  The study of nonmodal amplification of distributed body forces in channel
flows of viscoelastic fluids has provided useful insights into the mechanisms
that may govern the initial stages of transition to elastic turbulence.
However, distributed body forces are not easy to implement in experiments and
so there is a need to examine amplification of localized body forces. In this
work, we use the linearized governing equations to examine such amplification
in Poiseuille flow of FENE-CR fluids. We first identify the wall-normal
location at which the impulsive excitations experience the largest
amplification and then analyze the kinetic energy of the fluctuations and the
resulting flow structures. For both a Newtonian fluid at high Reynolds numbers
and a viscoelastic fluid at low Reynolds numbers, the largest amplification
occurs for disturbances that are located near the channel wall. Our analysis of
the energy evolution shows that a localized body force in the spanwise
direction has the largest impact and that the streamwise velocity component is
most affected. For viscoelastic fluids we observe the development of vortical
structures away from the source of impulsive excitation. This feature is less
prominent in Newtonian fluids and it may provide a mechanism for triggering the
initial stages of transition to elastic turbulence.

Fluid Dynamics