Although stably stratified shear flows, where the base velocity shear is quasi-continuously forced externally, arise in many geophysically and environmentally relevant circumstances, the emergent dynamics of their ensuing statistically steady stratified turbulence is still an open question. We address this phenomenon in a series of three-dimensional direct numerical simulations using spectral element methods. We consider a forced, stably stratified shear flow with an initial bulk Reynolds number $\textit{Re}_{0} = 50$, an initial bulk Richardson number $\textit{Ri}_{0} = 1/80$ (also corresponding to the initial minimum gradient Richardson number $\textit{Ri}_{{g}}$) and a fluid of Prandtl number ${\textit{Pr}} = 1$ in horizontally extended domains. Although the initial configuration is unstable to a primary Kelvin–Helmholtz instability, the ensuing turbulence is sustained by continuously relaxing the resulting flow back towards the initial profiles of streamwise velocity and buoyancy. We study statistical as well as structural aspects of the final statistically steady flows, including the flux coefficient $\varGamma _{\chi }$ and dynamically emergent length scales $\varLambda$ associated with the large-scale dynamics, respectively. Despite the ongoing stirring and mixing, we find that the shear layer half-depth converges to a finite value of $d \approx 8$ (i.e. $\varLambda _{z} \approx 16$) once the horizontal extent of the domain $L_{{h}} \gtrsim 96$. While this implies a final ${{Re}} \approx 400$ and ${Ri} \approx 0.1$, we hypothesise that such forced flows ‘tune’ themselves eventually to a state of a gradient Richardson number $\textit{Ri}_{{g}} \lesssim 0.2$, consistently with several previous studies. Moreover, provided sufficiently extended domains, we observe the emergence of large-scale flow structures with spanwise $\varLambda _{\!y} \approx 50$ and streamwise $\varLambda _{x} \lesssim 115$. Clearly, these observations demonstrate the marked anisotropy of characteristic emergent length scales, even for such ‘weakly stratified’ forced shear flows. We conjecture that the actual emergent streamwise structures are a vestigial ‘imprint’ in the sheared turbulent flow of the primary linear instability of the converged deepened turbulent shear layer.

In this study, we investigate the dynamic behaviour of reconfigurable circular plates under acceleration as a model problem to understand the interplay between kinematics and shape deformation in biological propulsion. A high-resolution force transducer and time-resolved particle image velocimetry were employed to simultaneously capture both hydrodynamic forces and vortex dynamics. The results reveal that, unlike rigid plates that exhibit Reynolds number independence, the force evolution of reconfigurable plates is governed by the dimensionless bending stiffness ${\textit{EI}}^*$. A distinct load-shifting phenomenon is observed – characterized by a reduction in peak force amplitude and an elevation of the postpeak force trough, contrasting with the ‘peak-valley’ behaviour typical of rigid plates. Based on ${\textit{EI}}^*$, reconfigurable plates are classified into three regimes: extra-flexible (${\textit{EI}}^* \lt 2.28 \times 10^{-3}$), flexible ($2.28 \times 10^{-3} \leqslant {\textit{EI}}^* \leqslant 0.143$) and rigid (${\textit{EI}}^* \gt 0.143$). Notably, only plates within the flexible regime exhibit the load-shifting phenomenon. Flow visualizations show that the flexible plates, due to their shape reconfiguration, produce flow fields with two distinct features: initially, the formation of three-dimensional, non-axisymmetric vortex rings; subsequently, vortex breakdown occurs due to instability. By applying the vorticity moment theorem, force generation is accurately estimated from the flow field. Using a vortex-based low-order force model, the radial distribution of vorticity is identified as the key mechanism underlying the load-shifting phenomenon. This finding suggests that biological morphing structures in real propulsion scenarios can reduce force fluctuations without compromising average thrust by ‘load-shifting’, offering insights into efficient propulsion strategies.

