Characteristics of Separation-Bubble Formation on Finite Circular Cylinder in Critical Transition Range

Experiments were conducted for three finite circular cylinder models for which the aspect ratios were two, three and four, respectively, in the critical transition range. Oil-film flow visualization was performed at Reynolds numbers at the initiation of the critical transition range, which unveiled the three-dimensionality and nonstationarity of multiple separation lines on each of the model surfaces due to intermittent formation of a separation bubble. Moreover, based on the time-averaged pressure coefficients obtained at three heights on each of the models, it was seen that the symmetry of flow varied with Reynolds number. A modified peak-valley method was proposed to identify the intermittent events of separation-bubble formation. By this method, pronounced variations in the real-time signals due to intermittent formation of the separation bubble were identified, which were referred as to the characteristic events. Interesting findings included that the average normalized timescales of the characteristic events fell within 0.1 and 0.6, whereas the average normalized timescales obtained from the real-time traces of the lift coefficient were mostly around 0.08. The difference between these is discussed in this paper.

Automated summary

This study conducted experiments on three finite circular cylinder models with different aspect ratios in the critical transition range. Oil-film flow visualization revealed the three-dimensionality and nonstationarity of multiple separation lines on each model surface due to intermittent formation of a separation bubble. The symmetry of flow was found to vary with Reynolds number based on the time-averaged pressure coefficients obtained at different heights on each model. A modified peak-valley method was proposed to identify the intermittent events of separation bubble formation.