Active Control of Vortex Induced Vibrations of a Tethered Sphere in a Uniform Air Flow

VIV of two heavy tethered spheres ($D$ = 40 mm, $m^{\ast }$ = $m_{sphere}$/\textit{$\rho $}$_{f}V_{sphere}$ = 21 and 67, $L^{\ast }=L/D$ = 2.50) were studied in a wind tunnel under uniform free stream velocities up to $U^{\ast }=U$/$f_{n}D$ = 15.9, with and without acoustic control. Control was achieved using two speakers mounted on either side of the spheres and driven in-phase at $f $= 35Hz ($f^{\ast }$ = 22.3). In the non-controlled case, the bifurcation map of transverse sphere oscillation amplitude, $A_{y}$, showed stationary motion as well as periodic and non-stationary oscillations with increasing $U^{\ast }$. For $m^{\ast }$ = 21, $A_y^{\max } $ was about twice as large as for $m^{\ast }$ = 67. Acoustic control dampened $A_y^{\max } $ in the periodic region ($m^{\ast }$ = 67) and increased $A_y^{\max } $ in the non-stationary region for both spheres. Sphere boundary layer dynamics in the three different bifurcation regions were studied using time resolved PIV with a horizontal laser sheet positioned at the center of the sphere. The field of view was 55x55 mm$^{2}$ containing one quarter of the sphere. Results will be presented on the vortex dynamics near the sphere's surface with and without acoustic control.