Velocity-derivative skewness in small Reynolds number, nearly isotropic turbulence

Previous measurements in the moderate to small Reynolds number range of isotropic turbulence have all shown the skewness factor $S\equiv -{\overline{(\partial u/\partial x)^3}}/[\overline{(\partial u/\partial x)^2}]^{\frac{3}{2}}$ of the streamwise velocity derivative to increase with decreasing Reynolds number. This ‘paradoxical’ trend was found for 150 ≥ R_λ ≥ 4. New data covering the range 4 ≥ R_λ ≥ 1 show a maximum S for R_λ between 4 and 3 and a rapid decrease for R_λ < 2.