Incompressible Navier-Stokes equation is numerically simulated for investigating the character-. istics of the energy-dissipation s field and the vorticity-square a/ field of homogeneous isotropic turbulence. The concentration of ai into a volume is higher than that of s. The sizes of the concentrating regions (clusters) of a/ have almost the same power-law distribution function as that of s. The vorticity stretching in regions of large a/ or s has two directions of expansion and one direction

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... f contraction. On the other hand, in the regions of small ai, two directions show expansion and contraction respectively, but the remaining third direction is inactive, where a twodimensional stretching appears to occur.