The supersonic wake of a circular cylinder in Mach 3 flow was studied through spectral proper orthogonal decomposition (SPOD) of high-speed focussing schlieren datasets. A wavenumber decomposition of the SPOD eigenvectors was found to be an effective tool for isolating imaging artefacts from the flow features, resulting in a clearer interpretation of the SPOD modes. The cylinder wake consists of both symmetric and antisymmetric instabilities, with the former being the dominant type. The free shear layers that form after the flow separates from the cylinder surface radiate strong Mach waves that interact with the recompression shocks to release significant disturbances into the wake. The wake shows a bimodal vortex shedding behaviour with a purely hydrodynamic instability mode around a Strouhal number of 0.2 and an aeroacoustic instability mode around Strouhal number of 0.42. The hydrodynamic mode, which is presumably the same as the incompressible case, is weaker and decays rapidly as the wake accelerates due to increasing compressibility. The aeroacoustic mode is the dominant shedding mode and persists farther into the wake because of an indirect energy input received through free-stream acoustic waves. A simple aeroacoustic feedback model based on an interaction between downstream propagating shear-layer instabilities and upstream propagating acoustic waves within the recirculation region is shown to accurately predict the shedding frequency. Based on this model, the vortex shedding in supersonic flows over a circular cylinder occurs at a universal Strouhal number (based on approach free-stream velocity and feedback path length) of approximately 0.3.

In this work we focus on expected flow in porous formations with highly conductive isolated fractures, which are of non-negligible length compared with the scales of interest. Accordingly, the definition of a representative elementary volume (REV) for flow and transport predictions may not be possible. Recently, a non-local kernel-based theory for flow in such formations has been proposed. There, fracture properties like their expected pressure are represented as field quantities. Unlike existing models, where fractures are assumed to be small compared with the scale of interest, a non-local kernel function is used to quantify the expected flow transfer between a point in the fracture domain and a potentially distant point in the matrix continuum. The transfer coefficient implied by the kernel is a function of the fracture characteristics that are in turn captured statistically. So far the model has successfully been applied for statistically homogeneous cases. In the present work we demonstrate the applicability for heterogeneous cases with spatially varying fracture statistics. Moreover, a scaling law is presented that relates the transfer coefficient to the fracture characteristics. Test cases involving discontinuously and continuously varying fracture statistics are presented, and the validity of the scaling law is demonstrated.

Fluids at supercritical pressures exhibit large variations in density near the pseudo-critical line, such that buoyancy plays a crucial role in their fluid dynamics. Here, we experimentally investigate heat transfer and turbulence in horizontal hydrodynamically developed channel flows of carbon dioxide at $88.5$ bar and $32.6\,^{\circ }\rm C$, heated at either the top or bottom surface to induce a strong vertical density gradient. In order to visualise the flow and evaluate its heat transfer, shadowgraphy is used concurrently with surface temperature measurements. With moderate heating, the flow is found to strongly stratify for both heating configurations, with bulk Richardson numbers $Ri$ reaching up to 100. When the carbon dioxide is heated from the bottom upwards, the resulting unstably stratified flow is found to be dominated by the increasingly prevalent secondary motion of thermal plumes, enhancing vertical mixing and progressively improving heat transfer compared with a neutrally buoyant setting. Conversely, stable stratification, induced by heating from the top, suppresses the vertical motion, leading to deteriorated heat transfer that becomes invariant to the Reynolds number. The optical results provide novel insights into the complex dynamics of the directionally dependent heat transfer in the near-pseudo-critical region. These insights contribute to the reliable design of heat exchangers with highly property-variant fluids, which are critical for the decarbonisation of power and industrial heat. However, the results also highlight the need for further progress in the development of experimental techniques to generate reliable reference data for a broader range of non-ideal supercritical conditions.

