Papers

Here, we investigate the motion of a thin circular quantum vortex filament near the infinite planar surface. The fluid surrounding this surface moves with a non-zero velocity $\bf{v}$, which is parallel to the surface. We study the specific features of this quantum system and show that they are quite suitable for the boundary layer theory. The developed model allows us to calculate the vortex energy spectrum, $E = E({\bf p})$, where ${\bf p}$ is the total momentum of a vortex ring. We have demonstrated that this function has complex non-trivial dependence on the velocity $\bf{v}$. It is stated that the inverse effective mass of a vortex under consideration is of a tensorial nature. In certain quantum states, the system shows the possibility of both negative and positive effective mass existing. This study employs a novel quantization method for classical closed vortex filaments, developed by the author earlier.

Drag reduction is achieved by a steady square-wave forcing of the spanwise wall velocity, based on the experiments in Knoop et al. (Phys. Rev. Fluids, 10, 2025). Particle tracking velocimetry data are analyzed for a non-actuated reference case and actuation at forcing amplitude $A^+ = 12$ ($+$ denotes viscous scaling) for three streamwise forcing wavelengths, which correspond to sub-optimal ($\Lambda_x^+ = 471$), near-optimal ($\Lambda_x^+ = 942$), and post-optimal ($\Lambda_x^+ = 1884$) drag reduction conditions. Conditionally averaged fields on large-scale bursts of turbulent kinetic energy show that forcing suppresses near-wall ejections across all cases, while outer-layer sweep suppression strengthens with $\Lambda_x^+$. Post-optimal forcing exhibits streamwise-periodic attenuation and recovery of turbulence. The recovery phenomenon is caused by an enhancement of very small scales, significantly smaller than those typically energetic in wall turbulence, and is linked to the emergence of small-scale bursting events. While these small-scale bursts are statistically insignificant in the non-actuated and sub-optimal cases, their frequency increases by a factor of four between the near-optimal and post-optimal cases. The small-scale bursts exhibit uniform signatures and intensities, hinting at the possible universality of the recovery phenomenon. A wavelet analysis shows that, in the post-optimal case, very small scales increase progressively in the streamwise direction in regions where the wall velocity remains constant, driving a cyclic pattern of small-scale re-energization and suppression consistent with earlier statistical analysis. This turbulence modulation mechanism is scale-selective: while small-scale structures emerge periodically, large-scale motions are more effectively suppressed as the forcing wavelength increases.

Three-dimensional particle-tracking velocimetry (3D-PTV) measurements were used to compute the spectral transport of the Reynolds-stress tensor. The experimental framework is validated for a zero-pressure-gradient (ZPG) turbulent boundary layer (TBL) at a friction Reynolds number $Re_\tau = 1020$, demonstrating that the dominant non-linear energy transfer mechanisms are adequately resolved to draw flow physics-based conclusions. For the streamwise Reynolds stress in the ZPG TBL, a component-wise decomposition of the non-linear transport term is considered for the first time, which reveals distinct energy transfer mechanisms associated with the spanwise and wall-normal advection. The same experimental framework was applied to a drag-reduced ($\approx 38\%$) TBL flow, achieved by imposing a steady streamwise-alternating spanwise wall velocity. This wall forcing causes a strong attenuation of non-linear energy transfer and its shift away from the wall. The energy transfer mechanisms remain qualitatively similar to those of the canonical ZPG TBL, suggesting that the existing mechanisms simply readjust to their new low-turbulent-energy state.

A volatile droplet gently deposited on a superheated substrate can sit on a thin film of its own vapour, which prevents contact between the drop and surface. This phenomenon is called the Leidenfrost effect. In this paper, through direct numerical simulations, we analyse characteristics of Leidenfrost water droplets with a single computational model over four decades of droplet radius, stitching together previous works in the limit of large and small droplets. Using the model, we show that the ambient humidity, an underappreciated factor in the Leidenfrost system, in combination with the flow in the drop has a significant impact on the geometry and drying kinetics. Our results imply the inadequacies of commonly made assumptions of a pure vapour phase and an isothermal droplet. When modelling large Leidenfrost droplets with an axisymmetric model, large discrepancies between experiments and the computational results occur. Through azimuthal stability analysis, we show that this is due to the unrealistic constraint of axisymmetry. This finding is supported by 3D simulations of a simplified model. Finally, some hypotheses are explored to account for the remaining discrepancies with experimental data.

Foam scraping exhibits a dynamic transition between a slip state, in which the foam moves beneath a plate, and a scraping state, in which the foam is expelled from the confined region. Although this transition has been associated with the propagation of local T1 rearrangements, the physical parameter controlling their propagation remains unclear. Here, we investigate the dependence of the critical scraping velocity on the liquid fraction, bulk viscosity, and surfactant system. For all examined conditions, the critical capillary number follows $\mathrm{Ca}_c\propto\phi^{-1}$, apart from a solution-dependent prefactor. We propose a local fluid–structure-coupling model in which viscous work transmitted through the Plateau-border network competes with the effective energetic cost required for one T1 event to trigger the next. The observed scaling implies an effective energetic cost governed by the Laplace pressure and is consistent with a small local compressive component of the bubble deformation. These results identify local coupling between interstitial flow and bubble deformation as a mechanism controlling the macroscopic slip–scraping transition.