抄録
High-Reynolds-number turbulence consists of a hierarchy of multi-scale vortex structures. In order to investigate generation mechanism of the hierarchy relevant to the physical-space energy cascade, we have conducted large-eddy simulations (LES) of incompressible hyperbolic stagnation-point flow driven by steady external body force in the form of quadruple vortices. Under this steady external force, four large-scale vortex tubes are sustained in a periodic domain. The energy cascade process that smaller-scale vortex tubes are created by being stretched in straining regions around larger-scale vortex tubes (Goto 2008, 2012) is identified in the flow. In our LES, such a generation process of smaller vortex tubes by larger-scale strain is observed in a wider inertial range than in direct numerical simulations with the comparable number of grid points. One of the most interesting observations in this flow is that this generation process of smaller vortices takes place in a quasi-periodic manner. For example, global flow quantities such as mean energy and its transfer rate to subgrid scale vary periodically in time with the period of O(10T ), where T is the eddy turnover time, even in the high-Reynolds-number case. In this cycle, the activity of the hierarchy of coherent vortices changes cyclically in time as well. Therefore, this cycle might be regarded as an elementary process of the energy cascade in isotropic turbulence.
