Abstract
Direct numerical simulation (DNS) of decaying isotropic turbulence with a passive scalar, say temperature, has been done on 1283-5123 grids of the Numerical Wind Tunnel at National Aerospace Laboratory. With the use of the alias-free Fourier spectral method, the solenoidal field decomposition of velocity was made to exactly satisfy the imcompressibility condition. The simulation covers the range of Taylor-microscale Reynolds number from about 90 to 160 for the fixed Prandtl number equal to 1. The universal spectral forms for velocity (noticed by She et al.) and temperature are verified, irrespective of Reynolds number. The fields of vorticity and temperature gradient are visualized to clarify their local structures such as “worms” and sheets, respectively. The statistics of velocity, its longitudinal and lateral gradients, temperature and its gradient are clarified at the fully-developed state. Particularly, temperature distributes in a non-Gaussian way, resembling “hard turbulence” in the Reyleigh-Benard flow. Finally, the multifractal nature of energy-dissipation in our DNS is discussed in comparison with existing theories.
