Laser-Induced Control of Laminar Separation Bubbles

Controlled investigations of laminar–turbulent transition require the introduction of well-defined disturbances with prescribed amplitude, frequency, wavelength, waveform, and geometry (0D, 1D, 2D, 3D). Classical excitation techniques, such as oscillating wires, are limited in spectral bandwidth and may introduce an undesired bias to the mean flow due to their intrusive nature. To overcome these limitations, the present study investigates pulsed laser excitation as a non-intrusive method for controlled disturbance generation in water flows. While the feasibility of laser-based excitation has previously been demonstrated in air, its applicability to liquid flows remains largely unexplored. The present work systematically examines the interaction between pulsed laser energy deposition and a water boundary layer, focusing on disturbance characteristics and controllability. Different materials, laser pulse energies, and excitation frequencies are evaluated to assess their influence on disturbance amplitude and repeatability. The methodology is applied to the controlled investigation of transition mechanisms in laminar separation bubbles (LSBs). Measurements demonstrate that laser-induced perturbations produce a clearly detectable and reproducible impact on the transition process over an LSB. The excitation allows targeted modification of instability growth without introducing mechanical interference into the flow. The results indicate that pulsed laser excitation constitutes a versatile and broadband disturbance generation technique for hydrodynamic stability experiments in liquids. This approach expands the experimental toolbox for transition research and provides new opportunities for controlled studies of instability amplification and transition control in separated flows.