Evaluation of a Serrated Edge to Mitigate the Adverse Effects of a Backward-Facing Step on an Airfoil

Abstract

Backward-facing steps are commonly formed on wings and blades due to misalignment between segments or the addition of protective films. A backward-facing step (BFS) is known to degrade the airfoil performance. To mitigate these adverse effects, a three-dimensional low-profile serrated pattern (termed sBFS) was applied downstream of a BFS on an LA203A profile airfoil. The model drag was determined from wake surveys using a traversing Pitot-static probe within a subsonic wind tunnel operating at a chord-based Reynolds number of 300,000. The airfoil spanned the wind tunnel width (914 mm) and had a 197 mm chord length. Four different sBFS configurations were tested, each formed by applying a 1 mm thick film around the model leading edge. In addition, a BFS at various chord locations and a clean wing (i.e., no film applied) were tested for reference. The sBFS was able to reduce the drag relative the BFS by up to 8–10%, though not outperforming the clean wing configuration. In addition, the wake surveys showed the sBFS produced strong coherent structures that persist into the far-wake region (five chord length downstream of the model) with a scale that was much larger than the step height. Additionally, a computational study was carried out to further examine the flow behavior on the airfoil that produced the coherent structures. This showed that fluid near the surface gets entrained towards the sBFS downstream tip of the sBFS, which creates the initial rotation of these coherent structures that persist into the far-wake region.

Laminar Boundary Layer over a Serrated Backward-Facing Step

“…Finally, the experimentally and computationally observed spanwise variation induced by the sBFS was shown to be associated with the pumping of near-wall fluid towards the downstream sBFS peak, where it would rapidly transition to turbulent flow after the step. Of note, the spanwise variation was consistent with the work of KC et al [25] that examined the wake induced by an airfoil with a similar sBFS design to mitigate the impacts of leading-edge protective films on wind turbine blades.…”

“…Even the flow that goes over the edges between the peak and valley is then moved along the sBFS face towards the peak location. This is very similar to that observed in a similar sBFS simulation on an airfoil [25], which noted that the near-wall flow was pumped towards the peak, while flow farther from the wall showed relatively little spanwise motion. In addition, this observation is consistent with the low-shear diamonds.…”

“…Preliminary results showed an increase in the wind turbine performance of up to 7.4% when using sBFS compared to the standard film. This observation motivated the study by KC et al [25], which experimentally and computationally investigated the bulk flow modifications induced by sBFS on an airfoil shape. That study showed that the sBFS could reduce drag relative to a standard BFS by up to 8-10%.…”