  We investigate the fluctuations of pion elliptic flow, triangular flow, and
Hanbury-Brown-Twiss (HBT) correlation functions for the hydrodynamic sources
with fluctuating initial conditions in the heavy ion collisions of the Au-Au at
$\sqrt{s_{NN}}=200$ GeV and the Pb-Pb at $\sqrt{s_{NN}} =2.76$ TeV. A method
based on event-subcollection analysis is used to detect these fluctuations in
ultrarelativistic heavy ion collisions. We introduce a granularity length to
describe the granular inhomogeneity of the initial sources, and investigate its
relationships with the fluctuations of the flow harmonics and HBT correlation
functions. Our investigations indicate that the fluctuations of the triangular
flow of event subcollections are sensitive to the granularity length of the
initial source. This dependence may provide a way to study the granular
inhomogeneity of the initial source through the analyses of the fluctuations of
triangular flow in ultrarelativistic heavy ion collisions.
