Receptivity to thermal noise of the boundary layer over a swept wing

The commonly accepted description of spatial transition to turbulence in a convectively unstable boundary layer involves as one of its stages the permanent excitation of instabilities by external noise (receptivity). The idea that thermal fluctuationsofmicroscopic origin providea sufficient amountof permanent noiseto playarolein this process has made onlyscant and isolatedappearancesinthe literature. Yet, the contributionofthermal noisetoreceptivity has its amplitude determined from physical first principles; it provides the lower bound beyond which disturbances cannotbe reduced (and the upper bound beyond which transition cannot bedelayed) lest thermodynamics isviolated. As it is also not significantly harder to compute than a typical N-factor plot, thermal-noise receptivity can be adopted as a standard against whichto compare other formsof receptivity througha noise figure analogoustothe one thatisof commonplace use in electronics. Elaborating on previous computations of the effect of thermal noise upon the twodimensional boundary layer past a flat plate, it is shown by a few real-world examples that the receptivity to thermal noise of the crossflow boundary layer over a swept wing is even larger, and by a substantial amount.