  Embedded vortices in turbulent wall-bounded flow over a flat plate, generated
by a passive rectangular vane-type vortex generator with variable angle $\beta$
to the incoming flow in a low-Reynolds number flow ($Re=2600$ based on the
inlet grid mesh size $L=0.039\;$m and free stream velocity $U_{\infty} =
1.0\;$m s$^{-1}$) have been studied with respect to helical symmetry. The
studies were carried out in a low-speed closed-circuit wind tunnel utilizing
Stereoscopic Particle Image Velocimetry (SPIV). The vortices have been shown to
possess helical symmetry, allowing the flow to be described in a simple
fashion. Iso-contour maps of axial vorticity revealed a dominant primary vortex
and a weaker secondary one for $20^{\circ} \leq \beta \leq 40^{\circ}$. For
angles outside of this range, the helical symmetry was impaired due to the
emergence of additional flow effects. A model describing the flow has been
utilized, showing strong concurrence with the measurements, even though the
model is decoupled from external flow processes that could perturb the helical
symmetry. The pitch, vortex core size, circulation and the advection velocity
of the vortex all vary linearly with the device angle $\beta$. This is
important for flow control, since one thereby can determine the axial velocity
induced by the helical vortex as well as the swirl redistributing the axial
velocity component for a given device angle $\beta$. This also simplifies
theoretical studies, \eg to understand and predict the stability of the vortex
and to model the flow numerically.
