Pressure-Strain Interaction: I. On Compression, Deformation, and Implications For Pi-D

TL;DR

This paper explores the pressure-strain interaction in plasmas, proposing an alternative decomposition to better interpret energy conversion processes, especially in collisionless conditions, and examines the physical mechanisms behind negative Pi-D values.

Contribution

It introduces a new decomposition of pressure-strain interaction separating normal and shear deformation effects, enhancing understanding of energy transfer in collisionless plasmas.

Findings

Proposes grouping pressure dilatation with normal deformation for better physical interpretation.

Develops a kinetic theory framework for compression, normal, and shear deformation.

Identifies mechanisms leading to negative Pi-D in weakly collisional plasmas.

Abstract

The pressure-strain interaction describes the rate per unit volume that energy is converted between bulk flow and thermal energy in neutral fluids or plasmas. The term has been written as a sum of the pressure dilatation and the collisionless analogue of viscous heating referred to as ${\rm Pi-D}$, which isolates the power density due to compressible and incompressible effects, respectively. It has been shown that ${\rm Pi-D}$ can be negative, which makes its identification as collisionless viscous heating troubling. We argue that an alternate decomposition of pressure-strain interaction can be useful for interpreting the underlying physics. Since ${\rm Pi-D}$ contains both normal deformation and shear deformation, we propose grouping the normal deformation with the pressure dilatation to describe the power density due to converging/diverging flows, with the balance describing the power density purely due to shear deformation. We then develop a kinetic theory interpretation of compression, normal deformation, and shear deformation. We use the results to determine the physical mechanisms that can make ${\rm Pi-D}$ negative. We argue that both decompositions can be useful for the study of energy conversion in weakly collisional or collisionless fluids and plasmas, and implications are discussed.