Turbulence subject to axisymmetric expansion is experimentally investigated using opposed multiple-jet arrays. For each array, jet interaction generates decaying, nearly homogeneous and isotropic turbulence within a duct. The turbulent flows from the opposed arrays collide and spread radially, forming a mean-flow stagnation point with associated mean strain. Flow properties are examined using particle image velocimetry. The mean velocity gradient tensor, $\mathsf{A}_{\textit{ij}} = \partial \langle u_i \rangle / \partial x_{\!j}$, satisfies $\mathsf{A}_{\textit{xx}} : \mathsf{A}_{\textit{yy}} : \mathsf{A}_{zz} = -2 : 1 : 1$ with $\mathsf{A}_{\textit{xx}} \lt 0$, indicating axisymmetric expansion. Turbulence is strongly influenced by this expansion, becoming increasingly anisotropic towards the stagnation point, suggesting a cumulative effect of mean strain. The ratios of streamwise to transverse root-mean-square velocity fluctuations, $u_{\textit{rms}}/v_{\textit{rms}}$, and of their integral scales both increase relative to an isotropic state, consistent with rapid distortion theory (RDT). However, because the strain time scale is comparable to that of large-scale motions, deviations from RDT arise, including larger $u_{\textit{rms}}/v_{\textit{rms}}$ values and a steeper decay of energy spectra in the inertial subrange than the $-5/3$ law. The spectral slope change is opposite to that reported for axisymmetric contraction, suggesting a common mechanism for spectral modification in both strain types, since both are described by the same tensor form with opposite signs of $\mathsf{A}_{\textit{ij}}$. Consistently, the scaling exponents of velocity structure functions differ from predictions based on Kolmogorov’s second similarity hypothesis, even for low-order functions. These results confirm that axisymmetric mean strain significantly modifies turbulence properties, some of which are considered universal for other turbulent flows.

Underground gas storage is a critical technology in global efforts to mitigate climate change. In particular, hydrogen storage offers a promising solution for integrating renewable energy into the power grid. When injected into the subsurface, hydrogen’s low viscosity compared with the resident brine causes a bubble of hydrogen trapped beneath caprock to spread rapidly into an aquifer through release of a thin gas layer above the brine, complicating recovery. In long aquifers, the large viscous pressure drop between source and outlet induces significant pressure variations, potentially leading to substantial density changes in the injected gas. To examine the role of gas compressibility in the spreading dynamics, we use long-wave theory to derive coupled nonlinear evolution equations for the gas pressure and gas/liquid interface height, focusing on the limit of long domains, weak gas compressibility and low gas/liquid viscosity ratio. Simulations are supplemented with a comprehensive asymptotic analysis of parameter regimes. Unlike the near-incompressible limit, in which gas spreading rates are dictated by the source strength and viscosity ratio, and compressive effects are transient, we show how compression of the main gas bubble can generate dynamic pressure changes that are coupled to those in the thin gas layer that spreads over the liquid, with compressive effects having a sustained influence along the layer. This coupling allows compressibility to reduce spreading rates and gas pressures. We characterise this behaviour via a set of low-order models that reveal dominant scalings, highlighting the role of compressibility in mediating the evolution of the gas layer.

In this study, we investigate the transition from laminar to turbulent flow in boundary layers within nozzles used as standard flow meters. Experiments are conducted to analyse the effects of surface roughness, the radius of curvature at the nozzle throat and the level of free stream turbulence. The results indicate that, for industrial nozzles with standard roughness levels, surface roughness does not significantly influence the transition process. Additionally, tests on three different nozzles reveal that a smaller throat radius corresponds to a higher Reynolds number at the transition point. To understand this behaviour, we develop a theoretical model where the laminar boundary layer profiles are derived using an analytical formulation that accounts for compressibility and favourable pressure gradients in these nozzles. The stability characteristics of this boundary layer are examined within the framework of linear stability theory, assuming parallel flow. Since Tollmien–Schlichting waves are stabilised by the favourable pressure gradient, a spatial transient growth analysis is employed to explore the role of bypass mechanisms. Similar to the flat-plate boundary layer case, the optimal perturbations are identified as streaks. Due to the nozzle wall’s convex curvature, the highest growth is observed at low but non-zero frequencies, leading to unsteady streak formation. A transition model, based on the N-factor of optimal unsteady streaks, is used to estimate the onset of transition. This model successfully reproduces the experimentally observed effects of curvature and can be used to optimise the design of mass-flow-meter nozzles. Finally, the critical roughness size required to trigger a roughness-induced transition is estimated theoretically and compared with experimental observations. The impact of distributed roughness on transition is examined in relation to stability theory, showing that the tested roughness levels are ineffective in triggering transient growth mechanisms.

