Effect of flow shear on the onset of dynamos

Abstract

Understanding the origin and structure of mean magnetic fields in astrophysical conditions is a major challenge. Shear flows often coexist in such astrophysical conditions, and the role of flow shear on the dynamo mechanism is of great interest. Here, we present a direct numerical simulation study of the effect of flow shear on dynamo instability for EPI2D flows [Yoshida et al., Phys. Rev. Lett. 119, 244501 (2017)] with controllable mirror symmetry (i.e., fluid helicity). Our numerical observations suggest that for helical base flows, the effect of shear is to reduce the small-scale dynamo (SSD) growth rate moderately. For non-helical base flows, flow shear has the opposite effect of amplifying the SSD action. The magnetic energy growth rate (γ) for non-helical base flows has been found to follow an algebraic nature of the form, γ=−aS+bS23, where a,b>0 are real constants, S is the shear flow strength, and γ is found to be independent of the scale of flow shear. Studies with different shear profiles and shear scale lengths for non-helical base flows have been performed to test the universality of our finding.