On the fine structure of turbulent flows

The turbulence produced by a multiplicity of small air jets has been investigated and comparisons are made with other turbulent flows. The eddy Reynolds number is large ( R λ = 250). The energy spectrum was measured, as well as the skewness and kurtosis of ∂ u /∂ x . The effect of the finite length of the hot wire is considered and the corrected results indicate that the spectrum follows the law of Kolmogoroff in an intermediate range and appers to fall off with the (−6)-power of the frequency in the viscous range. This range is limited at the upper end of the frequency range by electrical noise. Special precautions reduced this noise to the level of thermal agitation in the hot wire. The ultimate limit to spectral analysis is imposed by the molecular agitation of the gas. This limit is evaluated and compared with the spectrum of turbulence. It appears that the spectrum of ∂ 3 u /∂ x 3 merges with the spectrum of molecular agitation without a distinctive separation.