Numerical and experimental study of the tolerance of natural laminar flow on a wing to TS destabilisation at the leading edge/wing-box junction

The junction between the leading edge and wing box components of natural laminar flow wings presents a feature to the flow that can potentially enhance the growth of transition causing Tollmein Schlichting (TS) instabilities leading to a significant forward movement of transition. Even when a filler is applied to the gap between the components, subsequent curing will lead to a shallow cavity formed by the filler surface. This paper describes a detailed experimental and numerical study of flow over realistic filler shapes under conditions representative of a natural laminar flow wing. Wind tunnel results show that for the width of gap examined there is an initial forward movement of transition by a quite significant amount for very shallow filled gaps but essentially no further movement as the depth is increased. PSE based stability analysis reveals that there is a destabilisation of modes near the gap. The degree of destabilisation appears to plateau with increasing depth due to recirculation of flow inside the gap which maintains an almost constant effective depth. Compensating stabilisation soon after the gap means however that there is very little lasting effect of the gap on the magnitude of the transition causing modes further downstream and stability analysis suggests there is little movement in transition as seen beyond a certain depth. The initial movement observed in the tunnel tests may be due to enhanced receptivity to TS waves at the site of the gap which is not captured by the stability analysis.