Reorganisation of turbulence by large and spanwise-varying riblets

Endrikat, Sebastian; Newton, R.; Modesti, Davide; García-Mayoral, Ricardo; Hutchins, Nicholas; Chung, Daniel

doi:10.1017/jfm.2022.897

Cited by 21 publications

(19 citation statements)

References 77 publications

(212 reference statements)

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“…Moreover, for studies with δ + ≈ 1000-10 000 and h/δ 1, a decrease in κ is still observed with an increase in Reynolds number for the same roughness, suggesting an in-depth modification of the flow by the substrates (Suga et al 2010;Manes et al 2011;Fang et al 2018;Okazaki et al 2021Okazaki et al , 2022. Some studies have observed that permeable roughness could lead to an approximately 50 % drop in κ, which is more substantial than that induced by the impermeable roughness with the same geometry, implying that permeability may enhance the extent and intensity of roughness effects (Okazaki et al 2021(Okazaki et al , 2022Esteban et al 2022;Karra et al 2022). Nevertheless, the prediction of u τ , which is of great importance for the assessment of outer-layer similarity, remains a challenge in experiments (Chung et al 2021).…”

Section: Introductionmentioning

confidence: 93%

“…where k s is the equivalent sand grain roughness height) and Kármán constant (κ). The abbreviations for the studies are B06 (Breugem et al 2006), S10 (Suga et al 2010), M11 (Manes et al 2011), R17 (Rosti & Brandt 2017), K17 (Kuwata & Suga 2017), S20 (Shen, Yuan & Phanikumar 2020), K21 (Kazemifar et al 2021), K23 (Karra et al 2022), E22 (Endrikat et al 2022), F18 (Fang et al 2018, O21 (Okazaki et al 2021) and O22 (Okazaki et al 2022). Note that δ + denoted by * is estimated from k + s and k s /δ from S10.…”

Section: Introductionmentioning

confidence: 99%

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Examination of outer-layer similarity in wall turbulence over obstructing surfaces

Chen,

García-Mayoral

2023

J. Fluid Mech.

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Turbulent flows over canopies of rigid filaments with different densities are studied using direct numerical simulations at moderate Reynolds numbers $Re_\tau \approx 550\unicode{x2013}1000$ . The canopies have heights $h^+\approx 110\unicode{x2013}220$ , and are used as an instance of obstructing substrate for the assessment of outer-layer similarity. We show that conventional methods used to determine the zero-plane displacement $\Delta y$ can be at odds with proper outer-layer similarity and may not be applicable for flows at moderate $Re_\tau$ . Instead, we determine $\Delta y$ and the length and velocity scales that recover outer-layer similarity by minimising the difference between the smooth-wall and canopy diagnostic function everywhere above the roughness sublayer, not just in the logarithmic layer. In addition, we explore the possibility of the zero-plane displacement and the friction velocity being set independently, but find that outer-layer similarity is recovered more consistently when they are coupled. We observe that although the Kármán constant, $\kappa$ may not have smooth-wall-like values, the flow statistics are smooth-wall-like in the logarithmic layer and above if the surface effect is limited within the near-wall region. This suggests a modified outer-layer similarity, where $\kappa$ is not 0.39, but turbulence is otherwise smooth-wall-like. When the canopy is dense, the flow above the tips is essentially smooth-wall-like, with smooth-wall-like $\kappa \approx 0.39$ and origin essentially at the tip plane. For canopies with intermediate density, the overlying flow perceives a deeper zero-plane displacement into the canopy, which is consistent with observations reported by previous studies, but exhibits a lower Kármán constant, $\kappa \approx 0.34\unicode{x2013}0.36$ . For sparse canopies, $\kappa$ tends back to its smooth-wall value, and the zero-plane displacement height is at the canopy bed. For all canopies studied, the decrease in $\kappa$ never exceeds $15\,\%$ , which is significantly less than that obtained in some previous works using conventional methods to assess outer-layer similarity.

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Section: Introductionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

Examination of outer-layer similarity in wall turbulence over obstructing surfaces

Chen,

García-Mayoral

2023

J. Fluid Mech.

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“…Depending on the application, saving both energy and time could be important. Frohnapfel et al (2012) propose that operation of a drag-reducing mechanism with matched power input (τ w U b = const.) leads to simultaneous saving of pumping energy (due to the reduction in τ w ) and time (due to the increase in U b ).…”

Section: Simulation Casesmentioning

confidence: 99%

“…Techniques for flow control cover a variety of fields, and have been extensively reviewed (White & Mungal 2008;Dean & Bhushan 2010;Luchini & Quadrio 2022). Flow control devices are usually divided into two groups: passive devices that are fixed in place and do not change their shape or function in time, such as vortex generators (Lin 2002;Koike, Nagayoshi & Hamamoto 2004;Aider, Beaudoin & Wesfreid 2010) and riblets (García-Mayoral & Jiménez 2011Endrikat et al 2021a,b;Modesti et al 2021;Endrikat et al 2022;Rouhi et al 2022), and active devices that can be actuated in some way, such as targeted blowing (Abbassi et al 2017) or intermittent blowing and suction (Segawa et al 2007;Hasegawa & Kasagi 2011;Yamamoto, Hasegawa & Kasagi 2013;Schatzman et al 2014;Kametani et al 2015).…”

Section: Introductionmentioning

confidence: 99%

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Turbulent drag reduction by spanwise wall forcing. Part 1. Large-eddy simulations

Rouhi

Chandran

et al. 2023

J. Fluid Mech.

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Turbulent drag reduction (DR) through streamwise travelling waves of the spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the DR at friction Reynolds numbers $Re_\tau = 951$ and $4000$ . The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where DR is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence DR, is found to change with the near-wall Stokes layer protrusion height $\ell _{0.01}$ . A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer; in this range, the DR increases as $\ell _{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in DR with increasing $\ell _{0.01}$ . We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any DR savings. This motivates the study of the outer-scaled actuation pathway in Part 2, where DR is achieved through actuating the outer-scaled motions.

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