On the separation of air flow during phonation

The separation of air flow inside the vocal tract has been noticed experimentally in many works. In this work, some sort of such separation can be computed numerically using a simplified linear form of Navier-Stokes equations inside the vocal tract. The simplified set of flow equations preserves both the linearity of simple models and viscous losses of the natural phonation process. These equations are solved within two contiguous domains based on the boundary-layer theorem for compressible, low density and low viscosity fluids. The solution within the bulk of the vocal tract is easily conducted using the reflection-type line analog model. The momentum equation is a second order linear partial differential equation since it contains the viscosity effect. This equation, is, then, solved within the boundary layer which is adjacent to the vocal tract walls with time varying boundary conditions. This solution gives the velocity distribution of fluid flow which exhibits a separation. This approach afford a simple and fast linear solution which elaborates such complex behavior of air flow inside the vocal tract.