The circulation of an airfoil starting vortex obtained from instantaneous vorticity measurements over an area
Journal
1st National Fluid Dynamics Conference, 1988
Date Issued
1988-01-01
Author(s)
Abstract
By dissolving a photochromic chemical in an organic liquid and exciting it with ultraviolet light, a colored line can be created anywhere in the liquid. A device has been constructed which divides a laser beam into ‘n’ lines which are made to intersect to form a grid. Introducing two sets of these lines into the flowing fluid produces a grid of 100/µm colored fluid lines with mesh spacing of approximately 1mm. The color change persists long enough so that the grid lines are distorted by the flow before disappearing. The process is completely non-intrusive. The lines are analyzed by an image processing algorithm to allow the grid intersection points to be located quickly and with high precision. The movement of the intersection points in a unit of time results in temporal gradients of position, which in conjunction with estimates of the circulation around each grid box can be used to obtain the vorticity. Additionally, differencing again across a grid box results in spatial gradients of velocity. Experiments in a Stokes’ flow indicate that the vorticity can be measured to ± 1/sec. Application to the measurement of the formation of lift on an impulsively started airfoil has been made and compared with known results. It appears that the separating streamline is fixed to the trailing edge significantly before the Kutta condition is established, resulting in a starting vortex with less circulation than expected.
Type
conference paper