The effect of vortex generators on the flow around a circular cylinder

Kurzfassung

The effect of circular low-profile vortex generators for rotorcraft application has been investigated on a circular cylinder at a high Reynolds number. These so called Leading Edge Vortex Generators (LEVoGs) have shown significant reductions on the impact of dynamic stall on a pitching helicopter airfoil. However, the principle of operation of such devices remained unclear. In order to understand the aerodynamic phenomena, a wide range of experimental techniques such as oil flow visualization, infrared thermography, surface pressure measurements and stereo particle image velocimetry were employed. In addition, several post-processing approaches such as the proper orthogonal decomposition method were selected. The passive devices were found to significantly alter the flow around the cylinder. Depending on the azimuth angle of the vortex generators, the flow conditions could be classified into three categories, each with characteristic features. Similar to previous experiments on a helicopter rotor blade section, bundling of vortex generator wakes could be found under certain conditions. The vortex generators reduced the periodic Reynolds stress components v'v'/U∞2 and u'v'/U∞2 for all cases. For the cases where bundling was present, the size and peak values of the periodic u'u'/U∞2 component was increased significantly compared to the clean case. This means that the vortex generators almost only influence the periodic motion of the flow under these conditions. Even though the principle of operation couldn't be entirely cleared, the present study sheds some light onto the various mechanisms in the flow around such devices.