We numerically investigate the steady and unsteady wakes of three-dimensional permeable disks over Reynolds number ($\textit{Re}$) range 100–300 and Darcy number ($Da$) range $10^{-9}$–$10^{-3}$. For disks with low permeability ($Da\le 8\times 10^{-5}$), the dynamical transition route is the same as that of impervious disks, with the critical $\textit{Re}$ for all bifurcations increasing with decreasing permeability. In contrast, for disks with high permeability ($Da\ge 2\times 10^{-4}$), all unsteady bifurcations are suppressed, and the wake remains in a steady regime throughout the $\textit{Re}$ range considered. Interestingly, at moderate $Da$, permeability gives rise to two previously unreported flow regimes. The first is the ‘SVR breathing’ regime, occurring at $Da\approx 10^{-4}$ and $\textit{Re}\approx 200$, and is attributed to the subharmonic lock-in between two distinct unsteady dynamics: the shedding of hairpin vortices and the low-frequency unsteadiness of the near-wake recirculation regions. The second is the ‘intermittency’ regime, which occurs at $Da\approx 1.5\times 10^{-4}$, $\textit{Re}\approx 200$; the wake alternates irregularly between two periodic modes with orthogonal planes of symmetry. Future work might include verifying whether intermittency arises from the energy competition between two modes, as the vortices lack sufficient energy to sustain stable single-mode harmonic oscillations. These findings demonstrate that permeability can fundamentally alter wake dynamics and introduce new wake structures that do not occur on an impervious disk.

The drag wake of a towed inclined 6 : 1 prolate spheroid in unstratified and stratified ambients is investigated experimentally using stereoscopic particle image velocimetry. Tow speed, stratification strength and inclination angle are varied independently, resulting in a parameter space spanning Reynolds numbers $\textit{Re} = (1.25{-}20) \times 10^3$, Froude numbers $\textit{Fr}= U/\textit{ND} = 2{-}32$ and $\infty$, and inclination angles $\theta = 0^\circ$ and $20^\circ$. Measurements are repeated at each parameter combination to obtain converged wake statistics for $3 \leqslant x/D \leqslant 40$. Unstratified measurements provide a baseline experimental dataset for inclined spheroids that has not previously been reported. In the absence of stratification, inclination generates persistent wake asymmetries and a net vertical impulse that deflects the wake trajectory. Although inclined configurations exhibit larger initial wake heights than axisymmetric cases, the early wake evolution collapses when scaled by an effective body diameter, indicating that this increase is geometric in origin. Regular vertical velocity protrusions are observed in inclined wakes, with a characteristic spacing that depends on Reynolds number but shows no measurable dependence on Froude number. At sufficiently low Froude number, buoyancy influences the near-body flow, and modifies the wake trajectory and streamwise velocity profiles. For $\textit{Re} = 5000$, wake heights for both axisymmetric and inclined configurations collapse across stratification strengths when scaled by an effective diameter. In this regime, the wake trajectory exhibits oscillations with period $2\pi /N$, in agreement with previously reported stratified wake dynamics.

The interaction between acoustic waves and turbulent grazing flow over an acoustic liner is investigated using lattice-Boltzmann very-large-eddy simulations. A single-degree-of-freedom liner with 11 streamwise-aligned cavities is studied in a grazing flow impedance tube. The conditions replicate reference experiments from the Federal University of Santa Catarina. The influence of grazing flow (with a centreline Mach number of 0.32), acoustic wave amplitude, frequency and propagation direction relative to the mean flow is analysed. Impedance is computed using both direct (i.e. the in situ method) and model-fitting inference (i.e. the mode-matching) methods. The former reveals strong spatial variations; however, averaged values throughout the sample show minimal differences between upstream- and downstream-propagating waves, in contrast to what is obtained with the latter method. Flow analyses reveal that the orifices displace the flow away from the face sheet, with this effect amplified by acoustic waves and dependent on the wave propagation direction. Consequently, the boundary layer displacement thickness ($\delta ^*$) increases along the streamwise direction compared with a smooth wall and exhibits localised humps downstream of each orifice. The growth of $\delta ^*$ alters the flow dynamics within the orifices by weakening the shear layer at downstream positions. This influences the acoustic-induced mass flow rate through the orifices at equal sound pressure level, suggesting that acoustic energy is dissipated differently along the liner. The asymmetry of the flow field experienced by the acoustic wave, depending on its propagation direction, highlights the need to consider a spatially evolving turbulent flow when studying the acoustic–flow interaction and measuring impedance.

