  We investigate how momentum and kinetic energy is transferred between Fourier
components (the so-called triad interactions) in measured turbulent flow fields
with finite spatial and temporal resolution. We consider the effect of finite,
digitally sampled velocity records on the triad interactions and find that
Fourier components may interact within a broadened frequency window (finite
overlap widths), as compared to the usual integrals over infinite ranges where
Fourier components interact by overlapping Dirac delta functions. We also see
how the finite domain (temporal and/or spatial) of velocity records has a
significant effect on the efficiency of the triad interactions and thereby on
the shape and development of measured velocity power spectra. These finite
spatial/temporal domains thus influence the measured spatial and temporal
development of turbulence, the possibility for non-local interactions and hence
also non-equilibrium turbulence, e.g. fractal grid generated turbulence.
Finally, we quote results from other companion papers that report results from
laboratory and numerical experiments of the time development of velocity power
spectra in a turbulent jet flow into which a single or a couple of Fourier
modes are injected.
