3D global optimal forcing and response of the supersonic boundary layer

3D optimal forcing and response of a 2D supersonic boundary layer are obtained by computing the largest singular value and the associated singular vectors of the global resolvent matrix. This approach allows to take into account both convective-type and component-type non-normalities responsible for the non-modal growth of perturbations in noise selective amplifier flows. It is moreover a fully non-parallel approach that does not require any particular assumptions on the baseflow. The numerical method is based on the explicit calculation of the Jacobian matrix proposed by Mettot et al. [1] for 2D perturbations. This strategy uses the numerical residual of the compressible Navier-Stokes equations imported from a finite-volume solver that is then linearised employing a finite difference method. Extension to 3D perturbations, which are expanded into modes of wave number, is here proposed by decomposing the Jacobian matrix according to the direction of the derivatives contained in its coefficients. Validation is performed on a Blasius boundary layer and a supersonic boundary layer, in comparison respectively to global and local results. Application of the method to a boundary layer at M=4.5 recovers three regions of receptivity in the frequency-transverse wave number space. Finally, the energy growth of each optimal response is studied and discussed. •A numerical method to compute 3D global perturbations in 2D compressible, fully non-parallel baseflows is proposed.•A singular value analysis of the global resolvent matrix is carried out to characterise the flow response to a forcing field.•Optimal gain and optimal forcing and response fields are computed for the supersonic boundary layer at M=4.5.•The first and second mode instabilities as well as the non-modal growth of streaks are identified as optimal responses.•The analysis of their energy profiles reveals the role of the generalised inflection point and the region of supersonic relative Mach number.•Analysis of the 3D dynamics and developing control strategies of 2D complex compressible flows are promising perspectives.