Vortex Breakdown: Mechanism of Initiation and Change of Mode
Recent results of numerical and experimental investigations of the breakdown process of slender vortices are discussed. First the upstream motion of the flow, necessary for the destruction of the core is explained with a solution of a model equation. It is shown, how the axial pressure gradient at large radial distances influences the formation of stagnant flow on the axis. The conclusions drawn from these considerations are qualitatively confirmed by experimental flow visualization studies. Finally, a method is described through which the unknown side boundary conditions needed for computation of such flows can be determined. This procedure was successfully incorporated in a numerical solution of the Navier-Stokes equations for determining the time dependent, three-dimensional flow in slender vortices during breakdown and the changeover from bubble to spiral-type breakdown as it occurs on delta wings at high angles of attack.
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