Electroosmotic mixer stands as an energy efficient process for mixing biofluids. The present study explores a micropillar-laden microchannel under electroosmotic flow settings to modulate throughput and enhance mixing in viscoelastic medium. Through rigorous numerical simulations solving coupled Poisson–Nernst–Planck and generalized momentum conservation equation for an incompressible viscoelastic fluid modeled using Phan-Thien and Tanner constitutive relation, we report the effect of micropillar configuration, Deborah number (De), Péclet number (Pe), dimensionless Debye length, and surface charge density on the mixing efficiency, scalar dissipation rate, and flow throughput. Our study identifies that the flow vortices induced by virtue of the micropillar geometry play a critical role in mixing and throughput in the system. We observe that the vortices generally appear at thicker electrical double layer (EDL) across all Pe and surface charge densities, while thinner EDL and stronger viscoelasticity suppress these flow structures. In addition, higher Pe induces stronger vortices in thinner EDL. Notably, an increment in the elastic parameters (De and ϵ) results in enhanced net throughput as compared to Newtonian medium, with a non-intuitive mechanism in the context of viscoelastic medium. Mixing quality is reported to be high in configurations delineating stronger vortices. Biofluids generally exhibit thicker EDL and moderate De; thus, the present configuration may prove to be efficient in carrying out mixing of such systems along with reasonable throughput. We envisage that the present study will help to conceptualize the flow actuation and mixing enhancement phenomena in viscoelastic biofluids in micro-confinement comprising patterned micropillars.

Comparative assessment of nonpremixed and beta-premixed flamelet progress variable models: Effect of presumed probability density function strategies and scalar dissipation closures

• A rigorous two-stage DNS framework is developed to provide high-quality reference data for shock-turbulence-flame interactions in supersonic reacting shear layers using a moderately complex hydrocarbon fuel. • The flow is shown to be remarkably robust to variations in mean scalar dissipation rate, with reaction rates governed by localized regions of high scalar dissipation, implying accurate modelling is possible at lower grid resolutions than traditionally expected. This study examines critical aspects of Direct Numerical Simulations (DNS) of ethylene-air combustion for hypersonic propulsion applications. A combustion mechanism was selected based on a comparative analysis of multiple models, balancing chemical fidelity and computational cost. Detailed configurations of two DNS cases are presented, focusing on high-speed reacting turbulent shear layers including interactions with an oblique shock wave ( M s = 1.3 , inflow angle 5 ∘ ). A lower and standard Reynolds number case are employed to ensure mesh convergence while maintaining computational feasibility. Although most statistical quantities exhibit strong convergence trends, full convergence was not achieved for the scalar dissipation rate. This study investigates the underlying physical mechanisms responsible for this behaviour and demonstrates their limited impact on all other key variables. These results advance the understanding of turbulence-combustion interactions and will be used to guide new developments in numerical modelling for next-generation air-breathing hypersonic propulsion systems.

ABSTRACT We present results of a pilot observational wide-field VLBI (very long baseline interferometry) campaign on probing scattering properties of the partly ionized interstellar medium towards the Galactic plane sky region between $28^\circ \lt l\lt 36^\circ$ and $|b|\lt 1^\circ$. This covers the region where the Galactic bar connects to the spiral arms and where a lot of star formation is currently ongoing. The VLBA (Very Long Baseline Array) observations of the whole region were performed in a special mode with multiple phase centres at Lband (1.4–1.8 GHz) during 2022 April–June, and a year later, complemented by sessions at S band (2.2–2.4 GHz) and C band (4.6–5.0 GHz) partially covering the pilot region. We found compelling evidence that target sources are subject to scattering. The total detection rate in L, S, and C bands is 1.5, 3.4, and 9.2 per cent, respectively, and approximately scales with the square of the observation frequency. The low rate values imply that scattering is strong. Its power is non-uniform across the Galactic plane, and it can be approximated by a Gaussian with a width of about $2^\circ$ peaking at the Galactic mid-plane. One of the brightest sources of the field shows anisotropic scattering, with a $\lambda ^2$ dependence of its observed angular size, along a position angle of $26^\circ$ aligned with the line of constant Galactic latitude. We estimate the turbulence dissipation scale $r_\text{in}\approx 1500$ km towards the source J1833+0015.

