Stochastic models of near-wall turbulence commonly rely on the Markovian assumption, despite evidence that coherent structures induce long-lived temporal correlations. Here, we test the validity of this assumption using micron-sized particle resuspension from the viscous sublayer. Analysis of direct numerical simulation (DNS) data reveals that while high- and low-drag events occur with Poissonian statistics, their internal dynamics is strongly persistent, with a Hurst exponent $H \approx 0.84$, indicating intrinsic non-Markovian behaviour. We therefore develop a non-Markovian resuspension model based on a fractional Ornstein–Uhlenbeck process, with physical parameters extracted directly from the DNS flow. Comparative simulations show that the empirical success of classical Markovian models arises not from an accurate description of the near-wall dynamics, but from their free parameter $C_{0}$ acting as a phenomenological surrogate for unresolved flow memory. We further identify a critical regime transition controlled by the event decay rate $\lambda$: strong intermittency ($\lambda \lt 0.2$) invalidates the Markovian approximation, whereas weak intermittency ($\lambda \gt 0.2$) renders it physically justifiable. These results define quantitative limits on stochastic modelling in near-wall turbulence.

The nonlinear evolution of free-stream vortical disturbances entrained in the entrance region of a channel is investigated using asymptotic and numerical methods, building on the linear framework developed by Ricco & Alvarenga (2021 J. Fluid Mech., vol. 927, A18). The focus is on low-frequency disturbances that induce streamwise-elongated structures at Reynolds numbers for which the entrance flow is locally stable according to classical linear stability theory. The perturbation flow along the channel entrance is generated by free-stream vortical disturbances located at the channel inlet. These disturbances are symmetric or antisymmetric with respect to the centreplane and their amplitude is sufficiently intense to provoke nonlinear interactions within the channel. The formation and evolution of the perturbation flow are described by the nonlinear unsteady boundary-region equations. Combined with physically realistic initial conditions, the resulting initial-boundary-value problems are solved numerically using a streamwise integration method. A parametric study is conducted to elucidate how the nonlinear channel flow is influenced by the Reynolds number and the inlet-disturbance properties, i.e. the amplitude and the streamwise, wall-normal and spanwise wavelengths. Nonlinearity is found to stabilise the intense algebraic growth and to drive the formation of elongated channel-entrance structures that span the entire cross-section. These structures, characterised by low- and high-speed regions and streamwise vortices, meander along the streamwise direction and persist even when the base flow is fully developed. They exhibit a half-turn rotational symmetry with respect to the vortex centres. These properties emerge downstream regardless of the symmetry of the initial perturbation flow, provided nonlinear interactions are sufficiently intense. The occurrence of travelling waves is detected sufficiently downstream, and their similarity to those found in the fully developed region by other researchers is discussed. Our results show good agreement with theoretical predictions, numerical results and experimental measurements for both the mean flow and the perturbation flow.

An experimental study is conducted investigating the characteristics of tones due to ‘guided jet waves’ in the forward arc of jet noise radiation fields. These spectral peaks disappear in the far acoustic field in the aft and sideline directions, which is why they went unnoticed in decades of jet noise measurements. However, it is clearly shown in the present study that they radiate in the forward arc (shallow upstream polar locations in the approximate range θ < 45°). Jet noise spectra in the forward arc, data on which had been lacking in the literature, are not smooth but are characterised by these peaks. This is found for high subsonic to supersonic jets up to the highest jet Mach number covered in the experiment (MJ ≈ 1.9), for round and rectangular as well as convergent–divergent nozzles. In heated jets, these spectral peaks appear to get weakened especially around transonic conditions, whereas they clearly persist in supersonic conditions.

