Journal: Journal of Fluid Mechanics

Abbreviation

J. Fluid Mech.

Publisher

Cambridge University Press

Journal Volumes

ISSN

0022-1120
1469-7645

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Publications 1 - 10 of 247
  • Interplay between advective, diffusive and active barriers in (rotating) Rayleigh-Benard flow
    Item type: Journal Article
    Aksamit, Nikolas O.; Hartmann, Robert; Lohse, Detlef; et al. (2023)
    Journal of Fluid Mechanics
    Our understanding of the material organization of complex fluid flows has benefited recently from mathematical developments in the theory of objective coherent structures. These methods have provided a wealth of approaches that identify transport barriers in three-dimensional (3-D) turbulent flows. Specifically, theoretical advances have been incorporated into numerical algorithms that extract the most influential advective, diffusive and active barriers to transport from data sets in a frame-indifferent fashion. To date, however, there has been very limited investigation into these objectively defined transport barriers in 3-D unsteady flows with complicated spatiotemporal dynamics. Similarly, no systematic comparison of advective, diffusive and active barriers has been carried out in a 3-D flow with both thermally driven and mechanically modified structures. In our study, we utilize simulations of turbulent rotating Rayleigh-B & eacute;nard convection to uncover the interplay between advective transport barriers (Lagrangian coherent structures), material barriers to diffusive heat transport, and objective Eulerian barriers to momentum transport. For a range of (inverse) Rossby numbers, we identify each type of barrier and find intriguing relationships between momentum and heat transport that can be related to changes in the relative influence of mechanical and thermal forces. Further connections between bulk behaviours and structure-specific behaviours are also developed.
  • Pressure corrections for the effects of viscosity on the irrotational flow outside Prandtl’s boundary layer
    Item type: Journal Article
    Wang, Jing; Joseph, Daniel D. (2006)
    Journal of Fluid Mechanics
  • Exact theory of material spike formation in flow separation
    Item type: Journal Article
    Serra, Mattia; Vétel, Jérôme; Haller, George (2018)
    Journal of Fluid Mechanics
    We develop a frame-invariant theory of material spike formation during flow separation over a no-slip boundary in two-dimensional flows with arbitrary time dependence. Based on the exact curvature evolution of near-wall material lines, our theory identifies both fixed and moving flow separation, is effective also over short time intervals, and admits a rigorous instantaneous limit. As a byproduct, we derive explicit formulae for the evolution of material line curvature and the curvature rate for general compressible flows. The material backbone that we identify acts first as the precursor and later as the centrepiece of unsteady Lagrangian flow separation. We also discover a previously undetected spiking point where the backbone of separation connects to the boundary, and derive wall-based analytical formulae for its location. Finally, our theory explains the perception of off-wall separation in unsteady flows and provides conditions under which such a perception is justified. We illustrate our results on several analytical and experimental flows.
  • Behaviour of finite-size floating particles in free-surface turbulence
    Item type: Journal Article
    Sanness Salmon, Henri; Chang, Kelken; Mucignat, Claudio; et al. (2025)
    Journal of Fluid Mechanics
    Motivated by the need for a better understanding of marine plastic transport, we experimentally investigate finite-size particles floating in free-surface turbulence. Using particle tracking velocimetry, we study the motion of spheres and discs along the quasi-flat free-surface above homogeneous isotropic grid turbulence in open channel flows. The focus is on the effect of the particle diameter, which varies from the Kolmogorov scale to the integral scale of the turbulence. We find that particles of size up to approximately one-tenth of the integral scale display motion statistics indistinguishable from surface flow tracers. For larger sizes, the particle fluctuating energy and acceleration variance decrease, the correlation times of their velocity and acceleration increase, and the particle diffusivity is weakly dependent on their diameter. Unlike in three-dimensional turbulence, the acceleration of finite-size floating particles becomes less intermittent with increasing size, recovering a Gaussian