The evolution of an acoustic disturbance up to transition in the boundary-layer on an airfoil

An experiment has been conducted at the NASA Langley Research Cent& “2 ft by 3 ft Low Speed Wind Tunnel Facility” in order to examine the generation and evolution of boundary-layer disturbances on an unswept two-dimensional wing up through transition to turbulent flow The results are intended to be used to generate a comprehensive database that includes the effect of the external disturbance environment on the transition process and serves as a benchmark for future transition prediction tools. The model was a 6% thick, 1.2 m chord symmetric wing, and it was designed to provide an extensive region of boundary-layer instability growth, with the lower branch upstream of 20% chord and transition as far downstream as 80% chord. A flow speed of 20 m/s was examined, which corresponded to chord Reynolds number of 1.68 million. The experiment consisted of establishing the mean flow conditions, forcing two-dimensional TolhnienSchlichting boundary-layer waves using modulated acoustic bursts in the free-stream, and acquiring the mean boundary-layer data and fluctuating disturbance data using hot-wire probes. Acoustic receptivity due to surface roughness near Branch I was examined. The surface roughness consisted of two-dimensional strips of tape ‘applied at and symmetrically spaced about Branch I, where repeated roughness elements were spaced one disturbance wavelength apart based upon linear-stability theory. Surface-normal modeshapes and constant boundary-layer height chordwise traverses were acquired and examined. The experimental results match well with linear stability theory, indicating breakdown of disturbances between N-factors of 7 and 11 with surface roughness on the model. Overall, the effects of surface roughness and free-stream acoustic forcing on boundary-layer eceptivity and stability w&e examined in a well-documented disturbance environment. These results will be used to validate and refine non-linear flow theories as well as help to provide an improved understanding and improved methods to control flow transition. Knowledge of the stability. and transition to turbulence of boundary layers is a highly desired goal in the fields of fluid-mechanics and aerodynamics’. Laminar flow is sometimes a desired flow condition because a laminar boundary layer exhibits significantly less drag than a turbulent boundary layer, except in the case of flow separation, where a more energetic turbulent boundary layer can delay or prevent flow separation. Once an understanding of the flow physics is fully obtained, then control of the flow can be effectively completed, and performance benefits can be achieved. The physics of flow transition consists of several processes: receptivity, linear growth, non-linear breakdown and finally turbulent flow. Flow transition can occur over an extended distance, as will be examined herein, as well as over an immeasurably small distance via a bypass mechanism. A significant application of using the knowledge of transitional flow physics is Laminar Flow Control (LFC), the process of delaying boundary-layer transition’. The development of Laminar Flow Control technology for application to transport aircraft3 requires an understanding of boundary-layer transition physics and accurate transition prediction capabilities. While this paper will not go into a detailed discussion of the current status of LFC research, the writers will discuss the recent efforts in the more general field of boundarylayer stability from receptivity through transition, particularly with respect to Tollmien-Schlichting disturbances. Stability theory has been developing dramatically since the 1920’s, when Tollmien4# ‘, Schlichting6* ‘, .made their contributions, Tollmien and Schlichting examined solutions to the Orr-Sommerfeld equation using the method of small disturbances on flat plates and rotating cylinders. The Orr-Sommerfeld equation is a reduced form of the Navier-Stokes equations of motion, which assumes laminar, incompressible, two-dimensional, constant property and parallel flow. However, the OrrSommerfeld equation is used as a good approximation l Student Member AIAA + Professor, Fellow AL4A Copyright