We investigate the influence of side-wall wetting on the linear stability of falling liquid films confined in the spanwise direction. A biglobal stability framework is developed, capturing inertia, viscosity, gravity, capillarity and geometric confinement. The base flow exhibits a curved meniscus and a streamwise velocity overshoot near the side walls. Linear stability analysis based on the Navier–Stokes equations is performed in two limiting regimes. In confined channels, where spanwise confinement stabilises moderate-wavenumber perturbations via side-wall boundary layers, wetting weakens this stabilisation; as the contact angle decreases, the neutral curves shift towards the unconfined one-dimensional limit, thus wetting acts as a relative destabilising mechanism. In contrast, in weakly confined channels where side-wall boundary layers do not provide confinement-induced stabilisation, wetting produces a net long-wave stabilisation ($k \rightarrow 0$), significantly increasing the critical Reynolds number. This effect strengthens as the contact angle decreases, indicating a competition between destabilising inertia and stabilising wetting-induced capillary forces. The predicted long-wave stabilisation effect is compared quantitatively with available experimental measurements, showing consistent trends and comparable magnitudes within the accessible parameter range. Perturbation eigenmode structures show that, in confined channels, the relative destabilisation is associated with near-wall vortical structures induced by the meniscus elevation and velocity overshoot, which reduce effective viscous damping. In contrast, in weakly confined channels, stabilisation is consistent with interface tensioning through strong anchoring of the perturbations at the side walls.

Attached cavitation is likely the most common form of developed hydrodynamic cavitation, yet the reason for its dominance remains unclear. From the experimental side, a natural approach is to seed controllable nuclei and observe their evolution. We propose a laser-based on-demand nucleation method that generates micro- and nanobubbles as nuclei in Venturi flows, enabling unprecedented spatio-temporal control of hydrodynamic cavitation inception. For single-bubble cases, we find that attached cavitation occurs when the bubble surface enters the boundary layer of the channel where the pressure is below the vapour pressure. Based on it, we construct a phase diagram of cavitation regimes as a function of cavitation number and non-dimensional wall distance. Extending to multiple bubbles, assuming a random spatial distribution of nuclei within the laser-illuminated region, we develop a simple model to estimate the probability of attached cavitation. Results show that, at typical cavitation numbers, only a few bubbles suffice for attached cavitation to occur with nearly 100 % probability. Our finding provides new insights into why nuclei in hydrodynamic processes tend to develop into attached cavitation.

Bubble pairs are effective modulators of liquid jets. We investigate the jetting of an air bubble driven by a laser-induced cavitation bubble using high-speed imaging, compressible volume-of-fluid (VoF) simulations and theoretical analysis. Three distinct jet types emerge, depending on the stand-off distance $\gamma$ and size ratio $\eta$ between the bubbles. Jet formation proceeds through two stages: an initial shock-induced acceleration followed by flow focusing on the concave liquid–air interface. We derive scaling relations, $V_0=1.1 p_0R_0/(\rho cR_l)((\gamma (1+\eta )-1)/\eta )^{-1.6}$ for the shock-driven stage and $V_m={}(1+(0.8-0.5\gamma )\eta ^{0.75})V_0$ for the flow focusing stage in the strong jet regime, both of which agree closely with experimental and numerical measurements. Here, $V_0$ and $V_m$ denote the velocity increments associated with shock-wave-induced acceleration and flow focusing stages, respectively. The variables $p_0$, $R_0$, $\rho$, $c$ and $R_l$ represent the initial pressure and radius of the cavitation bubble, the fluid density, the speed of sound in the liquid and the maximum volume-equivalent radius of the cavitation bubble, respectively. A $(\eta ,\gamma )$ phase diagram delineates the weak, strong and explosive jets, with regime boundaries accurately captured by the theoretically derived transitions.