This study experimentally investigates the aerodynamic drag reduction capabilities of distributed micro-roughness (DMR) coatings on a streamlined model, utilising the 1-m magnetic suspension and balance system (MSBS) at Tohoku University. Previous direct numerical simulations indicated that DMR can mitigate turbulent-energy growth by suppressing Tollmien–Schlichting waves and influencing the breakdown of streamwise vortices. The present work provides the first experimental validation of these effects using an interference-free MSBS, which is essential for accurate measurement in the laminar and transitional regimes. A streamlined model was tested with two rows of artificial tripping tape to induce transition; the DMR height was approximately 1 % of the local boundary layer thickness, significantly smaller than typical roughness elements. Direct aerodynamic drag measurements using the MSBS revealed a substantial reduction of up to 43.6 % within the transitional flow regime. Crucially, integrated analysis using wall-resolved large eddy simulations (LES) and dynamic oil-flow visualisation confirmed that this benefit does not mainly originate from the suppression of flow separation. The LES drag decomposition established that the total pressure-drag budget is subordinate to skin friction, a finding complemented by oil-flow observations, which revealed qualitatively similar flow patterns regardless of the surface condition. Consequently, the observed drag reduction is primarily ascribed to friction drag reduction achieved through the modification of the boundary layer state. These findings provide compelling experimental evidence for the efficacy of DMR and offer valuable insights for optimising surface designs for passive flow control.

This work presents a predictive framework for energy harvesting of two tandem and staggered flapping foils based on four canonical modes of vortex–foil interaction. The role of the incoming vortex generated by the leading foil in modulating the hydrodynamic load of the trailing foil is systematically analysed. Four canonical interaction modes are classified by the vortex rotation and its interaction position relative to the leading-edge vortex (LEV). The most effective configuration occurs when the foil encounters a counter-rotating vortex on the pressure side, which strengthens the LEV and consequently enhances the lift magnitude, with maximum efficiency achieved when vortex merging occurs near stroke reversal. A second constructive mode occurs when a co-rotating vortex on the suction side promotes LEV roll-up through favourable induced velocities. Force decomposition reveals that in both constructive modes, the incoming vortices improve the efficiency of the trailing foil by enhancing the unsteady lift through altering the local velocity to strengthen the LEV or promote its roll-up, while their low-pressure cores contribute marginally to the unsteady force. Two destructive modes are also observed: direct interaction of a counter-rotating vortex on the suction side leads to only a transient lift increase; a co-rotating vortex on the pressure side reduces the effective angle of attack and leads to the poorest performance. Building on these insights, a mechanism-based predictive framework is established to rapidly identify high-performance configurations without exhaustive parametric exploration. The framework applies broadly to different wake conditions and trailing-foil kinematics and guides the design of multi-foil energy-harvesting systems.

Torsional vibration and galloping of a triangular prism (TP) in steady flow is investigated numerically at mass ratio 2.5, low Reynolds number 150, three angles of attack, and reduced velocities up to 40. The vibration of the TP is torsional galloping characterised by monotonic increase of the angular amplitude with the increase of reduced velocity. The angular displacement and amplitude are non-dimensionalised by 2π/3, which is the geometrical period in the rotation direction. The response of the TP is well correlated to the direction of the fluid moment coefficient on a stationary TP with a constant rotation angle. The rotation angles are consistently divided into excitation and damping ranges where the directions of the mean fluid moment of a stationary TP and the rotational angle are the same, and opposite to each other, respectively. When the reduced velocity is less than a critical value, the vibration amplitude falls into a damping range, and it increases with the increase of reduced velocity. When the reduced velocity is greater than this critical value, the galloping of the TP is strong and very aperiodic. The vibration amplitude switches very frequently between multiple amplitudes. Every identified amplitude is very close to the upper boundary of a damping range. Multiple-amplitude torsional galloping is a distinct feature that was not found in transverse galloping in the crossflow direction.

