Søren Ott

Lagrangian multi-particle statistics

Combined measurements of the Lagrangian evolution of particle constellations and the coarse-grained velocity derivative tensor ∂ũ i /∂ x j are presented. The data are obtained from three-dimensional particle tracking measurements in a quasi isotropic turbulent flow at an intermediate Reynolds number. Particle constellations are followed for as long as one integral time and for several Batchelor times. We suggest a method to obtain ∂ũ i /∂ x j from velocity measurements at discrete points. Based on an analytical result and on a sensitivity analysis, both presented here, we estimate the accuracy for filtered strain, ᵴ 2, and enstrophy, 2, at around 30%. The accuracy improves with higher tracer seeding density and with smaller filter scale Δ. We obtain good scaling with t* = √2r 2/15S 2(r) for filtered strain and vorticity and present filtered R–Q invariant maps with the typical ‘tear drop’ shape that is known from velocity gradients at viscous scales. Lagrangian results are given for the growth of particle ...

Field experiments with dispersion of pressure liquefied ammonia

Abstract This paper presents dispersion experiments with continuous releases of liquefied ammonia carried out during the CEC ENVIRONMENT project “Fladis Field Experiments”. The source was a horizontal flash boiling jet with release rates of 0.25–0.5 kg s−1. The objective was to study the dispersion in all its stages at the source, in the heavy jet with aerosols, the slightly stabilized plume, and further downstream into the regime of passive dispersion. The concentration field is analyzed in a fixed frame of reference as well as a frame of reference moving with the instantaneous plume centre-line. Empirical probability functions and a spatial correlation of the concentration fluctuations are found, and the enthalpy balance of the cold heavy jet and the aerosol composition are evaluated.

Experimental studies of occupation and transit times in turbulent flows

The motion of passively advected particles is studied experimentally in approximately homogeneous and isotropic turbulent flows. The turbulence is generated in water by two moving grids. The simultaneous trajectories of many small passively advected, neutrally buoyant polystyrene particles are followed in time by a particle tracking technique. We estimate the probability distribution of the transit times of such particles in spherical volumes with given radius. A particle which is passively advected by the flow is selected to define the center of a reference sphere, with the transit time being defined as the difference between entrance and exit times of surrounding particles advected through this sphere by the turbulent motions. Simple scaling laws are obtained for the probability density of the transit times in terms of the basic properties of the turbulent flow and the geometry. Also other formulations of the problem have been considered, by assuming, for instance, that particle positions are uniformly ...

Experimental investigation of Lagrangian structure functions in turbulence.

Lagrangian properties obtained from a particle tracking velocimetry experiment in a turbulent flow at intermediate Reynolds number are presented. Accurate sampling of particle trajectories is essential in order to obtain the Lagrangian structure functions and to measure intermittency at small temporal scales. The finiteness of the measurement volume can bias the results significantly. We present a robust way to overcome this obstacle. Despite no fully developed inertial range, we observe strong intermittency at the scale of dissipation. The multifractal model is only partially able to reproduce the results.

The Correlation Between Velocity and Acceleration in Turbulence

This relation seems also fulfilled in real turbulent flows [3, 2, 4]. The Eulerian velocity–acceleration structure function 〈δv · δa〉, where δv = v(x+r)−v(x) and likewise for a, can be shown theoretically to be −2ε independent of r = |r| in the inertial subrange. This is true for homogeneous, stationary turbulence, where the body forces act on larger scales, as well as for locally homogeneous flows. For real flows and for Direct Numerical Simulation (DNS) the relation

Turbulent Pair Dispersion: A PTV Experiment

Particle Tracking Velocimetry (PTV) is an experimental technique used to obtain Lagrangian statistics in a turbulent flow[1, 2, 3]. Compared to the more widely used Particle Image Velocimetry (PIV) we track fluid elements in time and space and hence dispersion between more than one fluid element (particle hereinafter) can be observed. Dispersion of such particles is related to many applications and is governing the spreading of pollutants, combustion and prey-predator encounters in a turbulent sea, etc. In this paper we will focus on dispersion of two particles in a homogeneous and isotropic turbulent flow and present some preliminary results. Dispersion of two particles in the inertial subrange where viscous effects can be neglected is described by the Richardson-Obukhov law [4],