Experimental Study of Turbulent Boundary Layer Separation Induced by a Backward-Facing Ramp

The importance of turbulent separated flow is undoubted for a wide range of applications, and in particular at the borders of the aircraft flight envelope. At the same time, turbulence modeling in Reynolds-Averaged Navier-Stokes equations still requires improvements to correctly predict the shape and size of turbulent separation bubbles. For reliable turbulence mo-del validation and modeling extensions, high-quality experimental data are necessary. A new experiment, focusing on geometry-induced separation of a turbulent boundary layer at quasi-zero pressure gradient, was designed and conducted within the DLR project ADaMant to provide such validation data. The measurements were performed in the Large Water Tunnel of TU Braunschweig on a flat-plate configuration featuring a 25° Backward-Facing Ramp. The measurements were conducted using Lagrangian Particle Tracking by Shake-The-Box (STB) and Temperature-Sensitive Paint (TSP). The application of these measurement techniques was enabled by a modular model design: a module with a glass insert was used for the STB measurements, whereas a module with TSP coating and integrated electrical heating was used for the TSP measurements. The glass in the STB module allowed a minimization of light reflections at the wall during the STB measurements, while the integrated heating in the TSP module enabled an enhancement of the flow-induced thermal signatures at the surface, which were captured via TSP. The experiments were conducted at four different freestream velocities (in the range of 1.2–2.6 m/s), corresponding to Reynolds numbers Rexh = 1.2–2.7 million.