Transitional Phenomena on an Elliptic Profile

Abstract

Elliptic profiles have been the subject of fundamental flow research as “elliptic cylinders” since several decades, including the study of boundary-layer separation and transition. In recent times, elliptic profiles have become relevant also for practical applications, such as canard rotor/wing aircraft and hydrokinetic energy harvesting technologies. In these cases, they are operated at moderate Reynolds numbers, at which their fluid dynamic behavior is strongly influenced by boundary-layer transition and separation. Therefore, information on these crucial phenomena is of fundamental importance for the experimental evaluation of the aero/hydrodynamic performance of elliptic profiles and the validation and improvement of the numerical prediction of the involved fluid dynamics. The transition of the boundary layer developing on an elliptic profile with 16\% relative thickness is the focus of the present work. The experiments were conducted in the Large Watertunnel of the Technical University of Braunschweig (GWB) on a two-dimensional model with a chord length of 0.2 m. The model upper surface was equipped with a Temperature-Sensitive Paint (TSP) for the global, non-intrusive, time-resolved measurement of the surface temperature distribution, and thus of boundary-layer transition. The TSP data were acquired over a mid-span region of approx. 0.2 m x 0.3 m at a frequency of 1 kHz. The model was also equipped with a row of pressure taps to measure the surface pressure distribution. The experiments were performed at two chord Reynolds numbers (Re = 2.5 and 5.0 x 10^5) for a wide range of angles-of-attack (-4° <= AoA <= 30°), thus enabling the study of the boundary-layer evolution for a wide range of stability situations. Two exemplary TSP results show the intermittent character of the transition front in the presence of a weak adverse pressure gradient and transition occurring over a laminar separation bubble close to the leading edge, as induced by a stronger adverse pressure gradient. Turbulent spots were identified in both cases. The final contribution will provide the temporal development of the transitional phenomena and a detailed analysis of the transition evolution for the examined flow conditions.