The present study investigates the scale-dependent links between turbulent structures and wall-pressure fluctuations in the turbulent boundary layer close to the NACA0012 trailing edge. The three-dimensional velocity fields and wall-pressure signal are simultaneously measured at ${\textit{Re}}_{\tau } = 216$. The velocity and wall-pressure fluctuations are decomposed into intrinsic mode functions (IMFs) of increasing scales using empirical mode decomposition. The most correlated IMFs for wall-pressure and streamwise velocity fluctuations occur when they share similar length scales. The correlation patterns for the smaller scales indicate that hairpin vortices and hairpin packets are the dominant pressure sources. The conditional averaged velocity fluctuations based on the zero-crossing events and peaks of the wall-pressure IMFs are analysed, revealing the spatial–temporal signature of turbulent structures on wall-pressure fluctuations. High scale-dependence and convection nature are detected for responsible turbulent structures. For the high-energy wall-pressure IMFs, the pressure peaks are caused by the shear layer induced by the impinging and splitting of alternating positive and negative motions. Conversely, the zero-crossing events are related to a single large-scale motion.

We investigate the triangular instability of a Batchelor vortex subjected to a stationary triangular strain field generated by three satellite vortices, in the presence of weak axial flow. The analysis combines theoretical predictions with numerical simulations. Theoretically, the instability arises from resonant coupling between two quasi-neutral Kelvin modes with azimuthal wavenumbers $m$ and $m+3$ with the background strain. Numerically, we solve the linearised Navier–Stokes equations around a quasi-steady base flow to identify the most unstable modes, and compare their growth rates and frequencies with theoretical predictions for a Reynolds number $\textrm{Re} = 10^4$ and a straining strength $\epsilon = 0.008$. In the absence of axial flow, only the mode pair $(m_A, m_B) = (-1,2)$ (and its symmetric counterpart) is unstable. However, we show that additional combinations such as $(0,3)$, $(1,4)$ and $(2,5)$, which are otherwise strongly damped by the critical layer in the absence of axial flow, also become unstable once axial flow exceeds a certain threshold, as the critical-layer damping is significantly reduced. Furthermore, we show that the most unstable mode in the no-axial-flow case, originating from the second branch of $m = -1$ and the first branch of $m = 2$, becomes less unstable as axial flow increases. It is eventually overtaken by a mode from the first branches of both wavenumbers, which then remains the dominant unstable mode across a wide range of axial flow strengths, Reynolds numbers and straining strengths. A comprehensive instability diagram as a function of the axial flow parameter is presented.

Similarities and differences between Kolmogorov scale-by-scale equilibria/non-equilibria for velocity and scalar fields are investigated in the intermediate layer of a fully developed turbulent channel flow with a passive scalar/temperature field driven by a uniform heat source. The analysis is based on intermediate asymptotics and direct numerical simulations at different Prandtl numbers lower than unity. Similarly to what happens to the velocity fluctuations, for the fluctuating scalar field Kolmogorov scale-by-scale equilibrium is achieved asymptotically around a length scale $r_{\textit{min}}$, which is located below the inertial range. The length scale $r_{\textit{min}}$ and the ratio between the inter-scale transfer and dissipation rates at $r_{\textit{min}}$ vary following power laws of the Prandtl number, with exponents determined by matched asymptotics based on the hypothesis of homogeneous two-point physics in non-homogeneous turbulence. The inter-scale transfer rates of turbulent kinetic energy and passive scalar variance are globally similar but show evident differences when their aligned/anti-aligned contributions are considered.

