Experimental assessment of fractal scale similarity in turbulent flows. Part 4. Effects of Reynolds and Schmidt numbers

摘要

Experimental results are presented for the influence of Reynolds number on multifractal scale similarity in turbulent flows. These are obtained from single-point measurements of a dynamically passive Sc approximate to 1 conserved scalar quantity zeta(t) in a turbulent shear flow at outer-scale Reynolds numbers of 14000 less than or equal to Re-delta less than or equal to 110000. Statistical criteria based on the maximum allowable scale-to-scale variation L-1(6) in multiplier distributions P(M-epsilon) from multifractal gauge sets allow accurate discrimination between multifractal and non-multifractal scaling. Results show that the surrogate scalar energy dissipation rate chi(s)(t) = (d zeta/dt)(2) is found to display a scale-invariant similarity consistent with a random multiplicative cascade characterized by a bilinear multiplier distribution P(M-6) over a range of scales extending downward from the outer scale T-delta. For a range of scales extending upward from the inner (diffusive) scale T-D,T- the dissipation rate displays a different scale-invariant similarity characterized by a uniform multiplier distribution. The former scale-invariance becomes evident in the present Sc approximate to 1 data only when Re-delta is sufficiently large. Comparisons with results from Sc much greater than 1 data indicate that this scale-invariant similarity applies when the outer-to-inner scale ratio T-delta/T-D approximate to 0.09 (RedeltaSc1/2)-Sc-3/4 is greater than about 400. In contrast to the scalar dissipation rate field, the scalar field is found to lack any multifractal scale similarity. [References: 26]