Ice shelves that spread into the ocean can develop rifts that can trigger iceberg calving and enhance ocean-induced melting. Fluid mechanically, this system is analogous to an extensionally dominated radial spreading of a non-Newtonian fluid into a relatively inviscid and denser ambient fluid. Laboratory experiments have shown that rift patterns can emerge when the spreading fluid is shear thinning. Linear stability analysis supports these findings, predicting that while the instability mechanism is active in Newtonian fluids, it is suppressed by stabilising secondary-flow cellular vortices. Here, we explore the fully nonlinear evolution of a radially spreading Newtonian fluid, assessing whether large-amplitude perturbations could drive an instability. We use a quasi-three-dimensional numerical simulation that solves the full nonlinear shallow-shelf approximation, tracing the evolving fluid front, and validate it with known axisymmetric solutions and predictions from linear-stability theory. We find that large-amplitude perturbations induce nonlinear effects that give rise to non-axisymmetric patterns, including cusp-like patterns along the fluid front and complex secondary-flow eddies, which have neither been predicted theoretically nor observed experimentally. However, despite these nonlinear effects, large-amplitude perturbations alone are insufficient to induce rift-like patterns in Newtonian fluids. Strain-rate peaks at the troughs of the fluid front suggest that shear-thinning fluids may become more mobile in these regions, potentially leading to rift formation. This coincides with the likely weakening of stabilising forces as the fluid becomes more shear-thinning. These findings elucidate the critical role of nonlinear viscosity on the formation of rift-like patterns, which is the focus of Part 2 of this study.

Characteristics of the turbulent/non-turbulent interface (TNTI) and entrainment in separated and reattaching flows induced by an oscillating fence are investigated using time-resolved particle image velocimetry. Disturbed flows are classified into subcritical, transitional, critical and supercritical cases based on the ratio of the oscillation frequency to the natural vortex shedding frequency. In the recirculation zone, distinct vortices across different cases lead to significant variations in TNTI characteristics. In the subcritical case, the TNTI evolution resembles that in the stationary fence case but with intensified height fluctuations due to the undulation of separated shear layer. For other cases, the mean TNTI height increases with the oscillation frequency, while height fluctuation diminishes. The TNTI thickness varies with nearby vortices, scaling with the Taylor microscale. After the reattachment, TNTI height distributions converge into two groups: subcritical and transitional cases exhibit larger fluctuations and positively skewed probability density functions (PDFs), while critical and supercritical cases show smaller fluctuations and basically symmetric PDFs. The TNTI thickness becomes consistent across various cases, matching the adjacent small-scale vortex size. Besides, the nibbling mechanism of entrainment aligns well with the flow development. The minimum mean entrainment velocity coincides with the strongest prograde vortex while the maximum occurs at $x\approx 1.2x_{{r}}$ (where $x$ denotes the streamwise coordinate and $x_{{r}}$ is the mean reattachment position) in all cases. Engulfment is enhanced near the reattachment location by oscillations in the transitional and critical cases, but is suppressed in the supercritical cases due to the weakness of vortex structures at higher oscillation frequencies.

We examine how ambient temperature $T$ (23–90 $^\circ \mathrm{C}$) alters the dynamics of spark-induced cavitation bubbles across a range of discharge energies. As $T$ rises, the collapse of an isolated spherical bubble weakens monotonically, as quantified by the Rayleigh collapse factor, minimum volume and maximum collapse velocity. When the bubble is generated near a rigid wall, the same thermal attenuation is reflected in reduced jet speed and diminished migration. Most notably, at $T \gtrsim 70\,^\circ \text{C}$, we observe a previously unreported phenomenon: secondary cavitation nuclei appear adjacent to the primary bubble interface where the local pressure falls below the Blake threshold. The pressure reduction is produced by the over-expansion of the primary bubble itself, not by rarefaction waves as suggested in earlier work. Coalescence between these secondary nuclei and the parent bubble seeds pronounced surface wrinkles that intensify Rayleigh–Taylor instability and promote fission, providing an additional route for collapse strength attenuation. These findings clarify the inception mechanism of high-temperature cavitation and offer physical insight into erosion mitigation in heated liquids.