  We have studied the dependence of azimuthal anisotropy $v_2$ for inclusive
and identified charged hadrons in Au$+$Au and Cu$+$Cu collisions on collision
energy, species, and centrality. The values of $v_2$ as a function of
transverse momentum $p_T$ and centrality in Au$+$Au collisions at
$\sqrt{s_{_{NN}}}$=200 GeV and 62.4 GeV are the same within uncertainties.
However, in Cu$+$Cu collisions we observe a decrease in $v_2$ values as the
collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the
more peripheral collisions. By examining both Au$+$Au and Cu$+$Cu collisions we
find that $v_2$ depends both on eccentricity and the number of participants,
$N_{\rm part}$. We observe that $v_2$ divided by eccentricity ($\varepsilon$)
monotonically increases with $N_{\rm part}$ and scales as ${N_{\rm
part}^{1/3}}$. The Cu$+$Cu data at 62.4 GeV falls below the other scaled
$v_{2}$ data. For identified hadrons, $v_2$ divided by the number of
constituent quarks $n_q$ is independent of hadron species as a function of
transverse kinetic energy $KE_T=m_T-m$ between $0.1<KE_T/n_q<1$ GeV. Combining
all of the above scaling and normalizations, we observe a near-universal
scaling, with the exception of the Cu$+$Cu data at 62.4 GeV, of
$v_2/(n_q\cdot\varepsilon\cdot N^{1/3}_{\rm part})$ vs $KE_T/n_q$ for all
measured particles.
