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Series of concentration and velocity patterns are found for the rotating suspension of non-Brownian settling particles in a completely filled horizontal cylinder. Individual flow states, or phases, are studied using both side and cross-sectional imaging to examine the detailed flow structures. The overall steady-state phase diagram of the system is mapped over a wide range of the rotation rate and fluid viscosity. Effects of the particle radius a, volume fraction φ, and cylinder radius R on the transition boundaries are examined. It is found that the phase diagram of the rotating suspensions can be divided into three regions, in which the transition boundaries obey different scaling laws. A theoretical attempt is made to understand the scaling behaviour of the transition boundaries. The theoretical understanding is achieved at three different levels: a general dimensional consideration, a scaling analysis on the continuum equations of motion, and a specific instability calculation for the transition boundary at the centrifugal limit.