Cavitation bubble induced wall shear stress

The shear flow generated by a collapsing cavitation bubble near a rigid boundary is relevant in surface cleaning, cell membrane poration and enhanced cooling among other applications. However, neither the spatio-temporal distribution nor the magnitude of the shear stress is well known. Here we report the recent simulations of the wall shear stress induced by a single bubble. The simulation is done based on a compressible two-phase Volume of Fluid (VOF) solver from the OpenFOAM framework. We focus on the result for a non-dimensional distance, γ≈1.0 (γ=h/R_max, where h is the distance of the initial bubble center to the boundary, and R_max is the maximum spherical equivalent radius of the bubble). The flow region with constant shear rate in the boundary layer is reproduced with a locally refined mesh spacing Δx=0.05 μm. Very high stresses of 100 kPa are found during the early spreading of the high-speed transient jet from the collapsing bubble. Later the main spreading flow and the re-expansion of the toroidal bubble together produce a vortex ring, which stabilizes the flow and thereby slows down the decay of the shear stress. In particular, a spatio-temporal map on the wall shear stress is provided, which summarizes the complex distribution of the shear stress.