The energy-harvesting performance of two oscillating hydrofoil turbines in tandem configuration is experimentally studied at a $Re$ of $20\,000$ to determine the array’s optimal kinematics. By characterising interactions between the leading foil’s wake and the trailing foil, the kinematic configuration required to maximise array power extraction is identified. This is done by prescribing leading-foil kinematics that produce specific wake regimes, identified by the maximum effective angle of attack, $\alpha _{T/4}$, parameter. The kinematics of the trailing foil are varied significantly from those of the leading foil, with heave and pitch amplitudes of $0.6c\lt h_{0,{\textit{tr}}}\lt 1.8c$ and $65^{\circ} \lt \theta _{0,{\textit{tr}}}\lt 75^{\circ}$, and inter-foil phase of $-110^{\circ} \lt \psi _{1-2}\lt 180^{\circ}$. Configurations with reduced frequencies of $0.11$ and $0.12$, and foil separations of $4c$ and $6c$ are tested within each wake regime. The power extracted by each foil over an oscillation cycle is measured through force and torque measurements. Wake–foil interactions that improve trailing foil performance are analysed with time-resolved particle image velocimetry. Constructive and destructive wake–foil interactions are compared, showing that trailing-foil performance improves by either avoiding wake vortices or interacting directly with them. By interacting with the primary wake vortex, the latter configuration sees no power loss during the cycle. System power from the two foils is found to be maximised when the leading foil operates at an intermediate $\alpha _{T/4}$ range, and when the trailing foil avoids wake vortices. This optimal array configuration sees both foils operating with different kinematics compared with the optimal kinematics of a single oscillating foil.

A spherical capsule (radius $R$) is suspended in a viscous liquid (viscosity $\mu$) and exposed to a uniaxial extensional flow of strain rate $E$. The elasticity of the membrane surrounding the capsule is described by the Skalak constitutive law, expressed in terms of a surface shear modulus $G$ and an area dilatation modulus $K$. Dimensional arguments imply that the slenderness $\epsilon$ of the deformed capsule depends only upon $K/G$ and the elastic capillary number ${Ca}=\mu R E/G$. We address the coupled flow–deformation problem in the limit of strong flow, ${Ca}\gg 1$, where large deformation allows for the use of approximation methods in the limit $\epsilon \ll 1$. The key conceptual challenge, encountered at the very formulation of the problem, is in describing the Lagrangian mapping from the spherical reference state in a manner compatible with hydrodynamic slender-body formulation. Scaling analysis reveals that $\epsilon$ is proportional to ${Ca}^{-2/3}$, with the hydrodynamic problem introducing a dependence of the proportionality prefactor upon $\ln \epsilon$. Going beyond scaling arguments, we employ asymptotic methods to obtain a reduced formulation, consisting of a differential equation governing a mapping field and an integral equation governing the axial tension distribution. The leading-order deformation is independent of the ratio $K/G$; in particular, we find the approximation $\epsilon ^{2/3} {Ca}\approx 0.2753\ln (2/\epsilon ^2)$ for the relation between $\epsilon$ and $Ca$. A scaling analysis for the neo-Hookean constitutive law reveals the impossibility of a steady slender shape, in agreement with existing numerical simulations. More generally, the present asymptotic paradigm allows us to rigorously discriminate between strain-softening and strain-hardening models.

The modification of a gravity current past a thin two-dimensional barrier is studied experimentally, focusing on propagation characteristics as well as turbulence and mixing at the gravity-current head near the obstacle. The broader aim is to develop an eddy-diffusivity parametrisation based on local governing variables to represent gravity-current/obstacle interactions in numerical weather prediction models. A gravity current is produced in a rectangular tank by releasing a salt solution via a lock-exchange mechanism into an aqueous ethanol solution with matched refractive index, and it is allowed to interact with the barrier. A combined particle image velocimetry and planar laser-induced fluorescence system is used to obtain instantaneous velocity and density fields. The experiments span two Reynolds numbers and four obstacle heights, with each case replicated ten times for conducting phase-aligned ensemble averaging. Four evolutionary stages of the front are identified: approach, vertical deflection, collapse and reattachment. Particular focus is placed on the vertical deflection and collapse stages (dubbed collision phase), which includes flow (hydraulic) adjustment, flow modulation over the obstacle, instabilities, turbulence and mixing, and relaxation to a gravity current downstream. The time scales for various flow stages were identified. The results demonstrate that the normalised eddy diffusivity changes significantly throughout these stages and with the dimensionless height of the obstacle.

