Experimental observation of a pathological laminar separation bubble over an aerofoil

It is a well-established fact that at a fixed angle of attack, with increasing Reynolds number,

, a laminar separation bubble (LSB) will shrink and eventually cease to exist, due to increase in the growth rate of hydrodynamic instability modes. However, during this wind tunnel test campaign, we observed that paradoxically the LSB increased in length from

italic Re equals 2 times 10 Superscript 5 Re = 2 × 10 5 ${\textit{Re}}=2\times 10^5$

to

italic Re equals 2.5 times 10 Superscript 5 Re = 2.5 × 10 5 ${\textit{Re}}=2.5\times 10^5$

. Further diagnostics showed that this was due to a modification of the background forcing environment, often neglected in the literature. The perturbation generated from potential aero–vibroacoustic interaction unexpectedly boosted Tollmien–Schlichting modes and their faster amplification even upstream of the LSB drove the rapid secondary growth and breakdown at

italic Re equals 2 times 10 Superscript 5 Re = 2 × 10 5 ${\textit{Re}}=2\times 10^5$

. At

italic Re equals 2.5 times 10 Superscript 5 Re = 2.5 × 10 5 ${\textit{Re}}=2.5\times 10^5$

, when the lock-in with background forcing modes was not significant, a well-behaved LSB was observed with spatially amplifying Kelvin–Helmhotz modes. This finding raises important questions on the mechanisms entailing the dynamics and instability of LSBs proposed from not only experiments in similar facilities in the literature, but also in other scenarios where background perturbation due vibration, noise and confinement are non-negligible.