Transition Prediction in External Flows via Linear Stability Theory

Paper summarizes progress in relating linear stability theory to transition location. Linear stability theory computations for both flight and low-disturbance wind tunnels, at low and high speeds, and for T-S, Gortler, and cross-flow modes indicate that, in the absence of Morkovin “bypasses,” transition at low background disturbance levels corresponds to N-factors the order of 9 to 11 (in agreement with the early two-dimensional low speed flow research of A. M. O. Smith and van Ingen) provided that the appropriate physics, such as curvature effects, are included. This work indicates a much wider range of applicability for the e N method than previously conjectured. Therefore, if roughness and waviness and background disturbance fields are small, the eN method can evidently be utilized to correlate transition location and parameterize for such effects as (1) Mach number, (2) pressure gradient, (3) wall temperature, (4) angle of attack, (5) wall mass transfer, (6) sweep, (7) flow history, (8) geometry/curvature, (9) body rotation, (10) flow chemistry, and (11) bluntness. The next stage of research in the relationship between stability theory and transition will prove extraordinarily difficult, as exquisite and often immeasurable details of the environmental disturbance fields will be required. Transonic specific and accentuated background disturbances and their influences upon transition are also discussed.