An asymptotic approach is presented for studying the diffraction problem of in-duct acoustic modes by the termination of a rigid, circular duct with negligible thickness, based on Keller’s (1957, 1958, 1962) geometrical theory of diffraction (GTD). The diffracted field is solved first for the unflanged duct case, followed by an extension to the flanged duct case for which no closed-form exact solutions are available. The GTD solution for the primary diffraction of unflanged ducts, which invokes the half-plane diffraction coefficient obtained from Sommerfeld’s exact solution to the half-plane diffraction problem, is shown to yield agreement with the leading term of the Wiener–Hopf solution, in which the split functions of the Wiener–Hopf kernel are replaced with their steepest-descent approximations. Despite being developed for high-frequency analysis, experimental data from an unflanged duct and the numerical solutions for a flanged duct, both including the radiation directivity and the reflection coefficient, indicate that GTD solutions perform reasonably well even for wavelengths smaller than the duct’s radius, provided the frequency does not approach the cutoff condition. A reciprocity relation, which couples the absorption and emission of the (un)flanged duct, is derived from the reciprocity principle and verified by the Wiener–Hopf (if available) and GTD solutions. Physical insights are supplied by the GTD to explain why, for example, only a plane wave would be excited within the duct by a plane wave incident normally from the exterior of the duct. In cases where the uniform flow is present, an extended GTD formulation is proposed by utilising the canonical solution to the half-plane diffraction problem. The resulting correction factor for the diffracted field of unflanged ducts that accounts for an arbitrary amount of shedding vortices is consistent with Rienstra’s (1984 J. Sound Vib. vol. 94 (2), pp. 267–288) Wiener–Hopf solution. Potential strategies for addressing variants and extensions of the current work are outlined.

Fluid pumping in a horizontal slot using the pattern interaction effect has been analysed. This pumping is of interest as it operates without external energy sources beyond those required to create the necessary heating patterns. Activation of this effect involves a combination of fixed surface topography and adjustable heating patterns. The flow rate, including its direction, can be controlled by moving the heating pattern relative to the groove pattern. This analysis extends that of Abtahi & Floryan (2017 J. Fluid Mech. vol. 826, pp. 553–582), who considered only small-amplitude grooves in which the achieved flow rate is proportional to the groove amplitude. Grooves with arbitrary amplitudes spanning the slot were considered in the current analysis, and their most effective heights and distributions have been identified. A detailed analysis was conducted of groove and heating patterns described by a single Fourier mode applied to one or both plates bounding the slot. In all cases, the flow rate increased proportionally to the groove amplitude until an excessively large amplitude caused flow choking, and to the heating intensity until saturation was reached. The groove wavenumber of approximately 0.8 was found to be the most effective in the case of one groove plate, and 0.5–0.7 in the case of two groove plates. The flow rate decreases rapidly at both smaller and larger wavenumbers. The largest flow rate was achieved by placing grooves on both plates to form a wavy slot, with hot spots positioned halfway between the groove peaks and troughs.

Non-equilibrium evaporative flows play a central role in many nanoporous membrane technologies, where transport of fluids is confined by solid surfaces at the nanoscale. In this work, we propose a molecular kinetic model that consistently resolves the coupled interactions among vapour, liquid and solid surfaces in such flows. As a direct consequence of this bottom-up approach, the liquid–vapour, liquid–solid and vapour–solid interfaces form autonomously, and the effects of non-equilibrium and real fluids can be captured simultaneously, which does not need empirical models depending on ad hoc parameters such as the evaporation/condensation coefficients and contact angle. Accuracy of the model improves further by including the soft-collision effect in the pair correlation function and applying a temperature-dependent correction to the mean-field Vlasov term, as validated against the experimental data and the molecular dynamics simulations. Furthermore, when applied to unsteady, evaporation-driven liquid–vapour flows, the model reveals distinct dynamics due to surface wettability: the hydrophilic surfaces exhibit phenomena such as liquid meniscus breakage and enhanced evaporation flux, whereas the hydrophobic surfaces lead to disappearance of liquid droplets. These findings highlight the potential of the proposed molecular kinetic model as a powerful design tool for next-generation nano-technologies that leverage nano-confined phase change.

Görtler vortices developing over a concave wall support rapidly oscillating wavelike disturbances through secondary instabilities. Although experiments indicate that the finite-amplitude evolution of these waves acts as a precursor to turbulence transition, accurate and efficient prediction has remained out of reach. We overcome this limitation by using the parabolised coherent structures (PCS) method of Song & Deguchi (2025 J. Fluid Mech., vol. 1025, A42), which incorporates the nonlinear vortex-wave interaction into a standard spatial-marching approach. Our computational results agree well with the wave amplitude and displacement thickness observed in the widely known experiments of Swearingen & Blackwelder (1987 J. Fluid Mech., vol. 182, pp. 255–290).

