Geometric optimization of riblet-textured surfaces for drag reduction in laminar boundary layer flows

Raayai-Ardakani, Shabnam; McKinley, Gareth H.

doi:10.1063/1.5090881

Cited by 45 publications

(25 citation statements)

References 42 publications

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“…This mechanism has also been observed in turbulent flows [4,8,10,11]. Using finite-volume simulations of the developing laminar boundary layer flow over sinusoidal riblets, we have recently shown that viscous drag reduction indeed depends on the local riblet geometry but also on the overall length of the riblet surface under study, because the kinematic features of the complex three-dimensional flow near the riblets evolve along the plate [12][13][14].…”

Section: Introductionmentioning

confidence: 67%

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Geometry mediated friction reduction in Taylor-Couette flow

Raayai-Ardakani

McKinley

2020

Phys. Rev. Fluids

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Periodic surface microtextures of different shapes such as V grooves, semicircular grooves, or rectangular grooves have been studied under laminar and turbulent flow conditions to offer guides for designing optimized low-friction surfaces. In this work we investigate the efficacy of periodic streamwise-aligned surface features in reducing the torque exerted on a steadily rotating cylinder in Taylor-Couette flow. Using threedimensional printed riblet-textured rotors and a bespoke Taylor-Couette cell, which can be mounted on a controlled stress rheometer, we measure the evolution in the torque acting on the inner rotor as a function of three different dimensionless parameters: (i) the Reynolds number characterizing the flow, (ii) the sharpness of the riblets, as defined by their aspect ratio (height to wavelength), and (iii) the axial scale of the riblets with respect to the size of the overall Taylor-Couette cell (the ratio of the riblet wavelength to the gap of the Taylor-Couette cell). Our experimental results in the laminar viscous flow regime show a reduction in torque up to 10% over a wide range of Reynolds numbers that is a nonmonotonic function of the aspect ratio of the grooves and independent of Re d (the gap-based Reynolds number). However, after the transition to the Taylor vortex regime, the modification in torque also becomes a function of the Reynolds number while remaining a nonmonotonic function of the aspect ratio. Using finite-volume simulation of the three-dimensional swirling flow in the annular gap, we discuss the kinematic changes to the Taylor-Couette flow in the presence of the riblets compared to the case of smooth rotors and compute the resulting torque reduction as a function of the parameter space defined above. Good agreement between experiments and computational predictions is found for both azimuthal Couette flow and the Taylor vortex regime.

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Section: Introductionmentioning

confidence: 67%

“…As a measure of the sharpness of the V groove, we use the dimensionless aspect ratio R, defined as R = A/(λ/2) = tan θ , to be consistent with our previous works [12,13]. Here θ is the side angle of the V groove as shown in Fig.…”

Section: Taylor-couette Flow and Riblets: Background And Theorymentioning

confidence: 99%

Geometry mediated friction reduction in Taylor-Couette flow

Raayai-Ardakani

McKinley

2020

Phys. Rev. Fluids

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“…The roughness of a surface in contact with a moving fluid can have substantial effects on the character of flow near that surface and on the nature of the shear stress in the boundary layer, which determines one component of drag (Schetz, 1993; Smits, 2000; Vogel, 1994). Effects of surface roughness are extremely complex, and depend on the scale of the roughness, orientation of surface elements, fluid velocity, Reynolds number, and the movement of the surface (Raayai‐Ardakani & McKinley, 2019). This latter feature, how any particular surface moves in flow, is particularly difficult to correlate with specific effects on the boundary layer: analyses of moving biological surfaces and how motion affects near‐surface flow dynamics have proven to be challenging to perform (Anderson, McGillis, & Grosenbaugh, 2001; Eloy, 2013; Lauder et al, 2016; Taneda & Tomonari, 1974; Yanase & Saarenrinne, 2015).…”

