  An epitaxial transition from a bicontinious double gyroid to a hexagonally
packed cylinder structure induced by an external flow is simulated using
real-space dynamical self-consistent field technique. In order to simulate the
structural change correctly, we introduce a system size optimization technique
by which emergence of artificial intermediate structures are suppressed. When a
shear flow in [111] direction of the gyroid unit cell is imposed, a nucleation
and growth of the cylinder domains is observed. We confirm that the generated
cylindrical domains grow epitaxially to the original gyroid domains as gyroid
$d_{\{220\}}$ $\to$ cylinder $d_{\{10\}}$. In a steady state under the shear
flow, the gyroid shows different reconnection processes depending on the
direction of the velocity gradient of the shear flow. A kinetic pathway
previously predicted using the self-consistent field theory where three fold
junctions transform into five fold junctions as an intermediate state is not
observed.
