  The receptivity of a laminar boundary layer flow to small amplitude wall
roughness is investigated on an ONERA-D swept aerofoil by introducing a
dedicated resolvent operator based on linearised small amplitude wall
displacements. The singular value decomposition of this operator for a given
spanwise wavenumber provides optimal wall roughness and flow responses that
maximise an input-output gain. At the most receptive spanwise wavenumber, the
optimal response is a cross-flow mode associated with an optimal roughness
located close to the attachment-line and presenting a wavy shape with a
wavevector nearly orthogonal to the external streamlines. The method therefore
allows direct identification of the location and structure (chordwise and
spanwise wavenumbers) of the most receptive roughness. For various given wall
roughness shapes and locations (periodic or compact in the chordwise and/or
spanwise directions), an approximation of the response based on the dominant
optimal response is shown to accurately match the total response downstream of
the roughness. The method therefore allows a straightforward computation of the
response of the flow to any given small amplitude roughness.
