  Pore-scale observation of vortical flow structures in porous media is a
significant challenge in many natural and industrial systems. Vortical
structure dynamics is believed to be the driving mechanism in the transition
regime in porous media based on the pore Reynolds number, $Re_p$. To examine
this assertion, a refractive-index matched randomly packed porous medium is
designed to measure the scale of vortical flow structures in transition from
unsteady laminar to turbulent using two-dimensional time-resolved Particle
Image Velocimetry (PIV). Planar PIV data for $Re_p$ from 100 to 948 is used to
quantify the scale in terms of the size, strength, and number density using two
different scalings (i) $Re_p$ macroscopic (global), and (ii) $Re_{\langle
p\rangle}$ microscopic (local). Direct measurement of vortex scale is
quantified by employing swirl strength, vortex core, and enhanced swirl
strength vortex identification methods. These scales are compared with
turbulent integral scales. Shear-dominant vortical structures in moderate
unsteady laminar Reynolds numbers ($Re_p <300$) were observed, while the
swirl-dominant flow structures appeared in weak turbulent Reynolds numbers
($Re_p>500$). From the macroscopic point of view, increasing Rep resulted in,
(i) reduction in the sizes of vortical structures to asymptotically to reach
20$\%$ of the global hydraulic diameter, (ii) growth in strength of vortices,
and (iii) increase in the average number densities of vortices. From the
pore-scale (local) point of view, increasing $Re_{\langle p\rangle}$ led to
decrease in the size of vortical structures monotonically, (ii) rise of the
strength of vortices, and (iii) invariance in the number density of vortical
structures. These findings suggest pore versus macro-scale coupling exists for
the scale of vortical flow structures in the transition regime.
