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Mystery of The Miniaturized Velocity Profile Interrogator Negative Velocity Profile Solved Remotely
Nearly all undergraduate engineering courses, including laboratories, were moved online in 2020 Fall due to COVID-19. As no fully online ABET-accredited mechanical engineering bachelor’s degree programs exist, faculty developed creative methods for remote lab instruction. One approach to maintain experiential learning was sending kits by mail to students. This work describes a mailed prototype fluids kit.
To illustrate internal and external flows, a 1-meter-long velocity profile interrogator was developed from 100-mm-ID PVC pipe and driven by a variable speed fan. Half-meter-long sections are joined to achieve variable length. The deconstructed apparatus fits in a 40-quart shipping container. Internal velocity profiles are measured via a pitot-static probe rastered across the end diameter while total / static pressure difference at each location is measured.
A dead spot at the flow field’s core was anticipated because no flow is induced by the fan’s hub. Initial experiments suggested negative velocity along the pipe’s central axis, a logical impossibility. Preliminary corrective measures failed, leading to the conclusion that the apparent negative velocity was anomalous. Further investigation revealed this phenomena was caused by fan-inducted vorticity approaching perpendicular to the probe’s static port. Addition of a hexagonal mesh downstream of the fan eliminated this anomaly.
The underlying mystery and its solution were realized by an undergraduate student working remotely from home and interacting with an instructor over Zoom. The problem would never have occurred in a CFD simulation with preset simple boundary conditions or, likewise, in a premanufactured laboratory-scale wind tunnel where vanes and straighteners eliminate flow rotation. While anecdotal, this experience illustrates how mailed experimental laboratory kits are critical to honing students’ engineering skills. Moreover, it shows that remote learning can provide richer experiences than brick-and-mortar labs, suggesting take-home kits are an important engineering teaching tool worth retaining after the global pandemic ends.