Investigation of a Multiple-Timescale Turbulence-Transport Coupling Method in the Presence of Random Fluctuations

TL;DR

This paper evaluates a numerical coupling method for turbulence-transport modeling in fusion reactors, focusing on the impact of turbulent fluctuations and how to mitigate their effects through averaging.

Contribution

It introduces an approach to assess the performance of a coupling method in turbulent conditions using stochastic fluctuation modeling.

Findings

Residual monitoring can be misleading as an error proxy.

Averaging reduces fluctuation error to improve accuracy.

Predicts the averaging needed for desired precision.

Abstract

One route to improved predictive modeling of magnetically confined fusion reactors is to couple transport solvers with direct numerical simulations (DNS) of turbulence, rather than with surrogate models. An additional challenge presented by coupling directly with DNS is that the inherent fluctuations in the turbulence, which limit the convergence achievable in the transport solver. In this article, we investigate the performance of one numerical coupling method in the presence of turbulent fluctuations. To test a particular numerical coupling method for the transport solver, we use an autoregressive-moving-average model to efficiently generate stochastic fluctuations with statistical properties resembling those of a gyrokinetic simulation. These fluctuations are then added to a simple, solvable problem, and we examine the behavior of the coupling method. We find that monitoring the residual as a proxy for the error can be misleading. From a pragmatic point of view, this study aids us in the full problem of transport coupled to DNS by predicting the amount of averaging required to reduce the fluctuation error and obtain a specific level of accuracy.