  Interscale energy transfer in wall turbulence has been intensively studied in
recent years, and both the forward (i.e. from larger to smaller scales) and
reversed transfers of turbulent energy have been found whereas their
corresponding physical phenomena have not been revealed. In the present study,
we perform DNS of turbulent plane Couette flow with reduced-size computational
domains, where either the streamwise or spanwise domain size is reduced to
their minimal lengths, aiming at elucidating the role of scale interactions in
each direction. Our computational results with the streamwise-minimal domain
suggest that the interplays between streamwise-elongated streaks and vortices
smaller than the streamwise minimal length are the essential scale interactions
for both the inner and outer structures. We further show that these
streamwise-independent and smaller-scale structures exchange energy through
forward and reversed interscale energy transfers, and the reversed energy
transfer maintains the energy production at larger scales. Based on the
resemblance of the observed Reynolds-stress transport and the scenario of the
self-sustaining cycle, we conjecture that the forward and reversed energy
transfers mainly represent the streak instabilities and regeneration of
streamwise vortices, respectively. Furthermore, the computation with the
spanwise-minimal domain indicates that the interscale energy transfers observed
by one-dimensional spanwise spectral analysis are likely related to individual
dynamics of each inner and outer structure, rather than represent their
interactions.
