The paper by Donald Coles discusses two distinct types of transition in Couette flow between concentric rotating cylinders: spectral evolution and catastrophic transition. Spectral evolution occurs when the inner cylinder has a higher angular velocity, leading to a sequence of secondary modes, including Taylor motion and traveling waves, with higher modes corresponding to harmonics of the fundamental frequencies. This transition is a cascade process where energy is transferred through non-linear interactions to higher frequencies. Catastrophic transition, characteristic of the outer cylinder having a higher angular velocity, involves the division of the fluid into distinct regions of laminar and turbulent flow, separated by interfaces that may propagate in either direction. The paper also highlights the complexity of spiral turbulence, a common pattern in the transition region, and the challenges in determining the factors controlling the direction and velocity of interface propagation. The research uses both hot-wire anemometry and flow visualization techniques to study these transitions, providing detailed observations and experimental data.The paper by Donald Coles discusses two distinct types of transition in Couette flow between concentric rotating cylinders: spectral evolution and catastrophic transition. Spectral evolution occurs when the inner cylinder has a higher angular velocity, leading to a sequence of secondary modes, including Taylor motion and traveling waves, with higher modes corresponding to harmonics of the fundamental frequencies. This transition is a cascade process where energy is transferred through non-linear interactions to higher frequencies. Catastrophic transition, characteristic of the outer cylinder having a higher angular velocity, involves the division of the fluid into distinct regions of laminar and turbulent flow, separated by interfaces that may propagate in either direction. The paper also highlights the complexity of spiral turbulence, a common pattern in the transition region, and the challenges in determining the factors controlling the direction and velocity of interface propagation. The research uses both hot-wire anemometry and flow visualization techniques to study these transitions, providing detailed observations and experimental data.