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Magnetohydrodynamical (MHD) dynamos emerge in many different astrophysicalsituations where turbulence is present, but the interaction between large-scale(LSD) and small-scale dynamos (SSD) is not fully understood. We performed asystematic study of turbulent dynamos driven by isotropic forcing in isothermalMHD with magnetic Prandtl number of unity, focusing on the exponential growthstage. Both helical and non-helical forcing was employed to separate theeffects of LSD and SSD in a periodic domain. Reynolds numbers (Rm) up to$\approx 250$ were examined and multiple resolutions used for convergencechecks. We ran our simulations with the Astaroth code, designed to accelerate3D stencil computations on graphics processing units (GPUs) and to employmultiple GPUs with peer-to-peer communication. We observed a speedup of$\approx 35$ in single-node performance compared to the widely used multi-CPUMHD solver Pencil Code. We estimated the growth rates both from the averagedmagnetic fields and their power spectra. At low Rm, LSD growth dominates, butat high Rm SSD appears to dominate in both helically and non-helically forcedcases. Pure SSD growth rates follow a logarithmic scaling as a function of Rm.Probability density functions of the magnetic field from the growth stageexhibit SSD behaviour in helically forced cases even at intermediate Rm. Weestimated mean-field turbulence transport coefficients using closures like thesecond-order correlation approximation (SOCA). They yield growth rates similarto the directly measured ones and provide evidence of $\alpha$ quenching. Ourresults are consistent with the SSD inhibiting the growth of the LSD atmoderate Rm, while the dynamo growth is enhanced at higher Rm.

Interaction of large- and small-scale dynamos in isotropic turbulent flows from GPU-accelerated simulations