This study investigates the multiscale complexity and wave-number partitioning of airfoil boundary-layer fluctuating pressures measured in the NF-3 wind tunnel at Northwestern Polytechnical University. A multifractal asymmetric correction model is established. The energy cascade characteristics of transition and turbulence are studied via proper orthogonal decomposition (POD). The results indicate that during the transition, the multifractal spectrum width of fluctuating pressure expands to approximately 0.39, and the asymmetry increases to approximately 1.9, signifying a significant enhancement of multiscale intermittency. POD analysis confirms that the transition energy is concentrated in large-scale coherent structures governed by linear wave-number partitioning, whereas turbulence follows self-similar cascade laws based on geometric series partitioning. Furthermore, intermediate turbulent modes (n=5–20) exhibit multiple wave-number partitioning mechanisms.

The turbulence kinetic energy dissipation rate ϵ, from a turbulent combustion computation using either Reynolds-averaged Navier-Stokes (RANS) or large-eddy simulation (LES), is proposed for closure with a sub-grid non-premixed flamelet model. The intentions are to avoid the creation of artificial tracking or progress variables and to relate accurately the physics of turbulent non-premixed combustion at the mean length scales to the small-scale physics where the mixing and chemical reactions occur. The analysis addresses the relations between ϵ and the strain rate, vorticity, viscous dissipation rate, scalar gradients, scalar dissipation rate, and burning rate at the smallest turbulence length scales where diffusion-controlled burning is faster than at larger length scales and thereby dominant. The imposed strain rate and vorticity on these smallest eddies are determined from the kinetic energy dissipation rate. Thus, an ϵ value at a specific time and location determines the two mechanical constraints (vorticity and strain rate) on the inflow to the counterflow flamelet. ϵ affects the sign of the Laplacian of pressure, which must be negative to allow the existence of the counterflow. Using different flamelet models, with and without vorticity, different results for maximum flamelet temperature, integrated flamelet burning rate, and maximum flamelet scalar dissipation rate are obtained. Flamelet models that consider the centrifugal effect of vorticity produce substantial enhancements in the accuracy and completeness of information for a turbulent combustion computation. ϵ may be used as a tracking variable that connects the sub-grid flamelet model to RANS or LES computations.

Direct numerical simulations are used to investigate the impact of compressibility and compositional variable-density (VD) effects on the growth of instability in buoyancy-driven shear layers (BSL), which exhibit features common to both Rayleigh–Taylor (RTI) and Kelvin–Helmholtz (KHI) instabilities. The two-dimensional simulations are performed in a periodic domain with alternating high- and low-density fluid columns under neutral background stratification. This study expands on previous work by Gat et al. [Incompressible variable-density turbulence in an external acceleration field. J Fluid Mech. 2017;827:506–535. doi: 10.1017/jfm.2017.490] conducted in the incompressible regime. The Atwood number, A, and the isothermal Mach number, Ma, are varied to isolate VD and compressibility effects. Two flow regimes are identified. At early times, shear dominates and the evolution resembles KHI with minimal sensitivity to A or Ma. Later, buoyancy becomes dominant and the flow exhibits RTI-like behaviour; increasing A accelerates this transition. Compressibility suppresses the growth of the mixing-layer, reduces bubble and spike penetration, and amplifies the VD-induced mixing asymmetry. Enhanced molecular mixing weakens the baroclinic torque, shifting vorticity production toward dissipative scales and eventually suppressing mixing-layer growth at sufficiently large Ma. The suppressive effect of compressibility strengthens with increasing A, indicating that VD and compressibility are coupled yet competing influences.

