Spectral treatment of gyrokinetic shear flow

Sheared E×B flow in a tokamak, driven by external torque from neutral beam injection, is known to have an important stabilizing effect on drift-wave turbulence. In gyrokinetic codes, flow shear can be implemented directly on a radial mesh with nonperiodic boundary conditions. The mesh-based implementation is straightforward, but carries the possibility of spurious effects related to simulation boundaries. Alternatively, flow shear has been implemented in spectral solvers using a wavenumber shift method. Although the spectral representation has numerous computational benefits, the wavenumber shift method for treating flow shear is of questionable accuracy. Efforts to compare mesh-based solutions with spectral ones have met with limited success. In particular, significant differences in the critical shear required to stabilize turbulence are sometimes observed. We outline a new approach to treat flow shear spectrally. The method is simple to implement, matches the nonperiodic results more closely, and predicts a critical shear that is less sensitive to radial wavenumber resolution. •New method to treat flow shear in gyrokinetic systems.•Improved agreement with global gyrokinetic simulations.•Method based on Fourier series for triangle wave.•Potential application to other profiles shearing (density, temperature).