The physics of settling suspensions under shear are investigated by theoretical and numerical analyses of unstable equilibrium solutions to the incompressible Navier–Stokes equations, coupled with an advection–diffusion–settling equation for a dilute phase of particles. Two cases are considered: the ‘passive scalar’ regime, in which the sediment is advected by the fluid motion, but concentrations are too dilute to affect the flow; and the ‘stratified’ regime, where a non-uniform vertical distribution of sediment due to particle settling leads to a bulk stratification that feeds back on the flow via buoyancy. In the passive regime, we characterise the structure of the resultant sediment concentration fields and derive formulae for transport fluxes of sediment with asymptotically low and high settling velocities. In the stratified regime, parametric continuation is employed to explore the dependence of states upon the bulk Richardson number $ \textit{Ri}_b$. Symmetry breaking in the governing equations leads to travelling wave solutions with a rich bifurcation structure. The maximum $ \textit{Ri}_b$ attained by these states depends non-monotonically on settling velocity and obeys asymptotic scalings that have also been observed to capture the dependence of the laminar–turbulent boundary in direct numerical simulations.

The incipient cavitation of a pair of unequal strength counter-rotating vortices undergoing the long-wavelength Crow instability is examined with high-speed video, acoustic measurements and volumetric particle tracking velocimetry. This work expands upon the previous studies of Chang & Ceccio (J. Acoust. Soc. Am., vol. 130, 2011, pp. 3209–3219) and Chang et al. (Phys. Fluids, vol. 24, 2012, 014107). Volumetric velocimetry results presented by Knister et al. (J. Fluid Mech., 2026) were used to predict the core pressures of the stretched secondary vortices. These data are combined with free-stream nuclei measurements to predict the rates of cavitation inception, which compared well with the directly measured inception rates. The acoustic emissions of incipient cavitation events are also related to the vortex properties and the nuclei content of the water. The reduced pressure in the stretched vortices is shown to be related primarily to the reduction in the core radius of the secondary vortex and not due to axial jetting or straining. The measured vortex dynamics indicates that the process leading to the pressure drop in the secondary vortex core is a transient process but not more rapid than the development of the Crow instability. In conclusion, these results show that a relatively simple model of cavitation inception in a stretched secondary vortex captures the essential physics connecting the nuclei population and the underlying vortical flow field, enabling prediction of the resulting observed inception rates. These results also indicate that the reduced pressures in vortical flow leading to inception are primarily due to the reduction in the core of the vortices and not due to substantive axial jetting. The pressure drop in the vortex cores is accordingly a transient process, but despite the appearances of cavitation inception it does not proceed faster than the development of the Crow instability in the secondary vortices.

The hydrodynamics of vortex-induced vibration of a flexible pipe in bidirectionally sheared flows are investigated through combined experimental and numerical approaches. Such bidirectionally sheared flows are inspired by subsurface currents induced by internal solitons widely occurring in the ocean, which feature oppositely directed flow velocities along the flexible pipe. Experiments are conducted in an ocean basin using a tensioned pipe with distributed strain sensors, and numerical simulations are conducted by a validated strip method based framework. The mean in-line displacement, mean drag, shear force and bending moment are characterised. The mean in-line displacement, maximised over the spanwise direction, exhibits an approximately quadratic dependence on the maximum velocity in bidirectionally sheared flow. In contrast, the mean drag coefficient remains nearly unchanged with increasing flow velocity. Both trends are cross-validated via experiments and simulations. Furthermore, empirical expressions are proposed to describe the influence of background flow velocities on the shear force and bending moment. Excitation and added mass coefficients associated with dominant frequency response and time-varying hydrodynamic coefficients considering multi-frequency responses are extracted and analysed. The results show that the phase difference between the cross-flow and in-line responses retains an antisymmetric distribution, whereas the excitation coefficients exhibit quasi-symmetric patterns around the mid-span. The time-dependent added mass coefficient is strongly correlated with the wake pattern, with negative values occurring predominantly during P + S and 2P vortex shedding patterns. Moreover, spectral proper orthogonal decomposition is employed to link the dominant vortex-induced vibration frequencies of the flexible pipe with the surrounding flow field. The results indicate a clear transition in which the dominant flow field frequency follows the Strouhal frequency at lower velocity, but aligns with the structural vibration frequency at higher velocity associated with the occurrence of the lock-in phenomenon.