LAUSR

Abstract The phenomenon of foam formation by air entrainment of impinging water jet is reported in this paper. An effort has been made to understand the physics of the phenomenon through visual and photographic observations. The formation of an air sheath and downward motion of the impinging jet are reported. Experimental visualization of the mechanism of air entrainment by foam formation at low jet flowrates of $$8.33 \times 10^{ - 6} - 1.83 \times 10^{ - 6} {\text{m}}^{3} /{\text{s}}$$ 8.33 × 10 - 6 - 1.83 × 10 - 6 m 3 / s for jet lengths of 13, 18, 23 and 25 cm, respectively, was performed with a vertically free-falling jet. Foam formation via air entrainment has been classified into four types: no entrainment, incipient entrainment, intermittent entrainment and continuous entrainment with increases in jet impinging velocity with fixed jet length. Foam formation depends on the impinging jet velocity, nozzle design, impinging jet length and physical property of the jet such as the surface tension which constitutes the major controlling factors for foam formation via air entrainment of impinging jet. Graphic abstract