On the Co-existence of Transonic Buffet and Separation-Bubble Modes for the OALT25 Laminar-Flow Wing Section

Transonic buffet is an unsteady flow phenomenon that limits the safe flight envelope of modern aircraft. Scale-resolving simulations with span-periodic boundary conditions are capable providing new insights into its physics. The present contribution shows co-existence multiple modes unsteadiness over unswept laminar-flow wing section, appearing in following order increasing frequency: (a) a low-frequency transonic mode, (b) intermediate-frequency separation bubble and (c) high-frequency wake associated vortex shedding. Simulations run range Reynolds Mach numbers to connect lower frequency from moderate high pre-buffet established conditions. intermediate mode found be more sensitive Reynolds-number effects compared those number, which opposite trend observed for buffet. Spectral proper orthogonal decomposition used extract spatial structure modes. involves coherent oscillations suction-side shock structure, consistent previous studies including global analysis. laminar separation-bubble at fundamentally different, phase relationship between reattachment does not correspond simple `breathing' same Strouhal number shock-induced bubbles. Instead, based on length reverse magnitude independent within cases studied.