Raceways are nowadays the most used large-scale reactors for microalgae culture. This paper focuses on the hydrodynamics in such a reactor, and emphasizes on the effects of the paddlewheel geometry (two impeller configurations are tested). The global hydrodynamics behavior of the raceway (flow velocity, mixing time) is characterized experimentally by the tracer pulse injection method, and local velocity measurements are acquired by Pulsed Ultrasonic Doppler Velocimetry. Finally, the flow is modeled by using the sliding-mesh CFD technique, a method overcoming many limitations of the simulation approaches used in the literature. CFD simulations successfully estimate the flow rate in the reactor and the power consumption of the paddlewheel. It is shown that the mixing efficiency in the raceway reactor is closely related to the low-frequency flow unsteadiness that arises from the periodic motion of the blades. Concerning local velocities, CFD and experimental data are in good agreement at some positions in the reactor, but a significant disagreement is observed at some other locations. Taking into account the wind presence in the simulations reduces the discrepancy between the experimental and numerical results, showing that, even with a moderate intensity, wind has an important effect on the hydrodynamics in the reactor. (C) 2014 Elsevier B.V. All rights reserved.