Three-dimensional modes in a periodically driven elongated cavity.

Three-dimensional instability modes of the periodic flow in a rectangular cavity driven by the harmonic sliding oscillation of its floor are explored experimentally. Theory for a cavity with infinite span predicts two synchronous modes and a quasiperiodic traveling-wave mode as primary transitions from two-dimensional to three-dimensional flow for different combinations of floor oscillation amplitude and frequency. Previously, only one of the two synchronous modes had been found experimentally. Here, we provide experimental details of both synchronous modes and a quasiperiodic mode. All three modes appear in the parameter regimes predicted by the theory; however, in the finite-span experiments, the traveling wave nature of the quasiperiodic mode is replaced by a nonpropagating mode with spatial features similar to those of the traveling mode.