Experimental investigations of controlled transition of a laminar separation bubble in an axisymmetric diffuser

The instability of a pressure-induced laminar separation bubble is examined experimentally on an axisymmetric diffuser for a Reynolds number range 7,800 ≤ ≤ 11,400 for an inlet pipe diameter D ₁ (50 mm) and as mean input flow velocity 4.2 m/s ≤ u _m ≤ 6.1 m/s. A characterization of the base flow shows a wide-spread separation at the smooth diverging contour which gives rise to a massive amplification of instabilities. Controlled disturbances are introduced by means of a slot and a membrane actuator to trigger the transition, and the receptivity of the perturbations to the laminar boundary layer is evaluated. Different axisymmetric and azimuthal disturbances are applied in order to study their influence on the laminar–turbulent transition. The measurements show a clear dependence of the transition scenario and the reattachment length on the actuation mode.