A Modal One-Way Navier-Stokes Approach to Modelling Non-Modal Boundary Layer Instabilities

TL;DR

This paper introduces a modal OWNS method that improves robustness and efficiency in modeling boundary layer instabilities, capturing both modal and non-modal disturbances with finer resolution and greater stability.

Contribution

The paper develops a novel modal OWNS approach that overcomes step-size limitations of PSE, enabling stable, high-resolution analysis of boundary layer disturbances.

Findings

Retains non-modal disturbance capturing capability

Allows arbitrarily small streamwise step-sizes

Demonstrates improved robustness over non-modal OWNS

Abstract

This paper presents a method to solve the modal form of the linearised one-way Navier-Stokes (OWNS) equations for investigating disturbance development in developing subsonic and supersonic boundary layers. The modal framework offers significant advantages in robustness and computational efficiency over the conventional non-modal OWNS framework. Notably, we demonstrate that modal OWNS (M-OWNS) retains the capability to capture non-modal disturbance development. Our technique leverages the modal ansatz of parabolised stability equations (PSE) whilst employing the recursion-parameter parabolisation strategy of non-modal OWNS to stabilise the streamwise-marching modal algorithm. A key contribution is that we overcome the minimum streamwise step-size requirement that constrains conventional PSE in capturing short-scale disturbance evolution. We demonstrate through canonical test cases that fine-scale variations in the baseflow can be captured effectively. The M-OWNS procedure permits arbitrarily small streamwisemarching step-sizes whilst maintaining stability. Crucially, we find the algorithm to be more robust than conventional non-modal OWNS, whilst recovering identical modal and non-modal phenomena.