Two- and three-dimensional instabilities in the wake of a circular cylinder near a moving wall

Two-dimensional (2D) and three-dimensional (3D) instabilities in the wake of a circular cylinder placed near to a moving wall arc investigated using direct numerical simulation (DNS). The study covers a parameter space spanning a non-dimensional gap ratio (G*) between 0.1 to 19.5 and Reynolds number (Re) up to 300. Variations in the flow characteristics with Re and 0 are studied, and their correlations with the hydrodynamic forces on the cylinder are investigated. It is also found that the monotonic increase of the critical Re for 2D instability (Re-cr2D)) with decreasing G* is influenced by variations in the mean flow rate around the cylinder, the confinement of the near-wake flow by the plane wall and the characteristics of the shear layer formed above the moving wall directly below the cylinder. The first factor destabilizes the wake flow at a moderate G* while the latter two factors stabilize the wake flow with decreasing G* In terms of 3D instability, the flow transition sequence of '2D steady -> 3D steady -> 3D unsteady' for small gap ratios is analysed at G* = 0.2. It is found that the 3D steady and 3D unsteady flows are triggered by Mode C instability due to wall proximity. However, the Mode C structure is not sustained indefinitely, since interference with the shear layer leads to other 3D steady and unsteady flow structures.