We use direct and stochastic numerical simulations of the magnetohydrodynamic equations to explore the influence of turbulence on the dynamo threshold. In the spirit of the Kraichnan-Kazantsev model, we model the turbulence by a noise, with given amplitude, injection scale and correlation time. The addition of a stochastic noise to the mean velocity significantly alters the dynamo threshold. When the noise is at small (resp. large) scale, the dynamo threshold is decreased (resp. increased). For a large scale noise, a finite noise correlation time reinforces this effect.PACS numbers: 47.27. Eq, 47.27.Sd, 47.65.+a, 91.25.Cw The process of magnetic field generation through the movement of an electrically conducting medium is called a dynamo. When this medium is a fluid, the instability results from a competition between magnetic field amplification via stretching and folding, and damping through magnetic diffusion. This is quantified by the magnetic Reynolds number Rm, which must exceed some critical value Rm c for the instability to operate. Despite their obvious relevance in natural objects, such as stars, planets or galaxies, dynamos are not so easy to study or model. Computer resources limit the numerical study of dynamos to a range of either small Reynolds numbers Re (laminar dynamo), modest Rm and Re [1] or small P m = Rm/Re using Large Eddy Simulation [2]. These difficulties explain the recent development of experiments involving liquid metals, as a way to study the dynamo problem at large Reynolds number. In this case, the flow has a non-zero mean component and is fully turbulent. There is, in general, no exact analytical or numerical predictions regarding the dynamo threshold. However, prediction for the mean flow action can be obtained in the so-called "kinematic regime" where the magnetic field back reaction onto the flow is neglected (see e.g. [3]). This approximation is very useful when conducting optimization of experiments, so as to get the lowest threshold for dynamo action based only on the mean flow Rm MF c [4,5,6,7]. It led to very good estimate of the measured dynamo threshold in the case of experiments in constrained geometries [8], where the instantaneous velocity field is very close to its time-average.In contrast, unconstrained experiments [7,9] are characterized by large velocity fluctuations, allowing the exploration of the influence of turbulence onto the meanflow dynamo threshold. Theoretical predictions regarding this influence are scarce. Small velocity fluctuations produce little impact on the dynamo threshold [10]. Predictions for arbitrary fluctuation amplitudes can be reached by considering the turbulent dynamo as an instability (driven by the mean flow) in the presence of a multiplicative noise (turbulent fluctuations) [11]. In this context, fluctuations favor or impede the magnetic field growth depending on their intensity or correlation time. This observation is confirmed by recent numerical simulations of simple periodic flows with non-zero mean flow [12,13] showing that tur...

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