Abstract: Recent studies by the authors have shown that the
use of a localised asymmetrically located synthetic jet can improve the
aerodynamic performance of bluff bodies such as the sphere by altering the
three-dimensional aerodynamics of the fluid flow over the body. Moreover these
studies have alluded to the use of these localized synthetic jets in improving
the performance of aerodynamically shaped bodies such as aircraft wings with
further exciting prospects of improving problematic flow areas such as at the
junction between the wing and the fuselage of aircraft. Thus, the present study
has focused on presenting the results for this continued experimental work
performed on the wake region of a bluff body, a sphere of 80mm diameter designed
with an asymmetrically localized synthetic jet placed in a crossflow. The
airflow was generated in an open
circuit wind tunnel that was operated at a Reynolds Number of corresponding
to an airspeed of 10m/s. Wake surveys were conducted using five-hole pressure
probe instrumentation, initially for the standard sphere case which was then
used as a base to compare three different angles of incidence with the
introduction of active flow control of the localised synthetic jet. Measurement
data from the five-hole probe confirmed the expected highly three-dimensional
flow in the wake region of the sphere indicating reversal of flow. Hot-wire
instrumentation was used to confirm this data. Criteria were then formulated to
determine the regions were reversal of flow occurs in the wake. The criteria
were also tested against a NACA23012 airfoil. Pressure contour and vorticity
contour plots showed that the synthetic jet increased the streamwise velocity
component and decreased the size of the wake. Further to this, the present work
seeks to characterise the wake region using the concepts of displacement and
momentum thickness and shape factor. These boundary-layer properties have been
used effectively in the wake region characterising the extent of the wake and
the energy loss. Moreover, these properties have proven to be useful measures of
synthetic jet performance for wake modification. They can indicate improvements
and differences due to synthetic jet actuation and angle of incidence. Synthetic
jet flow control effected a decrease in the size of the wake and a concomitant
streamlining of the flow as was further made evident using flow visualisation
and force balance measurements.
