Numerical studies of dynamo action in a turbulent shear flow
by
DrNishant Singh(Nordita)
→
Europe/Stockholm
122:026
122:026
Description
We perform numerical experiments to study the shear dynamo problem where we look for the growth of large-scale magnetic field due to non-helical stirring at small scales in a background linear shear flow, in previously unexplored parameter regimes. We demonstrate the large-scale dynamo action in the limit when the fluid Reynolds number (Re) is below unity whereas the magnetic Reynolds number (Rem) is above unity; the exponential growth rate scales linearly with shear, which is consistent with earlier numerical works. The limit of low Re is particularly interesting, as seeing the dynamo action in this limit would provide enough motivation for further theoretical investigations, which may focus the attention to this analytically more tractable limit of Re < 1 as compared to more formidable limit of Re > 1. We also perform simulations in the limits when, (i) both (Re, Rem) < 1; (ii) Re > 1 & Rem < 1, and compute all components of the turbulent transport coefficients (\alpha_{ij} and \eta_{ij}) using the test-field method. A reasonably good agreement is seen between our results and the results of earlier analytical works (Sridhar & Singh 2010; Singh & Sridhar 2011) in the similar parameter regimes. For all the simulations performed in different parameter regimes, we estimate the dynamo number (D_{\alpha S}), which was empirically defined in Brandenburg et al. (2008) corresponding to the incoherent alpha-shear mechanism, and find that D_{\alpha S} is always supercritical for cases in which we see dynamo growth, a result which is in agreement with Brandenburg et al. (2008). This seems to suggest that the fluctuations in \alpha_{ij} (which is ultimately related to the turbulent helicity fluctuations; see e.g. Kraichnan (1976)) in conjunction with mean background shear might have significant effect on the generation of large-scale magnetic field in such systems.
