Experimental investigation of slug initiation by controlled disturbances in gas-liquid horizontal pipe flow

•Experimental determination of limiting conditions for slug onset.•Slug initiation due to controlled disturbances.•Weakly nonlinear effects accounts for a great part of conditionally unstable regime.•Stuart-Landau model allows to estimate qualitatively the flow behavior. The work reports an investigation of slug initiation in horizontal gas-liquid pipe flow induced by controlled interfacial disturbances. The idea is to investigate the limiting conditions for slug onset. To this end, disturbances with varying amplitudes were introduced at the fluid interface. An oscillating paddle was employed to generate controlled interfacial waves. These perturbations were introduced in the flow as short wavepackets of large spectral bandwidth. Thereby, disturbances in nearly all relevant wavenumbers are introduced in the flow hence providing seeds for instability mechanisms. The driving signal of the oscillating paddle was controlled and synchronized with acquisitions, enabling phase-locked measurements. The experiments were conducted at conditions close to the transition from stratified to slug/plug flow regimes. According to current findings, the disturbance amplitudes required to initiate slug increase for constant superficial gas velocity and decreasing liquid velocities. The variation of superficial liquid velocities required to initiate the slugs with respect to the disturbance amplitude displays two distinct behaviours. For small amplitudes (Amplitude/Diameter