Non-Newtonian fluid flow in porous media results in spatially varying viscosity, driven by flow–pore–geometry interactions, potentially leading to non-monotonic dispersion. In this work, using high-resolution micro-particle image velocimetry, we present a direct experimental observation of shear-viscosity-distribution-dependent transport with non-Newtonian fluid flows in porous media. We experimentally investigate dispersion in porous media in a microfluidic chip featuring a physical rock geometry, comparing a shear-thinning, non-Newtonian fluid with its Newtonian analogue at various Péclet numbers. We demonstrate that, in the absence of advective fluxes driven by elastic instabilities, non-Newtonian fluid flows at either extreme of the shear-dependent viscosity ($\eta _0,\eta _{\infty }$) converge to the Newtonian analogue. In contrast, for flows between these extremes, the non-Newtonian velocity fields are broadly distributed along the streamline curvature, leading to a larger enhancement in dispersion.

We revisit the interaction of an initially uniform near-inertial wave (NIW) field with a steady background flow, with the goal of predicting the subsequent organisation of the wave field. To wit, we introduce an exact analogy between the Young–Ben Jelloul (YBJ) equation and the quantum dynamics of a charged particle in a steady electromagnetic field, whose potentials are expressed in terms of the background flow. We derive the time-averaged spatial distributions of wave kinetic energy, potential energy and Stokes drift in two asymptotic limits. In the ‘strongly quantum’ limit, where the background flow is weak compared with wave dispersion, we compute the wave statistics by extending a strong-dispersion expansion initially introduced by YBJ. In the ‘quasi-classical’ limit, where the background flow is strong compared with wave dispersion, we compute the wave statistics by leveraging the equilibrium statistical mechanics of classical systems. We compare our predictions with numerical simulations of the YBJ equation, using an instantaneous snapshot from a two-dimensional turbulent flow as the steady background flow. The agreement is very good in both limits. In particular, we quantitatively describe the preferential concentration of NIW energy in anticyclones. We predict weak NIW concentration in both asymptotic limits of weak and strong background flow, and maximal anticyclonic concentration for background flows of intermediate strength, providing theoretical underpinning to observations reported by Danioux, Vanneste and Bühler (2015 J. Fluid Mech., vol. 773, R1).

This study investigates the aerobreakup mechanisms of a liquid droplet initially at a temperature below its critical point impacted by a shockwave in a supercritical environment, i.e. transcritical conditions, occurring in high-pressure/speed liquid-fuelled propulsion systems. Aerobreakup droplet breakup mechanisms have been extensively studied at atmospheric conditions, not considering the significant changes in fluid properties past the critical point that occur within very short breakup time scales in shock-dominated flows. Furthermore, the effects of decreased surface tension forces due to the weakening of intermolecular forces at supercritical conditions on the droplet breakup behaviour have not been resolved to date. This study aims to address these major gaps by developing a direct numerical simulation method to investigate the governing mechanism of droplet aerobreakup at transcritical conditions considering the changes in surface tension. A diffuse interface method coupled with a real-gas equation of state is developed to capture the fluid behaviour beyond the critical point. The results show that simultaneous changes in surface tension and density ratio unique to transcritical flows dictate the droplet aerobreakup mechanisms and the resultant breakup modes. This study presents the first transcritical droplet breakup regime map as a function of Weber number and density ratio compared with the classical breakup criteria commonly accepted for subcritical conditions, proving that the breakup is facilitated at supercritical conditions. The findings are expected to significantly contribute to the development of transcritical droplet aerobreakup models to enable the simulation of spray-shock interaction needed for designing new high-speed/pressure liquid fuel injection systems.

Despite its significance in biology and materials science, the dynamics of multicomponent vesicles under shear flow remains poorly understood because of its nonlinear and strongly coupled nature, especially regarding the role of membrane heterogeneity in driving non-equilibrium behaviour. Here we present a thermodynamically consistent phase-field model, which is validated against experiments, for the quantitative investigation of the dynamics. While prior research has primarily focused on viscosity or bending rigidity contrasts, we demonstrate that surface tension heterogeneity can also trigger swinging and tumbling in vesicles under shear. Additionally, our systematic phase diagram reveals three previously unreported dynamical regimes arising from the interplay between bending rigidity heterogeneity and shear flow. Overall, our model provides a robust framework for understanding multicomponent vesicle dynamics, with findings offering new physical insights and design principles for tuneable vesicle-based carriers.