The failure resistance of polymer networks dictates their utility as material candidates across industries. However, relating the key length scales driving crack growth to molecular mechanisms remains a key bottleneck in predicting and designing against fracture. The fractocohesive length—defined in terms of the ratio of fracture energy to the specific work to rupture—of a material correlates with the length scale of energy dissipation and controls fracture resistance. Although the Lake–Thomas model predicts the fractocohesive length of a perfect polymer network to match the undeformed mesh size, real soft materials exhibit values that far exceed this prediction. Here, we report extraordinarily high fractocohesive lengths in polymer-like networks with and without defects. We find that even perfect networks can have fractocohesive lengths orders of magnitude higher than the undeformed mesh size due to highly nonlinear chain behavior giving rise to nonlocal effects during fracture. Introducing defects further increases the fractocohesive length. We identify quantitative relations between nonlinear chain mechanics, defect length, defect density, and fractocohesive length. Overall, strain-stiffening chain behavior, defect density, and defect size independently correlate with larger fractocohesive lengths in polymer-like networks, and their individual effects can be collapsed into a single power law scaling. These outcomes point the way towards improved physics-informed design of soft yet tough polymers and metamaterials.

Slender fibres, including textile-derived microplastics, are abundant in aquatic environments and often extend beyond the Kolmogorov length scale. While breakup at dissipative scales has been characterised by velocity-gradient statistics, no closure existed for inertial-range spans where eddy turnover sets the clock. Here we develop a turbulence-informed kinetic theory of fibre fragmentation bridging turbulence forcing and slender-beam mechanics. First, we derive a load-to-curvature mapping showing that spanwise forcing generates peak bending moments scaling as $\sim U_L L^2$ , with $U_L$ the velocity increment across fibre length $L$ . Second, we construct a breakup hazard $h(L)$ from curvature-threshold exceedances over eddy-time blocks, which identifies a turbulence-defined critical span $\ell _c$ . For $L\gt \ell _c$ , breakup is eddy-time-limited, $h(L)=O(\bar \varepsilon ^{1/3}L^{-2/3})$ with $\bar \varepsilon$ the mean turbulent energy dissipation rate, whereas for $L\lt \ell _c$ , it is a rare-event process with $h(L)\propto L^{5/3+\alpha }$ , $\alpha$ denoting the small correction from intermittency. Embedding this hazard in a self-similar binary kernel yields a closed population-balance equation for the fragment distribution $n(L,t)$ with sources and sinks. The framework produces explicit predictions: intermittency-corrected curvature scalings, critical spans set by material and flow parameters, start-up and halving times linked to surf-zone conditions and scaling profiles in the cascade. The steady-state bulk distribution on the subcritical branch, with vertical removal induced by horizontal convergence, follows $n(L)\propto L^{-8/3-\alpha }\simeq L^{-2.7}$ , in striking agreement with the mean slope $\simeq -2.68$ observed for environmental microfibres in recent surveys. The reported variability of slopes is naturally explained in our framework by the coexistence of supercritical and subcritical branches together with $L$ -dependent removal-driven sinks.

Abstract Kinetic energy (KE) transfer between spatial scales contributes to the ocean's energy budget by linking scales of KE supply and KE dissipation. Numerical simulations have indicated that for scales smaller than the baroclinic deformation radius, cross‐scale KE transfer has complex spatial and temporal variability, modulated by mixed layer properties, fronts, and eddies. Here, over a decade of upper‐ocean surface velocity data, collected from high‐frequency radar within the Santa Barbara Channel, are used to estimate cross‐scale KE transfer. The transfer of KE across 7 km has strong seasonal and interannual variations linked to energy exchange with the atmosphere. This study observationally confirms (a) the importance of the surface divergence field in determining the direction of the KE transfer and (b) the equi‐partitioning of KE transfer between divergent and straining motions. The temporal variability in KE transfer suggests that surface forcing influences the long‐term redistribution of energy between scales.

