  This paper develops a high-density air corridor traffic flow model for
Uncrewed Aircraft System (UAS) operation in urban low altitude airspace. To
maximize throughput with safe separation guarantees, we define an airspace
spatiotemporal planning problem. For the spatial planning, we propose a
multi-floor UAS coordination structure divided into a finite number of air
corridors safely wrapping buildings and obstacles. We use the USGS Lidar data
to map buildings and in turn generate air corridors by modeling UAS
coordination as ideal fluid flow with the streamlines obtained by solving the
Laplace partial differential equation (PDE). Proper boundary conditions for the
differential equations are imposed to direct air corridors along the floors
desired motion direction. For temporal planning, we use 4-dimensional
path-finding through the corridor network with A* search to maximize airspace
usability given each UAS initial and destination waypoint pair.