Rogue waves are associated with various ocean processes, both at the coast and in the open ocean. In either zone, inhomogeneities in the wave field caused by shoaling, crossing seas or current interactions disturb the wave statistics, increasing the rogue wave probability and magnitude. Such amplification of the frequency of rogue waves and their intensity, i.e. the maximum normalised height, have been attested to in numerical simulations and laboratory studies, in particular for wave–current interactions. In this study, we investigate the effect of the current intensity and direction on rogue wave probability, by analysing long-term observations from the southern North Sea. We observe that the amplification is similar for opposing and following currents. Despite the sea states being dominantly broadbanded and featuring a large directional spread, the anomalous statistics are of the same order of magnitude as those observed in unidirectional laboratory experiments for stationary currents.

Local eddy viscosity and diffusivity models are widely used to understand and predict turbulent flows. However, the local approximations in space and time are not always valid for actual turbulent flows. Recently, a non-local eddy diffusivity model for turbulent scalar flux was proposed to improve the local model and was validated using direct numerical simulation (DNS) of homogeneous isotropic turbulence with an inhomogeneous mean scalar (Hamba 2022 J. Fluid Mech. 950, A38). The model was modified using the scale-space energy density in preparation for application to inhomogeneous turbulence (Hamba 2023 J. Fluid Mech. 977, A11). In this paper, the model is further improved by incorporating the effects of turbulence anisotropy, inhomogeneity and wall boundaries. The needed inputs from the flow to evaluate the model are the Reynolds stress and the energy dissipation rate. With the improved model, one- and two-dimensional profiles ofthe non-local eddy diffusivity in turbulent channel flow are evaluated and compared with the exact DNS values. The DNS results reveal a contribution to the scalar flux from the mean scalar gradient in a wide upstream region. Additionally, the temporal profile of the non-local eddy diffusivity moves downstream, diffuses anisotropically and is tilted towards the bottom wall. The model reproduces this behaviour of mean flow convection and anisotropic turbulent diffusion well. These results indicate that the non-local eddy diffusivity model is useful for gaining insights into scalar transport in inhomogeneous turbulence.

A ground vortex engendered by the interaction of uniform flow over a plane surface with suction into a cylindrical conduit whose axis is normal to the cross-flow and parallel to the ground plane is investigated in wind tunnel experiments. The formation and evolution of the columnar vortex and its ingestion into the conduit’s inlet are explored using planar/stereo particle image velocimetry over a broad range of formation parameters that include the speeds of the inlet and cross-flows and the cylinder’s elevation above the ground plane with specific emphasis on the role of the surface vorticity layer in the vortex initiation and sustainment. The present investigations show that the appearance of a ground vortex within the inlet face occurs above a threshold boundary of two dimensionless formation parameters, namely the inlet’s momentum flux coefficient and its normalised elevation above the ground surface. Transitory initiations of wall-normal columnar vortices are spawned within a countercurrent shear layer that forms over the ground plane within a streamwise domain on the inlet’s leeward side by the suction flow into the duct. At low suction speeds, these wall-normal vortices are advected downstream with the cross-flow but when their celerity is reversed with increased suction, they are advected towards the cylinder’s inlet, gain circulation and stretch along their centrelines and become ingested into the inlet at a threshold defined by the formation parameters. Finally, the present investigations demonstrated that reduction of the countercurrent shear within the wall vorticity layer by deliberate, partial bypass of the inlet face flow through the periphery of the cylindrical duct can significantly delay the ingestion of the ground vortex to higher level thresholds of the formation parameters.