This study investigates convection in a non-isothermal spherical Taylor–Couette flow (sTC) under the influence of the dielectrophoretic (DEP) force. The convective flow is driven by differential rotation of the inner and outer boundaries rotating with $\varOmega$ and $\Delta {\varOmega }$ in combination of an electric tension applied between both shells to induce thermo-electrohydrodynamic (TEHD) convection. To understand the interaction between DEP force-driven and rotation-driven mechanisms, we first analysed TEHD convection and non-isothermal sTC flow independently. For the TEHD case, we establish scaling relations for heat transport by expressing the Nusselt number, ${\textit{Nu}}$, as a function of the electric Rayleigh number, ${\textit{Ra}}_{{E}}$, and the kinetic energy density, $\tilde {E}_k$. These relations are evaluated against classical models of convection to assess consistency and deviations. A similar approach was applied to the non-isothermal sTC flow in the absence of the DEP force, where we identified axisymmetric and non-axisymmetric flow regimes which were classified by ${\textit{Nu}}$, $\tilde {E}_k$ and $\Delta {\varOmega }$, and developed corresponding scaling relations. When both mechanisms were active, ${\textit{Nu}}$ generally increased, however, the DEP force locally suppressed angular momentum transport, especially near the equator. This interplay revealed three distinct regimes: (A) DEP force-dominated TEHD convection, (C) rotation-dominated non-isothermal sTC flow and (B) a transitional regime with reduced heat transport. A decomposition of a derived inflow Nusselt number, ${\textit{Nu}}^q$, based on conductive and convective contributions, further elucidated the underlying heat transport mechanism.

We report the first streamwise-localised travelling-wave solution in square-duct flow that acts as an edge state in the full phase space, without any imposed spatial symmetries. Performing edge tracking and Newton iteration, we identify a steady travelling wave that possesses a codimension-one stable manifold, which (at least locally) forms the boundary between the basins of laminar and turbulent attractors. Parametric continuation identifies this solution as the lower branch of a saddle-node bifurcation pair. Perturbation analysis places both solutions on the laminar–turbulent boundary and uncovers a heteroclinic connection that links the two branches and is likewise confined to the basin boundary. This symmetry-free, localised edge state expands the catalogue of invariant solutions in wall-bounded shear flows and provides a geometric framework for understanding the transition dynamics in extended systems.

Turbulence-induced friction is a significant contributor to energy consumption in the fluid-transport and piping industries. Here, we describe a passive approach to suppress turbulence and reduce friction: we show that a local increase in streamwise flow curvature, combined with changing a circular cross-section to an oval, relaminarises turbulent flow in curved pipes. We exemplify this effect in a $180^{\circ }$ bend at ${\textit{Re}}_D=10\,000$ and $20\,000$ (based on bulk velocity $U_B$ and pipe diameter $D$), well above the limit for sustained turbulence in straight pipes and the linear stability limit in $180^{\circ }$ bends. Curvature inhibits streamwise Reynolds stresses, and cross-sectional modifications weaken the unstable secondary flow, together disrupting the near-wall regeneration cycle and collapsing turbulence. Simulations and experiments confirm that these geometric modifications suppress turbulence and reduce pressure loss by 53 % and 36 % compared with the baseline $180^{\circ }$ bend and a fully developed straight pipe of equal length, respectively. The results establish a passive, mechanism-based route to relaminarisation in curved pipes with implications for energy-efficient control in other wall-bounded flows with curvature.

In this work, we revisit the Generalised Navier Boundary Condition (GNBC) introduced by Qian et al. in the sharp interface volume-of-fluid context. We replace the singular uncompensated Young stress by a smooth function with a characteristic width $\varepsilon \gt 0$ that is understood as a physical parameter of the model. Therefore, we call the model the ‘contact region GNBC’ (CR-GNBC). We show that the model is consistent with the fundamental kinematics of the contact angle transport described by Fricke, Köhne and Bothe. We implement the model in the geometrical volume-of-fluid solver Basilisk using a ‘free angle’ approach. This means that the dynamic contact angle is not prescribed, but reconstructed from the interface geometry and subsequently applied as an input parameter to compute the uncompensated Young stress. We couple this approach to the two-phase Navier–Stokes solver and study the withdrawing tape problem with a receding contact line. It is shown that the model allows for grid-independent solutions and leads to a full regularisation of the singularity at the moving contact line, which is in accordance with the thin film equation subject to this boundary condition. In particular, it is shown that the curvature at the moving contact line is finite and mesh converging. As predicted by the fundamental kinematics, the parallel shear stress component vanishes at the moving contact line for quasi-stationary states (i.e. for $\dot \theta _d=0$), and the dynamic contact angle is determined by a balance between the uncompensated Young stress and an effective contact line friction. Furthermore, a nonlinear generalisation of the model is proposed, which aims at reproducing the molecular kinetic theory of Blake and Haynes for quasi-stationary states.

