  Local phonon motion underneath a surface interacting with a flow may cause
the flow to passively stabilize, or destabilize, as desired within the region
adjacent to the subsurface motion. This mechanism has been extensively analyzed
over only a spatial region on the order of the instability wavelength along the
fluid-structure interface. Here we uncover fundamental relations between the
behavior of flow instabilities and the frequency response characteristics of
the phononic subsurface structure admitting the elastic motion. These relations
are then utilized to demonstrate the possibility of extensive spatial expansion
of the control regime along the downstream direction with minimal loss of
performance--potentially covering the entire surface exposed to the flow.
