Ignition is an important process in many practical devices. Most studies of ignition have been performed in stagnant flows only, using asymptotic methods, numerical techniques or experiments. Although the importance of the minimum energy and ignition delay times was shown by these studies, the effects of turbulence on these processes remain unknown. In this paper Direct Numerical Simulation (DNS) is used to provide precise data on one of the mechanisms controlling ignition, i.e., the effect of a large flow velocity at the point where the spark is produced. Spark ignition in a constant speed flow is simulated using a thermal model for the spark. Results show that a minimal power is necessary for successful ignition in nonzero mean flows. This minimal power depends nearly linearly on the flow speed (except for very low speeds). For very high heating powers the ignition delay times become independent from the flow speed. A characteristic flame radius may be used to describe the ignition limit. For zero mean flows this radius is close to the one resulting from asymptotic analysis. It increases with increasing flow speed but seems to be independent from spark duration. Finally a model based on a simplified one-dimensional configuration is developed to predict ignition in a nonzero mean flow. It is shown that this model may be used over a large range of parameters to replace DNS results.