Experimental investigation of secondary pulsation and load characteristics of spark-generated bubbles adjacent to a rigid horizontal wall

Underwater bubble–wall interactions critically affect impulsive loads on marine structures, influencing their safety and operational reliability. While first-cycle bubble collapses have been extensively studied, the physical mechanisms and engineering implications of secondary pulsation remain underexplored. This study systematically investigates the dynamics and load characteristics of spark-generated bubbles near a rigid horizontal wall over 17 dimensionless standoff parameters (γ = D/Rmax = 0.33–3.00), using synchronized high-speed imaging (22 500 fps) and piezoelectric pressure measurements. Results reveal that within a critical γ range (0.83 ≤ γ ≤ 2.17), the secondary pulsation peak pressure can surpass that of the initial collapse (e.g., 25.1 vs 12.02 MPa at γ = 1.17), primarily due to bubble migration reducing collapse distance and a transition from radiative collapse to direct wall impact loading modes. The secondary pulsation phase is more complex due to the interaction between bubble fragmentation, re-coalescence, and wall adhesion/de-adhesion. These factors contribute to higher peak pressures and more localized impulsive loads. The transition from radiative collapse to direct wall impact is critical in amplifying the overall loading effects, which are not accounted for in traditional first-collapse-based models. Morphological analysis identifies five bubble motion patterns (adherent jets, non-adherent jets, and free-field behavior), and quantifies the second-pulsation impulse as contributing up to 66.9% of total energy at γ = 1.67, significantly accelerating structural damage accumulation. These findings provide new experimental and theoretical insights into underwater explosion load amplification mechanisms, offering valuable guidance for the design of blast-resistant marine structures and optimization of non-contact cleaning and rock fragmentation technologies. Methodologically, the study combines high-speed camera imaging with synchronized pressure sensor measurements, which enables accurate, real-time characterization of bubble migration and secondary pulsation effects near boundaries.