The indication of boundary-layer transition on aerofoils in the N.P.L. 20 in. x 8 in. high speed wind tunnel

Abstract

The "liquid film" evaporation method developed by Gray for indicating the position of boundary-layer transition on models in low speed wind tunnels has proved satisfactory in the N.P.L. 20 in. x 8 in. high speed tunnel. Observation of transition over the complete span of the model can be simultaneous with that of surface pressures, profile drag and flow pattern. Direct shadow photographs taken in this tunnel also give an approximate indication of the transition point in certain cases, due to the change in density profile accompanying the change from laminar to turbulent flow. Thus, except when transition occurs near the leading edge,a laminar layer is indicated by a bright line parallel to, but separated from, the dark shadow of the aerofoil and converging on to this shadow to moot it just downstream of the transition point.