We report experimental evidence of an Eulerian-mean flow, $\overline {u}(z)$, created by the interaction of surface waves and tailored ambient sub-surface turbulence, which partly cancels the Stokes drift, $u_s(z)$, and present supporting theory. Water-side turbulent velocity fields and Eulerian-mean flows were measured with particle image velocimetry before vs after the passage of a wave group, and with vs without the presence of regular waves. We compare different wavelengths, steepnesses and turbulent intensities. In all cases, a significant change in the Eulerian-mean current is observed, strongly focused near the surface, where it opposes the Stokes drift. The observations support the picture that, when waves encounter ambient sub-surface turbulence, the flow undergoes a transition during which Eulerian-mean momentum is redistributed vertically (without changing the depth-integrated mass transport) until a new equilibrium state is reached, wherein the near-surface ratio between $|{\rm d}\overline {u}/{\rm d}z|$ and $|{\rm d}u_s/{\rm d} z|$ approximately equals the ratio between the streamwise and vertical Reynolds normal stresses. This accords with a simple statistical theory derived here and holds regardless of the absolute turbulence level, whereas stronger turbulence means faster growth of the Eulerian-mean current. We present a model based on Rapid distortion theory which describes the generation of the Eulerian-mean flow as a consequence of the action of the Stokes drift on the background turbulence. Predictions are in qualitative, and reasonable quantitative, agreement with experiments on wave groups, where equilibrium has not yet been reached. Our results could have substantial consequences for predicting the transport of water-borne material in the oceans.

In this study, direct numerical simulation of a turbulent flame–wall interaction (FWI) has been done for premixed H$_2/$air and NH$_3/$H$_2/$air flames in a fully developed channel flow at Re$_\tau$ $\approx$ 300. Both isothermal and adiabatic walls are considered. The results contribute to further clarification of the underlying mechanisms of FWIs. First, the underlying mechanism for the rapid increase of chemical flame thickness near the wall is found to be the zero-flux boundary condition for diffusion. Effects of wall heat loss and wall turbulence are minor. Then, a ridge-based flame surface identification method is proposed to track the flame front, which is found to be more accurate than an isosurface of $C$ (the progress variable), especially during FWIs. Using this technique, the near-wall flame geometry and orientation are correctly captured. It is found that the flames are laminarised near the wall and almost parallel to the isothermal wall shortly before quenching. Flame–vortex interactions lead to entrained flame pockets for H$_2$ as a fuel and to a distributed reaction zone for the case of NH$_3/$H$_2$. Finally, the turbulent combustion regime is investigated by checking wall-distance-dependent Reynolds number and Karlovitz number. It is found that the flames enter the laminar flame regime shortly before wall quenching, instead of the broken reaction regime suggested in previous studies. To support the analysis, the turbulent flame dynamics, including turbulent burning rate, turbulent flame surface area, flame stretch factor, local displacement speed, flame dilatation, flame strain rate (both tangential and normal) and flame alignment with the principal strain rate are quantified, providing a full picture of near-wall turbulent flames for the considered conditions.

Compost amendments are a promising tool for building productivity in degraded rangelands, but the effect on biological soil crusts (biocrusts), the surface microbial communities found in drylands, has not been investigated. Biocrusts contribute both carbon uptake and other ecosystem services in drylands. We investigated how 6.3 mm of surface-dressed compost at a Tribal rangeland in central New Mexico, USA, affected temperature, carbon and nitrogen characteristics, the relative abundance of biocrust microbial communities (fungi and bacteria) – specifically cyanobacterial communities – as well as the resulting aggregate stability at the soil surface after 1 year. Surface temperature maxima increased with compost addition in cooler ambient conditions, and the δ13C signatures of the soils from compost addition plots were >1‰ lighter compared to controls, indicating >35% of soil carbon was compost-derived, but organic C, total N percentage and aggregate stability did not differ among compost treatments. Several compost-derived taxa became indicator species in the amended plots, and compost addition decreased cyanobacteria relative abundance up to 58%. While previous results show that compost may benefit plants from a slow-release fertilization effect and soil carbon in deeper soil layers increases, there could be complex impacts on biocrust organic carbon with changing temperature and microbial community.

