Transient thermocapillary convection flows near a suddenly heated vertical wire are widely present in nature and industrial systems. The current study investigates the dynamical evolution and heat transfer for these transient flows near a suddenly heated vertical wire, employing scaling analysis and axisymmetric numerical simulation methodologies. Scaling analysis indicates that there exist four possible scenarios of the dynamical evolution and heat transfer for these transient flows, dependent on the wire curvature, Marangoni number and Prandtl number. In a typical scenario of the dynamical evolution and heat transfer, heat is first conducted into the fluid after sudden heating, resulting in an annular vertical thermal boundary layer around the wire. The radial temperature gradient may generate a thermocapillary force on the liquid surface, dragging the liquid away from the wire. The pressure gradient also drives a vertical flow along the wire. Further, the current study analyses and derives the scaling laws of the velocity, thickness and Nusselt number for the surface and vertical flows in different scenarios. Additionally, a number of two-dimensional axisymmetric numerical simulations are performed. The flow structure around the suddenly heated vertical wire is characterised under different regimes and the validation for the proposed scaling laws in comparison with numerical results is presented.

The dispersion of solutes has been extensively studied due to its important applications in microfluidic devices for mixing, separation and other related processes. Solute dispersion in fluids can be analysed over multiple time scales; however, Taylor dispersion specifically addresses long-term behaviour, which is primarily influenced by advective dispersion. This study investigates Taylor–Aris dispersion in a viscoelastic fluid flowing through axisymmetric channels of arbitrary shape. The fluid’s rheology is described using the simplified Phan-Thien–Tanner (sPTT) model. Although the channel walls are axisymmetric, they can adopt any geometry, provided they maintain small axial slopes. Drawing inspiration from the work of Chang & Santiago (2023 J. Fluid Mech. vol. 976, p. A30) on Newtonian fluids, we have developed a governing equation for solute dynamics that accounts for the combined effects of fluid viscoelasticity, molecular diffusivity and channel geometry. This equation is expressed using key dimensionless parameters: the Weissenberg number, the Péclet number and a shape-dependent dimensionless function. Solving this model allows us to analyse the temporal evolution of the solute distribution, including its mean and variance. Our analysis shows that viscoelasticity significantly decreases the effective solute diffusivity compared with that observed in a Newtonian fluid. Additionally, we have identified a specific combination of parameters that results in zero or negative transient growth of the variance. This finding is illustrated in a phase diagram and provides a means for transient control over dispersion. We validated our results against Brownian dynamics simulations and previous literature, highlighting potential applications for the design and optimisation of microfluidic devices.

The spatial organisation of a passive scalar plume originating from a point source in a turbulent boundary layer is studied to understand its meandering characteristics. We focus shortly downstream of the isokinetic injection ($1.5\leqslant x/\delta \leqslant 3$, $\delta$ being the boundary-layer thickness) where the scalar concentration is highly intermittent, the plume rapidly meanders and breaks up into concentrated scalar pockets due to the action of turbulent structures. Two injection locations were considered: the centre of the logarithmic region and the wake region of the boundary layer. Simultaneous quantitative acetone planar laser-induced fluorescence and particle image velocimetry were performed in a wind tunnel, to measure scalar mixture fraction and velocity fields. Single- and multi-point statistics were compared with established works to validate the diagnostic novelties. Additionally, the spatial characteristics of plume intermittency were quantified using ‘blob’ size, shape, orientation and mean concentration. It was observed that straining, breakup and spatial reorganisation were the primary plume-evolution modes in this region, with little small-scale homogenisation. Further, the dominant role of coherent vortex motions in plume meandering and breakup was evident. Their action is found to be the primary mechanism by which the injected scalar is transported away from the wall in high concentrations (‘large meander events’). Strong spatial correlation was observed in both instantaneous and conditional fields between the high-concentration regions and individual vortex heads. This coherent transport was weaker for wake injection, where the plume only interacts with outer vortex motions. A coherent-structure-based mechanism is suggested to explain these transport mechanisms.

New experimental results on gas flow through a long tube in the viscous, slip and transitional regimes are presented, obtained using an improved constant-volume measurement technique. This method is based on measuring the pressure variation in the inlet tank while the outlet tank is evacuated to a low pressure. Experimental pressure data for helium, neon, argon, nitrogen, krypton and xenon are used to extract the Poiseuille coefficient through a newly developed methodology. The obtained values show good agreement with theoretical predictions. Additionally, the velocity slip coefficient is also extracted from the same pressure data for all tested gases.