Section: Discussionmentioning

confidence: 99%

“…(a) denticle length (μm), (b) denticle width (μm), (c) ridge height (μm), (d) ridge spacing (μm), (e) roughness (μm), (f) kurtosis (Sku), (g) skew (Ssk), (h) average denticle aspect ratio. Details on how each variable was measured are given in the methods the surface (Raayai-Ardakani & McKinley, 2019). This latter feature, how any particular surface moves in flow, is particularly difficult to correlate with specific effects on the boundary layer:…”

Section: Denticle Roughness Gradients and Hypothesized Functionmentioning

confidence: 99%

The denticle surface of thresher shark tails: Three‐dimensional structure and comparison to other pelagic species

Popp

White

Bernal

et al. 2020

Journal of Morphology

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Shark skin denticles (scales) are diverse in morphology both among species and across the body of single individuals, although the function of this diversity is poorly understood. The extremely elongate and highly flexible tail of thresher sharks provides an opportunity to characterize gradients in denticle surface characteristics along the length of the tail and assess correlations between denticle morphology and tail kinematics. We measured denticle morphology on the caudal fin of three mature and two embryo common thresher sharks (Alopias vulpinus), and we compared thresher tail denticles to those of eleven other shark species. Using surface profilometry, we quantified 3D-denticle patterning and texture along the tail of threshers (27 regions in adults, and 16 regions in embryos). We report that tails of thresher embryos have a membrane that covers the denticles and reduces surface roughness. In mature thresher tails, surfaces have an average roughness of 5.6 μm which is smoother than some other pelagic shark species, but similar in roughness to blacktip, porbeagle, and bonnethead shark tails. There is no gradient down the tail in roughness for the middle or trailing edge regions and hence no correlation with kinematic amplitude or inferred magnitude of flow separation along the tail during locomotion. Along the length of the tail there is a leading-to-trailing-edge gradient with larger leading edge denticles that lack ridges (average roughness = 9.6 μm), and smaller trailing edge denticles with 5 ridges (average roughness = 5.7 μm). Thresher shark tails have many missing denticles visible as gaps in the surface, and we present evidence that these denticles are being replaced by new denticles that emerge from the skin below.

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“…3,4 More recent studies have optimized riblet geometry to minimize drag in a laminar boundary layer. 5 Despite their academic interest, riblets have remained largely out of favor in many engineering applications due to their high cost, weight, and limited range of operation. 6,7 Several other studies have investigated anisotropic wall porosity, 8 micro-cavities for turbulence attenuation, 9 and superhydrophobic roughness elements 10 as well as active approaches such as the use of streamwise aligned plasma actuators 11 to directly reduce drag or through a spatial arrangement of the pulsatile dielectric barrier discharge array in order to create a Stokes-layer in the flow.…”

Section: Introductionmentioning

confidence: 99%

Targeted turbulent structure control in wall-bounded flows via localized heating

et al. 2020

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A targeted turbulent flow control strategy, based on selective heating of streamwise-aligned heat strips, is assessed for drag reduction using direct numerical simulations of variable viscosity and compressible turbulent channel flows. As increasing the temperature of a gas increases its viscosity, heating is generally an unfavorable drag mitigation approach. However, through a selective spatial arrangement of the heating array, the slight increase in viscosity and decrease in density can serve to modify the organization of the streamwise-aligned structures and the likelihood of the ejection and sweep events near the wall. This can, under specific conditions, lead to a very modest drag reduction. The optimal spatial arrangement is identified using a bidimensional empirical mode decomposition and targets the near-wall, large-scale turbulent motion. The drag coefficient, at constant mass flow rate, remains unchanged with heating despite up to an 11% increase in the local viscosity above the heating strips. When accounting for the viscosity variation in the drag reduction calculation, an effective drag reduction of 6% is observed.

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Geometric optimization of riblet-textured surfaces for drag reduction in laminar boundary layer flows

Cited by 45 publications

References 42 publications

Geometry mediated friction reduction in Taylor-Couette flow

Geometry mediated friction reduction in Taylor-Couette flow

The denticle surface of thresher shark tails: Three‐dimensional structure and comparison to other pelagic species

Targeted turbulent structure control in wall-bounded flows via localized heating

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