Turbulent flows are composed of a wide range of large- and small-scale coherent motions in both the momentum field and the particle field. These motions are responsible for practical engineering effects, like the generation of turbulent drag and particle mixing. Scale interactions between large- and small-scales in turbulent wall-bounded flows have historically been investigated by linear analysis which requires a filtering and envelope procedure. The phase delay between large and small scales was explained as a spatial delay in the flow. However, this linear analysis depends on the cutoff wavelength in the filtering process and the use of an artificial envelope of small scales that does not exist in real data. A nonlinear diagnostic, the bispectrum, is introduced and applied to a numerical turbulent channel flow. The bispectrum shows that large-scale structure that is most strongly coupled, in a triadic sense, to small scales, is the very-large-scale-motion (VLSM). The large scale structures can not only be detected in the momentum field, but also in the particle field. The wavelet transform was applied to extract the statistically significant, spatial particle clusters. Inertial particles were seeded in the high speed water tunnel facility, and two-color PIV/PTV measurement was performed to investigate the coherent motions in both the momentum and particle field.