Abstract Fluctuation dynamos provide a robust mechanism for amplifying weak seed magnetic fields in turbulent astrophysical plasmas. However, their behaviour in the highly compressible regimes characteristic of the interstellar medium remains incompletely understood. Using high-resolution 3D magnetohydrodynamic simulations of supersonic turbulence with rms Mach number M rms ≈ 11 , we explore fluctuation dynamos across magnetic Prandtl numbers Pm = 1–10. At Pm = 1, dynamo growth is slower and saturates at lower magnetic-to-kinetic energy ratios, with amplification in the kinematic phase dominated by compression rather than line stretching. In contrast, at Pm = 10, vortical stretching emerges as the dominant mechanism, yielding faster growth, higher saturation levels, and stronger suppression of density–magnetic field correlations by magnetic pressure. This transition is reflected in the correlation coefficient between density and magnetic field strength, which is strongly positive at Pm = 1 but decreases significantly at higher Pm. Across all runs, the ratio of velocity-to-magnetic integral scales is ∼3.4, in the saturated phase, independent of Pm, while the ratio of viscous to resistive dissipation scales increases with the increase in Pm. Synthetic Faraday rotation measures reveal coherence lengths of ∼one-fourth to one-third of the forcing scale across the range of Pm explored. Using these coherence scales, we discuss the potential contribution of fluctuation dynamos to Faraday rotation expected from turbulent, gas-rich young disk galaxies.

The mixing mechanism within a single vortex has been a theoretical focus for decades, while it remains unclear especially under the variable-density (VD) scenario. This study investigates canonical single-vortex VD mixing in shock–bubble interactions (SBI) through high-resolution numerical simulations. Special attention is paid to examining the stretching dynamics and its impact on VD mixing within a single vortex, and this problem is investigated by quantitatively characterising the scalar dissipation rate (SDR), namely the mixing rate, and its time integral, referred to as mixedness. To study VD mixing, we first examine single-vortex passive-scalar (PS) mixing with the absence of a density difference. Mixing originates from diffusion and is further enhanced by the stretching dynamics. Under the axisymmetry and zero diffusion assumptions, the single-vortex stretching rate illustrates an algebraic growth of the length of scalar strips over time. By incorporating the diffusion process through the solution of the advection–diffusion equation along these stretched scalar strips, a PS mixing model for SDR is proposed based on the single-vortex algebraic stretching characteristic. Within this framework, density-gradient effects from two perspectives of the stretching dynamics and diffusion process are discovered to challenge the extension of the PS mixing model to VD mixing. First, the secondary baroclinic effect increases the VD stretching rate by the additional secondary baroclinic principal strain, while the algebraic stretching characteristic is still retained. Second, the density source effect, originating from the intrinsic nature of the density difference in the multi-component transport equation, suppresses the diffusion process. By accounting for both the secondary baroclinic effect on stretching and the density source effect on diffusion, a VD mixing model for SBI is further modified. This model establishes a quantitative relationship between the stretching dynamics and the evolution of the mixing rate and mixedness for single-vortex VD mixing over a broad range of Mach numbers. Furthermore, the essential role of the stretching dynamics on the mixing rate is demonstrated by the derived dependence of the time-averaged mixing rate $\overline {\langle \chi \rangle }$ on the Péclet number ${\textit{Pe}}$ , which scales as $\overline {\langle \chi \rangle } \sim {\textit{Pe}}^{{2}/{3}}$ .

Abstract We develop a generalized wave kinetic theory for weakly nonlinear wave
systems with stochastic dissipation. Starting from the Zakharov equation
with multiplicative Gaussian noise, interpreted in the Itô sense, we derive
the statistical evolution of the wave action using action--angle variables
and a systematic multiscale expansion. Two distinct dissipation scalings are
identified. For $\mu\sim\epsilon^2$, stochastic dissipation does not affect
the leading--order kinetics and the classical Hasselmann equation is
recovered. For $\mu\sim\epsilon$, stochastic fluctuations survive averaging
and induce an exponential modulation of resonant wave--wave interactions,
together with a noise--induced drift term. For constant dissipation
coefficients this reduces to a rescaling of the kinetic time, while in the
genuinely stochastic regime temporal fluctuations reshape energy transfer in
wave turbulence.

