We introduce a simple and powerful new process for mixing two fluid streams introduced into a pipe via a splitter plate. We demonstrate that the fluids can be thoroughly mixed (at least on large scale) within one pipe diameter downstream of the splitter. Mixing is controlled via periodic velocity forcing of one of the input streams, and is due to amplification of the perturbation as described by instability and receptivity theory. The effectiveness of the mixing depends strongly and non-trivially on the perturbation frequency and amplitude, which distinguishes this method from simple mechanical stirring. The method is effective even at relatively low Reynolds numbers, where laminar flow is observed in the absence of the forcing. Fast mixing can be obtained for different initial velocity ratios of the two inlet streams, rendering the mixing process relatively versatile. Here the mixing method is demonstrated, and potential issues for industrial application are discussed. (C) 2003 Elsevier Ltd. All rights reserved.