We show that both temporal and spatial symmetry breaking in canonical K-type boundary layer transition arise as organised structures with quantifiable energetic pathways rather than unstructured noise. Before the skin-friction maximum, the flow is described by a periodic, spanwise-symmetric fundamental harmonic response (FHR) to the Tollmien–Schlichting wave. The FHR is spatially compact, produces hairpin packets and remains fully harmonic despite a turbulence-like appearance, thereby delimiting the deterministic regime. Past this point, a distinct regime change occurs: a hierarchy of quasi-periodic and aperiodic structures emerges, followed shortly by anti-symmetric structures that develop similarly despite no anti-symmetric inputs. We identify these structures as symmetry-decomposed spectral and space–time proper orthogonal modes that resolve the progression from deterministic harmonics to broadband dynamics. We introduce inter-modal and inter-symmetry energy budgets derived from symmetry-decomposed Navier–Stokes equations. They reveal a directed energy transfer from the FHR into the leading temporal and spatial symmetry breaking modes and, subsequently, into broadband residual fluctuations, showing that broadband dynamics grow only once inter-modal transfer is active, while inter-symmetry transfer also strongly amplifies broadband anti-symmetric fluctuations once asymmetry is present. These key insights support a view of laminar–turbulent transition as a sequence of symmetry breaking events, energetically driven by dominant space–time modes that route energy from harmonic flow to broadband turbulence.

This study experimentally investigates bubble size evolution and void fraction redistribution in an unexplored, coalescence-dominated regime of a decaying turbulent bubbly flow. The flow is generated downstream of a regenerative pump in a duct, with bulk Reynolds number (Re) $\sim \mathcal{O}(10^5)$, Taylor-scale Reynolds number (Re$_\lambda$) $\sim \mathcal{O}(10^3)$ and void fraction ($\phi$) $\sim \mathcal{O}(1\,\%)$, where the inlet turbulence is extremely intense (turbulence intensity $\gt 30\,\%$) but decays rapidly along the duct. Shadowgraph imaging and particle shadow velocimetry are used for measurements. The experimentally obtained turbulent dissipation in the duct flow decays as $\varepsilon \sim \mathcal{L}^{-2}$, where $\mathcal{L}$ is the axial position, in close agreement with the homogeneous isotropic turbulence prediction of $\varepsilon \sim \mathcal{L}^{-2.2}$. High-speed imaging and statistical analysis reveal that bubble coalescence dominates over breakup across most of the domain, leading to monotonic growth in the Sauter mean diameter ($d_{32}$) and progressive broadening of the bubble size distribution. The normalised extreme-to-mean diameter ratio ($\mathcal{D}$) increases axially and asymptotically from ${\sim} 1.9$ (breakup regime) and saturates at ${\sim} 2.2$ (coalescence regime), indicating the emergence of a quasi-self-similar bubble size distribution. The probability density function of the bubble diameter exhibits a dual power-law tail with exponents $-10/3$ and $-3/2$ near the duct inlet. However, after a few hydraulic diameters, a single $-3/2$ power-law scaling emerges, indicating a regime of pure coalescence in which all bubbles are smaller than the Hinze scale. The cumulative distribution plotted against $d/d_{32}$ shows that the slope decreases and the distribution width increases with both axial position and void fraction $(\phi )$. Although classical Hinze scaling gives $d_{\textit{H}} \propto \mathcal{L}^{0.9}$, our theory for $d_{32}$ and $d_{99.8}$ (99.8th percentile bubble diameter) in a pure-coalescence regime predicts the slower law $\propto \mathcal{L}^{0.5}$, which our experimental results confirm – indicating negligible breakup and sub-Hinze growth. Concurrently, in contrast to current models, transient $\phi$ profiles evolve from nearly uniform to sharply core-peaked Gaussian distributions in the developing regime, with increasing centreline values and decreasing near-wall values, due to lift-force reversal. These results provide the first spatially resolved characterisation of coalescence-dominated bubbly flows at high Re, advancing the design of industrial systems as in nuclear cooling and multiphase forming processes (e.g. paper manufacturing, chemical reactors).

