Self-consistent feedback mechanism for the sudden viscous dissipation of finite-Mach-number compressing turbulence

TL;DR

This study investigates how finite Mach numbers influence the sudden viscous dissipation of compressing turbulence, revealing that the feedback mechanism is weak at subsonic speeds, which impacts inertial confinement fusion strategies.

Contribution

It extends previous zero-Mach-limit models by incorporating finite Mach numbers, providing a self-consistent analysis of the feedback loop in compressing turbulence.

Findings

Sudden viscous dissipation occurs at finite Mach numbers.

Oscillations in dilatational TKE are observed during compression.

Dissipated TKE has minimal impact on temperature at low Mach numbers.

Abstract

Previous work (S. Davidovits and N. J. Fisch, "Sudden viscous dissipation of compressing turbulence," Phys. Rev. Lett., 116(105004), 2016) demonstrated that the compression of a turbulent field can lead to a sudden viscous dissipation of turbulent kinetic energy (TKE), and suggested this mechanism could potentially be used to design new fast-ignition schemes for inertial confinement fusion. We expand on previous work by accounting for finite Mach numbers, rather than relying on a zero-Mach-limit assumption as previously done. The finite-Mach-number formulation is necessary to capture a self-consistent feedback mechanism in which dissipated TKE increases the temperature of the system, which in turn modifies the viscosity and thus the TKE dissipation itself. Direct numerical simulations with a tenth-order accurate Pad\'e scheme were carried out to analyze this self-consistent feedback loop for compressing turbulence. Results show that, for finite Mach numbers, the sudden viscous dissipation of TKE still occurs, both for the solenoidal and dilatational turbulent fields. As the domain is compressed, oscillations in dilatational TKE are encountered due to the highly-oscillatory nature of the pressure dilatation. An analysis of the source terms for the internal energy shows that the mechanical work term dominates the viscous turbulent dissipation. As a result, the effect of the suddenly dissipated TKE on temperature is minimal for the Mach numbers tested. Moreover, an analytical expression is derived that confirms the dissipated TKE does not significantly alter the temperature evolution for low Mach numbers, regardless of compression speed. The self-consistent feedback mechanism is thus quite weak for subsonic turbulence, which could limit its applicability for inertial confinement fusion.