Inverse and Direct Energy Cascades in Three-Dimensional Magnetohydrodynamic Turbulence at Low Magnetic Reynolds...

Area of Science:

• Fluid Dynamics
• Plasma Physics
• Magnetohydrodynamics (MHD)

Background:

• Turbulence exhibits complex energy transfer dynamics, often categorized as upscale (inverse) or downscale (direct) cascades.
• The dimensionality of turbulent structures (e.g., 2D vs. 3D) is a critical factor influencing these energy transfer mechanisms.
• Magnetohydrodynamics (MHD) describes the behavior of electrically conducting fluids, such as plasmas, in magnetic fields, offering a framework to study turbulence.

Purpose of the Study:

• To investigate how the dimensionality of turbulence affects energy transfer (upscale vs. downscale).
• To analyze the interplay between different dimensional structures (3D and quasi-2D) within a turbulent system.
• To determine if turbulence dynamics are universally dictated by dimensionality or forcing scale.

Main Methods:

• Experimental study of low magnetic Reynolds number (Rm) magnetohydrodynamic (MHD) turbulence.
• Forcing turbulence within a confined channel geometry.
• Precisely controlling system dimensionality using an externally applied magnetic field.

Main Results:

• Identification of a critical length scale separating 3D and quasi-2D turbulent structures.
• Observation of an inverse energy cascade of horizontal kinetic energy at large scales, extending into 3D regions.
• Detection of a direct energy cascade confined to small, strongly 3D scales.

Conclusions:

• Turbulence energy transfer is not solely determined by the dimensionality of individual scales or the forcing scale.
• The relationship between kinematics and dynamics in turbulence is non-universal.
• Forcing and dissipation mechanisms significantly influence the observed turbulence dynamics.