Abstract
Three-dimensional wake transitions for a rectangular cylinder near a moving wall at the gap ratio G/D = 0.4, 0.5 and 1.0 have been studied for 0.5 ≤ AR (aspect ratio, width/height of cylinder) ≤ 2.0 using DNS and linear-stability analysis. Mode A for all G/D values, Mode B for G/D = 0.4 and Mode C for G/D = 0.4 and 0.5 are observed for different values of AR. A change in the flow rate near the cylinder and shear-layer interactions are identified as the causes of wake transitions. Interaction of shear-layers is a dominant factor for low values of G/D (≤0.5) and high values of AR (≥1.25). For G/D = 0.5, three-dimensionality first decreases for AR ≤ 1.25, then increases with AR and flows are periodic for all values of AR. For G/D = 0.4 and 1.0 the flows are periodic for AR ≤ 1.25 and AR ≤ 0.75, respectively. The flow transition is supercritical for all values of AR, at AR ≤ 1.5 and at AR = 0.5 for G/D = 0.4, 0.5 and 1.0, respectively. The wave number of the unstable mode has been obtained using Floquet analysis and is quite close to the DNS results.
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