Multiple laminar-turbulent transition cycles around a swept leading edge

• Published in: Experiments in fluids Vol. 53; no. 6; pp. 1915 - 1927
• Main Authors: Mukund, R., Narasimha, R., Viswanath, P. R., Crouch, J. D.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.12.2012; Springer
• Subjects: Aerodynamics; Applied fluid mechanics; Computational fluid dynamics; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Exact sciences and technology; Fluid dynamics; Fluid flow; Fluid- and Aerodynamics; Fundamental areas of phenomenology (including applications); Heat and Mass Transfer; Leading edges; Nose; Physics; Research Article; Swept wings; Transition to turbulence; Turbulence; Turbulent flow; Turbulent flows, convection, and heat transfer; Acceleration Parameter; Wall Shear Stress; Turbulent Boundary Layer; Separation Bubble; Freestream Turbulence; Hot film anemometer; Test facilities; Separation bubble; Pressure distribution; Flow visualization; Aerodynamics; Swept wing; Transition flow; Model test; Boundary layers; Turbulent laminar transition; Intermittency
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-012-1405-2

Certain interesting flow features involving multiple transition/relaminarization cycles on the leading edge of a swept wing at low speeds are reported here. The wing geometry tested had a circular nose and a leading edge sweep of 60°. Tests were made at a chord Reynolds number of 1.3 × 10 6 with model incidence α varied in the range of 3°−18° in discrete steps. Measurements made included wing chord-wise surface pressure distributions and wall shear stress fluctuations (using hot-film gages) within about 10 % of the chord in the leading edge zone. Results at α  = 16° and 18° showed that several (often incomplete) transition cycles between laminar-like and turbulent-like flows occurred. These rather surprising results are attributable chiefly to the fact that the Launder acceleration parameter K (appropriately modified for swept wings) can exceed a critical range more than once along the contour of the airfoil in the leading edge region. Each such crossing results in a relaminarization followed by direct retransition to turbulence as K drops to sufficiently low values. It is further shown that the extent of each observed transition zone (of either type) is consistent with earlier data acquired in more detailed studies of direct transition and relaminarization. Swept leading edge boundary layers therefore pose strong challenges to numerical modelling.