Effect of non-ideally manufactured riblets on airfoil and wind turbine performance

Tiainen, Jonna; Grönman, Aki; Jaatinen-Värri, Ahti; Pyy, Lauri

doi:10.1016/j.renene.2020.03.102

Cited by 10 publications

(7 citation statements)

References 23 publications

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“…For the trapezoidal riblets, g,i /s can be calculated for any ratio k 1 /k by substituting (4.4) with k = 0.5s into (4.2). As an example of manufacturing inaccuracies, we consider a hypothetical surface with the two-scale trapezoidal riblets for which every other riblet k 1 is normal distributed with average k 1 = k and most k 1 ∈ [0.7, 1.3]k (two standard deviations equal 0.3k), which is loosely based on the geometry measurements by Tiainen et al (2020). With that distribution of k 1 /k, the average g,i /s = 0.468 is roughly 9 % greater than g,i /s = 0.428 for the perfectly manufactured surface with all k 1 = k. The imperfections therefore shift the surface to the right along the drag curve in figure 11(d).…”

Section: Garcíamentioning

confidence: 99%

“…As an example of manufacturing inaccuracies, we consider a hypothetical surface with the two-scale trapezoidal riblets for which every other riblet is normal distributed with average and most (two standard deviations equal ), which is loosely based on the geometry measurements by Tiainen et al. (2020). With that distribution of , the average is roughly 9 % greater than for the perfectly manufactured surface with all .…”

Section: Drag-curve Scaling For Imperfectly Repeated Ribletsmentioning

confidence: 99%

“…Possible manufacturing techniques are limited by difficulties in fabricating a large area of accurately repeated riblets with minimal variation (West, Sammut & Tang 2018), but accepting a lower quality makes manufacturing faster and more economical (Kaakkunen et al 2018). Tiainen et al (2020) measured shape variation statistics of imperfect riblets manufactured with nanosecond laser ablation on a curved airfoil. Their riblet heights are in the range 0.71-1.28 times the nominal height and they verified in laboratory experiments that the surface nevertheless reduces drag.…”

Section: Introductionmentioning

confidence: 99%

“…Tiainen et al. (2020) measured shape variation statistics of imperfect riblets manufactured with nanosecond laser ablation on a curved airfoil. Their riblet heights are in the range times the nominal height and they verified in laboratory experiments that the surface nevertheless reduces drag.…”

Section: Introductionmentioning

confidence: 99%

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Reorganisation of turbulence by large and spanwise-varying riblets

et al. 2022

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We study the flow above non-optimal riblets, specifically large drag-increasing and two-scale trapezoidal riblets. In order to reach large Reynolds numbers and large scale separation while retaining access to flow details, we employ a combination of boundary-layer hot-wire measurements and direct numerical simulation (DNS) in minimal-span channels. Although the outer Reynolds numbers differ, we observe fair agreement between experiments and DNS at matched viscous–friction-scaled riblet spacings $s^+$ in the overlapping physical and spectral regions, providing confidence that both data sets are valid. We find that hot-wire velocity spectra above very large riblets with $s^+ \gtrsim 60$ are depleted of near-wall energy at scales that are (much) greater than $s$ . Large-scale energy likely bypasses the turbulence cascade and is transferred directly to secondary flows of size $s$ , which we observe to grow in strength with increasing riblet size. Furthermore, the present very large riblets reduce the von Kármán constant $\kappa$ of the spanwise uniform mean velocity in a logarithmic layer and, thus, reduce the accuracy of the roughness-function concept, which we link to the near-wall damping of large flow structures. Half-height riblets in the groove, which we use as a model of imperfectly repeated (spanwise-varying) riblets, impede in-groove turbulence. We show how to scale the drag optimum of imperfectly repeated riblets based on representative measurements of the true geometry by solving inexpensive Poisson equations.

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Section: Garcíamentioning

confidence: 99%