We perform numerical simulations of forced homogeneous isotropic turbulence over a range of bulk viscosities, Reynolds numbers and Mach numbers to investigate the scaling of key flow statistics. Using the Helmholtz decomposition, we analyse the scalings of Favre-averaged turbulent kinetic energy (TKE), root-mean-square (r.m.s.) pressure, pressure dilatation, dilatational dissipation and higher-order velocity-gradient moments. Additionally, new models are proposed for the pressure-dilatation term and the bulk-viscosity dependence of dilatational dissipation. Although the solenoidal and dilatational components of the Favre-averaged TKE are not strictly orthogonal, our numerical results demonstrate that their ratio is well approximated by the squared ratio of the corresponding r.m.s. velocities. The r.m.s. pressure approaches the pseudo-sound scaling as bulk viscosity increases. Within the Donzis r.m.s. pressure model (Donzis & John 2020 Phys. Rev. Fluids 5(8), 084609), we find that the solenoidal contribution becomes dominant for large bulk viscosity. Pressure dilatation is found to depart systematically from pseudo-sound predictions: without bulk viscosity it favours transfer from kinetic to internal energy, while finite bulk viscosity can reverse this transfer at high Mach numbers. The scaling exponent of dilatational dissipation is shown to vary with bulk viscosity, enabling a corrected model for its exponent and prefactor. Velocity-gradient skewness and flatness reveal that the onset of shocklet-induced divergence is delayed with increasing bulk viscosity and may be suppressed entirely. The results extend recent velocity-ratio-based scaling frameworks and provide modelling insights into compressible turbulence.

An experimental study was conducted to investigate the characteristics of unsteady oblique shock trains in a constant-area rectangular duct under an asymmetric incoming boundary layer. High-speed schlieren techniques and high-frequency pressure measurements were utilised in this study. The results indicate that the oblique shock train mainstream leans significantly towards the thin-boundary-layer side. Under downstream periodic excitation, the shock train moves periodically, and its shape changes during the movement. This phenomenon occurs to match the downstream pressure by altering the relative Mach number in front of the shock train, with the average pressure rise slope along the thick-boundary-layer side changing periodically. Additionally, unlike a normal shock train, the pressure rise distribution along the thick-boundary-layer side is nearly linear, and the correlation coefficients between the transducers on this side and the most downstream transducer are higher than those on the thin-boundary-layer side. Due to differences in flow structure and pressure rise distribution, the existing amplitude prediction model proposed by Xiong et al. (J. Fluid Mech. vol. 846, 2018, pp. 240–262) for the unsteady normal shock train is no longer applicable to the unsteady oblique shock train. Therefore, a new prediction model is derived and verified by experiments. Moreover, it is found that using only the downstream pressure transducer information on the thick-boundary-layer side can effectively predict the amplitude of the shock-train motion. Combined with the prediction model, a novel method is proposed to estimate the amplitude of the shock-train motion conveniently.

