This paper presents an experimental investigation focusing on the impact of structural damping on the flow-induced vibration (FIV) of a set of generic three-dimensional bodies, in this case, elastically mounted oblate spheroids. The objective is to identify and analyse the two primary FIV responses: vortex-induced vibration (VIV) and galloping, and how these vary with structural damping ratio. The VIV response has similarities to that observed for a sphere, reaching a maximum amplitude of approximately one major diameter. However, and not seen in the sphere case, a galloping-like response exhibits a linear amplitude growth as the reduced velocity is increased beyond the normal resonant range, akin to the transverse galloping response seen for a D-section or elliptical cross-section cylinder. By increasing the damping ratio, this aerodynamic-instability-driven response is effectively suppressed. However, increased damping also significantly reduces the VIV response, decreasing its maximum amplitude and contracting the VIV synchronisation, or lock-in, region. These results suggest that three-dimensional spheroids, as for two-dimensional cylindrical bodies such as D-section and elliptical cylinders, can encounter asymmetric aerodynamic forces that support movement-induced vibration, resulting in substantial body oscillation – beyond that expected under VIV alone. The study indicates that modifying the structural damping ratio can facilitate a transition between the VIV and galloping responses. These findings offer novel insights into the dynamics of fluid–structure interactions and have potential implications for designing structures and devices that can experience resonant flow conditions. Additionally, the energy harvesting performance of oblate spheroids has been evaluated, revealing that the afterbody significantly influences energy harvesting capabilities. Notably, an oblate spheroid can extract up to $50\,\%$ more power from the fluid flow than a sphere.

Using pore-resolved direct numerical simulation (DNS), we investigate passive scalar transport at a unit Schmidt number in a turbulent flow over a randomly packed bed of spheres. The scalar is introduced at the domain’s free-slip top boundary and absorbed by the bed, which maintains a constant and uniform scalar value on the sphere surfaces. Eight cases are analysed, which are characterised by friction Reynolds numbers of ${\textit{Re}}_\tau \in [150, 500]$ and permeability Reynolds numbers of ${\textit{Re}}_{{\kern-1pt}K} \in [0.4, 2.8]$, while flow depth-to-sphere-diameter ratios vary within $h/D \in \{ 3, 5, 10 \}$ and the roughness Reynolds numbers lie within $k_s^+ \in [20,200]$. For cases with comparable ${\textit{Re}}_\tau$, the permeable wall behaviour enhances scalar absorption, as indicated by increases in the Sherwood number and the scalar roughness function $\Delta c^+$ over ${\textit{Re}}_{{\kern-1pt}K}$. At progressively higher ${\textit{Re}}_{{\kern-1pt}K}$, the scalar absorption diverges increasingly from the momentum absorption, as its distribution peaks deeper below the crests of the sphere pack and spreads over a wider vertical region. The fixed-value scalar boundary condition emphasises certain similarities between the scalar and velocity fields. Near-interface scalar fluctuations are correlated with streamwise velocity fluctuations, and the turbulent Schmidt number remains close to its value in the free-flow region. Compared with zero-flux scalar boundary conditions, prescribing a uniform scalar value on the sphere surfaces reduces spatial heterogeneity within the pore space, thereby limiting both dispersive transport and the form-induced production of temporal scalar fluctuations.

