This work investigates the mechanism of deflagration-to-detonation transition (DDT) in a closed channel filled with a stoichiometric hydrogen–air mixture under knocking-relevant conditions (10 atm, 1100 K). We first revisit the low-Mach-number model for a planar flame propagating towards an inert end-gas, deriving an analytical solution that reveals a finite-time singularity in temperature, which remains physically irrelevant without end-gas reactions. The core analysis demonstrates that the transition is triggered by the breakdown of the low-Mach-number approximation due to weak heat release during the end-gas autoignition induction period. The extreme thermal sensitivity of the induction delay to a small increase in temperature (resulting from both adiabatic compression and autoignition reactions) causes a drastic reduction in the time scale of flame dynamics. This leads to acceleration-induced compression waves which quickly steepen into shocks propagating in a combustible gaseous mixture prompt to detonate. A thermal feedback loop between the self-accelerating flame and the compression waves culminates in the catastrophic breakdown of the laminar flame structure associated with the spontaneous onset of a detonation burning quickly the end-gas in a shorter time than the bulk thermal explosion by autoignition. Supporting numerical simulations, in which the induction chemistry is artificially modified without modifying the chemical kinetics controlling the flame propagation, confirm that the DDT is intrinsically linked to the high thermal sensitivity of the reaction scheme controlling the induction delay in the end-gas.

The mechanical properties of confining boundaries can fundamentally alter the flow behaviour of shear-thickening suspensions. We study a dense cornstarch suspension sheared beneath a viscous silicone oil layer, using the oil viscosity to tune boundary compliance. Flow visualisation and rheometry reveal two distinct regimes. With compliant boundaries, long-lived heterogeneities emerge via density waves or persistent clusters, maintained by a balance between interface deformation and particle rearrangement. With more resistant confinement, we observe transient jamming events, marked by abrupt spanning of load-bearing structures across the suspension thickness and the emergence of secondary stress waves. The onset stress of these events remains constant at the discontinuous shear thickening (DST) threshold, independent of bounding viscosity. Our results reveal that boundary compliance selects the lifetime and morphology of heterogeneous structures, offering a means to amplify otherwise short-lived microscopic processes and providing new insight into the interplay between shear thickening, shear jamming and confinement mechanics.

Understanding the vortex interactions and wake transitions for flapping flexible foils is important because of their increased usage in bioinspired aquatic and aerial robotic propulsors. Although wake transitions have been studied for rigid foils, we experimentally investigate how flexibility alters the transitions and vortex interactions for flexible foils, which are closer to the natural flapping foils in fish, birds and insects. We conduct the experiments in a flowing soap film on a pitching airfoil with a flexible filament at its trailing edge (TE). We find that, apart from the Strouhal number (${\textit{St}}$), flexural rigidity (${\textit{EI}}$) is important to determine the transitions. We vary ${\textit{EI}}$ of the flexible filament by three orders of magnitude and also investigate an extreme case of ${\textit{EI}} \rightarrow \infty$. Flexibility triggers the shedding of multiple small ‘secondary vortices’ (SVs) along with big ‘primary vortices’ (PVs), unlike only PVs for the rigid foil. Continuous deformations of the flexible filament play crucial roles in determining the interaction of boundary layer vortices and trailing edge vortices and, ultimately, the generation and evolution of PVs and SVs. We identify five vortex interaction mechanisms (VIMs). Depending on how SVs interact with PVs, the wake assumes different patterns. We construct the ${\textit{St}}$–${\textit{EI}}$ phase maps for wake transitions and newly identified VIMs. We devise a non-dimensional parameter $\varUpsilon$, referred to as ‘Yashavant number’. One order increase in $\varUpsilon$ reduces the number of VIMs by one. Instead of following the usual transition route, the flexible foil reveals counterintuitive transition trends that strongly depend on the filament ${\textit{EI}}$.

Steady tip streaming in the limit of vanishing flow rate has been experimentally and numerically documented, yet theoretical solutions describing local conical Stokes flows have remained elusive. Here, we derive approximate analytical solutions for local conical flows in liquid–liquid flow focusing scenarios, addressing the limit of negligible emitted flow rate. Our analysis demonstrates the existence of a universal relationship between the inner-to-outer liquid viscosity ratio and the cone angle, establishing the theoretical underpinning for precise control of microscopic jet formation. A posteriori comparison with previously published experiments reveals that digitised cusp-like meniscus profiles collapse quantitatively onto the predicted slender-body similarity solution. These findings pave the way for technologies that require exact manipulation of fluid flows at nearly molecular dimensions.

Energy transfer across scales is fundamental in fluid dynamics, linking large-scale flow motions to small-scale turbulent structures in engineering and natural environments. Triadic interactions among three wave components form complex networks across scales, challenging understanding and model reduction. We introduce triadic orthogonal decomposition (TOD), a method that identifies coherent flow structures optimally capturing spectral momentum transfer, quantifies their coupling and energy exchange in an energy-budget bispectrum and reveals the regions where they interact. Triadic orthogonal decomposition distinguishes three components – a momentum recipient, donor and catalyst – and recovers laws governing pairwise, six-triad and global triad conservation. We apply TOD to three examples: the classical cylinder wake, experimental wind turbine wake data and a direct numerical simulation of isotropic turbulence. Energy transfer can be spatially distributed but vanish upon integration or spatially localised but facilitate net interscale exchange, so a complete characterisation of nonlinearity requires examination of both integral and local transfers. In the cylinder wake, we link backscatter of energy from high to low frequencies to a compact attenuation region downstream of the cylinder. In the turbine wake, we confirm the known association between energy amplification and decay and vortex tilting, but observe more complex secondary mechanisms in suboptimal modes. For isotropic turbulence, we derive and confirm inertial-range frequency scaling for convective–recipient covariances, then demonstrate self-similar energy transfer at each rank.