17 research outputs found
Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow
Embedded vortices in turbulent wall-bounded flow over a flat plate, generated
by a passive rectangular vane-type vortex generator with variable angle
to the incoming flow in a low-Reynolds number flow ( based on the
inlet grid mesh size m and free stream velocity m s) 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 . 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 . 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 . This also simplifies
theoretical studies, \eg to understand and predict the stability of the vortex
and to model the flow numerically
Instabilities in the wake of an inclined prolate spheroid
We investigate the instabilities, bifurcations and transition in the wake
behind a 45-degree inclined 6:1 prolate spheroid, through a series of direct
numerical simulations (DNS) over a wide range of Reynolds numbers (Re) from 10
to 3000. We provide a detailed picture of how the originally symmetric and
steady laminar wake at low Re gradually looses its symmetry and turns unsteady
as Re is gradually increased. Several fascinating flow features have first been
revealed and subsequently analysed, e.g. an asymmetric time-averaged flow
field, a surprisingly strong side force etc. As the wake partially becomes
turbulent, we investigate a dominating coherent wake structure, namely a
helical vortex tube, inside of which a helical symmetry alteration scenario was
recovered in the intermediate wake, together with self-similarity in the far
wake.Comment: Book chapter in "Computational Modeling of Bifurcations and
Instabilities in Fluid Dynamics (A. Gelfgat ed.)", Springe