We study the force exerted by the uniform flow of a Bingham fluid around two- and three-dimensional particles in the regime of slow creeping flow and relatively weak yield stress. Matched asymptotic expansions are employed to couple a viscously dominated Stokes flow close to the particle with a far field in which the yield stress and viscous stresses are comparable. The far-field region is therefore modelled as a Bingham fluid driven by a point force at the origin (i.e. a viscoplastic Stokeslet). It features the full nonlinearity of the viscoplastic rheology, and its solution is computed through direct numerical simulation. Asymptotic matching then leads to a quasi-analytical expression for the drag force in terms of the dimensionless Bingham number ${\textit{Bi}}$, which measures the magnitude of the yield stress relatively to viscous effects at the particle scale. We deploy this methodology to determine the drag force on a sphere in three dimensions, and circular and elliptic cylinders in two dimensions, confirming our asymptotic predictions by comparison with full numerical simulations of the motion. We also generalise the three-dimensional result to arbitrary particles. The viscoplastic correction to the Newtonian drag in three dimensions scales as ${\textit{Bi}}^{1/2}$. In two dimensions, however, the effects of viscoplasticity are non-negligible at leading order. The drag varies with $[\ln (1/{\textit{Bi}})]^{-1}$, but this asymptotic result is only approached very slowly. Instead, an accurate representation of the drag is derived in terms of a single algebraic relation between the drag and the Bingham number.

We present an acoustic characterisation of a model-scale wind turbine using large eddy simulation and the acoustic analogy. The analysis is representative of medium-sized turbines with low tip Mach number (${\sim} 0.10$). The fluid dynamic analysis revealed: a turbulent boundary layer over the blades, together with a trailing edge vortex sheet; a complex near-wake structure, including tip and root vortices; an intermediate wake with vortex instabilities triggering leap-frogging and vortex grouping mechanisms; and a far wake characterised by fully developed turbulence. Two primary noise generation mechanisms were identified. The unsteady pressure field over the turbine surface generates tonal noise at the blade passing frequency and a high-frequency broadband noise, associated with the trailing edge vortex sheet (linear-noise contribution). The turbulent wake generates broadband low-frequency noise, driven by the complex fluid-dynamic processes outlined previously (nonlinear noise contribution). The linear part of the noise was found to dominate over the nonlinear one in the acoustic far field, while the opposite is true in the acoustic near field. As a composition of the two contributions to the noise, the directivity exhibits a non-symmetric dipole shape oriented along the flow direction, with lobes recovering symmetry moving from the near to the far field. Finally, analysis of the acoustic decay rates reveals that the linear term in the near field decays according to an $r^{-(n+1)}$ law within the rotor plane, where n is the number of blades, consistent with recent findings on the acoustics of rotating sources.

One of the challenges with modelling subsurface flows is the uncertainty in measurements of geological properties, mostly due to limited resolution in observation methods. Many subsurface flows can be modelled as a gravity current, which, for uniform material properties and power-law injection rate, has a well-characterised similarity solution. The similarity solution forms a dynamical attractor that is typically approached rapidly from a host of initial conditions. Here, we consider the impact of transverse variations to the permeability field by performing a perturbation analysis of the self-similar spreading. This treats the response as perturbations to the self-similar flow. We restrict our focus to permeability fields that vary laterally, or across the flow, starting with the simple case of a sinusoidal perturbation to a uniform permeability. At early times, the height and nose position of the current are determined by the local permeability, and deviations to the height and nose grow at the same rate as the mean, and proportional to the amplitude, of the permeability variation. The transition between the early and late time regimes is governed by the wavelength of the permeability. At late times, lateral spreading between high and low permeability streaks is dominant; the height deviations decay, and the nose deviations approach a steady state. The magnitudes of both depend on the product of the wavelength and amplitude of the permeability. The single mode sets the groundwork for examining more complex, multimodal permeabilities, which are more representative of real geological structures.

