Abstract Details
Abstracts
Author: Bindesh Tripathi
Requested Type: Consider for Invited
Submitted: 2023-03-24 12:23:55
Co-authors: A.E. Fraser, P.W. Terry, E.G. Zweibel, and M.J. Pueschel
Contact Info:
University of Wisconsin-Madison
Thomas C Chamberlin Hall, 1150
Madison, Wisconsin 53706
USA
Abstract Text:
Shear flows are a critical element of transport barriers in fusion devices, and as such have been investigated both experimentally and theoretically. Though often stable, shear flows can become unstable, driving momentum transport through the Reynolds stress. Reduced models of shear-flow-instability-driven momentum transport have considerable potential, but have received only limited attention.
We develop a nonlinear saturation theory in both 2D magnetized and 3D hydrodynamic driven shear-flow turbulence. Using nonlinear eigenmode-coupling and energy transfer analyses, we show the unstable modes nonlinearly excite and continuously feed almost all their energy to large-scale linearly stable modes, which then return the turbulent energy to the background gradient [1]. Although the energy cascade to small scales and subsequent dissipation exist, they are severely subdued, and thus have reduced impact on momentum transport. These effects persist even with a magnetic field strong enough to nearly suppress the instability. We build a reduced model of transport where even a few unstable and stable modes, taken together, accurately reproduce the transport rates from full-scale turbulence simulations; neglect of stable modes results in severe overestimation of transport [2]. We further probe the nonlinear coupling of eigenmodes and show that, in three-dimensional turbulence, secondary-instability zonal flows, with zero frequency, act as an efficient intermediary in near-resonantly transferring energy from the unstable to conjugate-stable modes. We discuss differences between two- and three-dimensional instability saturation, e.g., zonal flows.
[1] Tripathi, Fraser, Terry, Zweibel, and Pueschel, Phys. Plasmas 29, 070701 (2022).
[2] Tripathi, Fraser, Terry, Zweibel, and Pueschel, Phys. Plasmas 29, 092301 (2022).
-Work funded by the Department of Energy [DE-SC0022257] through the NSF/DOE Partnership.
Comments:
1,4