Response of a flat plate laminar separation bubble to Reynolds number, free-stream turbulence and adverse pressure gradient variation

An extensive data set describing separated flows with different Reynolds numbers (Re), free-stream turbulence intensities (Tu) and adverse pressure gradients (APG) is used for the statistical characterization of laminar separation bubbles (LSBs). Measurements were performed by means of time-resolved particle image velocimetry along a flat plate installed between two adjustable endwalls, providing a test matrix of 3 Re, 4 Tu levels and 4 APGs. In this paper, 22 flow conditions are considered, which are related to the occurrence of boundary layer separation for each combination of the flow parameters. Boundary layer integral parameters, the root mean square of velocity fluctuations and the intermittency function distributions were computed for each condition. The connections between the main quantities that are commonly adopted for the statistical characterization of separated flows are clearly highlighted, thus providing the description of a LSB by means of a reduced set of these parameters. Empirical correlations predicting the transition onset location as well as the main dimensions (i.e., length and height) describing the geometry of a LSB are included in this work. Proper orthogonal decomposition is also adopted for the statistical characterization of the shedding process developing as a consequence of the Kelvin–Helmholtz instability. The frequency, wavelength and propagation velocity of the large-scale structures shed near the bubble maximum displacement are computed for each condition, showing the relation between the dominant shedding frequency and the average flow field at the separation position, and how these quantities are affected by Re, Tu and APG.