On Closing the Streamwise BiGlobal Stability Problem: The Effect of Boundary Conditions

The modal streamwise BiGlobal stability approach introduces problems regarding the specification of in- and outflow boundary conditions (BCs). Several conditions linked to lower hierarchical stability frameworks are elaborated and are applied to a freestream and boundary layer flow. The former case is used to demonstrate the odd-even decoupling of the spectrum with the streamwise node number. The latter case illustrates that the spatial growth varies widely with different BCs, while specific Robin BCs yield the largest amplification near a target frequency. Combined, the cases show that the spectra might vary widely while corresponding to very similar spatial growth characteristics and vice versa.