Three-dimensional flow reversal and wake characterisation of a sphere modified with active flow control using synthetic jet

Recent studies by the authors have shown that the use of a localized asymmetrically located synthetic jet can improve the aerodynamic performance of bluff bodies such as the sphere by altering the threedimensional 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 18" × 18" open circuit wind tunnel that was operated at a Reynolds Number of 5 × 10⁴ 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 boundarylayer 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.