ABSTRACT This study presents a numerical investigation of passive scalar mixing in homogeneous isotropic turbulence (HIT). Different volumetric forcing schemes have been used in the literature, but the side effects are rarely discussed, either because these are assumed irrelevant or because it is too costly to conduct such an analysis with a high‐fidelity model. In this study, we have used One‐Dimensional Turbulence (ODT) model to compare forcing schemes at low Reynolds numbers (upto ). Our analysis reveals critical flaws in the linear forcing model when applied to ODT. While both schemes exhibit spectral deviations from direct numerical simulation (DNS), the stochastic forcing scheme demonstrates superior dynamic fidelity, better capturing the turbulent energy cascade. In contrast, the linear forcing scheme suffers from a non‐physical energy deficit at large scales and is approximately 10 times more computationally expensive. These artefacts directly impact scalar mixing: The stochastic scheme produces classic, multi‐scale intermittency, whereas linear forcing generates extreme gradients confined only at the dissipative scales. These results demonstrate that the choice of forcing is a critical modelling decision in ODT, leading to fundamentally different model‐dependent artefacts in both turbulence dynamics and scalar mixing statistics, at least in low Reynolds number regimes.

In Navier–Stokes (NS) turbulence, large-scale turbulent flows inevitably determine small-scale flows. Previous studies using data assimilation with the three-dimensional (3-D) NS equations indicate that employing observational data resolved down to a specific length scale, $\ell ^{\rm 3\text{-}D}_{\ast }$ , enables the successful reconstruction of small-scale flows. Such a length scale of ‘essential resolution of observation’ for reconstruction $\ell ^{\rm 3\text{-}D}_{\ast }$ is close to the dissipation scale in three-dimensional NS turbulence. Here, we study the equivalent length scale in two-dimensional (2-D) NS turbulence, $\ell ^{\rm 2\text{-}D}_{\ast }$ , and compare with the three-dimensional case. Our numerical studies using data assimilation and conditional Lyapunov exponents reveal that, for Kolmogorov flows with Ekman drag, the length scale $\ell ^{\rm 2\text{-}D}_{\ast }$ is actually close to the forcing scale, substantially larger than the dissipation scale. Furthermore, we discuss the origin of the significant relative difference between the length scales, $\ell ^{\rm 2\text{-}D}_{\ast }$ and $\ell ^{\rm 3\text{-}D}_{\ast }$ , based on inter-scale interactions, ‘cascades’ and orbital instabilities in turbulence dynamics.

We study the statistics of dynamical quantities associated with magnetic reconnection events embedded in a sea of strong background magnetohydrodynamic turbulence using direct numerical simulations. We focus on the relationship of the reconnection properties to the statistics of global turbulent fields. We show that the distribution in turbulence of reconnection rates (determined by upstream fields) is strongly correlated with the magnitude of the global turbulent magnetic field at the correlation scale. The average reconnection rates, and associated dissipation rates, during turbulence are thus much larger than predicted by using turbulent magnetic field fluctuation amplitudes at the dissipation or kinetic scales. Magnetic reconnection may, therefore, be playing a major role in energy dissipation in astrophysical and heliospheric turbulence.

The distinct polar vortex dynamics observed on Jupiter and Saturn may provide insights into their interiors. In this study, we examine how the number and structure of polar vortices vary with forcing strength, dissipation rate, and interior stratification using a 1.5-layer quasi-geostrophic model. This simplified setup enables a broad exploration of the parameter space, revealing that vortex characteristics are determined by the sequence in which three key length scales-the deformation radius [Formula: see text], the zonostrophic scale [Formula: see text], and the dissipative scale [Formula: see text]-are encountered as energy cascades from small to large scales. Four distinct vortex patterns are identified, including a vortex crystal resembling Jupiter's polar vortices and a single-vortex state akin to that of Saturn. The conditions under which these patterns emerge provide constraints on the stratification of Jupiter and Saturn.