We investigate the scale-by-scale transfers of energy, enstrophy and helicity in homogeneous and isotropic polymeric turbulence using direct numerical simulations. The study relies on the exact scale-by-scale budget equations, derived from the governing model equations, that fully capture the back-reaction of polymers on the fluid dynamics. Polymers act as dynamic sinks and sources and open alternative routes for interscale transfer whose significance is modulated by their elasticity, quantified through the Deborah number (${\textit{De}}$). Polymers primarily deplete the nonlinear energy cascade at small scales, by attenuating intense forward and inverse transfer events. At sufficiently high ${\textit{De}}$, a polymer-driven flux emerges and dominates at small scales, transferring on average energy from larger to smaller scales, while allowing for localised backscatter. For enstrophy, polymers inhibit the stretching of vorticity, with fluid–polymer interactions becoming the primary enstrophy source at high ${\textit{De}}$. Accordingly, an analysis of the small-scale flow topology reveals that polymers promote two-dimensional straining states and enhance the occurrence of shear and planar extensional flows, while suppressing extreme rotation events. Helicity, injected at large scales, exhibits a transfer mechanism analogous to energy, being dominated by nonlinear dynamics at large scales and by polymer-induced fluxes at small scales. Polymers enhance the breakdown of small-scale mirror symmetry, as indicated by a monotonic increase in relative helicity with ${\textit{De}}$ across all scales.

Contact between fluctuating, fluid-lubricated soft surfaces is prevalent in engineering and biological systems, a process starting with adhesive contact, which can give rise to complex coarsening dynamics. One representation of such a system, which is relevant to biological membrane adhesion, is a fluctuating elastic interface covered by adhesive molecules that bind and unbind to a solid substrate across a narrow gap filled with a viscous fluid. This flow is described by the stochastic elastohydrodynamic thin film equation, which incorporates thermal fluctuations into the description of viscous nanometric thin-film flow coupled to elastic membrane deformation. The average time it takes the fluctuating elastic membrane to adhere is predicted by the rare event theory, increasing exponentially with the square of the initial gap height. When the forces arising from spring-like adhesive molecules are included in the simulations, thermal fluctuations initiate phase separation of domains of bound and unbound molecules. The coarsening process of these unbound pockets displays close similarities to classical Ostwald ripening; however, the inclusion of hydrodynamics affects power-law growth. In particular, we identify a new bending-dominated coarsening regime, which is slower than the well-known tension-dominated case.

A large-scale parametric study of the flow over the prolate spheroid is presented to understand the effect of Reynolds number and angle of attack on the separation, the wake formation and the loads. Large-eddy simulation is performed for six Reynolds numbers ranging from ${\textit{Re}} = 0.15\times 10^6$ to $4 \times 10^6$ and for eight angles of attack ranging from $\alpha = 10^\circ$ to $\alpha = 90^\circ$. For all the cases considered, the boundary layer separates symmetrically and forms a recirculation region. Several distinct flow topologies are observed that can be grouped into three categories: proto-vortex, coherent vortex and recirculating wake. In the proto-vortex state, the recirculation does not have a distinct centre of rotation, instead, a two-layer detached flow structure is formed. In the coherent vortex state, the separated shear layer rolls into a three-dimensional vortex that is aligned with the axis of the spheroid. This vortex has a clear centre of rotation corresponding to a minimum of pressure and transforms the transverse momentum from the separated shear layer into axial momentum. In the recirculating wake regime, the recirculation is incoherent and the primary separation forms a dissipative shear layer that is convected in the direction of the free stream. This symmetric pair of shear layers bounds a low-momentum recirculating cavity on the leeward side of the spheroid. The properties of these states are not constant, but evolve along the axis of the spheroid and are dictated by the characteristics of the boundary layer at separation. The variation of the flow with Reynolds number and angle of attack is described, and its connection to the loads on the spheroid are discussed.

Not all particulate matter carried by fluid flows has constant buoyancy. In some cases, the buoyancy of a particle can change dynamically based on the local flow. We refer to this phenomenon as ‘active buoyancy.’ Although actively buoyant particles are found throughout nature, their dynamics is not well understood, particularly when they are also highly inertial. Motivated by the problem of the transport of firebrands in wildfires, whose effective buoyancy is modulated by conductive and convective heat transfer to the surrounding fluid, we conducted a series of experiments to investigate the effects of active buoyancy on particle settling in quiescent fluid. We find that, depending on the control parameters, active buoyancy can either hinder or enhance settling, in some cases to a large extent. The details of this settling modulation, however, cannot be simply captured by any single control parameter. Analysis of the trajectories of the falling particles showed that they fall along nearly sinusoidal paths even though the particle Reynolds number is higher than expected for this regime, suggesting that active buoyancy may act to stabilise their wakes. Our results suggest both that models of actively buoyant particles such as firebrands must account for the effects of active buoyancy and that there is still much to be understood about the behaviour of these complex particles.