Heat-transfer measurements published in the literature seem to be contradictory, some showing a transition for the dependance of the Nusselt number (${\textit{Nu}}$) with the Rayleigh number (${\textit{Ra}}$) behaviour at ${\textit{Ra}} \approx 10^{11}$, some showing a delayed transition at higher ${\textit{Ra}}$, or no transition at all. The physical origin of this discrepancy remains elusive, but is hypothesised to be a signature of the multiple possible flow configurations for a given set of control parameters, as well as the sub-critical nature of the transition to the ultimate regime (Roche 2020 New J. Phys. vol. 22, 073056; Lohse & Shishkina 2023 Phys. Today vol. 76, no. 11, 26–32). New experimental and numerical heat-flux and velocity measurements, both reaching ${\textit{Ra}}$ up to $10^{12}$, are reported for a wide range of operating conditions, with either smooth boundaries, or mixed smooth–rough boundaries. Experiments are run in water at $40\,^\circ \textrm {C}$ (Prandtl number, ${\textit{Pr}}$ is 4.4), or fluorocarbon at $40\,^\circ \textrm {C}$ (${\textit{Pr}}$ is 12), and aspect ratios 1 or 2. Numerical simulations implement the Boussinesq equations in a closed rectangular cavity with a Prandtl number 4.4, close to the experimental set-up, also with smooth boundaries, or mixed smooth–rough boundaries. In the new measurements in the rough part of the cell, the Nusselt number is compatible with a ${\textit{Ra}}^{1/2}$ scaling (with logarithmic corrections), hinting at a purely inertial regime. In contrast to the ${\textit{Nu}}$ vs ${\textit{Ra}}$ relationship, we evidence that these seemingly different regimes can be reconciled: the heat flux, expressed as the flux Rayleigh number, ${\textit{Ra}}\textit{Nu}$, recovers a universal scaling with Reynolds number, which collapses all data, both our own and those in the literature, once a universal critical Reynolds number is exceeded. This universal collapse can be related to the universal dissipation anomaly, observed in many turbulent flows (Dubrulle 2019 J. Fluid Mech. vol. 867, no. P1, 1).

Whilst surface-stress integration remains the standard approach for fluid force evaluation, control-volume integral methods provide deeper physical insights through functional relationships between the flow field and the resultant force. In this work, by introducing a second-order tensor weight function into the Navier–Stokes equations, we develop a novel weighted-integral framework that offers greater flexibility and enhanced capability for fluid force diagnostics in incompressible flows. Firstly, in addition to the total force and moment, the weighted integral methods establish, for the first time, rigorous quantitative connections between the surface-stress distribution and the flow field, providing potential advantages for flexible body analyses. Secondly, the weighted integral methods offer alternative perspectives on force mechanisms, through vorticity dynamics or pressure view, when the weight function is set as divergence-free or curl-free, respectively. Thirdly, the derivative moment transformation (DMT)-based integral methods (Wu et al., J. Fluid Mech. vol. 576, 2007, 265–286) are generalised to weighted formulations, by which the interconnections among the three DMT methods are clarified. In the canonical problem of uniform flow past a circular cylinder, weighted integral methods demonstrate advantages in yielding new force expressions, improving numerical accuracy over original DMT methods, and enhancing surface-stress analysis. Finally, a force expression is derived that relies solely on velocity and acceleration at discrete points, without spatial derivatives, offering significant value for experimental force estimation. This weighted integral framework holds significant promise for flow diagnostics in fundamentals and applications.

The merging of two turbulent fronts without mean shear is investigated by direct numerical simulations. The turbulent streams are created by prescribing instantaneous velocity fields from precursor simulations of homogeneous isotropic turbulence (HIT) as inlet conditions for spatially evolving turbulent merging. The fronts are initially separated by a distance $H$ and convected with a uniform free stream velocity $U_{\infty }$. The inlet turbulence intensity varies in the range of $0.24 \leqslant u^{\prime}/U_{\infty } \leqslant 0.47$, while the inlet Taylor-scale Reynolds number is in the range of $151 \leqslant \textit{Re}_{\lambda } \leqslant 317$. As the flow develops in the streamwise direction, two distinct regions are identified: (i) an initial linear decay region, where the two turbulent fronts gradually approach each other without noticeable interaction; and (ii) a rapid decay region, where the opposing turbulent fronts influence one another and eventually merge. The flow statistics collapse once the streamwise coordinate is rescaled as $x^{+} = (x/H) (u^{\prime}/U_{\infty })$, suggesting that the merging location is imposed by large scales. An analysis conditioned to the developing turbulent/non-turbulent interfaces (TNTIs) reveals that, within the merging region, conditional mean enstrophy profiles deviate from those observed in ‘classical’ TNTIs, indicating a locally more homogenous flow. Within this region of interaction, the surface area of the TNTI increases while the volume of irrotational fluid steadily decreases, resulting in the generation of fine-scale structures. These findings support that turbulent merging is a multiscale process, where both the largest and smallest scales of motion intervene.