This study examines the cross-flow vortex-induced vibration (VIV) of a circular cylinder in combined current–oscillatory inflows, revealing a distinct multi-frequency response characterised by beat-like modulation. Systematic water-channel experiments were conducted across a range of reduced velocities, inflow oscillation intensities and frequency ratios to investigate the synchronisation mechanisms among inflow velocity variations, cylinder motion and hydrodynamic loading. Results show that the presence of oscillatory inflow can lead to significant deviations of vibration amplitudes from quasi-steady predictions within the upper-branch regime. At a given reduced velocity, the cylinder motion is dominated by a primary frequency component similar to that observed in steady flow, but accompanied by two secondary components. The contributions of these supplementary frequencies increase with inflow oscillation intensity but diminish as the oscillation frequency rises. Analysis of time-varying hydrodynamic forces reveals that, in the upper-branch regime, the vortex-force phase angle deviates substantially from quasi-steady estimation based on instantaneous reduced velocity, which is associated with non-quasi-steady vortex-shedding patterns. Particle image velocimetry measurements reveal that when the minimum vortex-force phase angle lies between 0$^\circ$ and 180$^\circ$ over the inflow oscillation cycle, a mixed vortex-shedding mode emerges. This mode is characterised by a vortex sequence resembling the ‘2P’ (two-pair) shedding pattern but with negligible secondary vortices, occurring predominantly during intervals of low inflow velocity. A theoretical framework incorporating nonlinear damping and excitation coefficients assuming quasi-steady response well predicts VIV amplitudes and elucidates the influence of inflow oscillation intensity and frequency on the emergence of supplementary vibration frequencies.

We examine elastic travelling-wave (‘arrowhead’) solutions in a viscoelastic, unidirectionally body-forced flow, focusing on their existence and morphological changes as the Weissenberg number ${\textit{Wi}}$ and streamwise duct length $L$ are varied. We find that, First, branch topology varies from an isola at low $L$ through a two-sided reconnection at intermediate $L$ to a branch that exists at asymptotically large ${\textit{Wi}}$ for larger $L$. At intermediate $L$, more than two arrowhead solutions can coexist at a given $({\textit{Wi}},L)$ choice due to extra saddle–node bifurcations. Second, the canonical arrowhead consists of two legs joined by an arched head that blocks throughflow and traps a counter-rotating vortex pair, while a polymer strand can emerge as a by-product of a strong extensional region attached to/detached from the arrowhead arch. Third, a minimal domain length $L_{min }$ required to sustain an arrowhead is found to vary non-monotonically with ${\textit{Wi}}$; for ${{\textit{Wi}}}\geqslant 20$, detached-strand states control $L_{min }$ with a relation $L_{min }\approx 0.125\,{{\textit{Wi}}}+1.5$. And fourth, in sufficiently long domains, the upper branch becomes a localised single arrowhead whose streamwise extent depends on ${\textit{Wi}}$, whereas the lower branch can proliferate into a train of arrowheads at high ${\textit{Wi}}$, a phenomenon not previously reported.

In a previous study, we have proposed a mechanism for simultaneous reduction of drag and lift by half-rotation at moderately low Reynolds numbers. The axis of rotation ($z$) is perpendicular to both the drag ($x$) and lift ($y$) directions, i.e. the rotation is transverse to the incoming flow direction. Under laminar flow conditions, force-element analysis indicates that a partially rotating sphere can significantly reduce both drag and lift with suppression of vortex shedding. This study extends investigation of the same mechanism of half-rotating a sphere to the turbulence regime at a Reynolds number $Re = 1 \times 10^4$. Similar to the laminar case, half-rotation of the sphere introduces a significant negative velocity drag term, which effectively counteracts the rapid increase in the volume- and surface-vorticity drag terms. Numerical simulations with delayed detached eddy simulation, aided by direct numerical simulation, show that the drag coefficient decreases monotonically with increasing the non-dimensional rotational speed $\alpha$, even becoming negative at $\alpha =10$, while the lift and side-force coefficients remain small for all $\alpha$. However, in contrast to laminar conditions, the turbulent regime is characterised by an earlier onset of shear-layer instabilities, which accelerates the transition of the wake into a fully turbulent state. The relative importance of volume- and surface-vorticity contributions to the drag and lift is the most outstanding difference between the laminar and turbulent flows. In turbulent flow, simultaneous reduction of drag and lift is more pronounced as the contributions of volume- and surface-vorticity lift terms almost cancel each other exactly. These mechanisms and characteristics are systematically compared with those observed in the flow around a fully rotating sphere at the same Reynolds number in terms of vorticity structures, force elements, pressure distributions as well as surface-vorticity distributions.

