  We attempt to address the old problem of plane shear flows: the origin of
turbulence and hence transport of angular momentum in accretion flows as well
as laboratory flows, such as plane Couette flow. We undertake the problem by
introducing an extra force in Orr-Sommerfeld and Squire equations along with
the Coriolis force mimicking the local region of the accretion disk. For plane
Couette flow, the Coriolis term drops. Subsequently we solve the equations by
WKB approximation method. We investigate the dispersion relation for the
Keplerian flow and plane Couette flow for all possible combinations of wave
vectors. Due to the very presence of extra force, we show that both the flows
are unstable for a certain range of wave vectors. However, the nature of
instability between the flows is different. We also study the Argand diagrams
of the perturbation eigenmodes. It helps us to compare the different time
scales corresponding to the perturbations as well as accretion. We ultimately
conclude with this formalism that fluid gets enough time to be unstable and
hence plausibly turbulent particularly in the local regime of the Keplerian
accretion disks. Repetition of the analysis throughout the disk explains the
transport of angular momentum and matter along outward and inward direction
respectively.
