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Article

Streamwise evolution of longitudinal vortices in a turbulent boundary layer

Authors: Lögdberg, O., Fransson, J. H. M., Alfredsson, P.H.
Document Type: Article
Pubstate: Published
Journal: J. Fluid Mech.
Volume: 623   27-58
Year: 2009

Abstract

In this experimental study both smoke visualisation and three component hot-wire measurements have been performed in order to characterize the streamwise evolution of longitudinal counter-rotating vortices in a turbulent boundary layer. The vortices were generated by means of vortex generators in different configurations. Both single pairs and arrays in a natural setting as well as in yaw have been considered. Moreover three different vortex blade heights $h$, with the spacing $d$ and the distance to the neighbouring vortex pair $D$ for the array configuration, were studied keeping the same $d/h$ and $D/h$ ratios. It is shown that the vortex core paths scale with $h$ in the streamwise direction and with $D$ and $h$ in the spanwise and wall-normal directions, respectively. A new peculiar {\emph{hook-like}} vortex core motion, seen in the cross flow plane, has been identified in the far region starting around $200h$ and $50h$ for the pair and the array configuration, respectively. This behaviour is explained in the paper. Furthermore the experimental data indicate that the vortex paths asymptote to a prescribed location in the cross flow plane, which first was stated as a hypothesis and later verified. This observation goes against previously reported numerical results based on inviscid theory. An account for the important viscous effects is taken in a pseudo-viscous vortex model which is able to capture the streamwise core evolution throughout the measurement region down to $450h$. Finally, the effect of yawing is reported and it is shown that spanwise averaged quantities such as the shape factor and the circulation is hardly perceptible. However, the evolution of the vortex cores are different both between the pair and the array configuration as well as in the natural setting versus the case with yaw. From a general point of view the present paper report on fundamental results concerning the vortex evolution in a fully developed turbulent boundary layer.