Turbulent drag reduction by the seal fur surface

Itoh, Motoyuki; Tamano, Shinji; Iguchi, Ryo; Yokota, Kazuhiko; Akino, Norio; Hino, Ryutaro; Kubo, Shinji

doi:10.1063/1.2204849

Cited by 65 publications

(46 citation statements)

References 29 publications

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“…We portray in figures 3(b)-3(d ) the histograms of the breakdown dimensions of several riblets, expressed as s + , h + and + g . We have omitted experiments for which the optimum performance could not be clearly defined, such as the measurements for fibres of Bruse et al (1993), or those for seal fur of Itoh et al (2006). Disregarding them, the histograms show that the optimum values of s + and h + have scatters of the order of 40 %, while the scatter for + g is only about 10 %.…”

Section: Scaling Of the Riblet Dimension At Breakdownmentioning

confidence: 92%

Hydrodynamic stability and breakdown of the viscous regime over riblets

2011

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The interaction of the overlying turbulent flow with a riblet surface and its impact on drag reduction are analysed. The ‘viscous regime’ of vanishing riblet spacing, in which the drag reduction produced by the riblets is proportional to their size, is reasonably well understood, but this paper focuses on the behaviour for spacingss+≃ 10–20, expressed in wall units, where the viscous regime breaks down and the reduction eventually becomes an increase. Experimental evidence suggests that the two regimes are largely independent, and, based on a re-evaluation of existing data, it is shown that the optimal rib size is collapsed best by the square root of the groove cross-section, ℓg+=Ag+1/2. The mechanism of the breakdown is investigated by systematic DNSs with increasing riblet sizes. It is found that the breakdown is caused by the appearance of long spanwise rollers belowy+≈ 20, with typical streamwise wavelengths λx+≈ 150, that develop from a two-dimensional Kelvin–Helmholtz-like instability of the mean streamwise flow, similar to those over plant canopies and porous surfaces. They account for the drag breakdown, both qualitatively and quantitatively. It is shown that a simplified linear instability model explains the scaling of the breakdown spacing with ℓg+.

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Section: Scaling Of the Riblet Dimension At Breakdownmentioning

confidence: 92%

Hydrodynamic stability and breakdown of the viscous regime over riblets

2011

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“…However, as with the experiments of Colebrook and White, 5 it is not clear how this study of two roughness scales of substantially different size translates to the case of practical roughness, which is marked by a broad spectrum of topographical scales. With respect to more irregular roughness, Itoh et al 31 measured the turbulent flow over the fur surface of a seal which exhibited a ribletlike character, though both the amplitude and the wavelength of these topographical features varied significantly in space. Itoh et al 31 also made measurements of turbulent flow over a model of the seal fur that consisted of ordered riblets manufactured with the dominant amplitude and wavelength of the seal fur.…”

Section: Introductionmentioning

confidence: 99%

Low-order representations of irregular surface roughness and their impact on a turbulent boundary layer

Mejía-Álvarez

Christensen

2010

Physics of Fluids

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The present effort explores the relative impact of various topographical scales present within irregular surface roughness on a turbulent boundary layer under both developing-and developed-flow conditions. Low-order representations of highly irregular surface roughness replicated from a turbine-blade damaged by deposition of foreign materials were generated using singular value decomposition to decompose the complex topography into a set of topographical basis functions of decreasing importance to the original ͑"full"͒ surface character. The low-order surface models were then formed by truncating the full set of basis functions at the first 5 and 16 modes ͑containing approximately 71% and 95% of the full surface content, respectively͒ so that only the most dominant and large-scale topographical features were included in the models, while the finer-scale surface details are excluded. Physical replications of the full surface and the two low-order models were created using rapid prototyping methods to generate short and long streamwise fetches of roughness, and particle-image velocimetry was used to acquire ensembles of instantaneous velocity fields in the streamwise-wall-normal plane for developing-and developed-flow conditions at moderate Reynolds number. Comparison of both single-and multipoint statistics ͑mean velocity and Reynolds normal and shear stresses͒ as well as quadrant analysis of the instantaneous events contributing to the mean Reynolds shear stress indicates that a 16-mode model of the full surface faithfully reproduces the characteristics of flow over the full surface for both developing-and developed-flow conditions. For the latter scenario, both the 5-and 16-mode models reproduce the outer-layer characteristics for flow over the full surface in accordance with Townsend's wall similarity hypothesis. However, neither low-order surface representation fully reproduces important details of the Reynolds-shear-stress-producing events within the roughness sublayer, particularly the contributions of the most intense ejection and sweep events.

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“…Bruse et al [7] conducted oil channel tests of shark-skin replicas, hairy surfaces and riblets with adjustable geometry, and Bechert et al [8] reviewed the drag-reduction properties of biological surfaces and their replicas. More recently, Itoh et al [9] tested the flow over seal fur, obtaining drag reductions of 12 per cent.…”

confidence: 99%

Drag reduction by riblets

García-Mayoral

Jiménez

2011

Phil. Trans. R. Soc. A.

311

201

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The interaction of the overlying turbulent flow with riblets, and its impact on their drag reduction properties are analysed. In the so-called viscous regime of vanishing riblet spacing, the drag reduction is proportional to the riblet size, but for larger riblets the proportionality breaks down, and the drag reduction eventually becomes an increase. It is found that the groove cross section A + g is a better characterization of this breakdown than the riblet spacing, with an optimum A + g 1/2 ≈ 11. It is also found that the breakdown is not associated with the lodging of quasi-streamwise vortices inside the riblet grooves, or with the inapplicability of the Stokes hypothesis to the flow along the grooves, but with the appearance of quasi-two-dimensional spanwise vortices below y + ≈ 30, with typical streamwise wavelengths l

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Turbulent drag reduction by the seal fur surface

Cited by 65 publications

References 29 publications

Hydrodynamic stability and breakdown of the viscous regime over riblets

Hydrodynamic stability and breakdown of the viscous regime over riblets

Low-order representations of irregular surface roughness and their impact on a turbulent boundary layer

Drag reduction by riblets

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