A flipped classroom approach was implemented in an undergraduate fluid mechanics course. Students watched short, online video lectures before class, participated in active in-class problem solving sessions (in pairs), and completed individualized online quizzes weekly. In-class activities were designed to develop problem-solving skills and teach subject content. The instructor and assistants provided critical "just-in-time tutoring" during the in-class problem solving sessions. Comparisons were made with a simultaneous section offered in a traditional mode by a different instructor. Students in the flipped-classroom section demonstrated greater gains on the Fluid Mechanics Concept Inventory (pre-test versus post-test). Multivariate regression analysis revealed that students in the flipped-classroom section had a significant relative gain on the Fluid Mechanics Achievement Score, which was an aggregative measure of problem solving ability and conceptual understanding. Student feedback on the flipped-classroom section was very positive both in terms of the course format and their perceptions of the amount learned in the course.