The actuator line model (ALM) is an approach commonly used to represent lifting and dragging devices like wings and blades in large-eddy simulations (LES). The crux of the ALM is the projection of the actuator point forces onto the LES grid by means of a Gaussian regularisation kernel. The minimum width of the kernel is constrained by the grid size; however, for most practical applications like LES of wind turbines, this value is an order of magnitude larger than the optimal value that maximises accuracy. This discrepancy motivated the development of corrections for the actuator line, which, however, neglect the effect of unsteady spanwise shed vorticity. In this work we develop a model for the impact of spanwise shed vorticity on the unsteady loading of an aerofoil modelled as a Gaussian body force distribution, where the model is applicable within the regime of unsteady attached flow. The model solution is derived both in the time and frequency domain and features an explicit dependence on the Gaussian kernel width. We verify the model with ALM-LES for both pitch steps and periodic pitching. The model solution is compared withTheodorsen theory and validated with both computational fluid dynamics using body fitted grids and experiment. It is concluded that the optimal kernel width for unsteady aerodynamics is approximately $40\,\%$ of the chord. The ALM is able to predict the magnitude of the unsteady loading up to a reduced frequency of $k\approx 0.2$.

We report experiments in a long tank showing that transverse Benjamin–Feir instability of Stokes waves can lead to a significant energy transfer into oscillations across the tank. We observe frequency downshift in the long-term evolution of Stokes waves essentially when significant energy is transferred to narrow-banded transverse modes. Experiments with Stokes waves are often carried out with wavelengths that are not long compared with the width of the tank, permitting transverse instabilities to be excited. With insufficient resolution of measurements across the tank, transfer of energy to transverse modes can be misinterpreted as dissipation. Our experiments suggest that the frequency downshift depends as much on energy-preserving transverse modulations of type I as it does on damping or wave breaking. Broad-banded unstable modulations of type II do not imply downshift.

The guided-jet waves (GJWs) that may be trapped into a jet are investigated by simulating the propagation of the waves generated by an acoustic source on the axis of a jet at a Mach number of 0.95. The flow is modelled as a cylindrical shear layer to avoid reflections in the axial direction. For the source frequencies considered, GJWs belonging to the first two radial GJW axisymmetric modes are observed. They propagate in the upstream or downstream directions, and are entirely or partially contained in the flow, depending on the frequency. Their amplitudes are quantified. In the frequency–wavenumber space, they lie along the GJW dispersion curves predicted using linear-stability analysis. At specific spatial locations, they vary strongly and sharply with the frequency, exhibiting tonal-like peaks near the frequencies of the stationary points in the dispersion curves where the GJWs are standing waves with zero group velocity. Given the flow configuration, these properties can be attributed to propagation effects not requiring axial resonance between upstream- and downstream-travelling waves. Finally, it can be noted that, upstream of the source, outside the jet, the GJW amplitudes fluctuate in a reverse sawtooth manner with very intense peaks up to 30 dB higher than the levels obtained without flow at 10 jet radii from the source, similarly to the GJW footprints in the near-nozzle spectra of high-subsonic jets.

