ABSTRAKT

A square-fractal-element grid and two regular grids, all with the same blockage, are used to investigate how the produced turbulence differs. The square-fractal-element grid is made-up of a 12×8 array of small square fractals mounted to a background mesh. For a constant inlet Reynolds number, Re_M, it is found that the turbulent kinetic energy powerlaw decay exponent is comparable for all three grids in the far-field, although the fractal-based grid produces an extended non-equilibrium region relative to the other two grids. The normalized dissipation, C_ε, is found to be approximately described by Re_M/Re_L in the non-equilibrium region, becoming approximately constant in the far-field. A correlation is also found between C_ε and normalized Reynolds stress, 〈uv〉 / u'v', across both the non-equilibrium region and the far-field. Finally, non-equilibrium and far-field spectra are compared at fixed Re_λ across all three grids. It is found that the far-field spectra are reasonably collapsed for all three grids for a given Re_λ. However, if a non-equilibrium spectrum at a location where 〈uv〉 ≠ 0 is compared at a given Re_λ to a far-field spectrum where 〈uv〉 ~ 0, then the non-equilibrium spectrum is nearer to k^−5/3. This result appears to be closely related to the existence of 〈uv〉 that is able to penetrate to scales associated with the scaling range.