Dispersion in turbulent flows is of broad interest in engineering and environmental processes, particularly for rivers, lakes and oceanic water bodies. Based on our streamwise dispersion model grounded in a Lagrangian perspective of convection–diffusion dynamics (Guan & Chen, 2024, J. Fluid Mech., vol. 980, A33), this work presents a comprehensive solution that consistently unifies dispersion across the Reynolds number spectrum, bridging laminar and turbulent regimes. The streamwise dispersion mechanism is general across time scales, yet its statistical behaviour cannot be fully described using conventional coarse-grained moments averaged over cross-sections. While classical drift–diffusion models that are effective for long-time asymptotics fail to capture the turbulent dynamics of the pre-asymptotic phase, our analytical model enables a complete spatio-temporal characterisation of concentration, and reveals how local statistics evolve towards their asymptotic, coarse-grained limits. Through asymptotic expansions and eigenfunction analysis, we quantify the time-dependent behaviour of phenomenological dispersion coefficients, and distinguish between local and mean statistics, which diverge significantly during the pre-asymptotic phase. The early regime exhibits robust features, including an overshoot in local dispersivity, asymmetric long tails in mean concentration, and island-shaped solute accumulation near the free surface. Three regimes are identified in the evolution of the local concentration: (i) an initially uniform line source, (ii) a transitional logarithmic profile shaped by vertical shear, and (iii) an emergent Gaussian dispersion regime approaching vertical uniformity. Comparisons of both local and mean concentration demonstrate quantitative agreement with finite difference and Monte Carlo simulations across all regimes. These findings clarify the interplay between shear and turbulent diffusion, laying a foundation for addressing more intricate and physically significant transport problems.

Shock–boundary-layer interactions on hypersonic cone-step flows exhibit a range of intrinsic unsteady behaviours, from shear-layer oscillations to large-scale pulsations. This work investigates the unsteadiness in a cone-step geometry at Mach 6 under quiet-flow conditions at different free-stream Reynolds numbers using time-resolved schlieren imaging and spectral proper orthogonal decomposition. Experimental results are compared with high-fidelity axisymmetric and three-dimensional simulations. Results demonstrate regime transition in the parameter space, across the unsteadiness boundary, all the way from shear-layer breakdown to shock system oscillations and ultimately to large-amplitude pulsations. The dominant mode in the experiments and the simulations corresponds to a Strouhal number St $\approx 0.17$ for small oscillations reducing to St $ \approx 0.13$ for large pulsations. A detailed description of the unsteady shock dynamics and an analysis of the nonlinear limit cycle is presented.

Supersonic diamond airfoils operating in ground effect exhibit choking phenomena, where slight variations in free-stream Mach number can induce significant alterations in the ground effect flow structure and consequently affect the aerodynamic loading on the airfoil. However, existing models for predicting the choking limit Mach number demonstrate systematic discrepancies. This study establishes a novel predictive model by analysing the steady inviscid supersonic flow field around a two-dimensional diamond airfoil in ground effect. Benchmarking against numerical simulations demonstrates that the prediction errors for the choking limit Mach number across various diamond airfoil geometries are all below 3.5 %. These results affirm the high accuracy of the proposed predictive model. Under critical choking conditions, the ground effect flow field manifests multiple shock structures, including regular reflection, curved reflection and strong Mach reflection. Crucially, all of these configurations share the characteristic feature of the reflected shock impinging on the lower vertex of the airfoil. Consequently, the problem of predicting the choking limit is reformulated as determining the free-stream Mach number at which the reflected shock strikes the lower vertex of the airfoil. To circumvent complications from the reflected shock curvature inherent to critical choking, the model solves mass and momentum conservation equations for a strategically defined control volume. This approach eliminates curvature-induced errors, enabling precise prediction of the choking limit Mach number for supersonic diamond airfoils in ground effect.

