M.Sc Thesis

M.Sc StudentGoyta Snir
SubjectTethered Cube Stabilization by Means of Active Flow
DepartmentDepartment of Mechanical Engineering
Supervisor PROF. David Greenblatt
Full Thesis textFull thesis text - English Version


A significant advantage of helicopters over fixed wing aircraft is their ability to lift and transport large and bulky loads from otherwise inaccessible locations. A well-known limitation associated with these so-called slung-loads is that they become unstable at moderate forward speeds. In the case of cube-shaped cargo containers, vortices are shed from the container’s leading-edges and these are synchronized by body’s angular motion. Many techniques have been proposed to stabilize the loads. In contrast to previous studies, the hypothesis advanced in this thesis is that active control of the leading-edge flow separation can be used to stabilize the load.

            In order to test this hypothesis, an idealized experimental investigation was carried out to assess the effectiveness of active control as a means of suppressing oscillations of cubes tethered in a low-speed wind tunnel. To achieve an idealized experimental representation of the problem, several 15cm square model cubes of different masses were constructed and two experimental configurations were considered: a static configuration involving surface pressure and particle image velocimetry (PIV) flowfield measurements, and a dynamic, tethered, configuration. Corner-mounted, pulsed dielectric barrier discharge (DBD) plasma actuators were used at the leading-edges in an attempt to suppress the oscillations. The actuators were driven at frequencies in the range of 10 kHz to 20 kHz and at 10 kV peak-to-peak. In addition, pulsation modulation frequencies were chosen to represent the reduced frequency range between O(0.1) and O(2), known to be effective for separation control. The duty cycle, namely the percentage (or fraction) of time that the actuator is operational, was varied in the range of 1% to 7 0%. On the static configuration, the results showed that actuation changed the direction of the side forces and virtually eliminated moment excursions. Surface pressure and flowfield measurements showed that control of separation bubbles on the surfaces, as well as control of the separated shear layer, were responsible for these effects. Phase-averaged PIV measurements elucidated the mechanism whereby actuation severs the leading-edge vortex that subsequently sheds downstream.

            For the tethered configurations, the longitudinal, lateral and angular motions of the tethered cubes were tracked photographically. It was observed that active separation control at the leading-edges of the tethered cube could dramatically reduce the cube rotational motions. Reduced frequency and duty cycle had a marked effect on control effectiveness. Furthermore, transients following initiation of control acted over a longer period than transients following termination of control.