Abstract
Cross-flow over a smooth circular cylinder was studied experimentally in a wind tunnel at Reynolds numbers in the range of 2-3.2 X 105. A discrete transition indicating the initial transition from the sub-critical to the critical states is noticed. This transition leads to a pre-critical state so-called. The flow state features two separation bubbles situated symmetrically and intermittently on the two sides of the circular cylinder. The results obtained from the pressure measurements and the MEMS thermal tuft sensors on the cylinder surface reveal a trend that the separation bubbles tend to move upstream as the Reynolds number get higher. Moreover, for the cases of the circular cylinder roughened by textile materials, it is found that this transition process still holds, but takes place at lower Reynolds numbers.
