Daniel P. Lathrop — Page 2

arXiv2004

Experimental observation and characterization of the magnetorotational instability

Differential rotation occurs in conducting flows in accretion disks and planetary cores. In such systems, the magnetorotational instability can arise from coupling Lorentz and centrifugal forces to cause large radial angular momentum fluxes. We present the first experimental observation of the magnetorotational instability. Our system consists of liquid sodium between differentially rotating spheres, with an imposed coaxial magnetic field. We characterize the observed patterns, dynamics and torque increases, and establish that this instability can occur from a hydrodynamic turbulent background.

physics.flu-dyn↗

arXiv2003

Turbulent intermittency and Euler similarity solutions

Self-similar Euler singularities may be useful for understanding some aspects of Navier-Stokes turbulence. Here, a causal explanation for intermittency is given, based on the control of the sudden growth of the gradients by the Euler equations. This explanation uses certain Euler solutions as intermediate asymptotics in Navier-Stokes turbulence \cite{barenblatt} -- controlling the dynamics over a limited spatial and temporal domain. These arise from an analysis of similarity equations, previously discussed by Pelz and Green \cite{pelz}, which yield experimentally testable predictions. Three main points are presented here: scalings of suitable characteristic lengths with time from a critical time $l \sim (t_\circ -t)^α$, $α>1$, a discussion of invariant sets of the similarity equations that result, and a discussion of cutoff mechanisms. The value $α= 3/2$ appears to correspond to Kolmogorov scaling for turbulence. Some limited experimental evidence is presented from Eulerian gradient measurements at the Kolmogorov scale showing $1<α<3$ values. Much testing is necessary to ascertain the final usefulness and validity of these ideas, as several conceptual obstacles remain.

cond-mat.soft↗