Clustering of swimming phytoplankton in turbulence

• Guido Boffetta (University of Torino)

Room: NORDITA seminar room

Joint Activity: Modelling of Fibre Suspension Flows Room: Lecture Room FR 4

Joint Activity: Modelling of Fibre Suspension Flows Room: FB 52

Entrainment and mixing at the edge of a cloud

• Joerg Schumacher (TU Ilmenau, Germany)

Room: FB 52 Responses of a droplet ensemble during initial entrainment and subsequent mixing of cloudy and clear air at the edge of a cloud are studied by 3d DNS which combine the Eulerian description of the turbulent velocity, temperature and vapor content fields with a Lagrangian ensemble of cloud water droplets. We discuss their size distribution, the supersaturation and the mixing properties. Furthermore we give an outlook of how these data might be matched to large eddy simulation which cover a whole cloud or clusters of clouds.

Aggregation, diffusion, and rheology in particulate suspensions at continuum scale

• Antti Puisto (Aalto University)

Room: FB 52 In this presentation we will discuss the modeling of aggregation and diffusion of particles under shear flow at continuum scale using population balance based models. First, we will concentrate on aggregation colloidal suspensions, specifically modeling the state of fractally growing aggregates, and its implications to the rheology of the fluid - for both orthokinetic and perikinetic aggregation [1]. Furthermore, we will discuss the consequences of such thixotropy caused by time-dependent aggregation to the rheological characterization of the fluid. Then, we turn the discussion to hard sphere suspensions. For these we have studied shear induced migration of particles [2] in simple geometries, and its influence on the experimentally measured flow curves. With repulsive wall-particle interaction we observe the appearance of shear rate dependent depletion layers near the device walls [3]. For long time, in structured fluids this has been modeled on sub-particle length scales, however, here we show slip-layers of several tens of particle diameters, comparable to experimental data observed, for instance, in stabilized emulsions [4]. Furthermore, we will discuss the influence of this wall depletion to the globally measured rheology. [1] M. Mohtaschemi et al. Soft Matter 10, 2971 (2014). [2] R. J. Phillips et al. Phys. Fluids A 4, 30 (1992). [3] M. Korhonen et al. Under preparation (2014). [4] S. Manneville et al. Eur. Phys. J. E 10, 209 (2003).

Advection of Active Particles

• Nick Ouellette (Yale)

Room: FB 52

Motion of fluid particles and the irreversibility of turbulence

• Haitao Xu (University of Gottingen)

Room: FB 52 In fluid turbulence, there is a wide separation between the scales at which the fluid is forced into motion and the scales at which the dissipation dominates. In between, energy is transferred through scales. This energy cascade dictates that turbulence statistics are not time-reversible, as reflected in the celebrated Karman-Howarth-Kolmogorov equation, which relates the energy flux with velocity differences in space (Eulerian statistics). This can be further extended to statistics along trajectories of fluid particles in turbulence (Lagrangian statistics). The energy flux can also be related to the relative motion between fluid particles. The interesting question is then: Can one detect irreversibility from the motion of single fluid particles, where an intrinsic length scale is missing? Using data from both experiments and direct numerical simulations in a large set of flow conditions, we show that the irreversibility induced by the energy flux through spatial scales can be revealed and quantified by following the change of the kinetic energy of single fluid particles. We find that fluid particles decelerate harder than they accelerate, i.e., they tend to lose kinetic energy faster than they gain it. The third moment of the power fluctuations along a trajectory, nondimensionalized by the energy flux, displays a remarkable power law as a function of the Reynolds number, both in two and in three spatial dimensions. This establishes a relation between the irreversibility of the system and the range of active scales.

