Transient growth in compressible boundary layer flow

The potential for transient growth in compressible boundary layers is studied. Transient amplification is mathematically associated with a non-orthogonal eigenvector basis, and can amplify disturbances although the spectrum of the linearized evolution operator is entirely confined to the stable half-plane. Compressible boundary layer flow shows a large amount of transient growth over a wide range of parameter values. The disturbance size is here measured by a positive definite energy like quantity that has been derived such that pressure-related transfer terms in its evolution equation mutually cancel. The maximum of the transient growth is found for structures which are independent of the streamwise direction and is found to scale with R2. This suggests that the transient growth originates from the same lift-up mechanism found to give large growth in incompressible shear flows. The maximum growth is also found to increase with Mach number. In compressible flow, disturbances that experience optimal transient growth can be excited naturally by a non-linear interaction of oblique unstable first mode waves. Thus, a triggering of transient growth may account for the difference in timescales between the fast oblique breakdown process and traditional secondary instability.