Relaminarization on swept leading edges under high-lift conditions

Abstract

Detailed experiments were made to gain an improved understanding of relaminarization on swept wings under high-lift conditions. Two swept wings with different airfoil sections and flaps were tested, and measurements were made consisting of surface-pressure distributions and wall shear-stress fluctuations in the leading-edge zone. Measurements were made at a chord Reynolds number of 1.3 × 10 with wing-sweep angles of 45 and 60 deg and model incidence varied in the range of 3 to 18 deg in discrete steps. Following the attachment-line transition, the turbulent boundary layer relaminarized under certain conditions due to the acceleration around the wing's leading edge. Broad features of relaminarization and subsequent retransition are discussed based on wall shear-stressfluctuation data. The present dataset shows that relaminarization is likely if the maximum value of the acceleration parameter K (evaluated along the external inviscid streamline) is greater than about 3 × 10, in agreement with earlier findings. Much higher maximum values of K result in significant reduction of intermittency during relaminarization. After relaminarization, the boundary layer retransitions across a separation bubble. Characteristics of the postrelaminarization separation bubbles are shown to be similar to those of bubbles occurring in more pristine laminar boundary layers.