33th Congress of the International Council of the Aeronautical Sciences

03.2 - Experimental Aerodynamics

STABILTY AND TRANSITION OF A LAMINAR SEPARATION BUBBLE SUBJECTED TO FREESTREAM TURBULENCE

T. Jaroslawski¹, O. Vermeersch¹, J.-M. Moschetta², M. Forte¹, E. Gowree²; ¹ONERA, France ;²ISAE-SUPAERO, France

Experiments are conducted on a two-dimensional laminar separation bubble subjected to a large range of free stream turbulence levels (0.18% < Tu < 6.26%) and integral length scales (8.31mm < Lu < 17.18mm). Spectral analysis of the unsteady velocity measurements and wall-normal disturbance pro les revealed the co-existence of two modes in the bubble: a modal instability attributed to a convective instability and a non-modal instability linked to streaks. The streamwise growth of the energy disturbances is exponential for the lowest levels of freestream turbulence and gradually becomes algebraic as the level of freestream turbulence increases. In occurrence with previous studies, an increase in the level of freestream turbulence, advances the transition position, decreasing the size of the bubble, with its eventual elimination at the highest levels. Finally, for a relatively fixed level of Tu, the variation of Lu has modest effects, however a slight advancement of transition with the decrease in Lu is observed.


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