Inverse cascade in decaying three-dimensional magnetohydrodynamic turbulence

Area of Science:

• Physics
• Astrophysics
• Fluid Dynamics

Background:

• Magnetohydrodynamics (MHD) describes electrically conducting fluids.
• Turbulence is a complex, chaotic fluid motion.
• Freely decaying turbulence involves the dissipation of turbulent energy over time.

Purpose of the Study:

• To investigate the behavior of three-dimensional freely decaying magnetohydrodynamic turbulence.
• To analyze energy transfer mechanisms, specifically the inverse cascade effect.
• To identify scaling regimes and power-law behaviors in decaying MHD turbulence.

Main Methods:

• Direct numerical simulations (DNS) were employed.
• The simulations focused on three-dimensional, freely decaying turbulence.
• Helical magnetic fields were specifically studied to observe their influence.

Main Results:

• An inverse cascade effect was observed for helical magnetic fields, transferring power from smaller to larger scales.
• The magnetic field exhibited a scaling regime with self-similar evolution.
• Power-law behavior was identified at high wave numbers for the magnetic field.
• Power-law decay was found in both magnetic and kinematic energies.
• The characteristic length scale of the magnetic field showed power-law growth.

Conclusions:

• Helical magnetic fields promote an inverse energy cascade in decaying MHD turbulence.
• The system approaches a self-similar state characterized by power-law scaling.
• Decaying MHD turbulence exhibits distinct energy decay and length scale growth patterns.