Cavitation bubble-induced shear flow and stress within grooved structures

The shear flow induced by a jetting cavitation bubble within grooved structures is systematically investigated using a three-dimensional compressible volume-of-fluid (VoF) solver combined with ultra-high-speed photography of one million frames per second (fps). The structured surface comprises twenty parallel micro-channels, and local mesh refinement down to 0.1 mu m 0.1 μ m 0.1\ [Arial]x03BCm near the groove bottom resolves the linear tangential velocity profile within the boundary layer. The simulations accurately reproduce experimental observations for two representative configurations, namely bubbles initiated above the groove and above the protrusion. In general, a microjet formed from the distal bubble surface impacts the lower bubble interface and subsequently spreads both along and across the groove channel, generating strong shear flow. Spatio-temporal wall-stress distributions indicate that the maximum shear stress occurs near the instant of jet impact, while the subsequent evolution is further modulated by daughter-bubble fragmentation and re-expansion. For a bubble initiated above the groove, the microjet produces a single impact, whereas for a bubble initiated above the protrusion, the jet becomes oblique: it first strikes the protrusion, spreads into the trapped bubble inside the groove and finally pierces the lower bubble surface at an angle. The influences of three geometric parameters, namely the horizontal offset d Superscript asterisk d ∗ d^* , vertical stand-off distance h Superscript asterisk h ∗ h^* and groove width w Superscript asterisk w ∗ w^* , are systematically examined, revealing their coupled effects on bubble deformation, jet orientation, fragmentation and the vortex dynamics. A peak wall-shear stress of approximately 150 kPa 150 kPa 150\,kPa is obtained under specific geometric conditions due to rapid jet spreading and flow refocusing within the groove. These results provide fundamental insight into the three-dimensional, unsteady wall-shear generation produced by jetting cavitation bubbles interacting with micro-structured surfaces.