The impact of freestream turbulence (FST) on the aerodynamic performance of a flexible finite wing and the produced wingtip vortex was investigated. The wing had a NACA 4412 airfoil profile and the chord-based Reynolds number was $1.4\times 10^{5}$. The experiments were conducted in a closed-loop wind tunnel with four different inflow turbulence intensities ($0.2\,\%$, $3\,\%$, $8\,\%$ and $13\,\%$) generated using an active turbulence grid. Force balance measurements revealed that increasing the scale of the FST increased the maximum lift and delayed stall. Digital image correlation (DIC) measured deflections of the wing’s structure. Spanwise bending was found to be the dominant deformation. While the wing vibrated at its natural frequency in all conditions, FST increased the amplitude of the vibrations. A similar spectral signature was observed in the lift force fluctuations as well. Stereoscopic particle image velocimetry measurements were obtained two chord lengths downstream of the trailing edge simultaneously with DIC. FST decreased the vortex strength, and marginally increased vortex diffusion and size. It also increased the vortex meandering amplitude, while reducing the meandering frequency band. For the cases with a turbulence intensity of $8\,\%$ and $13\,\%$, the frequency of meandering and the wing’s vibration were similar and a phase relation between the two motions was observed. Proper orthogonal decomposition of the vortex (after removing meandering) and the subsequent velocity field reconstruction revealed temporal fluctuations in the vortex strength at the same frequency as the wing’s vibration. This was linked to the lift force fluctuations induced by the wing’s deformations.

The effect of a horizontal magnetic field on heat transport and flow structures in vertical liquid metal convection (Prandtl number $Pr \approx 0.03$) is investigated experimentally. The experiments are carried out for Rayleigh numbers in the range of $1.48 \times 10^6 \leqslant Ra \leqslant 3.54 \times 10^{7}$ and Chandrasekhar numbers in the range of $2 \times 10^2 \leqslant Q \leqslant 1.86 \times 10^6$, as well as for the non-magnetic case ($Q=0$). Measurements of the heat transport show a rise in the Nusselt number at low and moderate magnetic field strengths up to an optimum value of $Q$, before a further increase in the magnetic field leads to a decrease in the transport properties. By applying simultaneous velocity and temperature measurements, we are able to identify three different oscillatory flow regimes for $10^{-5}\lt Q/Ra \lt 0.5$ and assign them to the respective heat transfer characteristics. In the range $10^{-5}\gt Q/Ra\gt 10^{-3}$, first evidence of a transition to anisotropic flow structures caused by the magnetic field is visible. Two strongly oscillatory regimes are identified, where the energy is either distributed around a dominant frequency ($10^{-3}\gt Q/Ra\gt 10^{-2}$), or strongly concentrated on a single frequency ($10^{-2}\gt Q/Ra\gt 0.5$). The dominating frequency increases with the Rayleigh number according to $Ra^{0.71\pm 0.02}$. This flow structure based regime separation correspond to changes of both the heat transfer through the Nusselt number and mass transfer through the Reynolds number.

Magnetohydrodynamic turbulence with Hall effects is ubiquitous in heliophysics and plasma physics. Direct numerical simulations reveal that, when the forcing scale is comparable to the ion inertial scale, the Hall effects induce remarkable cross-helicity. It then suppresses the cascade efficiency, leading to the accumulation of large-scale magnetic energy and helicity. The process is accompanied by the disruption of current sheets through the entrainment by vortex tubes or the excitation of whistler waves. Using the solar wind data from the Parker Solar Probe, the numerical findings are separately confirmed. These findings provide new insights into the emergence of large-scale solar wind turbulence driven by helical fields and Hall effects.

