Mode resonance and wavelength-halving instability in the Taylor-Dean system

We investigate experimentally the higher modes of the Dean vortex flow in the regime of resonant fundamental and second harmonic modes in the Taylor-Dean system. An unusual transition sequence to spatiotemporal chaos is observed: stationary Dean vortex flow gives way to a drift instability followed by a wavelength-halving instability. The resulting small rolls are modulated in space and time, and they form a roll packet that preserves the initial spatial period. The linear and nonlinear stability analysis of the flow shows that the interaction of resonant modes leads to some of the observed results. The wavelength-halving instability may also be attributed to a second harmonic generation.