Manifold-based models offer a computationally efficient alternative to directly transporting the thermochemical state in computational simulations of turbulent reacting flows, projecting the high-dimensional thermochemical state-space onto a low-dimensional manifold. Recent efforts have yielded a manifold-based model applicable to multi-modal combustion, enabling reconstruction of the thermochemical state from solutions to two-dimensional manifold equations in mixture fraction and generalized progress variable that are parameterised by three scalar dissipation rates. In coarse-grained simulations such as Large Eddy Simulation (LES), closure of the multi-modal manifold equations and subfilter variances/covariance requires closure of three filtered scalar dissipation rates. The present work adopts a data-based approach, providing closure for the three filtered scalar dissipation rates via deep neural networks (DNNs). High-fidelity datasets corresponding to an autoigniting n-dodecane jet flame and a bluff body swirl-stabilized confined lifted spray flame of two aviation fuels (Jet-A and C1) with different ignition propensities are leveraged to generate training data that spans a diverse range of thermodynamic conditions and combustion modes, including low- and high-temperature ignition regimes in addition to premixed and nonpremixed behaviour. A final DNN model is trained to enforce inherent physical constraints by learning nonlinear functional transformations of the three filtered scalar dissipation rates. The generalizability of this constrained DNN model is demonstrated a priori via conditional statistics evaluated on the lifted spray flame with C1–a configuration that had not been included in the training data. Excellent DNN agreement with conditional DNS statistics is observed, and integrated gradients are computed to identify the most sensitive input variables. The similarity of the marginal PDFs of the most informative input variables and outputs across configurations are quantified via the Wasserstein metric, demonstrating that data-based models may successfully generalize to unseen parametric conditions so long as the most informative input variables share similar distributions across training and testing datasets.

The classical scaling of wall turbulence is based on wall parameters (friction velocity, friction temperature, and kinematic viscosity). Recently, a novel scaling based on dissipative scales—the mean turbulent energy dissipation rate, the mean scalar dissipation rate, and kinematic viscosity—was put forward by Tang and Antonia [“Scaling of small-scale wall turbulence,” J. Fluid Mech. 948, A25 (2022) and “Similarity for dissipation-scaled wall turbulence,” J. Fluid Mech. 960, A18 (2023)]. This new scaling is supported by extensive wall turbulence data, related mainly to the second- and third-order statistics. In this paper, we systematically compare the classical wall scaling and the dissipative scaling in the context of fourth-order statistics, specifically, the fourth-order velocity and scalar structure functions in a fully developed turbulent channel flow at a molecular Prandtl number Pr=0.71. It is found that both the wall parameter- and dissipative scale-normalized fourth-order velocity and scalar structure functions exhibit similar degrees of collapse at moderate distances from the wall. However, in the very-near-wall region, dissipative scale-normalized structure functions collapse approximately at all scales, while wall parameter-normalized distributions vary systematically with the Reynolds number. Far from the wall, dissipative scale-normalized structure functions collapse approximately at small scales, while wall parameter-normalized distributions continue to evolve with the Reynolds number at all scales. Overall, the scaling based on dissipative scales is superior to that based on wall parameters over a significant portion of the channel, except at moderate distances from the wall, where both scalings are valid in the small-scale range.

Direct numerical simulation data of velocity- and vorticity-fluctuations are used to study the Coriolis-induced modulation of spectral-scales, spatiotemporal correlations, and vorticity-dynamics in turbulent channels. Although rotation-induced asymmetry is known, its scale-resolved manifestation and link between spectral-dynamics and local vortex-topology remain unclear. Energy spectra show a spectral shift toward larger scales and increased energy on the anti-cyclonic side, consistent with the emergence of large-scale structures, while energy is suppressed on the cyclonic side. Dissipation becomes asymmetric, but the characteristic small scales of dissipation are nearly unaffected by rotation. Vortex stretching is amplified and shifts toward larger streamwise scales on the anti-cyclonic side, indicating the formation of elongated structures, while the spanwise scales remain nearly unchanged, reflecting anisotropic modulation. Enstrophy production is enhanced and exhibits spectral similarity with viscous dissipation on the anti-cyclonic side, reflecting active small-scale turbulence, while the cyclonic side shows weakened small-scale turbulent activity, consistent with re-laminarization. Spatial two-point correlations reveal large-scale, laminar-like structures with intermittent wall-normal fluctuations on the cyclonic side. Temporal autocorrelations reveal enhanced coherence on the anti-cyclonic side and slowly evolving streamwise structures on the cyclonic side. Quadrant analysis reveals a sustained bursting cycle on the anti-cyclonic side and suppressed turbulence on the cyclonic side. Enstrophy production exhibits intermittent bursts on the anti-cyclonic side, marking regions of strong vorticity amplification.