Comparison of material erosion under air and submerged conditions using a high-energy self-excited modulated water jet

Abstract

This study examines the effect of the frequency of modulated water jet on material erosion in both air and submerged environments. Two prototypes of nozzles with the ability to self-excite flow oscillations, each featuring an identical outlet orifice (1 × 1 mm), were developed to modulate water jets at distinct oscillating frequency levels (from 6 to 45 kHz). Jet oscillation characteristics were predicted via computational fluid dynamics (CFD) and validated experimentally using direct pressure-sensor measurements and indirect optical frequency-monitoring techniques. Optimal standoff distance (SOD) between the selected nozzle and the target surface was found (from 10 to 15 mm). Erosion tests were conducted on pure copper samples in both air and underwater conditions. During erosion testing, flow rates (19 and 24 l/min) and pressures (80 and 140 MPa) were continuously recorded by diagnostic sensors to maintain constant hydraulic power (26 kW and 55 kW). The resulting erosion grooves formed on copper samples under air and submerged conditions by moving the nozzle above the target surface (1 mm/s) were characterized by optical profilometry and scanning electron microscopy (SEM). Quantitative analysis of removed volume, groove geometry, and surface defects was conducted to elucidate the relationship between the frequency of the modulated water jet and erosion performance. The results confirmed that an increase in the frequency of the modulated high-speed water jet significantly enhances erosion efficiency in both tested environments. However, in submerged conditions, a pronounced attenuation of the jet occurs, reflected in a lower volume of material removal. Further details are discussed in the subsequent sections of the manuscript.