Characterisation of backflow events over a wing section
2017 (English)In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 18, no 2, 170-185 p.Article in journal (Refereed) Published
Rare backflow (negative wall-shear stress) events have recently been found and quantified in the near-wall region of canonical wall-bounded turbulent flows. Although their existence and correlation with large-scale events have been established beyond numerical and measurement technique uncertainties, their occurrence at numerically high Reynolds numbers is still rare (less than 1 per thousand and 1 per million at the wall and beyond the viscous sublayer, respectively). To better quantify these rare events, the turbulent boundary layer developing over the suction side of a wing section, experiencing an increasing adverse pressure gradient (APG) without separation along its chord c, is considered in the present work. We find that the backflow level of 0.06% documented in turbulent channels and zero-pressure-gradient (ZPG) turbulent boundary layers is already exceeded on the suction side for x/c > 0.3, at friction Reynolds numbers three times lower, while close to the trailing edge the backflow level reaches 30%. Conditional analysis of extreme events indicates that for increasing Clauser pressure-gradient parameters (reaching β ≃ 35), the flow reaches a state in which the extreme events are more likely aligned with or against the freestream, and that the otherwise strong spanwise component of the wall-shear stress reduces towards the vicinity of the trailing edge. Backflow events subjected to moderate up to strong APG conditions (0.6 < β < 4.1) exhibit an average width of Δz+ ≃ 20, and an average lifetime of Δt+ ≃ 2. This directly connects with the findings by Lenaers et al., and implies that there is a connection between high-Re ZPG and strong APG conditions.
Place, publisher, year, edition, pages
Taylor & Francis, 2017. Vol. 18, no 2, 170-185 p.
adverse pressure gradient, backflow, separation, Turbulent boundary layers, wings
IdentifiersURN: urn:nbn:se:kth:diva-202210DOI: 10.1080/14685248.2016.1259626ScopusID: 2-s2.0-84997235696OAI: oai:DiVA.org:kth-202210DiVA: diva2:1082973
Funding text: Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation. QC 201703202017-03-202017-03-202017-03-20Bibliographically approved