This paper derives the upper bound on heat transport in supergravitational turbulent thermal convection analytically and numerically. Using a piecewise background profile, the functional inequality analysis delivers a suboptimal bound $Nu\lesssim -({3\sqrt {3}}/{2})({\eta \ln (\eta )}/({1-\eta ^2}))\,Ra^{1/2}$ as $Ra\rightarrow \infty$, where $Nu$ is the Nusselt number, $\eta$ is the radius ratio of the inner cylinder to the outer cylinder ($0<\eta <1$), and $Ra$ is the Rayleigh number. A variational problem yielded from Doering–Constantin–Hopf formalism is solved asymptotically and numerically, which delivers a better upper bound than the piecewise background profile. The asymptotic analysis and numerical data indicate that the current best bound is given by $Nu\leqslant -0.106({\eta \ln (\eta )}/({(1-\eta )(1+\sqrt {\eta })^2}))\,Ra^{1/2}$. Both analytical and numerical results demonstrate that the upper bound can be significantly reduced by the curvature effect. Unlike the traditional Rayleigh–Bénard turbulence, in which the optimal perturbation yielded from the variational problem is always two-dimensional, the present study shows that three-dimensional perturbations, annular perturbations and axisymmetric perturbations can be induced by the curvature effect simultaneously. However, we show that the bound yielded from the three-dimensional variational problem is very close to the axisymmetric situation as $\eta$ increases and $Ra$ increases.

The lubricated motion of an object near a deformable boundary presents striking subtleties arising from the coupling between the elasticity of the boundary and lubricated flow, including but not limited to the emergence of a lift force acting on the object despite the zero Reynolds number. In this study, we characterize the hydrodynamic forces and torques felt by a sphere translating in close proximity to a fluid interface, separating the viscous medium of the sphere's motion from an infinitely more viscous medium. We employ lubrication theory and perform a perturbation analysis in capillary compliance. The dominant response of the interface owing to surface tension results in a long-ranged interface deformation, which leads to a modification of the forces and torques with respect to the rigid reference case, that we characterize in detail with scaling arguments and numerical integrations.

We study the first contact of an emulsion drop impacting on a smooth solid surface. The lubricating air layer causes rapid deceleration of the bottom tip of the drop as it approaches first contact, causing a dimple in the drop surface. When the dispersed emulsion droplets are of higher density than the drop's continuous phase, the rapid deceleration (${\sim }10^5$ m s$^{-2}$) induces the formation of narrow spikes extruding out of the free surface. These spikes form when the impact Weber number exceeds a critical value ${\simeq }10$. Time-resolved interferometric imaging, at rates up to 7 million frames per second, shows the emergence and shape of these spikes leading to the local contacts with the solid. We characterize the tip curvature and capillary pressure affecting their dynamics as they emerge and can touch the substrate before the main outer ring of contact.