The air-injection method of fixing boundary-layer transition and investigating scale effects

Experience with the air-injection method of fixing transition in wind tunnel tests on two-dimensional airfoils and a swept panel at high subsonic free-stream speeds is described. The method is shown to provide a repeatable level of disturbance which, for a given flow condition, may be generated without interrupting the test, and does not cause a significant excess drag. Examples illustrate the effectiveness of the technique in demonstrating scale-sensitive features of flows. A correlation which allows the minimum value of air-injection mass flow needed to fix transition to be estimated from a knowledge of the behavior of natural laminar flow on the wing is presented.