This paper reports the results of a study into the impact of computer simulations on the understanding of fluid mechanics by engineering students. A "lesson study" approach was taken, using constructivist educational theory combined with the variation theory of learning from phenomenography to inform the design of learning activities and to assess their impact. Student difficulties with fluid mechanics concepts were assessed using questions from the Fluid Mechanics Concept Inventory (FMCI). Students had the greatest difficulties with pressure measurement, fluid flow through pipes with changing diameter, and velocity profiles for fluid between flat plates. We developed a set of three simulations to address these difficulties. The impact of the simulations was gauged by a second administration of the FMCI. Most of the students in the sophomore fluid mechanics class participated in the whole of this exercise. Students showed significant improvement in two of the three areas of difficulty. Student feedback on this as an additional learning exercise was very positive.