  In this paper a model for viscous boundary and shear layers in
three-dimensions is introduced and termed a vortex-entrainment sheet. The
vorticity in the layer is accounted for by a conventional vortex sheet. The
mass and momentum in the layer are represented by a two dimensional surface
having its own internal tangential flow. Namely, the sheet has a mass density
per-unit-area making it dynamically distinct from the surrounding outer fluid.
The mechanism of entrainment is represented by a discontinuity in the normal
component of velocity across the sheet. The sheet mass is able to support a
pressure jump, which in turn may cause additional entrainment. This feature was
confirmed when the model was used to represent the Falkner-Skan boundary
layers. The velocity field induced by the vortex-entrianment sheet is given by
a generalized Birkhoff-Rott equation with a complex sheet strength. The model
was also applied to the case of separation at a sharp edge. There is no
requirement for an explicit Kutta condition in the form of a singularity
removal as this condition is inherently satisfied through an appropriate
balance of normal momentum with the pressure jump across the sheet. A pressure
jump at the edge results in the generation of new vorticity. The shedding angle
is dictated by the normal impulse of the intrinsic flow inside the bound sheets
as they merge to form the free sheet. When there is zero entrainment everywhere
the model reduces to the conventional vortex sheet with no mass. Consequently,
the pressure jump must be zero and the shedding angle must be tangential so
that the sheet simply convects off the wedge face. Lastly, the
vortex-entrainment sheet model was demonstrated on two shedding example
problems.
