Please use this identifier to cite or link to this item: http://dx.doi.org/10.18419/opus-13756
|Title:||Experiments on the laminar to turbulent transition under unsteady inflow conditions|
|Abstract:||Natural laminar ﬂow (NLF) airfoils are key to the performance of sailplanes and wind turbines. They provide a signiﬁcant reduction of friction drag by delaying the transition from laminar to turbulent boundary layer. However, the most common method for transition prediction in NLF airfoil design, the e^n method (Mack 1977), has limited capabilities for taking inﬂow turbulence into account. The current work employs wind tunnel experiments to study how the transition on an NLF airfoil is aﬀected by free-stream turbulence. The eﬀect of small- and large-scale turbulence is studied separately, as well as in combination. In the wind tunnel, turbulence grids generate small-scale turbulence, and a gust generator induces inﬂow angle oscillations corresponding to large-scale turbulence. The study includes a detailed characterization of the turbulence generated by grids placed in the settling chamber of the wind tunnel. The Reynolds number Re = 3400000 and the airfoil pressure distribution is matched to cruise or dash ﬂight of general aviation aircraft. The results are compared with direct numerical simulation, linear stability theory (LST) and ﬂight measurements. The results show that small-scale turbulence does have an inﬂuence on the transition location in the investigated range of turbulence level, 0.01% < Tu < 0.11%. The modiﬁed e^n method (Mack 1977) captures the general trend, but the sensitivity to Tu is airfoil dependent. The eﬀects of 2D, single-mode inﬂow angle oscillations are investigated in the range of reduced frequency 0.06 < kappa < 1.7. In this range, the transition process changes from quasi-steady to clearly unsteady, but a fully convective transition mode is not formed. This is an intermediate range of unsteady ﬂow in which trajectory-following LST is able to capture the main features of the unsteady transition process. No signiﬁcant interaction between the eﬀects of small- and large-scale turbulence are observed in the investigated range of Tu and kappa, indicating that the eﬀects can be superposed.|
|Appears in Collections:||06 Fakultät Luft- und Raumfahrttechnik und Geodäsie|
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|2023_Romblad_dissertation_transition_unsteady_inflow_final.pdf||18,45 MB||Adobe PDF||View/Open|
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