a1 Linné Flow Centre, SeRC, KTH Mechanics, SE-100 44 Stockholm, Sweden
a2 Swedish Defence Research Agency, FOI, SE-164 90 Stockholm, Sweden
Adjoint solutions of the linearized incompressible Navier–Stokes equations are presented for a cross-flow-dominated swept-wing boundary layer. For the first time these have been computed in the region upstream of the swept leading edge and may therefore be used to predict receptivity to any disturbances of the incoming free stream as well as to surface roughness. In this paper we present worst-case scenarios, i.e. those external disturbances yielding maximum receptivity amplitudes of a steady cross-flow disturbance. In the free stream, such an ‘optimal’ disturbance takes the form of a streak which, while being convected downstream, penetrates the boundary layer and smoothly turns into a growing cross-flow mode. The ‘worst-case’ surface roughness has a wavy shape and is distributed in the chordwise direction. It is shown that, under such optimal conditions, the boundary layer is more receptive to surface roughness than to incoming free stream disturbances.
(Received November 07 2011)
(Reviewed January 26 2012)
(Accepted March 20 2012)
(Online publication April 18 2012)