Direct numerical simulations with two-way coupled Lagrangian tracking are carried out to study the bubble preferential concentration and the flow field modification. Simulations are conducted in an upward vertical turbulent channel driven by a constant pressure gradient, corresponding to a friction Reynolds number $Re_{\tau 0}=180$. Micro-sized bubbles with diameters ranging from 0.72 to 1.43 wall units are considered. Competition between lift force and wall-lift force in the wall-normal direction leads to significant near-wall bubble accumulation and directly results in distinct preferential concentration patterns across the channel. Below (above) the peak concentration height, the wall-lift (lift) force dominates, driving bubbles to accumulate in regions of high-speed sweep (low-speed ejection) events. In the vicinity of the wall, the wall-normal lift force exhibits a strong correlation with the local streamwise flow velocity, further reinforcing the preferential concentration of bubbles in high-speed regions. Additionally, bubbles show a strong preference for the low-enstrophy and high-dissipation nodal topologies. Furthermore, small bubbles primarily accumulate in the vicinity of the wall, reducing the work done on the flow and leading to a decrease in bulk velocity and turbulence statistics. In contrast, the turbulence statistics of large bubbles are nearly identical to those of the unladen flow. The impact of large bubbles on the flow field primarily manifests as an effective increase in the mean pressure gradient. These findings demonstrate that bubbles in the upward vertical channel flow exhibit strong preferential concentration behaviours, whereas their ability to modulate turbulence remains limited.

Exact mathematical expressions are derived to predict the exponent $p$ observed in non-equilibrium turbulence, where the classical dissipation law is replaced by a new dissipation scaling law $C_{\varepsilon } \sim \textit{Re}_{\lambda }^p$. Here, $ \textit{Re}_{\lambda }$ is the Taylor-based Reynolds number and $C_{\varepsilon } = \varepsilon L_{11} / u^{\prime 3}$ is the non-dimensional dissipation rate, defined by the viscous dissipation rate, $\varepsilon$, longitudinal integral scale, $L_{11}$, and root-mean-square of the velocity fluctuations $u^{\prime} = \sqrt {\overline {u^{\prime 2}}}$ (Vassilicos, Annu. Rev. Fluid Mech., vol. 47, 2015, pp. 95–114). Assuming homogeneous and isotropic turbulence, it is shown that the exact value of $p$ involves only first-order derivatives of these variables; however, at very high Reynolds numbers, and under particularly strong changes in the power input of the external forcing (without changing the shape of the forcing spectrum), the exact expression simplifies to $p = 3\pi / 4\alpha L_{110} - 5 / 2$, where $L_{110}$ is the initial value of the longitudinal integral scale and $\alpha$ represents an effective forcing wavenumber. Thus, the main finding is that only large-scale effects are involved in the imposition of the non-equilibrium dissipation scaling law. The results are compared with direct numerical simulation (DNS) results of isotropic turbulence under abruptly changing forcing conditions and with experimental data of non-equilibrium decaying isotropic turbulence, showing consistent results.

Climate hazards impact pastoral communities due to their dependency on nature for their primary livelihoods. This study analyzes climate risk in ten pastoral livestock farming communities in Patagonia drylands of Argentina. A participatory impact chains (PICs) approach was used as a qualitative and participatory bottom-up methodology allowing for the identification and contextualization of climate hazards, exposure, intermediate impacts and vulnerability dimensions through knowledge co-production with local stakeholders. Results show that, although drought is the predominant climate hazard across the region, its impacts are heterogeneous and mediated by local socio-environmental conditions. The analysis underlines that vulnerability is not evenly distributed but is shaped by specific historical, political and environmental pathways. These findings challenge standardized top-down risk assessments, and highlight the need for adaptation strategies that are context-sensitive, territorially differentiated and that integrate local knowledge. The study also contributes to advancing qualitative participatory methodologies for climate risk assessment in pastoral systems in arid areas of Latin America, showing the regional heterogeneity and social inequalities.