Blowin' in the wind

Most at APS-DFD are well familiar with the Tacoma Narrows Bridge disaster. The operation of tethered observation balloons often suffer from similar vortex-induced oscillations, even at modest wind velocities. This talk will discuss two new feedback control techniques to subdue such wind-induced oscillations. In the case of operation with a single tether, a simple nonlinear (period doubling) feedback control strategy, synchronizing the motion of the winch (on the ground) controlling the tether with the wind-induced oscillations of the balloon, in a manner roughly opposite to what a child learns to do in order to drive up the oscillations of a playground swing, is found to be effective. In the case of operation with multiple (>6) tethers, analysis in a tensegrity-based framework is found to be effective, and a simple linear program (LP) may be formulated and solved to minimize the maximum tension, and to maximize the minimum tension, of the several tethers, thereby at any moment providing the most suitable set of tensions (driven by a set of winches) to stabilize the balloon. Ongoing efforts by USAFA cadets to validate such solutions experimentally will also be discussed.