A numerical study of the natural transition in a gas–liquid two-phase boundary layer over a flat plate taking account of interphase slip

The natural transition in a gas–liquid two-phase boundary layer over an underwater flat plate is studied, taking into account the interphase slip between gas and liquid. An interphase slip model is proposed to determine the dynamic viscosity of the two-phase flow based on the physical origins of fluid viscosity. The model is then applied to laminar flow calculation, instability analysis, transition prediction, and prediction of the spectrum of wall fluctuating pressure in the laminar region. Numerical calculations are conducted for boundary layers for different void fractions, including liquid single-phase flow at a zero void fraction. The results reveal the differences between two-phase and single-phase flows and show that these differences become more obvious as the void fraction increases: (i) the thickness of the two-phase laminar boundary layer becomes less; (ii) the unstable zone becomes larger; (iii) the transition location moves upstream, and the transition advance distance caused by the microbubbles becomes longer and is proportional to the void fraction; (iv) the dangerous frequency becomes higher, and the frequency bandwidth becomes wider; and (v) the wall fluctuating pressure in the laminar region becomes stronger, and its peak frequency becomes slightly higher. As the oncoming flow velocity increases, the transition advance coefficient becomes smaller, and the dangerous frequency becomes higher. The comparison of the results of the interphase slip model and those of the conventional homogeneous flow model implies that the above differences between two-phase and single-phase flows are caused by the interphase slip between gas and liquid.