Crossflow Instability Analysis for Swept Laminar Flow Wings Using Crossflow Pressure Gradient

Abstract

Crossflow instability (CFI) is crucial for triggering boundary-layer flow transition over a transonic natural laminar flow (NLF) wing, whose leading edge is swept for reducing wave drag. This paper proposes a new flow parameter called the nondimensionalized crossflow pressure gradient (CFPG) to combine the effects of both wing sweep and pressure distribution on crossflow, which enables an alternative interpretation of the mechanism for CFI suppression. At first, a self-contained derivation of nondimensional form of CFPG is presented, and the Falkner–Skan–Cooke boundary layers are employed to theoretically show that the nondimensionalized CFPG does relate to wing sweep and pressure gradients. Besides, infinite obliquely placed NLF wings with different sweep angles or pressure distributions are used to numerically validate our hypothesis. Second, examples of nontapered finite-span wings are used to demonstrate the advantage of using nondimensionalized CFPG. Finally, nondimensionalized CFPG is applied to analyze the crossflow on tapered NLF forward-swept wing (FSW) and backward-swept wing (BSW) configured with the same airfoil, and the underlying flow mechanism is exposed from a new perspective. It is observed by this paper that nondimensionalized CFPG can take into account the effects of both wing sweep and pressure distribution, which leads to a different perspective to analyze the crossflow on FSW as well as BSW.