painted fin and a smooth PVC cylinder, respectively. The study found that the shark scales do, in fact, help to control flow separation. However, in order for the scales to bristle and trap the reversing flow, a certain magnitude of reversed flow and shear is required. This phenomenon seems to be most effective at near stall conditions and at higher Reynolds numbers. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R25.00002: Controlling turbulent boundary layer separation using biologically inspired 2D transverse grooves Amy Lang, Emily Jones, Farhana Afroz It is theorized that the presence of grooves, such as the sinusoidal ones found on dolphin skin or the cavities that form between bristled shark skin scales, can lead to induced boundary layer mixing and result in the control of turbulent boundary layer separation. To test this hypothesis, a series of water tunnel experiments using DPIV studied the characteristics of a flat plate turbulent boundary layer whereby a rotating cylinder was used to induce an adverse pressure gradient and resulting flow separation. The experiments were repeated with the use of a plate covered with two types of grooves, rectangular and sinusoidal, with a spacing of 2 mm in size. Flow similarity of the cavity flow was preserved between the experiments and flow over bristled shark skin scales. Both geometries resulted in a reduction of flow separation as measured by backflow coefficient. In addition, Reynolds stress profiles showed that as the pressure gradient was increased, the sinusoidal geometry outperformed the rectangular grooves in terms of increased mixing close to the wall. The sinusoidal plate also generated a lower momentum deficit within the boundary layer which would indicate a smaller drag penalty. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R25.00003: A Novel Method to Induce Hydrodynamic Instability in Boundary Layer Flows Morteza Gharib, David Jeon, Francisco Pereira, Beverley McKeon We have developed a method to induce passive hydrodynamic displacement of boundary layer type flows by implementing spatially patterned hydrophobic patches in the form of bands and spots on the surface of a boundary layer plate. These patterns can be designed as parallel bands of a certain width, spacing and direction, or spots with a random or regular distribution of a certain shape, size and spatial density. We will present results from a series of experiments where the response of boundary layers in low to medium Reynolds number ranges to these spatial forcing will be demonstrated. Also, we will discuss~potential use of this novel~technique for drag reduction and separation delay applications where our technique could be used to replace riblets, trip wires and vortex generators. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R25.00004: Patterned Surface Roughness for Passive Transition Delay Robert Downs, Jens H.M. Fransson Surface roughness is demonstrably detrimental to boundary-layer stability in many scenarios; it is now known that sensibly chosen roughness can also delay the onset of transition, resulting in a drag reduction. The latest part of an ongoing research effort\footnote{Shahinfar et al. \emph{Phys. Rev. Lett.} 109, 074501 (2012).} exploring the use of streamwise streaks to attenuate growth of forced disturbances, the present experiments employ a spatially periodic surface pattern to modify the flow in a flat plate boundary layer. With respect to conventional cylindrical surface roughness, the critical roughness-height-based Reynolds number of the surface pattern is improved. Tollmien--Schlichting waves are excited via suction and blowing at the wall, to form a well-controlled disturbance. A parametric study reveals that patterned roughness inhibits the growth of these \mbox{T--S} waves and increases the transition Reynolds number by 70\% compared with the smooth plate reference case. Systematic changes to the pattern spacing demonstrate that the roughness can also accelerate the onset of transition. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R25.00005: Development of FDR-AF (Frictional Drag Reduction Anti-Fouling) Marine Coating Inwon Lee, Hyun Park, Ho Hwan Chun In this study, a novel skin-friction reducing marine paint has been developed by mixing fine powder of PEO(PolyEthyleneOxide) with SPC (Self-Polishing Copolymer) AF (Anti-Fouling) paint. The PEO is well known as one of drag reducing agent to exhibit Toms effect, the attenuation of turbulent flows by long chain polymer molecules in the near wall region. The frictional drag reduction has been implemented by injecting such polymer solutions to liquid flows. However, the injection holes have been a significant obstacle to marine application. The present PEO-containing marine paint is proposed as an alternative to realize Toms effect without any hole on the ship surface. The erosion mechanism of SPC paint resin and the subsequent dissolution of PEO enable the controlled release of PEO solution from the coating. Various tests such as towing tank drag measurement of flat plate and turbulence measurement in circulating water tunnel demonstrated over 10{\%} frictional drag reduction compared with conventional AF paint. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R25.00006: ABSTRACT WITHDRAWN |
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