  Flow around two rotating side-by-side circular cylinders of equal diameter D
is numerically studied at the Reynolds number Re=40-200 and various rotation
rate theta_i. The incoming flow is assumed to be two-dimensional laminar flow.
The governing equations are the incompressible Navier-Stokes equations and
solved by the finite volume method (FVM). The ratio of the center-to-center
spacing to the cylinder diameter is T/D=2. The objective of the present work is
to investigate the effect of rotational speed and Reynolds number on the
stability of the flow. The simulation results are compared with the
experimental data and a good agreement is achieved. The stability of the flow
is analyzed by using the energy gradient theory, which produces the energy
gradient function K to identify the region where the flow is the most prone to
be destabilized and the degree of the destabilization. Numerical results reveal
that K is the most significant at the separated shear layers of the cylinder
pair. With Re increases, the length of the wake is shorter and the vortex
shedding generally exhibits a symmetrical distribution for theta_i <theta_crit.
It is also shown that the unsteady vortex shedding can be suppressed by
rotating the cylinders in the counter-rotating mode.
