A narrow-channeled backward-facing step flow with or without a pin-fin insert: Flow in the separated region

This paper reports an experimental investigation of the separated flow region of a backward-facing step (BFS) turbulent flow. The goal was to characterize the three-dimensional flow field in a narrow BFS channel with or without a pin-fin insert. Consequently, a closed BFS channel of an expansion ratio 1.25 and aspect ratio 8.0 was tested. The pin-fin inserts employed were of 85 % porosity, and of rod height similar to the step height h. The Reynolds number of the in-coming flow based on the center-line mean streamwise velocity and h was maintained at 6000. The presence of the insert and its location behind the step were also varied from 0.69h to 15.58h. Using a planar particle image velocimetry, detailed velocity measurements were made across several spanwise planes within the entry and separated regions of the test section. The results show asymmetric qualities of reattachment length, vorticity and turbulent statistics about the spanwise plane. It is also evident that the use of inserts directly behind the step transforms the flow into one without any separated section. However, by varying the insert between 2.9h and 15.6h, the extent of the recirculation zone can be modulated between 56 % and 108 % of the length that exists without inserts. Additionally, the inserts may be used to enhance turbulent production while creating regions of energy sink. Apart from the physics unveiled in this study, the data presented provide valuable benchmark for validation, as well as suggestions to build more robust turbulent models to simulate narrow-channeled BFS flows.