Studies of the nonlinear turbulence stress response to arbitrary flows

To understand the nonlinear behavior of flows and Reynolds stress, the response of tokamak turbulence to a variety of zonal flow patterns has been measured in a series of computational experiments. Despite the seemingly erratic pattern, the Reynolds stress is surprisingly well described by a nonlinear functional, depending only on the turbulence level and the ambient zonal flows. The contributing terms can be understood qualitatively in terms of analytical models.