On Core and Curvature Corrections used in Straight-Line Vortex Filament Methods

Abstract

The convergence characteristics of two viscous core corrections as used in straight-line segmentation methods are rigorously analysed. These are \emph{curvature corrections} that account for the induced velocity contribution at a point on a vortex filament due to local curvature and \emph{core corrections} that remove unrealistically high induced velocities near vortex segments. Two alternative versions of the latter correction are studied: the original as introduced by Scully and a recently introduced improved correction by the authors. The problem is analysed using a viscous vortex ring. A high-order numerical filament method is presented that uses rational B-spline curves to model the vortex ring geometry exactly. First, the separate influence of the two corrections on induced velocity values are studied using the numerical filament method. Afterwards, the corrections are combined in four separate cases that are studied using the segmentation method. It is found that in order to get accurate results at coarse discretisations, curvature corrections cannot be neglected. When used together with the original core correction model, results start to diverge from the reference values for discretisations smaller than approximately ten degrees. Combining the curvature correction with the improved core correction extends the discretisation region for which accurate results are found down by approximately one order. Finally, it is shown that segmentation methods that use the original core correction model without curvature correction completely fail to converge to the reference values.