Bed shear stress is a key parameter governing sediment transport and fluxes at the sediment–water interface. In vegetated channels, predicting bed shear stress, especially for rough beds, remains a challenge. This study developed a unified theoretical model for bed shear stress that smoothly spans conditions from bare bed to vegetated bed for both smooth and rough beds. Building on phenomenological turbulence theory, the model relates bed shear stress to the characteristic velocities of the larger energy-containing eddies and the smaller, near-bed eddies, with the new assumption that the bottom boundary layer (BBL) thickness controls the larger, energy-containing eddy length scale. The BBL was defined as the region within which the bed shear stress contributed significantly, compared to vegetation drag, and a force balance predicted that the BBL thickness scales with the ratio of bed shear stress to vegetation drag. In the limit of zero vegetation density, the BBL thickness equals the water depth, and the bed shear stress model reduces to the classical bare bed formulation. With increasing vegetation density (drag), the thickness of the boundary layer decreases, and the bed friction coefficient increases, which is consistent with previous observations. For rough beds, the bed friction coefficient increases with bed roughness, but is not dependent on the mean velocity. In contrast, for smooth beds, the bed friction coefficient decreases with increasing mean velocity. The coupled models for bed shear stress and BBL thickness were compared against 114 physical and numerical experiments from multiple previous studies.

Birds extend their flight envelope and adapt to time-varying aerodynamic demands by actuating the shoulder and wrist joints to morph the local sweep angles of the inner and outer wings. However, the local sweep morphing may cause unfavourable unsteady lift on the lifting surface. We investigate these unsteady lift responses using an avian-inspired wing with local sweep morphing under different morphing strategies. The unsteady lift is computed through numerically solving the incompressible Navier–Stokes equations. The results show that the local sweep morphing wing experiences a substantial maximum lift overshoot when forward sweeping and a notable maximum lift undershoot when backward sweeping. These lift over/undershoot phenomena can be alleviated by three measures: adopting smooth nonlinear morphing kinematics, initiating morphing from lift extrema opposite to the over/undershoot direction and prolonging the morphing duration. For the lift over/undershoot, the component obtained by subtracting the extended pre-morphing lift is attributed to the modification induced by local sweep morphing. To predict such lift over/undershoot, we develop a reduced-order unsteady model for the sweeping-modification lift coefficient, where Prandtl’s lifting-line theory is extended to the regime of variable translational velocity. The proposed model captures the symmetry of the sweeping-modification lift coefficient, dominated by the horizontal morphing velocity. Additionally, the vertical morphing velocity is found to correlate with the asymmetry of the sweeping-modification lift coefficient by modulating the leading-edge vortices. This study is expected to improve the understanding of surging-wing flow physics and support the design of bio-inspired multifunctional aircraft.

The anomalous scaling of passive scalar fluctuations is experimentally investigated in turbulent pipe flow with a Taylor-microscale Péclet number of $\mathcal{O}(10^5)$, where the turbulence is known to deviate from the homogeneous isotropic turbulence. The scalar structure functions and intermittency in the mixing are examined. The experimental results consolidate that the scaling exponents of scalar structure functions saturate at high-order even moments, evidenced previously in homogeneous isotropic turbulence with a Taylor-microscale Péclet number of $\mathcal{O}(10^3)$. The saturation scaling exponent decreases to approach unity as the Taylor-microscale Péclet number increases. This saturation scaling exponent is further corroborated by the fractal codimension of sharp scalar fronts.