10:30
Lagrangian aspects of turbulence 2
Chair: Willem van de Water
10:30
15 mins
|
Longitudinal and transverse Lagrangian velocity increments
Emmanuel Leveque, Aurore Naso
Abstract: Longitudinal and transverse Lagrangian velocity increments are introduced as components along, and perpendicular to, the displacement of fluid particles during a time scale $\tau$. These increments provide a new path to the characterization of Lagrangian statistics in homogeneous and isotropic turbulence, and allows us to establish some bridge with Eulerian statistics. From direct numerical simulations, it is shown that the probability distributions of the longitudinal Lagrangian increment are negatively skewed at all time scales, which is a signature of time irreversibility in the Lagrangian framework. Transverse increments are found more intermittent than longitudinal increments. Eventually, transverse Lagrangian increments exhibit scaling properties that are very close to those of standard Cartesian Lagrangian increments.
|
10:45
15 mins
|
ENERGY EXCHANGES AND TIME ASYMMETRY IN 3D TURBULENT FLOWS
Alain Pumir, Haitao Xu, Rainer Grauer, Eberhard Bodenschatz
Abstract: In 3D turbulent flows, the direct cascade of energy, characterized by a flux through scales, ε, is a strong source of irre- versibility. This irreversibility manifests itself in the asymmetry between negative and positive kinetic energy differences along particle trajectories. In particular, it is observed that the odd moments of the power of the forces acting on a fluid particle, p ≡ a · V are negative, and that −⟨p3 ⟩/ε3 ∝ Rλ2 . This property rests on subtle correlations between acceleration and velocity. I will discuss two representations of the acceleration a, which shed light on the irreversibility of the flow.
|
11:00
15 mins
|
Clustering of particles driven by salinity gradients in turbulence
Lukas Schmidt, Itzhak Fouxon, Dominik Krug, Markus Holzner
Abstract: Experimental analysis of the diffusiophoretic effect in turbulence demonstrates that preferential concentration of particles driven by the combined action of turbulence and inhomogeneous distribution of salinity exists. Additionally, theoretical predictions on the particle behavior could be confirmed. We provide evidence that inhomogeneous salt distributions in turbulence produce particle clustering in fractal sets at scales below the Kolmogorov scale. This clustering mechanism may provide the key to understanding a multitude of processes such as formation of marine snow in the ocean and population dynamics of chemotactic bacteria.
|
11:15
15 mins
|
BENDING DYNAMICS OF SEMI-FLEXIBLE PARTICLES IN TURBULENT FLOWS
Aamir Ali, Emmanuel Lance Christopher VI Plan, Samriddhi Sankar Ray, Dario Vincenzi
Abstract: We study the Lagrangian dynamics of semi-flexible particles in laminar as well as in homogeneous and isotropic turbulent flows by means of analytically solvable stochastic models and direct numerical simulations. The statistics of the bending angle is qualitatively different in laminar and turbulent flows and exhibits a strong dependence on the topology of the velocity field. In particular, in two-dimensional turbulence, particles are either found in a fully extended or in a fully folded configuration; in three dimensions, the predominant configuration is the fully extended one.
|
11:30
15 mins
|
MEASURING THE ORIENTATION AND ROTATION RATE OF 3D PRINTED PARTICLES IN TURBULENCE
Stefan Kramel, Guy Geyer Marcus, Shima Parsa, Brendan Cole, Rui Ni, Greg Voth
Abstract: The orientation distribution and rotations of anisotropic particles in turbulent flows play a key role in many applications ranging from icy clouds to papermaking and drag reduction in pipe flow. However, experimental access to time-resolved orientations of anisotropic particles has not been easy to achieve. The use of 3D printing opens up the possibility to fabricate a wide range of particle shapes with smallest dimension down to 300 μm. So far, we have printed rods, crosses, jacks, triads, tetrads and helical particle shapes. We extract particle orientations from stereoscopic video images using a method of least squares optimization in Euler angle space. We find that in turbulence, the orientation and rotation-rate of many particles can be understood using a simple picture of alignment of both the vorticity and a long axis of the particle with the Lagrangian stretching direction of the flow.
