ESTUDIO EXPERIMENTAL DEL FLUJO SOBRE UN ESCALÓN MEDIANTE LA TÉCNICA PIV

Abstract

In this work an experimental study is presented. This study was carried out in the Labinthap, in a rectangular channel containing a step. This channel is 1400 mm length, 100 mm width, and 20 mm height; and it is built in cast acrylic with a walk thickness 0f 9 mm. The contained step is 700 mm length, 100 mm width and 10 mm height. This means its aspect ratio w/h=10. This aspect ratio value is the minimum value that allows a reduction of walls effects on the flow. Reynolds number was calculated using step height as characteristic length (Reh). In this work the following cases for Reh were studied: 281, 562, 843, 1124, 1404 and 1685. For the same flow conditions a different Reynolds number calculation was performed using hydraulic diameter as characteristic length obtaining: Reφ = 1021, 2043, 3064, 4086, 5107, and 6128. For the first value of Reφ, it was observed that average velocity profiles were similar to turbulent profiles because the test area was placed at a distance frow flow entrance that was not enough for fully developed laminar flow. The experiments were performed at two different zones: zone A placed at step base, and zone B separated 120 mm from the step base. The test area is 1600 x 1186 pix (22.77 x 16.89 mm) both, Forward facing step (FFS) and Backward facing step (BFS) flows were analyzed in both zones. For the FFS case, only a vortex appeared. Meanwhile, for the BFS case two vortices appeared: a main vortex and a secondary one. It was found that the vortex dimensions and core position change with Reφ. Velocity profiles at the step zone show negative velocities in both cases (FFS and BFS) which illustrates flow recirculation. urms and vrms at zone B turned out to be of similar magnitude for both cases. On the other hand, at zone A urms for BFS is larger than urms for FFS, which could be a sign of inestability for that case. Average vorticity at zone B is similar in both cases. However, at zone A FFS average vorticity is larger. It was observed that when vorticity increases, urms decreases. Vortex core position for the FFS case is directly related to Reφ. For the BFS case no tendency was found, but variation ranges for vortex core position were observed for different Reφ. For the interface layer observed between main and secondary vortices in the BFS case, horizontal position changes as Reφ increases