Institute for Advanced Study Astrophysics Seminar

ABSTRACT: Motivated by astronomical accretion disks in which magnetorotational instability (MRI) may be locally or episodically suppressed, such as protostellar disks, recent laboratory experiments have studied hydrodynamic turbulence in rotating shear flows at Reynolds numbers (Re) above one million. Positive radial angular-momentum gradients are found to inhibit turbulence, but to a degree that is disputed among experimenters, and it is not agreed whether hydrodynamic turbulence is possible for keplerian flow profiles. Numerical simulations, which better control boundary conditions but have been limited to lower Re, find strong inhibition. Complicating matters is the discovery that turbulence is transient, both numerically and experimentally, absent linear instabilities or forcing. Turbulent half-lives increase with Re, but there is no sharp threshold separating persistently laminar from persistently turbulent states. Transience is also seen in some zero-net-flux MRI simulations. One avenue for progress may have been opened by the discovery of self-sustaining nonlinear structures near turbulent "transition" in plane-Couette, pipe, and perhaps shearing-box flow. These matters will be reviewed, and preliminary results from a new suite of higher-Re shearing-box simulations will be presented.