Riblet-generated flow mechanisms that lead to local breaking of Reynolds analogy

We investigate experimentally the effects of micro-grooves on the development of a zero pressure gradient turbulent boundary layer at two different values of the friction Reynolds number. We consider both the well-known streamwise aligned riblets as well as wavy riblets, characterized by a sinusoidal pattern in the mean flow direction. Previous investigations by the authors showed that sinusoidal riblets yield larger values of drag reduction with respect to the streamwise aligned ones. We perform new particle image velocimetry experiments on wall-parallel planes to get insights into the effect of the sinusoidal shape on the near-wall organisation of the boundary layer and the structures responsible for the friction drag reduction and the turbulence generation. Conditional averages, aimed at identifying the topology of the low-speed streaks in the turbulent boundary layer, reveal that the flow is highly susceptible to wall manipulation. This is particularly evident in the cases that are associated with greater values of drag reduction. The results suggest a fragmentation and/or weakening of the streaks in the sinusoidal cases, that is triggered by the larger values of the wall-normal vorticity found at the streaks’ edges. The results are also confirmed by applying the variable interval spatial averaging events eduction technique. The turbulent kinetic energy budget also shows that the sinusoidal geometry significantly attenuates the turbulence production, hence supporting the idea of the manipulation of the turbulence regeneration cycle.

“…2021, 2022; Ran, Zare & Jovanović 2021; Rouhi et al. 2022; Von Deyn, Gatti & Frohnapfel 2022; Malathi Ananth et al. 2023; Zhang, Cai & Li 2024).…”

Section: Introductionmentioning

confidence: 99%

Manipulation of a turbulent boundary layer using sinusoidal riblets

Cafiero,

Amico,

Iuso

2024

“…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%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Rouhi

Chandran

et al. 2023

Self Cite

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.

“…2000; Nugroho, Hutchins & Monty 2013; Boomsma & Sotiropoulos 2016; Rouhi et al. 2022; Von Deyn et al. 2022).…”

Section: Introductionmentioning

confidence: 99%

“…Riblets are small-scale two-dimensional transverse grooves whose heights scale with the viscous scales of the flow, and can act as drag reduction devices (Walsh 1982;Park & Wallace 1994;García-Mayoral and Jiménez 2011;Cui et al 2019). However, if riblets are not viscous scaled and/or have other larger-scale features (such as a larger pattern of inclinations), then they create new flow features and perhaps increase drag (Bechert et al 2000;Nugroho, Hutchins & Monty 2013;Boomsma & Sotiropoulos 2016;Rouhi et al 2022;Von Deyn et al 2022). The sensitivity of riblets to these specific flow conditions calls into question their applicability to boost the performance of unsteady swimmers.…”

Section: Introductionmentioning

confidence: 99%

A systematic investigation into the effect of roughness on self-propelled swimming plates

Massey,

Ganapathisubramani,

Weymouth

2023

This study examines the effects of surface topography on the flow and performance of a self-propelled swimming (SPS) body. We consider a thin flat plate with an egg-carton roughness texture undergoing prescribed undulatory swimming kinematics at Strouhal number $0.3$ and tail amplitude to length ratio $0.1$ ; we use plate Reynolds numbers $Re=6$ , 12 and $24\times 10^3$ , and focus on $12\,000$ . As the roughness wavelength is decreased, we find that the undulation wave speed must be increased to overcome the additional drag from the roughness and maintain SPS. Correspondingly, the extra wave speed raises the power required to maintain SPS, making the swimmer less efficient. To decouple the roughness and the kinematics, we compare the rough plates to equivalent smooth cases by matching the kinematic conditions. We find that all but the longest roughness wavelengths reduce the required swimming power and the unsteady amplitude of the forces when compared to a smooth plate undergoing identical kinematics. Additionally, roughness can enhance flow enstrophy by up to 116 % compared to the smooth cases without a corresponding spike in forces; this suggests that the increased mixing is not due to increased vorticity production at the wall. Instead, the enstrophy is found to peak strongly when the roughness wavelength is approximately twice the boundary layer thickness over the $Re$ range, indicating the roughness induces large-scale secondary flow structures that extend to the edge of the boundary layer. This study reveals the nonlinear interaction between roughness and kinematics beyond a simple increase or decrease in drag, illustrating that roughness studies on static shapes do not transfer directly to unsteady swimmers.