Turbulent separating and reattaching flows are known to exhibit low-frequency fluctuations manifested in a large-scale contraction and expansion of the reverse-flow region. Previous experimental investigations have been restricted to planar measurements, while the computational cost to resolve the low-frequency spectrum with high-fidelity simulations currently appears to be unaffordable. In this article, we make use of volumetric measurements to reveal the low-frequency dynamics of a turbulent separation bubble (TSB) formed in the fully turbulent flow past a smooth backward-facing ramp. The volumetric velocity field measurements cover the entire separated flow region over a domain with a spanwise extent of $S=0.6\, {\textrm{m}}$. Spectral proper orthogonal decomposition (SPOD) of the velocity fluctuations reveals low-rank low-frequency behaviour at Strouhal numbers ${\textit{St}}\lt 0.05$, which was also observed in previous planar measurements. However, in contrast with the interpretation of a two-dimensional contraction/expansion motion, the low-frequency dynamics is shown to be inherently three-dimensional, and governed by large elongated structures with a spanwise wavelength of approximately $S/2$. A low-order model constructed with the leading SPOD mode confirms substantial changes of the TSB extent in the centre plane, linking it to the modal pattern that is strongly non-uniform in the spanwise direction. The findings presented in this study promote a more complete understanding of the low-frequency dynamics in turbulent separated flows, thereby enabling novel modelling and control approaches.

We present a mathematical solution for the two-dimensional linear problem involving acoustic-gravity waves interacting with rectangular barriers at the bottom of a channel containing a slightly compressible fluid. Our analysis reveals that, below a certain cutoff frequency, the presence of a barrier inhibits the propagation of acoustic-gravity modes. However, through the coupling with evanescent modes existing in the barrier region, we demonstrate the phenomenon of ‘tunnelling’ where the incident acoustic-gravity wave energy can leak to the other side of the barrier, creating a propagating acoustic-gravity mode of the same frequency. Notably, the amplitude of the tunnelling waves exponentially decays with the width of the barrier, analogous to the behaviour observed in quantum tunnelling phenomena. Moreover, a more general solution for multi-barrier and multi-modes is discussed. It is found that tunnelling energy tends to transform from an incident mode to the lowest neighbouring modes. Resonance due to barrier length results in more efficient energy transfer between modes.

This study explores interfacial waves in a three-layer fluid system, focusing on the coupling effects between the two interfaces. These effects include resonance induced by inertial coupling and damping caused by viscous coupling. A linear theoretical framework is developed to describe the coupled wave motion and evaluate the impact of interfacial coupling under viscous damping. Additionally, a semi-analytical model is introduced to accurately capture resonance frequency shifts and phase differences due to viscosity. The spiral structure of interfacial waves predicted by the models is confirmed experimentally using the background oriented Schlieren (BOS) method. Further, the model is validated by excellent agreement between theoretical predictions and ultrasonic measurements of wave amplitudes and phase differences. Finally, the study examines mechanical coupling and energy transfer between interfaces under external forcing, elucidating the formation of spiral waves. The accurate treatment of viscous boundary conditions by the semi-analytical model also enables its extension to multilayer fluid systems.

Large-scale circulation (LSC) dynamics have been studied in thermal convection driven by heat-releasing particles via the four-way coupled Euler–Lagrange approach. We consider a wide range of Rayleigh–Robert number (${\textit{Rr}}=4.97\times 10^{5} - 4.97 \times 10^{8}$) and density ratio ($\hat {\rho }_r=1- 1000$) that characterize the thermal buoyancy and the particle inertia, respectively. An intriguing flow transition has been found as $\hat {\rho }_r$ continuously increases, involving in sequence three typical LSC regimes, i.e. the bulk-flow-up regime, the marginal regime and the bulk-flow-down (BFD) regime. The comprehensive influence of the LSC regime transition is demonstrated by examining the key flow statistics. As integral flow responses, the heat transfer efficiency and flow intensity change substantially when the LSC regime transition happens, and the thermal boundary layer thicknesses at the top and bottom walls exhibit similar alterations. Significant local accumulation of particles occurs as $\hat {\rho }_r$ increases to a sufficiently high value, resulting in a great modification in the flow dynamics. Specifically, particles aggregate near the sidewalls and heat the local surrounding fluid to generate rising warmer plumes that drive the LSC regime transition. Of interest, well-patterned cellular structures of particles take place near the top wall and obtain notable deviation from the thermal convection cells for the BFD regimes. A mechanical interpretation is proposed and substantiated based on a conceptual vortex–particle model, namely, the centrifugal motion of heat-releasing particles that is confirmed to play a driving role for the LSC regime transition.