Hydrodynamic instability can occur when a viscous fluid is driven rapidly through a flexible-walled channel, including a multiplicity of steady states and distinct families of self-excited oscillations. In this study we use a computational method to predict the stability of flow through a planar finite-length rigid channel with a segment of one wall replaced by a thin pre-tensioned elastic beam of negligible mass. For large external pressures, this system exhibits a collapsed steady state that is unstable to low-frequency self-excited oscillations, where the criticality conditions are well approximated by a long-wavelength one-dimensional (1-D) model. This oscillation growing from a collapsed state exhibits a reduced inlet driving pressure compared with the corresponding steady flow, so the oscillating state is energetically more favourable. In some parameter regimes this collapsed steady state is also unstable to distinct high-frequency normal modes, again predicted by the 1-D model. Conversely, for lower external pressures, the system exhibits an inflated steady state that is unstable to another two modes of self-excited oscillation, neither of which are predicted by the lower-order model. One of these modes becomes unstable close to the transition between the upper and lower steady states, while the other involves small-amplitude oscillations about a highly inflated wall profile with large recirculation vortices within the cavity. These oscillatory modes growing from an inflated steady state exhibit a net increase in driving pressure compared with the steady flow, suggesting a different mechanism of instability to those growing from a collapsed state.

The effectiveness of polymer drag reduction by targeted injection is studied in comparison with that of a uniform concentration (or polymer ocean) in a turbulent channel flow. Direct numerical simulations are performed using a pseudo-spectral code to solve the coupled equations of a viscoelastic fluid using the finitely extensible nonlinear elastic dumbbell model with the Peterlin approximation. Light and heavy particles are used to carry the polymer in some cases, and polymer is selectively injected into specific flow regions in the other cases. Drag reduction is computed for a polymer ocean at a viscosity ratio of $\beta = 0.9$ for simulation validation, and then various methods of polymer addition at $\beta = 0.95$ are compared for their drag-reduction performance and general effect on the flow. It was found that injecting polymer directly into regions of high axial strain inside and around coherent vortical structures was the most effective at reducing drag, while injecting polymer very close to the walls was the least effective. The targeting methods achieved up to 2.5 % higher drag reduction than an equivalent polymer ocean, offering a moderate performance boost in the low drag-reduction regime.

This work investigates the Richtmyer–Meshkov instability (RMI) at gas/viscoelastic interfaces with an initial single-mode perturbation both experimentally and theoretically. By systematically varying the compositions and concentrations of hydrogels, a series of viscoelastic materials with tuneable mechanical properties is created, spanning from highly viscous to predominantly elastic. Following shock impact, the interface exhibits two distinct types of perturbations: small-amplitude, short-wavelength perturbations inherited from initial single-mode condition, and large-amplitude, long-wavelength perturbations arising from viscous effects. For hydrogels with high loss factors, viscosity dominates the interface dynamics, leading to pronounced V-shaped deformation of the entire interface accompanied by a rapid decay of the initial single-mode perturbation. In contrast, for hydrogels with low loss factors, elasticity plays a prominent role, leading to sustained oscillations of the single-mode perturbation. By employing the Maxwell model to simultaneously incorporate both viscous and elastic effects, a comprehensive linear theory for RMI at gas/viscoelastic interfaces is developed, which shows good agreement with experimental results in the early stages. Although deviations arise at later times due to factors such as the shear-thickening feature of hydrogels and three-dimensional effects, the model well reproduces the oscillation behaviour of single-mode perturbations. In particular, it effectively captures the trend that increasing elasticity reduces both oscillation period and amplitude, providing key insights into the role of material properties in interface dynamics.

This article follows on from Scott & Cambon (J. Fluid Mech., vol. 979, 2024, A17) and Scott (Phys. Rev. E, vol. 111, 2025, 035101). Like those articles, it concerns weak, decaying homogeneous turbulence in a rotating, stably stratified fluid with constant Brunt–Väisälä frequency, $N$. The difference is that here we consider the case in which $\beta =2{\varOmega} /N$ is close to $1$, where ${\varOmega}$ is the rotation rate. Because this renders inertial-gravity waves only weakly dispersive, wave-turbulence theory, which played a prominent role in the earlier studies, no longer applies. Indeed, wave-turbulence analysis does not appear here. Nonetheless, much of the analytical framework, based on modal decomposition, carries over, as do many of the conclusions. The flow is expressed as a sum of wave and non-propagating (NP) modes and their weak-turbulence mode-amplitude evolution equations are derived for small $\beta -1$. The NP component is found to evolve independently of the wave one, following an amplitude equation which is precisely that of the previous studies in the limit $\beta \rightarrow 1$. The NP component induces coupling between wave modes and, without it, the wave component has purely linear decay. The mode-amplitude equations are integrated numerically using a scheme similar to that of classical direct numerical simulation and results given. We find an inverse energy cascade of the NP component, whereas the presence of that component induces a forward cascade, hence significant dissipation, of the wave component. Detailed results are given for the energy, energy spectra and energy fluxes of the two components.