The growth of wall-mounted ice within channel flow which leads to a constriction is of significant practical relevance, especially in applications relating to aero-icing, large-scale pipe networks and mechanical systems. Whilst earlier works have treated ice constrictions as independent of the oncoming flow, few models explicitly account for the two-way coupling between the thermal and dynamical properties of the fluid and the evolving ice. To this end, the present work seeks to describe the interaction between high-Reynolds-number channel flow and constricting ice boundaries governed by Stefan conditions. Numerical simulations of the model indeed reveal that ice forming on the channel walls grows inwards towards the centreline and subsequently creates almost total constriction. In other parameter regimes, however, there is no ice formation. Using both a numerical and asymptotic approach, we identify regions of parameter space in which ice formation, and subsequently flow constriction, does or does not occur.

This work investigates the formation mechanism of the turbulent secondary vortex street (SVS) which appears in the far wake of bluff bodies, when the (primary) Kármán vortex street is absent. The turbulent wakes of four types of highly porous bluff bodies (plates/meshes) are characterised via time-resolved particle image velocimetry and large eddy simulations. The effect of ambient turbulence and initial conditions on SVS development is also examined, by installing a turbulence grid upstream of the bodies, and by varying the homogeneity of the bluff body porosity. Our results indicate that the SVS is a far-wake evolution of near-wake shear-layer vortices which, in the absence of the vortex shedding instability, continually grow and are finally arranged into alternating vortices. Free-stream turbulence and body inhomogeneity are found to significantly influence SVS development by amplifying mixing and attenuating the shear-layer instabilities of the near wake, which in turn lead to the formation of weaker and less coherent SVS structures further downstream.

Direct numerical simulations of two-phase, free-surface flow past a fully submerged, fixed circular cylinder are conducted for transitional Reynolds numbers $400 \leqslant {\textit{Re}} \leqslant 2000$, with Weber number ${\textit{We}} = 1000$, Froude number ${\textit{Fr}} = 1$ and a fixed gap ratio $G = 0.5$. This parameter combination corresponds to the gas entrainment regime characterised by the production of multiscale gas bubbles through interface breakup in the wake, which is of particular interest for its implications in enhancing gas transfer and mixing in environmental and engineering flows, such as air–water gas exchange processes in rivers and oceans, and the design and performance of naval and offshore structures. For ${\textit{Re}}= 400$, the jet forced through the $0.5D$ gap where $D$ is the diameter of the cylinder, efficiently convects opposite-signed vorticity downstream, suppressing the classical von Kármán instability and yielding a quasisteady recirculation bubble. The jet’s stabilising influence, however, breaks down once ${\textit{Re}} \approx 500$: periodic vortex shedding re-emerges and the wake becomes unsteady in spite of the continuing jet. The corresponding dimensionless shedding frequency Strouhal number $St$ grows with ${\textit{Re}}$ as $0.52-72.7{\textit{Re}}^{-1}$. The onset of unsteadiness first shortens the mean separation length but then drives it towards a saturation plateau for higher ${\textit{Re}}$ values. Surface rupture in the turbulent wake fragments entrained air into a multiscale bubble population whose number density follows $S_b(R_{\textit{eff}}) \propto R_{\textit{eff}}^{-6}$, consistent with gravity–capillary breakup in breaking waves, where $R_{\textit{eff}}$ represents the effective radii of the bubbles. Intermittency in entrainment corresponding to vortex shedding contrasts sharply with the finger-like structures observed under laminar conditions, underscoring the role of turbulent mixing. The coupled analysis of vorticity transport, shear-layer instability and bubble statistics elucidates how momentum exchange and air entrainment over a submerged body are governed under non-turbulent and turbulent conditions.

This work presents a comprehensive analysis of steady cone-jet electrospray (SCJ-ES) that captures the full range of its steady jet scales within the Taylor-cone electric field. We identify three fundamental regions, each governed by distinct scaling laws and dominant physical mechanisms: (i) the transition region, characterised by the balances that fix the emitted current; (ii) the charge convection-dominated region, where surface charge transport dominates total charge transport and the Taylor field drives jet acceleration; and (iii) the ballistic region, where the jet attains a fixed cylindrical scale before undergoing Rayleigh breakup into charged droplets. This refined theoretical framework harmonises existing models, particularly those using the Taylor–Melcher leaky dielectric model as an electrokinetic approximation for SCJ-ES. Notably, our newly proposed spatial scales achieve a remarkable collapse of published experimental SCJ-ES jet profiles. We also apply this framework to study the charge of resulting droplets using extensive literature data, observing significant differences between weak and strong electrolytes, consistent with recent findings.

