Turbulent channel flow controlled by spanwise wall oscillations is studied using direct numerical simulations to improve how spanwise forcing reduces skin-friction drag. Harmonic wall oscillations generate a periodic transverse Stokes layer whose thickness $\delta$ is determined by the forcing period $T$. Although an optimal $T$ that maximises drag reduction is known to exist, its physical significance remains unclear. To elucidate it, we extend the spanwise Stokes layer by augmenting wall oscillation with an additional spanwise body force. In this formulation, $\delta$ and $T$ become decoupled and can be varied independently. The oscillating wall thus appears as a special and suboptimal case of spanwise forcing. Optimal performance is obtained for substantially smaller $T$ and larger $\delta$ than those of the classical Stokes layer. For the conditions examined, with Reynolds number and forcing amplitude held fixed, the maximum drag reduction increases by approximately one third, while the maximum net energy saving improves markedly from $-35\,\%$ to $+16\,\%$. These findings suggest that drag-reduction strategies based on spanwise forcing deserve renewed scrutiny: wall oscillation represents only one possible actuation method, and not necessarily the most effective one.

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.

In this study, we develop a super-resolution (SR) model for homogeneous isotropic turbulence (HIT) inspired by the recently proposed low-inference-cost ResShift diffusion model. The training data are obtained from direct numerical simulation of two three-dimensional HIT cases with varying grid resolutions and Reynolds numbers ($ \textit{Re}_\lambda = 94$ and 173) to increase the model’s generalisability. The model is trained on two-dimensional snapshots rather than full three-dimensional fields, as training and inference on three-dimensional data would increase the computational cost significantly. Both the data from the whole domain and the data from a quarter of the domain are considered in the dataset to increase the diversity and quantity of training samples. This strategy also helps the model learn more localised flow structures and reduces dependence on global domain-specific patterns. The model is trained using single snapshots of velocity components for three upsampling factors of 4, 8 and 16. To assess the generalisability of the trained model, it is tested for flows under conditions different from those of the training data. Additionally, the high-resolution reconstruction of flow fields from low-resolution turbulent boundary layer data is performed to evaluate the model’s performance in anisotropic turbulence. The results show that the diffusion model presented in this study performs well in predicting the velocity field even for high upsampling factors, and unlike bicubic interpolation, convolutional neural network (CNN)- and U-Net-based models, it does not generate a visually blurry flow field when applied to high upsampling factors. It also outperforms bicubic interpolation, CNN- and U-Net-based models, as well as the traditional conditional denoising diffusion probabilistic model designed for SR, in predicting flow statistics. The model effectively extracts flow features, generates flow structures of varying sizes and shows strong performance in predicting vorticity. It also reproduces the energy spectrum at high wavenumbers with reasonable accuracy, indicating the recovery of small-scale structures often lost in coarse data. This capability is valuable for subgrid-scale stress estimation and helps improve the physical fidelity of large eddy simulation frameworks.