Capacitance matrix technique for avoiding spurious eigenmodes in the solution of hydrodynamic stability problems by Chebyshev collocation method

TL;DR

This paper introduces a capacitance matrix technique to eliminate spurious eigenmodes in hydrodynamic stability calculations using Chebyshev collocation, demonstrated on the Orr-Sommerfeld equation for plane Poiseuille flow.

Contribution

The paper proposes a novel capacitance matrix approach that preserves matrix structure and effectively removes spurious eigenvalues in hydrodynamic stability analysis.

Findings

The method successfully removes zero eigenvalues by a slight modification.

Spurious eigenvalues are avoided with the proposed approach.

Demonstrated on Orr-Sommerfeld equation for plane Poiseuille flow.

Abstract

We present a simple technique for avoiding physically spurious eigenmodes that often occur in the solution of hydrodynamic stability problems by the Chebyshev collocation method. The method is demonstrated on the solution of the Orr-Sommerfeld equation for plane Poiseuille flow. Following the standard approach, the original fourth order differential equation is factorised into two second-order equations using a vorticity-type auxiliary variable with unknown boundary values which are then eliminated by a capacitance matrix approach. However the elimination is constrained by the conservation of the structure of matrix eigenvalue problem, it can be done in two basically different ways. A straightforward application of the method results in a couple of physically spurious eigenvalues which are either huge or close to zero depending on the way the vorticity boundary conditions are eliminated. The zero eigenvalues can be shifted to any prescribed value and thus removed by a slight modification of the second approach.