Wall shear stress from jetting cavitation bubbles: influence of the stand-off distance and liquid viscosity

We study systematically the cavitation-induced wall shear stress on rigid boundaries as a function of liquid viscosity mu. and stand-off distance gamma using axisymmetric volume of fluid (VoF) simulations. Here, gamma = d/R-max is defined with the initial distance of bubble centre from the wall d and the bubble equivalent radius at its maximum expansion R-m(ax). The simulations predict accurately the overall bubble dynamics and the time-dependent liquid film thickness between the bubble and the wall prior to the collapse. The spatial and temporal wall shear stress is discussed in detail as a function of gamma and the inverse Reynolds number 1/Re. The amplitude of the wall shear stress is investigated over a large parameter space of viscosity and stand-off distance. The inward stress is caused by the shrinking bubble and its maximum value tau(mn) follows tau Re-mn(0.35) = -70 gamma + 110 (kPa) for 0.5 < gamma < 1.4. The expanding bubble and jet spreading on the boundary produce an outward-directed stress. The maximum outward stress is generated shortly after impact of the jet during the early spreading. We find two scaling laws for the maximum outward stress tau(mp) with tau(mp) similar to mu(0)(.)(21)h(jet)(-0.3)U(jet)(1.5) for 0.5 <= gamma <= 1.1 and tau(mp) similar to mu(-0.)(25) h(jet)(-)(1.5)U(jet)(0.5) for gamma >= 1.1, where U-jet is the jet impact velocity and h(jet) is the distance between lower bubble interface and wall prior to impact.

Automated summary

The study systematically examines the cavitation-induced wall shear stress on rigid boundaries as a function of liquid viscosity and stand-off distance using axisymmetric VoF simulations. The simulations accurately predict the dynamics of bubbles and the liquid film thickness before collapse. The spatial and temporal wall shear stress is analyzed in detail, showing inward stress from shrinking bubbles and outward stress from expanding bubbles and jet spreading.