Size Of Turbulent Structures Research Articles

Rotational vector-based analysis of turbulent structures in channel flow using large eddy simulation simulation

Even with modern measurement techniques and data from direct numerical simulation (DNS), it is very difficult to identify the individual attached eddies and understand their dynamical behavior due to the multi-scale nature of the eddies in wall-bounded flows, which puts these issues at the center of the current debate. However, Liutex vector ( L for short), a rotational vector field with information on both the rotation axis and swirling strength, has recently been developed by Prof. Liu’s group as a more accurate and clear definition of a vortex. Combining conventional and L methods may provide more detailed information about the complex flow structures as well as insights into the flow’s mixing and transport features. We simulated the channel flow in large eddy simulation by implementing an inflow condition based on the box turbulence. After validating the results with DNS data, we used L isosurfaces and their vector profiles to track ordered flow structures in wall-bounded turbulence. Based on the data, we observe numerous turbulent phenomena that have been described in other works with different visualization techniques. Moreover, the shear contamination on the wall is the most severe while all the root-mean-square L component variations are negligible. Due to the presence of background shear, the peak location of vorticity fluctuation is closer to the wall than the corresponding L fluctuation, and the displacement of peak location brought on by shear contamination is greatest for the spanwise component (z-component) of the vorticity fluctuation. According to the two-point correlation of L components, the streamwise size of turbulent structures does not vary considerably with , however, the spanwise size of turbulent structures increases gradually as increases.

Analysis of locally-aligned and non-aligned discretisation schemes for reactor-scale tokamak edge turbulence simulations

Edge turbulence codes will play a key role in the interpretation of ITER data and for reliable predictions of EU-DEMO. At present, such codes are not yet capable of routine simulations at reactor scale, and instead focus on smaller experiments like TCV or ASDEX Upgrade. Numerical methods have to be identified that scale to reactor size, and are able to cope with the highly anisotropic turbulent structures and simultaneously with the complex magnetic geometry in the edge with X-point(s), the vessel wall and divertor targets present. Particularly for such conditions two main approaches, non-aligned discretisation schemes and a locally-aligned discretisation scheme (commonly referred to as Flux-Coordinate Independent approach (FCI)), have emerged. We analyse both schemes concerning their applicability and scalability to next generation fusion reactors. We find that the ratio of the computational cost of a non-aligned scheme compared to aligned scheme scales as ∝(R0qρs)2 for shear Alfvén dynamics, and as (R0qρs)3 for electron heat conduction, where R0 is the major radius of a tokamak, q an estimate for the safety factor at the edge and ρs is the drift scale representing the typical size of turbulent structures to be resolved. Locally-aligned schemes can therefore be considered strongly favourable for reactor scale tokamaks concerning computational performance. On the other hand, locally-aligned schemes suffer from a more complex treatment of boundary conditions, for which an immersed boundary approach (IBA) was recently proposed. We demonstrate numerically the validity of this method in combination with the FCI approach. Finally, we present a first attempt of an ITER edge turbulence simulation with the FCI code GRILLIX at realistic parameters and in realistic geometry.

Numerical investigation of the possibility of macroscopic turbulence in porous media: a direct numerical simulation study

AbstractWhen a turbulent flow in a porous medium is determined numerically, the crucial question is whether turbulence models should account only for turbulent structures restricted in size to the pore scale or whether the size of turbulent structures could exceed the pore scale. The latter would mean the existence of macroscopic turbulence in porous media, when turbulent eddies exceed the pore size. In order to determine the real size of turbulent structures in a porous medium, we simulated the turbulent flow by direct numerical simulation (DNS) calculations, thus avoiding turbulence modelling of any kind. With this study, which for the first time uses DNS calculations, we provide benchmark data for turbulent flow in porous media. Since perfect DNS calculations require the resolution of scales down to the Kolmogorov scale, often only approximate DNS solutions can be obtained, especially for high Reynolds numbers. This is accounted for by using and comparing two different DNS approaches, a finite volume method (FVM) with grid refinement towards the wall and a lattice Boltzmann method (LBM) with equal grid distribution. The solid matrix was simulated by a large number of rectangular bars arranged periodically. The number of bars in the solution domain with periodic boundary conditions was reduced systematically until a minimum size was found that does not suppress any large-scale turbulent structures. Two-point correlations, integral length scales and energy spectra were determined in order to answer the question of whether or not macroscopic turbulence can be found in porous media.

Transport of neutrals in turbulent Scrape-off-Layer plasmas

The effect of turbulence on the transport of neutral species (atom, molecules) in plasmas is investigated. A stochastic model relying on a multivariate gamma distribution is used to describe turbulent fluctuations. This model incorporates several important known features of Scrape-off-Layer turbulence, and has been implemented in the EIRENE Monte Carlo code. The effect of fluctuations on the neutral density, ionisation source and Balmer α radial profiles are investigated. Two important control parameters are identified, namely the ratio of the neutral mean free path to the size of turbulent structures, and the ratio of recycling time scales to the turbulent times.

Cleanability study of complex geometries: Interaction between B. cereus spores and the different flow eddies scales

Wall shear stress measurements and the distribution size of turbulent structures at the confined zone near the wall were used in order to investigate the cleanability of a part of a dairy processing line consisting in a manometer and a two-way valve. Geometry of equipment was discerned as an important factor governing the flow behavior and thereafter the initial contamination and the cleanability of the installation. Interactions between suspended spores and coherent turbulent structures of different sizes were used to explain the eventual re-adhesion of spores in confined zones of the loop. These interactions also helped to explain the role of the bursting phenomenon in the increase of the wall shear stress which induces spores detachment.

Measurement of Turbulence Decorrelation during Transport Barrier Evolution in a High-Temperature Fusion Plasma

A low power polychromatic beam of microwaves is used to diagnose the behavior of turbulent fluctuations in the core of the JT-60U tokamak during the evolution of the internal transport barrier. A continuous reduction in the size of turbulent structures is observed concomitant with the reduction of the density scale length during the evolution of the internal transport barrier. The density correlation length decreases to the order of the ion gyroradius, in contrast with the much longer scale lengths observed earlier in the discharge, while the density fluctuation level remain similar to the level before transport barrier formation.