Evolution of forced shear flows in polytropic atmospheres: A comparison of forcing methods and energetics

TL;DR

This paper compares different forcing methods used in numerical models of shear flows in polytropic atmospheres, analyzing their applicability, limitations, and energetic implications during instability growth and non-linear evolution.

Contribution

It provides a systematic comparison of various forcing techniques in shear flow simulations, highlighting their advantages and drawbacks in astrophysical contexts.

Findings

Different forcing methods have distinct applicability and limitations.

Energetic analysis reveals how each method influences energy exchange.

Comparison informs best practices for long-term shear flow modeling.

Abstract

Shear flows are ubiquitous in astrophysical objects including planetary and stellar interiors, where their dynamics can have significant impact on thermo-chemical processes. Investigating the complex dynamics of shear flows requires numerical calculations that provide a long time evolution of the system. To achieve a sufficiently long lifetime in a local numerical model the system has to be forced externally. However, at present, there exist several different forcing methods to sustain large-scale shear flows in local models. In this paper we examine and compare various methods used in the literature in order to resolve their respective applicability and limitations. These techniques are compared during the exponential growth phase of a shear flow instability, such as the Kelvin-Helmholtz (KH) instability, and some are examined during the subsequent non-linear evolution. A linear stability analysis provides reference for the growth rate of the most unstable modes in the system and a detailed analysis of the energetics provides a comprehensive understanding of the energy exchange during the system's evolution. Finally, we discuss the pros and cons of each forcing method and their relation with natural mechanisms generating shear flows.