distribution for diameters in the inertial subrange. These results are used to assess the applicability of two distinct frameworks: temporal filtering and spatial filtering. Neglecting preferential sampling and assuming an empirical linear relation between the particle size and its response time, the temporal filtering approach is found to correctly predict the main trends, though with quantitative discrepancies. However, the spatial filtering approach, based on the spatial autocorrelation of the free-surface turbulence, accurately reproduces the decay of the fluctuating energy with increasing diameter. Although the scale separation is limited, power-law scaling relations for the particle acceleration variance based on spatial filtering are compatible with the observations.
  • Modelling the breakup of solid aggregates in turbulent flows
    Item type: Journal Article
    Bäbler, Matthäus Ulrich; Morbidelli, Massimo; Baldyga, Jerzy (2008)
    Journal of Fluid Mechanics
  • Lagrangian measurement of vorticity dynamics in turbulent flow
    Item type: Journal Article
    Lüthi, Beat; Tsinober, Arkady; Kinzelbach, Wolfgang (2005)
    Journal of Fluid Mechanics
  • Scale analysis of miscible density-driven convection in porous media
    Item type: Journal Article
    Jenny, Patrick; Lee, Joohwa S.; Meyer, Daniel W.; et al. (2014)
    Journal of Fluid Mechanics
  • Joint reduced model for the laminar and chaotic attractors in plane Couette flow
    Item type: Journal Article
    Kaszás, Bálint; Haller, George (2025)
    Journal of Fluid Mechanics
    We use the theory of spectral submanifolds (SSMs) to develop a low-dimensional reduced-order model for plane Couette flow restricted to the shift-reflect invariant subspace in the permanently chaotic regime at studied by Kreilos & Eckhardt (2012, Chaos: Interdisciplinary J. Nonlinear Sci., vol. 22, 047505). Our three-dimensional model is obtained by restricting the dynamics to the slowest mixed-mode SSM of the edge state. We show that this results in a nonlinear model that accurately reconstructs individual trajectories, representing the entire chaotic attractor and the laminar dynamics simultaneously. In addition, we derive a two-dimensional Poincaré map that enables the rapid computation of the periodic orbits embedded in the chaotic attractor.
  • Enumeration, orthogonality and completeness of the incompressible Coriolis modes in a sphere
    Item type: Journal Article
    Ivers, David J.; Jackson, Andrew; Winch, Denis (2015)
    Journal of Fluid Mechanics
  • Small-scale dynamics and structure of free-surface turbulence
    Item type: Journal Article
    Qi, Yinghe; Li, Yaxing; Coletti, Filippo (2025)
    Journal of Fluid Mechanics
    The dynamics of small-scale structures in free-surface turbulence is crucial to large-scale phenomena in natural and industrial environments. Here, we conduct experiments on the quasi-flat free surface of a zero-mean-flow turbulent water tank over the Reynolds number range Reλ=207-312. By seeding microscopic floating particles at high concentrations, the fine scales of the flow and the velocity-gradient tensor are resolved. A kinematic relation is derived expressing the contribution of surface divergence and vorticity to the dissipation rate. The probability density functions of divergence, vorticity and strain rate collapse once normalised by the Kolmogorov scales. Their magnitude displays strong intermittency and follows chi-square distributions with power-law tails at small values. The topology of high-intensity events and two-point statistics indicate that the surface divergence is characterised by dissipative spatial and temporal scales, while the high-vorticity and high-strain-rate regions are larger, long-lived, concurrent and elongated. The second-order velocity structure functions obey the classic Kolmogorov scaling in the inertial range when the dissipation rate on the surface is considered, with a different numerical constant than in three-dimensional turbulence. The cross-correlation among divergence, vorticity and strain rate indicates that the surface-Attached vortices are strengthened during downwellings and diffuse when those dissipate. Sources (sinks) in the surface velocity fields are associated with strong (weak) surface-parallel stretching and compression along perpendicular directions. The floating particles cluster over spatial and temporal scales larger than those of the sinks. These results demonstrate that, compared with three-dimensional turbulence, in free-surface turbulence the energetic scales leave a stronger imprint on the small-scale quantities.
Publications 1 - 10 of 247