From particle lifting in atmospheric boundary layers to dust ingestion in jet engines, the transport and deposition of inertial particles in wall-bounded turbulent flows are prevalent in both nature and industry. Due to triboelectrification during collisions, solid particles often acquire significant charges. However, the impacts of the resulting electrostatic interaction on the particle dynamics remain less understood. In this study, we present four-way coupled simulations to investigate the deposition of charged particles onto a grounded metal substrate through a fully developed turbulent boundary layer. Our numerical method tracks the dynamics of individual particles under the influence of turbulence, electrostatic forces and collisions. We first report a more pronounced near-wall accumulation and an increased wall-normal particle velocity due to particle charging. In addition, contrary to predictions from the classic Eulerian model, the wall-normal transport rate of inertial particles is significantly enhanced by electrostatic forces. A statistical approach is then applied to quantify the contributions from turbophoresis, biased sampling and electrostatic forces. For charged particles, a sharper gradient in wall-normal particle fluctuation velocity is observed, which substantially enhances turbophoresis and serves as the primary driving force of near-wall particle accumulation. Furthermore, charged particles are found to sample upward-moving fluids less frequently than neutral particles, thereby weakening the biased-sampling effect that typically pushes particles away from the wall. Finally, the wall-normal electric field is shown to depend on the competition between particle–wall and particle–particle electrostatic interactions, which helps to identify the dominant electrostatic force across a wide range of scenarios.

Delaying the laminar–turbulent transition of a boundary layer reduces the skin-friction drag and can thereby increase the efficiency of any aerodynamic device. A passive control strategy that has reaped success in transition delay is the introduction of boundary layer streaks. Surface-mounted vortex generators have been found to feature an unstable region right behind the devices, which can be fatal in flow control if transition is triggered, leading to an increase in drag with respect to the reference case without devices. In a previous proof of concept study, numerical simulations were employed to place artificial vortices in the free stream that interact with the boundary layer and accomplish transition delay. In the current study, we present experimental results showing the feasibility of generating free-stream vortices that interact with the boundary layer, creating high- and low-speed boundary layer streaks. This type of streaky base flow can act as stabilizing if introduced properly. We confirm the success of our flow control approach by artificially introducing two-dimensional disturbances that are strongly attenuated in the presence of streaks, leading to a transition delay with respect to the reference case of approximately 40 %.

Wall turbulence consists of various sizes of vortical structures that induce flow circulation around a wide range of closed Eulerian loops. Here we investigate the multiscale properties of circulation around such loops in statistically homogeneous planes parallel to the wall. Using a high-resolution direct numerical simulation database of turbulent channels at Reynolds numbers of $Re_\tau =180$, 550, 1000 and 5200, circulation statistics are obtained in planes at different wall-normal heights. Intermittency of circulation in the planes of the outer flow ($y^+ \gtrsim 0.1Re_\tau$) takes the form of universal bifractality as in homogeneous and isotropic turbulence. The bifractal character simplifies to space-filling character close to the wall, with scaling exponents that are linear in the moment order, and lower than those given by the Kolmogorov paradigm. The probability density functions of circulation are long-tailed in the outer bifractal region, with evidence showing their invariance with respect to the loop aspect ratio, while those in the inner region are closely Gaussian. The unifractality near the wall implies that the circulation there is not intermittent in character.

The aspect ratio effect on side and basal melting in fresh water is systematically investigated across a range of Rayleigh numbers and ambient temperatures using direct numerical simulations. The side mean melt rate follows a ${Ra}^{1/4}\,\gamma ^{-3/8}$ scaling relation in the side-melting dominant regime, where ${Ra}$ is the Rayleigh number, and $\gamma$ is the width-to-height aspect ratio of the ice block. In the basal-melting dominant regime, the basal mean melt rate follows a ${Ra}^{1/4}\gamma ^{3/8}$ scaling relation at low Rayleigh numbers, but transitions to a ${Ra}^{1/3}\gamma ^{1/2}$ scaling relation at higher Rayleigh numbers. This scaling transition is attributed to the formation of a bottom cavity resulting from flow separation at high Rayleigh numbers. The overall mean melt rate exhibits a non-monotonic dependence on the aspect ratio, driven by the competition between side and basal melting. The proposed theoretical model successfully captures the observed non-monotonic behaviour, and accurately predicts the overall mean melt rate over the considered range of Rayleigh numbers and ambient temperatures, especially in the side- and basal-melting dominant regimes. More specifically, the side, basal and overall mean melt rates follow a linear ${St}$ scaling relation for ambient temperatures $T_{w}\geqslant 15^{\,\circ }\textrm {C}$, with ${St}$ being the Stefan number (the ratio between sensible heat and latent heat), but deviations from this scaling relation and a non-monotonic dependence on the ambient temperature are observed at lower ambient temperatures, which can be attributed to the density anomaly effect.

This paper investigates the behaviour of turbulence production in adverse pressure gradient (APG) turbulent boundary layers (TBLs), including the range of pressure gradients from zero-pressure-gradient (ZPG) to separation, moderate and high Reynolds numbers, and equilibrium and non-equilibrium flows. The main focus is on predicting the values and positions of turbulence production peaks. Based on the unique ability of turbulence production to describe energy exchange, the idea that the ratios of the mean flow length scales to the turbulence length scales are locally smallest near peaks is proposed. Thereby, the ratios of length scales are defined for the inner and outer regions, respectively, as well as the ratios of time scales for further consideration of local information. The ratios in the inner region are found to reach the same constant value in different APG TBLs. Like turbulence production in the ZPG TBL, turbulence production in APG TBLs is shown to have a certain invariance of the inner peak. The value and position of the inner peak can also be predicted quantitatively. In contrast, the ratios in the outer region cannot be determined with unique coefficients, which accounts for the different self-similarity properties of the inner and outer regions. The outer time scale ratios establish a link between mean flow and turbulence, thus participating in the discussion on half-power laws. The present results support the existence of a half-power-law region that is not immediately adjacent to the overlapping region.

This article delves into the dynamics of inviscid annular supersonic jets, akin to those exiting converging–diverging nozzles in over-expanded regimes. It focuses on the first azimuthal Fourier mode of flow fluctuations and examines their behaviour with varying mixing layer parameters and expansion regimes. The study reveals that two unstable Kelvin–Helmholtz waves exist in all cases, with the outer-layer wave being more unstable due to differences in the velocity gradient. The inner-layer wave is more sensitive to changes in base flow and extends beyond the jet, potentially contributing to nozzle resonances. The article also investigates upstream propagating guided-jet modes, which are found to be robust and not highly sensitive to changes in base flow, which makes them essential for understanding jet dynamics. A simplified model is used to obtain ideal base flows but with realistic shape in order to study the effects of varying nozzle pressure ratios on the dynamics of the waves supported by the jet.