Drag reduction of turbulent flow over thin rectangular riblets

Elsamni, Osama A.; Chun, Ho Hwan; Yoon, Hyun Sik

doi:10.1016/j.ijengsci.2007.03.002

Cited by 88 publications

(51 citation statements)

References 22 publications

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“…From a drag-reduction perspective, this geometry is far from optimal, with a maximum expected reduction of about 6 %, but it requires a lower numerical resolution than sharper configurations, reducing the computational costs and improving the accuracy of the results. Previous DNSs of riblets (Choi et al 1993;Chu & Karniadakis 1993;Stalio & Nobile 2003;Orlandi, Leonardi & Antonia 2006;El-Samni, Chun & Yoon 2007) were usually conducted in channels with riblets in only one wall, with the opposite wall used as the reference to measure friction. However, although that arrangement may be useful in comparing channel simulations among themselves, it is inconvenient when trying to relate computations to experiments on boundary layers.…”

Section: The Numerical Methodsmentioning

confidence: 99%

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: The Numerical Methodsmentioning

confidence: 99%

Hydrodynamic stability and breakdown of the viscous regime over riblets

2011

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“…The physical mechanism of the drag reduction by riblets has been investigated in detail, although some aspects remain controversial. In particular, mean and local velocity profiles and turbulent statistics within and above the riblet grooves have been reported for experiments in wind tunnels [17][18][19], water channels [20] and numerical experiments [21][22][23][24][25][26].…”

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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“…Instead, DNS was used first to predict drag reduction outcomes before the actual experimental studies were carried out to confirm DNS data. Recently, in a DNS study using the immersed boundary method El-Samni et al [88] succeeded in obtaining 11% drag reduction with fence type riblets. Coming 20 years after Wilkinson and his co-workers optimized the V-groove riblets to produce 8% drag reduction in 1987, El-Samni and team only managed to improve on those results by a meager 3%.…”

Section: Ribletsmentioning

confidence: 99%

“…Velocity contours showing invasion of vortical structures into riblet valleys for S + = 40.6 where y + is the non-dimensional height and z + is the distance in the spanwise direction[88].…”

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confidence: 99%