Desktop Flow Visualisation Experiments for Guided Discovery of Boundary Layers

Abstract The theory of boundary layers, which is well established and taught in all undergraduate fluid mechanics courses, can be challenging for first-time learners to comprehend. Three challenges are identified in this paper, namely to visualise the existence, thinness, and attachment/separation of boundary layers. We frame these challenges as threshold concepts that may benefit from discovery-based learning. We present a new desktop experiment, where water flow is visualised in a transparent flow loop, that supports a `guided discovery' approach. Student lab books and reports from the activity provide evidence that the first two challenges were broadly solved, while the nuance of separation requiring strong positive gradients was recorded by approximately half of the students. A survey on the student experience over four years (717 students, 331 replies --- 46\%) considered seven dimensions of the student experience. Overall results showed that students found the experiment engaging, and helpful in gaining a conceptual understanding of the boundary layer. Overall, on the three challenges we identified, the equipment and the `guided discovery' activity were judged to be successful. We also show prototype improvements for future, to aid with the third challenge, including equipment upgrades and the introduction of computational fluid dynamics (CFD).