We study the temperature–velocity (TV) relation for laminar adiabatic and diabatic hypersonic boundary layers. By applying an asymptotic expansion to the compressible boundary-layer temperature equation, we derive a first-order equation for the TV relation, where the zeroth-order solution is found to be the classical Crocco–Busemann quadratic relation. The ensuing relation predicts accurately the temperature profile by using the velocity for hypersonic boundary layers with Chapman, power and Sutherland viscosity laws, arbitrary heat capacity ratios, variable Prandtl numbers close to unity and Mach number of up to 10. The Mach-number- and wall-temperature-independent quantities in our relation are also investigated. The present relation has the potential to function as a temperature wall model for laminar hypersonic boundary layers, especially for cold-wall cases.

We highlight the complete transition from liquid-wall-film instability of an annular gas–liquid flow inside a nozzle to spray formation at the trailing edge, aiming to identify two distinct flow regimes of ripple waves and disturbance waves and to clarify their distinct fragmentation mechanisms. Experiments conducted under strictly controlled boundary conditions support our theoretical analysis, revealing that the onset of disturbance waves coincides with the liquid-film Weber number (${\textit{We}}$) of unity, marking a significant change in following fragmentation dynamics. For ${\textit{We}}\lt 0.5$, the liquid wall film forms three-dimensional ripple waves driven by the superposition of Kelvin–Helmholtz and Rayleigh–Taylor (RT) instabilities, with no disturbance waves present. At the trailing edge, the liquid film temporarily accumulates, extends into isolated ligaments along the axial direction via RT instability, and subsequently fragments into droplets through Plateau–Rayleigh instability, displaying a weak coupling between ripple wave dynamics and fragmentation. In contrast, for ${\textit{We}}\gt 0.5$, disturbance waves with long wavelengths and large amplitudes become prominent, superimposed on the base ripple waves. As these disturbance waves reach the trailing edge, they are spontaneously ejected as liquid sheets at the same frequency, forming transverse rims through RT instability and rapidly disintegrating into fine droplets. This regime demonstrates a direct coupling between disturbance-wave dynamics and fragmentation.

The interaction between a forward-facing step (FFS) and single-frequency Tollmien–Schlichting (TS) waves is investigated with experiments and two-dimensional (2-D) direct numerical simulations (DNS). Dedicated hot-wire anemometry and particle image velocimetry measurements in the vicinity of the FFS provide characterisation of the perturbation field, as well as validation of the DNS results. Comparison between experiments, 2-D DNS, and linear parabolised stability equations confirm the 2-D nature of the flow and the linearity of the instability mechanisms around the FFS. Upstream of the step, TS waves are gradually amplified by the increasing adverse pressure gradient. In the step vicinity, both mean flow and perturbation field exhibit abrupt distortion, with decoupling of the base flow-oriented growth rate components indicating significant non-modal evolution. Downstream of the step, the mean flow recovers to baseline conditions, but the perturbation field remains highly distorted. Linear stability theory results suggest the presence of superimposed modes on the original TS mode in this region. Despite their decay in the streamwise direction, their presence imprints modifications in the TS wave growth and shape, manifested as the tilting of the perturbation structure in and against the mean flow shear direction. This initiates a reversed Orr mechanism, characterised by a region of stabilisation followed by destabilisation further downstream. Eventually, the TS waves realign to their asymptotic (modal) behaviour. Overall, the FFS destabilises the TS wave far downstream. However, the streamwise extent and magnitude of the stabilisation downstream of the FFS remain significant.