PHYSICAL REVIEW FLUIDS 5, 124609 (2020) Effect of body shape on riblets performance Benedetto Mele * and Renato Tognaccini Department of Industrial Engineering, University of Naples Federico II, P.zzle Tecchio 80, Italy Pietro Catalano and Donato de Rosa CIRA Italian Aerospace Research Center, Capua (CE) 81043, Italy (Received 1 August 2020; accepted 4 December 2020; published 28 December 2020) The effect of partial slip ﬂow on airfoil performance at high Reynolds numbers is analyzed in this paper. The link between the physical mechanism of drag reduction attained by many devices and the slip length concept has been well assessed in the literature. A slip length model is therefore here adopted in large eddy simulations to quantify the effect of slip ﬂow on airfoil performance. The possibility to adopt a slip ﬂow boundary condition to simulate riblets on airfoil is veriﬁed. Their effectiveness in reducing friction drag in turbulent ﬂow has been well assessed since the end of the last century. Both theory and experiments proved that the effect of riblets only depends on the local Reynolds number. However, some experiments showed an increased effectiveness of riblets in the presence of pressure gradient. This secondary effect is still being debated and a physical explanation has not been found. This paper has the aim to provide a contribution to the understanding of this phenomenon. Large eddy simulations of ﬂows around airfoils are proposed with an extensive analysis of riblet performance, obtained by a proper slip ﬂow boundary condition. It is shown that riblets reduce the boundary layer displacement thickness inducing small but signiﬁcant modiﬁcations to the pressure distribution, in particular in the adverse pressure gradient region. The reduced thickening of the equivalent body is the reason for the reduced form drag. DOI: 10.1103/PhysRevFluids.5.124609 I. INTRODUCTION Experiments on ﬂows over a ﬂat plate and channel ﬂows [1,2] showed that streamwise mi- crogrooved surfaces (riblets) are able to reduce skin friction drag in the turbulent regime up to 8%. Theoretical studies [2,3] contributed to clarify that the involved drag reduction mechanism can be essentially reduced to a shift U + of the turbulent velocity proﬁle in the log-law region. Experiments and available theories proved that the effect of riblets is essentially local, i.e., only depends on the local Reynolds number [1–3]. Walsh [4] analyzed the effect of pressure gradient on riblets’ performance reporting that riblet data for pressure gradient ﬂows were contradictory, while Choi [5] found a negligible effect. However, some later experiments and numerical simulations investigating ﬂow over a ﬂat plate under an adverse pressure gradient showed increased effectiveness of riblets, see Debisschop and Nieuwstadt [6] for instance with the result conﬁrmed by large eddy simulations (LES) [7]. Other experiments in airfoil ﬂows [8] and wind turbine airfoils (Sareen et al. [9], Chamorro et al. [10]) also reported an increasing efﬁciency of riblets with the angle of attack implying an effect of pressure * benmele@unina.it 2469-990X/2020/5(12)/124609(13) 124609-1 ©2020 American Physical Society