Study of ramp-induced turbulent boundary layer separation and reattachment via temperature-sensitive paint and Lagrangian particle tracking

An extensive experimental investigation of geometry-induced flow separation and reattachment of a turbulent boundary layer at nearly-zero pressure gradient was conducted in the Large Water Tunnel Braunschweig on a flat-plate configuration featuring a 25° backward-facing ramp. The examined ramp Reynolds number, based on the freestream velocity and on the ramp height, was 8,800. Lagrangian particle tracks and surface temperature maps were highly resolved in space and time via 3D Shake-The-Box and temperature-sensitive paint, respectively, whereas the time-averaged surface pressure distribution was measured by means of pressure taps. This enabled a description of the evolution of the attached boundary layer approaching the ramp, of the shear layer, of the recirculation zone, of flow reattachment, and of the recovery region further downstream. The time-averaged map of the celerity of propagation of temperature fluctuations was determined via analysis of the time-resolved surface temperature maps, while time- and spanwise-averaged distributions of flow velocity, acceleration and Reynolds stresses were obtained via bin-averaging of the Lagrangian particle tracks. The distributions of temperature fluctuation celerity and flow velocity allowed the identification of the time-averaged reattachment location, which was found to be in mutual, reasonable agreement for the two different methods, and also in agreement with previous work on 25° backward-facing ramps. Furthermore, the instantaneous flowfield and surface temperature data enabled capturing the arrangement and evolution of the turbulent flow structures and of their wall thermal traces in the aforementioned flow regions. Spectral analysis of the fluctuating wall temperature and velocity fields showed consistent evolution of these flow features; in particular, in the attached flow regions populated by streamwise-oriented structures, reasonable agreement was found for the spanwise wavenumbers corresponding to the maxima of the respective power spectra.