On the flow mechanism of forward swept naturallaminar-flow wing for crossflow instability suppression

Crossflow instability (CFI) is crucial for laminar-to-turbulent transition of flow over a transonic natural-laminar-flow (NLF) wing, whose leading edge is swept to reduce wave drag. Existing research has shown that compared to conventional backward swept wing (BSW), a forward swept wing (FSW) is advantageous for suppressing CFI since the effective sweep angle at leading edge is smaller. However, it is still not completely clear about the “flow” mechanism that causes the CFI on a FSW to be weaker than that on a BSW. This paper proposes to explain the mechanism by examining crossflow pressure gradient (CFPG), which is obtained through a Reynolds-averaged Navier–Stokes flow solver that features automatic transition prediction via a full e^N method. It is observed that non-dimensional CFPG dominates crossflow velocity profiles that affect CFI. Moreover, CFPG is smaller in a longer distance on a FSW than that on a BSW, which explains the reason why the boundary layer of a FSW is more stable concerning transition due to CFI. This finding suggests that we should pay more attention to the control of CFPG when designing a NLF forward swept wing towards suppression of CFI to maintain extensive natural laminar flow.