A low-rank solver for the stochastic unsteady Navier–Stokes problem

We study a low-rank iterative solver for the unsteady Navier–Stokes equations for incompressible flows with a stochastic viscosity. The equations are discretized using the stochastic Galerkin method, and we consider an all-at-once formulation where the algebraic systems at all the time steps are collected and solved simultaneously. The problem is linearized with Picard’s method. To efficiently solve the linear systems at each step, we use low-rank tensor representations within the Krylov subspace method, which leads to significant reductions in storage requirements and computational costs. Combined with effective mean-based preconditioners and the idea of inexact solve, we show that only a small number of linear iterations are needed at each Picard step. The proposed algorithm is tested with a model of flow in a two-dimensional symmetric step domain with different settings to demonstrate the computational efficiency. •Development of low-rank approximate solutions of stochastic-Galerkin formulations.•Demonstration of the utility of these representations for efficiency.•Use of tensor formats for representation of the solutions.•New efficient algorithms for “all-in-one” formulation of the time-dependent problem.•New preconditioners for linear systems arising from implicit time discretization.