We have established a novel molecular kinetic model that addresses fundamental challenges in the non-equilibrium transport of nanoscale confined fluids, such as rarefaction and fluid inhomogeneities, which are crucial to a range of scientific and engineering fields. The proposed model explicitly considers fluid–solid molecular interactions in the transport equations, eliminating the reliance on predefined boundary conditions. By consistently accounting for molecular interactions between fluids and solids, the unified model captures both intrinsic and apparent non-hydrodynamic effects, as well as real fluid behaviours. Rigorous comparisons with molecular dynamics simulations demonstrate that the present model accurately predicts unique features of strongly inhomogeneous fluid flows, including fluid adsorption, solvation force, velocity slip and temperature jump. Therefore, this mesoscopic model bridges the gap between molecular-scale dynamics and macroscopic hydrodynamics, enabling a practical simulation tool for nanoscale surface-confined flows. Moreover, it offers valuable insights into the molecular mechanisms underlying anomalous transport phenomena observed in confined flows, such as the disappearance and re-emergence of the Knudsen minimum.

Large-eddy simulations have been conducted to investigate the decay law of homogeneous turbulence influenced by a magnetic field within a cubic domain, employing periodic boundary conditions. The initial integral Reynolds number is approximately 1000, while the initial interaction number $N$ ranges from 0.1–100. The results reveal that the Joule cone angle $\theta$, half of the Joule cone, decays as $\cos \theta \sim t^{-1/2}$ when $N \gg 1$. In the nonlinear stage, small-scale vortices gradually recover and restore three-dimensionality. Moreover, the corresponding critical state at small scales, marking the transition from quasi-two-dimensional structure to the onset of three-dimensionality, has been quantitatively defined. During the linear stage, based on the true magnetic damping number ($\tau _t = \rho / (\sigma {\boldsymbol{B}}^2 \cos ^2 \psi )$, where $\sigma$, $\boldsymbol{B}$ and $\psi$ denote the electrical conductivity, magnetic field and the angle between the wavevector and $\boldsymbol{B}$ in Fourier space, respectively), Moffatt’s decay law, $K \sim t^{-1/2}$, manifests at distinct times and zones in the Fourier space, with $K$ signifying turbulent kinetic energy. In the nonlinear stage, for $N \gg 1$, a $-3$ slope in the energy power spectrum is prominently observed over an extended period. The near-equivalence of the characteristic time scales of inertial and Lorentz forces in the inertial subrange suggests a quasiequilibrium state between energy transfer and Joule dissipation in Fourier space, thereby corroborating the hypothesis proposed by Alemany et al. 1979 Journal de Mecanique 18(2): 277–313. Additionally, it is observed that pressure mediates energy transfer from horizontal kinetic energy ($K_{\parallel }$) to vertical kinetic energy ($K_{\bot }$), accelerating the decay of $K_{\parallel }$. Notably, concurrent inverse and direct energy transfers emerge during the decay process. Our analysis reveals that the ratio $R$ of the maximum inverse to maximum direct energy flux correlates with the dimensionality of the turbulence, following the scaling law $R\sim (\cos \theta )^{-2.2}$.

Flow over bluff bodies encounters instability at supercritical Reynolds numbers, exhibiting the periodic vortex shedding that leads to structural vibrations and acoustic noise. In this paper, a new aerodynamic shape optimisation strategy based on resolvent analysis is proposed to passively control the vortex shedding over two-dimensional cylinders. Firstly, we show that when the flow satisfies the rank-1 approximation, minimizing the maximal resolvent gain enhances flow stability. Secondly, we formulate the geometry-constrained resolvent-based optimisation problem that can be solved by the nonlinear conjugate gradient algorithm. Compared with conventional stability-based optimisation, the proposed approach is more effective as it avoids the cumbersome eigendecomposition of the high-dimensional Jacobian matrix. The efficacy of the proposed resolvent-based optimisation is validated through improving the stability of the one-dimensional Ginzburg–Landau equation. Thirdly, this approach is applied to suppress the vortex shedding of bluff bodies, initialised by a circular cylinder. Although the optimisation is performed at a subcritical state $Re = 40$, reduced vortex shedding and drag forces can be achieved at supercritical Reynolds numbers, while the critical Reynolds number is extended from $47$ to $60$. Dynamic mode decomposition is then performed to reveal that the optimised system becomes more stable and satisfies the rank-1 approximation. Finally, we demonstrate that the combined effects of the flattened surface and the Coanda effect delay flow separation, keeping the separation point nearly unchanged at supercritical Reynolds numbers (e.g. between 80 and 140) for the optimised geometry. This results in a substantial reduction in the strength of vortex shedding, which in turn leads to decreased drag forces. The optimised shape still achieves drag reduction in turbulent flows at a relatively high Reynolds number.