We present a theoretical analysis of a gyroscopic wave energy converter (GWEC), which generates electricity via the precession induced by the flywheel’s rotation and the pitch motion of a floating body. The coupled wave–body–gyroscope interaction problem is formulated under the assumptions of linear waves and resulting linear motions of both the floating body and the gyroscope. Within this framework, we identify the optimal control parameters that maximise the energy absorption efficiency. The analysis reveals that the GWEC can theoretically achieve the maximum energy absorption efficiency of 1/2 at any wave frequency through appropriate tuning of the flywheel’s rotational speed and the generator parameters. The derived theory is verified through numerical simulations in both the frequency and time domains. Furthermore, time-domain simulations incorporating the nonlinear gyroscopic response are conducted to assess the limitations of the linear gyroscopic model. These findings provide valuable insights for the future design of wave energy harvesting technologies.

Simple analytical criteria are derived to determine whether axisymmetric base flows in annuli and pipes are stable or unstable. Both axisymmetric and non-axisymmetric inviscid disturbances are considered. Our sufficient condition for stability improves upon the classical result of Batchelor & Gill (1962) J. Fluid Mech. 14(4), 529–551 following the idea of the second Kelvin–Arnol’d stability theorem. A novel sufficient condition for instability is also derived by extending the recently proposed hurdle theorem for parallel flows (Deguchi et al. 2024 J. Fluid Mech. 997, A25). These analytical criteria are applied to annular and pipe model flows and are shown to effectively predict the neutral parameters obtained from eigenvalue computations of the stability problem.

We investigate the three-dimensional melting dynamics of an initially spherical particle translating in a warmer liquid using sharp-interface simulations that fully resolve both solid and fluid phases with the Stefan condition. A wide parameter space is explored, spanning initial Reynolds number ($\textit{Re}_0$), Stefan number ($\textit{St}$) and Richardson number ($\textit{Ri}$). In the absence of buoyancy ($\textit{Ri}= 0$), the interface evolution is governed by canonical wake bifurcations. Four regimes are identified: an axisymmetric regime ($\textit{Re}_0\lt 212$) with a rounded front and planar rear; a steady planar-symmetric regime ($212\lt \textit{Re}_0\lt 273$) with an inclined rear plane; a periodic planar-symmetric regime ($273\lt \textit{Re}_0\lt 355$) where vortex shedding emerges in the wake; and a chaotic regime ($\textit{Re}_0\gt 355$) with fluctuating stagnation points and a more rounded rear. Despite these differences, all regimes exhibit a tendency towards melt-rate homogenisation over time. Besides, we introduce an aspect-ratio-based surface-area formulation that yields a predictive model, accurately capturing volume evolution across regimes. Hydrodynamic loads also reflect the coupling between shape and flow: drag follows rigid-sphere correlations only at moderate $\textit{Re}_0$; planar rears enhance drag at higher $\textit{Re}_0$; lift appears only in symmetry-broken regimes and reverses late in time; torque reorients the rear plane towards vertical, consistent with free-body experiments. When buoyancy is included, assisting configurations ($\textit{Ri}\gt 0$) suppress recirculation and maintain quasi-spherical shapes, whereas opposing or transverse buoyancy ($\textit{Ri}\lt 0$) destabilises wakes and promotes tilted planar rears. These results provide a unified framework for convection-driven melting across laminar, periodic and chaotic wakes, with implications for geophysical and industrial processes.

When a fluid is exposed to acoustic actuations or harmonic boundary vibrations, a steady flow known as acoustic streaming is superimposed on the oscillatory motion. In resonating acoustofluidic devices, the manipulation of nanoparticles by acoustic radiation forces is often hindered by the presence of acoustic streaming. In this study, we demonstrate, both theoretically and numerically, that microscale acoustic streaming can be significantly reduced or even completely eliminated by creating specific acoustic resonances within well-designed fluid cavities. By suppressing acoustic streaming and the corresponding drag force it induces, we demonstrate the potential to use acoustic radiation forces for manipulating nanoparticles, regardless of their size. Additionally, building upon the theoretical findings, we present the experimental realisation of acoustophoretic patterning of polystyrene nanoparticles with diameters ranging from 100 nm to 1 $\unicode {x03BC}$m in a resonating wavelength-scale acoustofluidic device that operates at sub- or low-MHz frequencies.

The nonlinear interactions of compressional Alfvén wave and a steadily moving charged obstacle are examined in Hall magnetohydrodynamics (MHD). The interaction dynamics is shown to be described by a forced derivative nonlinear Schrödinger equation (fDNLSE). The steadily moving charged obstacle induced weak perturbation is responsible for the forcing term. The variational structure is used to investigate the exact solitary wave solutions of the fDNLSE for a special analytic form of the forcing term by constructing a proper Hamiltonian of the system. The conditions for the stability of these solitary waves are delineated through variational method. The numerical solutions using the split-step Fourier method confirm the analytical results representing the pinned solitons. The relevance and potential applications of the results in astrophysics are also discussed.