  This paper discusses new perspectives and approaches to the problem of disk
dynamics where, in this study, we focus on the effects of viscous instabilities
influenced by boundary effects. The Boussinesq approximation of the viscous
large shearing box equations is analyzed in which the azimuthal length scale of
the disturbance is much larger than the radial and vertical scales. We examine
the stability of a non-axisymmetric potential vorticity mode, i.e. a
PV-anomaly. in a configuration in which buoyant convection and the
strato-rotational instability do not to operate. We consider a series of
boundary conditions which show the PV-anomaly to be unstable both on a finite
and semi-infinite radial domains. We find these conditions leading to an
instability which is the disk analog of Tollmien-Schlichting waves. When the
viscosity is weak, evidence of the instability is most pronounced by the
emergence of a vortex sheet at the critical layer located away from the
boundary where the instability is generated. For some boundary conditions a
necessary criterion for the onset of instability for vertical wavelengths that
are a sizable fraction of the layer's thickness and when the viscosity is small
is that the appropriate Froude number of the flow be greater than one. This
instability persists if more realistic boundary conditions are applied,
although the criterion on the Froude number is more complicated. The unstable
waves studied here share qualitative features to the instability seen in
rotating Blasius boundary layers. The implications of these results are
discussed. An overall new strategy for exploring and interpreting disk
instability mechanisms is also suggested.