We derive boundary conditions for two-dimensional parallel and non-parallel flows at the interface of a homogeneous and isotropic porous medium and an overlying fluid layer by solving a macroscopic closure problem based on the asymptotic solution to the generalised transport equations (GTE) in the interfacial region. We obtained jump boundary conditions at the effective sharp surface dividing the homogeneous fluid and porous layers for either the Darcy or the Darcy–Brinkman equations. We discuss the choice of the location of the dividing surface and propose choices which reduce the distance with the GTE solutions. We propose an ad hoc expression of the permeability distribution within the interfacial region which enables us to preserve the invariance of the fluid-side-averaged velocity profile with respect to the radius $r_0$ of the averaging volume. Solutions to the GTE, equipped with the proposed permeability distribution, compare favourably with the averaged solutions to the pore-scale simulations when the interfacial thickness $\delta$ is adjusted to $r_0$. Numerical tests for parallel and non-parallel flows using the obtained jump boundary conditions or the generalised transport equations show quantitative agreement with the GTE solutions, with experiments and pore-scale simulations. The proposed model of mass and momentum transport is predictive, requiring solely information on the bulk porosity and permeability and the location of the solid matrix of the porous medium. Our results suggest that the Brinkman corrections may be avoided if the ratio $a=\delta /\delta _B$ of the thickness $\delta$ of the interfacial region to the Brinkman penetration depth $\delta _B$ is large enough, as the Brinkman sub-layer is entirely contained within the interfacial region in that case. Our formulation has been extended to anisotropic porous media and can be easily dealt with for three-dimensional configurations.

We examined theoretically, experimentally and numerically the origin of the acoustothermal effect using a standing surface acoustic wave-actuated sessile water droplet system. Despite a wealth of experimental studies and a few recent theoretical explorations, a profound understanding of the acoustothermal mechanism remains elusive. This study bridges the existing knowledge gap by pinpointing the fundamental causes of acoustothermal heating. Theory broadly applicable to any acoustofluidic system at arbitrary Reynolds numbers, going beyond the regular perturbation analysis, is presented. Relevant parameters responsible for the phenomenon are identified and an exact closed-form expression delineating the underlining mechanism is presented. We also examined the impact of viscosity on acoustothermal phenomena by modelling temperature profiles in sessile glycerol–water droplets, underscoring its crucial role in modulating the acoustic field and shaping the resulting acoustothermal profile. Furthermore, an analogy between the acoustothermal effect and the electromagnetic heating is drawn, thereby deepening the understanding of the acoustothermal process.

We investigate experimentally the effect of salinity and atmospheric humidity on the drainage and lifetime of thin liquid films motivated by conditions relevant to air–sea exchanges. We show that the drainage is independent of humidity and that the effect of a change in salinity is reflected only through the associated change in viscosity. On the other hand, film lifetime displays a strong dependence on humidity, with more than a tenfold increase between low and high humidities: from a few seconds to tens of minutes. Mixing the air surrounding the film also has a very important effect on lifetime, modifying its distribution and reducing the mean lifetime of the film. From estimations of the evaporation rate, we are able to derive scaling laws that describe well the evolution of lifetime with a change of humidity. Observations of the black film, close to the top where the film ruptures, reveal that this region is very sensitive to local humidity conditions.

