Investigation of Laminar Separation Bubble on Flat Plate with Adverse Pressure Gradient

The performance of turbomachinery blade profiles, at low Reynolds numbers, is influenced by Laminar Separation Bubbles (LSB). Such a bubble is caused by a strong adverse pressure gradient (APG), and it makes the laminar boundary layer to separate from the curved profile surface, before it becomes turbulent. The present dissertation consists on a joint experimental and numerical investigation on a flat plate with adverse pressure gradient. The experiment provides detailed results including distribution of wall pressure coefficient and boundary layer velocity and turbulence profiles for several values of typical influencing parameters on the behavior of the flow phenomena: Reynolds number, freestream turbulence intensity and end-wall opening angle, which determines the adverse pressure gradient intensity. The numerical work consists on carrying out a systematic analysis, both with Reynolds Average Navier-Stokes (RANS) and Unsteady Reynolds Average Navier-Stokes (URANS) simulations. The results of the numerical simulations are critically investigated and compared with the experimental ones in order to understand the effect of the main physical parameters on the LSB behavior. For RANS simulations, different turbulence and transition models are compared at first to identify the adaptability to the flow phenomena; then, the influence of the three aforementioned parameters on the LSB behavior, is investigated under a typical aggressive adverse pressure gradient. Boundary layer integral parameters are discussed for the different cases in order to understand the flow phenomena in terms of flow time-mean properties. For URANS simulations, the analysis focuses on the surveys of the instantaneous velocity vector, normalized velocity and vorticity maps to highlight the dynamics of the large-scale structures shedding from the bubble maximum thickness position. In addition, the response of shedding vortex phenomenon to the variation of Reynolds number and freestream turbulence intensity under fixed APG condition is also systematically investigated. By quantifying the spatial wavelength and frequency of shedding vortices with FFT (Fast Fourier Transport) to analyze the dynamic influence factor to K-H instability progress.