Identification of multi-scale coherent eddy structures in a homogeneous isotropic turbulence

Abstract

The coherent vortices of various scales are identified from a turbulent flow field obtained by a Direct Numerical Simulation (DNS). In this paper, the vorticity field of the database is filtered before the vortex-identifying method is applied, which enables the identification of large and middle scale coherent vortices from the turbulent flow field. The lattice Boltzmann method with a forcing scheme is used to obtain a statistically stationary three dimensional homogeneous and isotropic turbulence in a periodic box with a resolution of 128³. The vorticity field is filtered using sharp cut-off filters of different cut-off-wavenumbers to obtain different scales of coherent eddy structures. The Q criterion is applied to visualise the coherent vortices and the conditional sampling technique is used to investigate the energy spectrum of the coherent and incoherent parts, separately. By applying the filtering process prior to identification, flow fields of different scale structures are extracted and the energy spectrum of each is investigated.