Noise source identification has been a long-standing challenge for decades. Although it is known that sound sources are closely related to flow structures, the underlying physical mechanisms remain controversial. This study develops a sound source identification method based on longitudinal and transverse process decomposition (LTD). Large-eddy simulations were performed on the flow around a cylinder at a Reynolds number of 3900. Using the new LTD method, sound sources in the cylinder flow were identified, and the mechanisms linking flow structures with noise generation were discussed in detail. Identifying the physical sound sources from two levels, low-order theory and high-order theory, the physical mechanism of wall sound sources was also analysed. Results indicate that the sound sources in the flow field mainly come from the leading edge, shear layer and wake region of the cylinder. The high-order theory reveals that sound sources are correlated with the spatio-temporal evolution of enstrophy, vortex stretching and surface deformation processes, this reflecting the coupling between transversal and longitudinal flow fields. The boundary thermodynamic flux and boundary dilatation flux distribution of the cylinder were analysed. Results indicate that the wall sound sources mainly come from the separation point and have a disorderly distribution on the leeward side of the cylinder, which is the main region where longitudinal variables enter the fluid from the wall surface, and the wall sound source is related to the boundary enstrophy flux.

Numerous studies showed that the flow and transport phenomena in angstrom channels are different from existing understandings. In this work, we investigate the electrokinetic phenomena in a charged angstrom channel, including homogeneous and heterogeneous charge distributions at the wall to mimic the charging mechanisms of electrified metal-like surfaces and deprotonated dielectric surfaces, respectively. Our results show that both the streaming current and the flow velocity linearly increase as the applied pressure increases in a homogeneously charged system. However, in a heterogeneously charged system, the streaming current is activated only when the applied pressure exceeds a critical threshold. This behaviour arises from the strong Coulomb interactions between counterions and the surface charge, manifesting as an obvious nonlinear feature. The dissociation of counterions from the surface charge may not only cause pressure-dependent streaming conductance but also reduce the friction coefficient of the system, thus the flow resistance, when the system friction is governed by the bound ions. We found that such pressure-dependent streaming conductance gradually weakens as the channel size increases and reaches the regime of classical nanofluidic theories. Taking one-dimensional non-equilibrium statistics and Markov chains for the sequence evolution of bound-ion dissociation, our theory can well explain the pressure-dependent streaming conductance and water permeability in angstrom charged channels. Voltage-driven nonlinear ionic transport and electro-osmosis were also observed in heterogeneously charged systems. Our findings will be helpful for understanding the ionic transport in angstrom-scale channels and possibly useful in ion separations.

Embedding the intrinsic symmetry of a flow system in training its machine learning algorithms has become a significant trend in the recent surge of their application in fluid mechanics. This paper leverages the geometric symmetry of a four-roll mill (FRM) to enhance its training efficiency. Stabilising and precisely controlling droplet trajectories in an FRM is challenging due to the unstable nature of the extensional flow with a saddle point. Extending the work of Vona & Lauga (Phys. Rev. E, vol. 104(5), 2021, p. 055108), this study applies deep reinforcement learning (DRL) to effectively guide a displaced droplet to the centre of the FRM. Through direct numerical simulations, we explore the applicability of DRL in controlling FRM flow with moderate inertial effects, i.e. Reynolds number $\sim \mathcal{O}(1)$, a nonlinear regime previously unexplored. The FRM’s geometric symmetry allows control policies trained in one of the eight sub-quadrants to be extended to the entire domain, reducing training costs. Our results indicate that the DRL-based control method can successfully guide a displaced droplet to the target centre with robust performance across various starting positions, even from substantially far distances. The work also highlights potential directions for future research, particularly focusing on efficiently addressing the delay effects in flow response caused by inertia. This study presents new advances in controlling droplet trajectories in more nonlinear and complex situations, with potential applications to other nonlinear flows. The geometric symmetry used in this cutting-edge reinforcement learning approach can also be applied to other control methods.