|
11:45
15 mins
|
Influence of small-scale turbulence on spatial distribution of cloud-like particles
Katarzyna Karpinska, Szymon Malinowski
Abstract: Abstract Influence of small-scale turbulence on cloud droplets spatial distribution was analyzed by examination of motion of inertial
droplets in a simple model of a vortex tube aligned at arbitrary angle to gravity force. Both analytical calculations and numerical
simulations demonstrated characteristic features of the motion such as equillibrium points, limit cycles and stationary orbits, for which
conditions of existence were calculated. Simulations of motion of polydispersed droplets illustrate sorting effects of a vortex tube
|
12:00
15 mins
|
Turbulent super-diffusion as a ballistic cascade
Mickael Bourgoin
Abstract: Since the pioneering work of Richardson in 1926, later refined by Batchelor and Obukhov in 1950, it is predicted that the rate of separation of pairs of fluid elements in turbulent flows with initial separation at inertial scales, grows ballistically first (Batchelor regime), before undergoing a transition towards a super-diffusive regime where the mean-square separation grows as $t^3$ (Richardson regime). Richardson empirically interpreted this super-diffusive regime in terms of a non-Fickian process with a scale dependent diffusion coefficient (the celebrated Richardson's ``4/3rd'' law). However, the actual physical mechanism at the origin of such a scale dependent diffusion coefficient remains unclear. The present work proposes a simple physical phenomenology for the Richardson super-diffusivity in turbulence based on a scale dependent \emph{ballistic} scenario rather than a scale dependent \emph{diffusive} scenario. It is shown that this phenomenology elucidates several aspects of turbulent dispersion: (i) it gives a simple physical explanation of the origin of the super diffusive $t^3$ Richardson regime as an iterative cascade of scale-dependent ballistic separations, (ii) it simply relates the Richardson constant to the Kolmogorov constant (and eventually to a ballistic persistence parameter), (iii) it gives a simple physical interpretation of the non-Fickian scale-dependent diffusivity coefficient as originally proposed by Richardson and (iv) a further extension of the phenomenology, taking into account higher order corrections to the local ballisitic motion, gives a robust interpretation of the assymetry between forward and backward dispersion, with an explicit connection to the energy flux accross scales.
|
12:15
15 mins
|
THE EFFECT OF TEMPERATURE FLUCTUATIONS ON THE SPREAD OF A BUOYANT PLUME
Andrea Bisignano, Benjamin Devenish
Abstract: Emissions from many natural and anthropogenic sources are hot compared with the surrounding ambient air. Such buoyancy
effects cause the emitted plume to rise, increasing the effective source height and significantly decreasing the maximum ground level
concentrations (in the vicinity of the source). A major aspect that distinguishes buoyant and passive dispersion is that buoyant fluid
particles create their own turbulence and hence exchange processes between the plume and its environment need to be accounted for.
The inclusion of plume rise in Lagrangian stochastic models (LSMs) of turbulent dispersion has been considered by many authors but
the interaction of the buoyant plume with the environment (by means of entrainment) is difficult to model in a Lagrangian framework.
Webster and Thomson [8] formulated a hybrid model in which the mean flow is calculated from a simple plume model and the
fluctuations of velocity are calculated using an LSM. They model the effect of turbulence generated by the plume by an additional
random increment to the position of a particle. Here, instead of including this extra term, we add a stochastic differential equation
(SDE) for the temperature fluctuations suitably coupled with the SDE for the velocity fluctuations. The interaction of temperature
and velocity fluctuations, directly related to the turbulence within the plume, determines the plume’s spread. The results of the model
are compared with large-eddy simulation (LES) of buoyant plumes in a uniform crosswind and also with the plume generated by the
explosion and fire at the Buncefield oil depot in 2005 using realistic profiles of the wind speed and direction and thermal stratification.
|
|