508 research outputs found
Upstream open loop control of the recirculation area downstream of a backward-facing step
The flow downstream a backward-facing step is controlled using a pulsed jet
placed upstream of the step edge. Experimental velocity fields are computed and
used to the recirculation area quantify. The effects of jet amplitude,
frequency and duty cycle on this recirculation area are investigated for two
Reynolds numbers (Re=2070 and Re=2900). The results of this experimental study
demonstrate that upstream actuation can be as efficient as actuation at the
step edge when exciting the shear layer at its natural frequency. Moreover it
is shown that it is possible to minimize both jet amplitude and duty cycle and
still achieve optimal efficiency. With minimal amplitude and a duty-cycle as
low as 10\% the recirculation area is nearly canceled
A model for the symmetry breaking of the reverse Benard-von Karman vortex street produced by a flapping foil
The vortex streets produced by a flapping foil of span-to-chord aspect ratio
of 4:1 are studied in a hydrodynamic tunnel experiment. In particular, the
mechanisms giving rise to the symmetry breaking of the reverse B\'enard-von
K\'arm\'an vortex street that characterizes fish-like swimming and forward
flapping flight are examined. Two-dimensional particle image velocimetry
measurements in the mid-plane perpendicular to the span axis of the foil are
used to characterize the different flow regimes. The deflection angle of the
mean jet flow with respect to the horizontal observed in the average velocity
field is used as a measure of the asymmetry of the vortex street. Time series
of the vorticity field are used to calculate the advection velocity of the
vortices with respect to the free-stream, defined as the phase velocity
, as well as the circulation of each vortex and the spacing
between consecutive vortices in the near wake. The observation that the
symmetry breaking results from the formation of a dipolar structure from each
couple of counter-rotating vortices shed on each flapping period serves as
starting point to build a model for the symmetry breaking threshold. A symmetry
breaking criterion based on the relation between the phase velocity of the
vortex street and an idealized self-advection velocity of two consecutive
counter-rotating vortices in the near wake is established. The predicted
threshold for symmetry breaking accounts well for the deflected wake regimes
observed in the present experiments and may be useful to explain other
experimental and numerical observations of similar deflected propulsive vortex
streets reported in the literature.Comment: 10 page
Experimental scaling law for the sub-critical transition to turbulence in plane Poiseuille flow
We present an experimental study of transition to turbulence in a plane
Poiseuille flow. Using a well-controlled perturbation, we analyse the flow
using extensive Particule Image Velocimetry and flow visualisation (using Laser
Induced Fluorescence) measurements and use the deformation of the mean velocity
profile as a criterion to characterize the state of the flow. From a large
parametric study, four different states are defined depending on the values of
the Reynolds number and the amplitude of the perturbation. We discuss the role
of coherent structures, like hairpin vortices, in the transition. We find that
the minimal amplitude of the perturbation triggering transition scales like
Re^-1
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