Experimental and Numerical Methods for Transition and Drag Predictions of Laminar Airfoils

Abstract

The friction component is responsible for more than 40% of typical civil aircraft drag. As a consequence, the issue of laminar flow has been of prime importance in aeronautics for many years now. This article is focused on TollmienSchlichting-induced transition and drag predictions of two-dimensional laminar airfoils obtained with experimental and numerical methods. In 2012, a test campaign in the ONERA-S2MA wind tunnel, including infrared acquisitions, pressure sensors, and wake analyses, allowed substantial data to be obtained on such airfoils in transonic conditions. To complete this study, two-dimensional fluid dynamics computations have been performed, either with a Reynoldsaveraged Navier-Stokes solver using transition criteria or with a boundary-layer code combined with direct stability analysis. Furthermore, experimental (wake survey) and numerical (far-field theory) techniques allowing airfoil drag breakdown have been employed. Wind-tunnel and computational fluid dynamics transition predictions have been compared. Good agreement has been observed but the transition criteria may show some limitations in particular situations, such as long separation bubble development. The gains in lift and drag due to laminar flow have been quantified (natural vs triggered transition). Concerning drag reduction, the importance of the viscous pressure component has been highlighted. Finally, the effects of parameters such as angle of attack, Mach number, and Reynolds number on transition location and drag have been investigated.subscript for local state value f = frequency of boundary-layer instability M = freestream Mach number N T = value of factor N at transition onset P i = stagnation pressure Re = freestream Reynolds number based on the airfoil chord T i = stagnation temperature Tu = turbulence level U, V, W = x, y, z velocity components x, y, z = x, y, z coordinates Y = normalized first cell height Λ 2 = mean Pohlhausen parameter ρ = density Subscript 0 = freestream state value

Wind-Tunnel and CFD Investigations Focused on Transition and Performance Predictions of Laminar Wings

“…The latter depends on the upstream turbulence level Tu and the local pressure gradient (through the Pohlhausen parameter). More details about the AHD-Gleyzes (AHD-GL) criteria are available in the paper dealing with transition prediction and drag assessment for 2-D airfoils [10].…”

“…To calculate transition in elsA, the combined AHD-GL criterion with a turbulence level of 0.1%, which corresponds to a threshold transition factor close to 8 (see [10] for more details), and the C 1 criterion are used. Transition calculation is only applied to the wing while all the other aircraft elements are fully turbulent.…”

“…The ONERA-The French Aerospace Lab computational fluid dynamics (CFD) methods were then validated for this 2-D case both in terms of transition-location prediction and relative-performance-gain assessment. A paper presenting the results of this work was published in the AIAA Journal in 2015 [10].…”

“…In contrast with the 2-D study presented in [10], this paper considers a 3-D configuration, for which transition to turbulence will be driven not only by the amplification of Tollmien-Schlichting (TS) instabilities, but also by the crossflow (CF) mechanisms.…”

Shock Control of a Low-Sweep Transonic Laminar Flow Wing

“…In 2006, Cella et al [9][10] designed a transonic NLF wing, UW-5006, referring to a medium-sized business-jet-class aircraft and experiments were conducted in wind tunnel. Recently, Hue et al [11][12] and Xu et al [13] conducted some experimental and computational research of transonic NLF wing. The up to date "Breakthrough Laminar Aircraft Demonstrator in Europe" (BLADE) project tried to move from the aerodynamic concepts of natural laminar flow through to its industrialization and operational demonstration at full scale [14] [15].…”