  We study the influence of solid boundaries on dynamics and structure of
active fluids as the height of the container, $z$, changes. Along the varying
dimension, the geometry systematically increases, therefore, the confinement
($z$) transits from "strong confinement", to "intermediate confinement" and to
"weak confinement" (close to "unconfined"). In horizontal dimensions ($x,y$),
the system remains "unconfined". Through tracking the tracers dispersed in the
active fluids in three dimensions we observed that activity level,
characterized by velocity fluctuations of flow tracers, increases as system
size increases. Concomitantly, the velocity-velocity temporal correlation
changes from weak correlation to strong positive correlation, indicating
"memory" in active flows. We estimate the characteristic size of the flow
structure by integrating the velocity-velocity spatial correlation function.
The integral increases as confinement becomes weaker and saturates at
approximately 400 microns as the system becomes "unconfined". This saturation
indicates an intrinsic length scale which, along with the small-scale isotropy,
demonstrates the multi-scale nature of this kinesin-driven bundled microtubule
system.
