TRANSITION OF A LAMINAR SEPARATED BOUNDARY LAYER UNDER VARYING ADVERSE PRESSURE GRADIENT

This paper describes the effects of pressure gradients on the transition of a laminar separation bubble (LSB) formed over the flat plate. The measurements were performed using electronically scanned pressure (ESP) and hot-wire anemometer. The different adverse pressure gradients (APG) are achieved by changing the opening angle (a) of the upper boundary of the domain. The experiments were performed for alpha=7 degrees, 9.5 degrees and 11.5 degrees at a Re of 2x10(5) (based on the inlet freestream velocity and plate length) and freestream turbulence intensity (fst) of 1.02%. The onset of separation and transition occurs at x / L = 0.39,0.449 respectively with reattachment at 0.491 for alpha=7 degrees. As APG increases, the separation, transition and reattachment shift upstream with respect to alpha= 7 degrees leading to the reduction in bubble length. The decrease in the length of LSB is attributed to the earlier transition. Moreover, the growth rate of fluctuations increases with increasing APG. The power spectra of streamwise fluctuations illustrate that the transition occurs via K-H instability for all APG and the predominant shedding frequency increases with increasing pressure gradient. Moreover, the self-similarity of normalized u(rms) profiles in the second half of the bubble show the presence of viscous instability.