Analysis of Intermittency in Turbulent Flows by Way of Higher-order Spectral Moments

Book excerpt: "The analysis of turbulence by way of higher-order spectral moments is uncommon, despite the relatively frequent use of such statistical analyses in other fields of physics and engineering. In this work, higher-order spectral moments are used to investigate the internal intermittency of the turbulent velocity and passive scalar (temperature) fields. This research first introduces the theory behind higher-order spectral moments as they pertain to the field of turbulence. Then, a short-time-Fourier-transform-based method is developed to estimate the higher-order spectral moments and provide a relative, scale-by-scale measure of intermittency. Experimental data are subsequently analysed and consist of measurements of homogeneous, isotropic, high-Reynolds-number, passive and active grid turbulence and wall-bounded turbulence (fully developed turbulent channel flow) over Taylor microscale Reynolds numbers between 35 and 731. Emphasis is placed on third- and fourth-order spectral moments using the definitions formalised by Antoni (2006), as such statistics are sensitive to transients and provide insight into deviations from Gaussian behaviour in grid turbulence. The higher-order spectral moments are also used to investigate the Reynolds and Péclet number dependence of the internal intermittency of velocity and passive scalar fields, respectively. The results demonstrate that the evolution of higher-order spectral moments with Reynolds number is strongly dependent on wavenumber. Additionally, the relative levels of internal intermittency of velocity and passive scalar fields are compared and a higher level of internal intermittency in the inertial subrange of the scalar field is consistently observed whereas a similar level of internal intermittency is observed for the velocity and passive scalar fields for the high-Reynolds-numbers-cases as the Kolmogorov length scale is approached. Finally, higher-order spectral moments are shown to display increased levels in the near-wall region of a wall-bounded (channel) flow. The increased intermittent activity is believed to be caused by the presence of coherent structures in wall-bounded flows"--