A new experimental setup for studies on
wake flow instability and its control that successfully has been designed and
manufactured, is introduced and described. The main body is a dual-sided flat
plate with an elliptic leading edge and a blunt trailing edge. Permeable
surfaces enable boundary layer suction and/or blowing that introduce the unique
feature of adjusting the inlet condition of the wake created behind the plate.
This, in combination with a trailing edge that is easily modified, makes it an
ideal experiment for studies of different control methods for the wake flow
instability as well as extensive parameter studies. Experimental validation of
the setup has been performed by means of measurements of the wake symmetry and
boundary layer velocity profiles at the trailing edge. Some preliminary results
on the Strouhal number versus different inlet
conditions are reported.
Additionally, an in-house vortex detection
(VD) program has been developed in order to detect, analyse and compare
small-scale vortical structures in instantaneous
velocity fields from flow measurements. This will be a powerful tool for
comparison of wake characteristics for varying inlet conditions and control
methods in the new experimental setup. Measurements from three completely
separate experimental setups with different geometries and flow cases, have
been analysed by the VD-program.
i. In order to obtain improved
ventilation we have studied the effect of pulsating inflow into a closed volume
compared to having the inflow at a constant flow rate. We show that the number
of small-scale eddies is significantly increased and that the stagnation zones
are reduced in size, which enhances the mixing.
ii. Instantaneous velocity fields in
the wake behind a porous cylinder subjected to suction or blowing through the
entire cylinder surface have also been analysed using the VD-program. The
results show that the major change for different levels of blowing or suction
is the location of vortices while the most common vortex size and strength are
essentially unchanged.
iii. Another study on how the
geometry of a V-shaped mixer in a pipe flow affects the mixing have also been
examined, where no general differences were found between different
thicknesses, why a thickness that is favourable from an acoustic point of view
can be chosen.
We also
propose a new method, using global mode analysis on experimental data, showing
that randomly ordered snapshots of the velocity field behind the porous
cylinder can be re-ordered and phase-averaged.