Enhanced, passive transport is studied numerically in an oscillating vortex chain with stress-free boundary conditions. The long-range transport is found to be diffusive in the long-time limit with an effective diffusion coefficient D * that peaks dramatically in the vicinity of a few, well-defined resonant frequencies. Superdiffusive transients are also observed for frequencies near these resonant frequencies, with the duration of the transients diverging at the resonant frequencies. Standard analytical techniques based on the Melnikov approximation and on lobe dynamics fail to explain the behavior in the vicinity of these resonant peaks. An alternate explanation is provided, based on flights that have power-law scaling up to a maximum length that also diverges at the resonant frequencies. The long flights for frequencies near the resonant peaks occur because tracers in a lobe return (after an integer number of oscillation periods) to almost precisely the same location in the lobe of another vortex. These periodic orbits correspond to the formation -only at the resonant frequencies -of "tangle islands" within the chaotic region.
Long-range transport is studied numerically in a time-independent, three-dimensional (3D) fluid flow composed of the superposition of two chains of alternating vortices, one horizontal and the other vertical. Tracers in this flow follow chaotic trajectories composed of correlated Lévy flights with varying velocities. Locations of the chaotic regimes in the flow are compared with recent theories of chaos in non-Beltrami 3D flows. Growth of the variance of a distribution of tracers is divided into transient and long-term regimes, each with different growth exponents.
We propose here an experimental investigation of a vortex submitted to a radial perturbation while being compressed. This experiment reproduces a model situation of the complex flows that take place in a real engine cylinder. An isolated tumbling flow is first submitted to an injection of fluid and then compressed and measurements are realised by Particle Image Velocimetry (PIV). The Proper Orthogonal Decomposition is known to be an unbiased method to identify the coherent structures of turbulent flows. It is possible to make this decomposition for a given phase or to create a set of basis functions with the whole set of compression stroke velocity fields. First, the experimental set-up will be presented, then the second part provides the POD principle. Results of phased POD decomposition of the compression of the unperturbated vortex will be presented. Finally results from the time invariant POD will be discussed to study the influence of the jet perturbation in the compression of a vortex.
Both time-dependent and time-invariant Proper Orthogonal Decompositions are performed on LES and PIV data as an initial step in a study of tumble breakdown in in-cylinder flows. Evidence of tumble instability during compression is found in the time-dependent POD of both data sets. Time-invariant POD modes are presented which will be used later in low-dimensional models of these systems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.