The transient response of a laminar separation bubble to impulsive disturbances of varying amplitudes is examined by means of time-resolved PIV. The impulse is two-dimensional and introduced in spanwise invariant fashion by means of a DBD plasma actuator. As a result, a wave packet emerges and amplifies while travelling through the separated shear layer. Laminar-turbulent transition is advanced, causing size reduction or elimination of the LSB. The latter are accompanied by the ceasing of vortex shedding. As the bubble recovers, it bursts and significantly elongates. The time interval until full recovery is independent of the forcing amplitude. The conjecture that bursting and flapping mechanisms are due to the same feedback loop is confirmed. A sufficiently high amplitude disturbance distorts the mean flow reducing the amplification rate of following disturbances, delaying transition to turbulence. Similarly, small amplitude disturbances mildly affect the amplification rate at an intermittent rate and correspondingly alter the transition characteristics leading to the flapping behavior.
Nomenclaturea sampling frequency [Hz] δ * displacement thickness [m] f b burst frequency [Hz] ρ density [kg/m 3 ] f c carrier frequency [Hz] σ standard deviation h bubble height [m] τ ij Reynolds stress [kg/m 2 ] l bubble length [m] φ phase Re Reynolds number Φ POD spatial eigenfunction t time [s] ω vorticity [1/s] x, y, z space coordinates [m] u, v velocity components [m/s] Subscripts w wavelet coefficient s separation U 0 freestream velocity [m/s] r reattachment U ∞ local freestream velocity [m/s] ro reduced order φ phase-averaged 0 unforced