Turbulence modulations by small particles in gas-solid channel flow

• L Zhao (Norwegian University of Science and Technology)

Room: FB 52 Turbulence modulations and transfer of mechanical energy between solid spherical particles and a Newtonian carrier fluid has been explored in two-way coupled direct numerical simulations in a turbulent channel flow. At sufficiently large particle response times the Reynolds shear stress and the turbulence intensities in the spanwise and wall-normal directions were attenuated whereas the velocity fluctuations were augmented in the streamwise direction. The physical mechanisms involved in the particle–fluid interactions were analysed in detail, and it was observed that the fluid transferred energy to the particles in the core region of the channel whereas the fluid received kinetic energy from the particles in the wall region. A local imbalance in the work performed by the particles on the fluid and the work exerted by the fluid on the particles was observed. This imbalance gave rise to a particle induced energy dissipation which represents a loss of mechanical energy from the fluid–particle suspension. Additionally Reynolds budgets stresses are analyzed and compared with the particle-unladen flow.

Mathematical models & measures of mixing

• Charles R. Doering (University of Michigan)

Room: FB 52 Mixing passive tracers by stirring in a fluid can be measured in a variety of ways including particle dispersion, via the flux-gradient relationship, or by suppression of scalar concentration variations in the presence of inhomogeneous sources and sinks. The mixing efficiency or efficacy of a particular flow is often expressed in terms of enhanced diffusivity and quantified as an effective diffusion coefficient. In this work we compare and contrast several notions of effective diffusivity. We thoroughly examine the fundamental case of a steady sinusoidal shear flow mixing a scalar concentration sustained by a steady sinusoidal source-sink distribution to explore apparent quantitative inconsistencies among the measures. Ultimately the conflicts are attributed to the noncommutative asymptotic limits of large Peclet number and large length-scale separation. We then propose another approach, a generalization of Batchelor's 1949 theory of diffusion in homogeneous turbulence, that helps unify the particle dispersion and concentration variance suppression mixing measures.

Turbulent cascades & inviscid invariants

• S Musacchio

Room: FB 52 I will present two examples of three-dimensional turbulent flows in which the presence of a second positive-defined inviscid invariant beside kinetic energy causes a reversal of the direction of the turbulent cascade of energy. I will first discuss a non- isotropic case, in which the flow is confined in a thin layer. Then I will show that a similar phenomenon can occur also in three- dimensional isotropic flows with breaking of parity invariance.

Hertz theory, collisional fusion and protoplanets

• John Wettlaufer (Yale University)

Room: FB 52

Conference on Dynamics of Particles in Flows Room: FD 5

Conference on Dynamics of Particles in Flows Room: FR 4

Conference on Dynamics of Particles in Flows Room: FD 5

Conference on Dynamics of Particles in Flows Room: FD 5

TBA

• Gino ALMONDO

Room: NORDITA seminar room

Discussion session on Atmospheric Clouds Room: NORDITA seminar room

Statistical Properties of Inertial-Particle Trajectories in Turbulent Flows

• Rahul PANDIT

Room: NORDITA seminar room

Triangular Constellations in Fractal Measures

• John GRANT

Room: NORDITA seminar room The local structure of fractal sets may have important implications for properties such as light scattering or network connectivity. It is therefore of interest to understand the statistics of the internal shape structures within a fractal. In this presentation I will summarise the findings from recent investigations into the distribution of triangle shapes within fractals formed by particles in a turbulent flow.

Lattice Boltzmann simulations of finite-size fibres in a turbulent channel

• Geert Brethouwer (KTH)

Room: NORDITA seminar room

Discussion session on Planet Formation Room: NORDITA seminar room

Turbulent broadening of a scalar spot

• E Calzavarini

Room: NORDITA seminar room

Bending dynamics of semiflexible particles in turbulent flows

• Dario Vincenzi

Room: FB 52

Discussion session on Particle Clustering Room: NORDITA seminar room

Energy conservation law in the free atmosphere

• Kulyash Kaliyeva

Room: FB 52

Turbulence-induced relative velocity of inertial particles

• Liubin Pan

Room: NORDITA seminar room

Fossil and renewable electricity production: the importance of particle impaction

• Nils Erland L. Haugen (SINTEF Energy Research)

Room: NORDITA seminar room

Understanding droplet collisions in turbulent flows

• Vincent Perrin

Room: FB 52

Concluding discussion Room: FB 52