Wind-tunnel experiments on boundary layer transition caused by coarse distributed roughnessRobin Joseph – PhD scholar, Department of Aerospace Engineering, IISc-Bangalore 
P Phani Kumar – Scientist D, Indian Space Research Organization 
Sourabh S. Diwan – Assistant Professor, Department of Aerospace Engineering, IISc-BangaloreDistributed roughness in the form of ingested aerosols, dust, combustion products, fouling etc occur on compressor and gas turbine blades (typically near the leading edge) during their runtime resulting in early boundary layer transition over the blade surface. The early transition causes increased skin friction, reduced lift-to-drag ratio and higher heat transfer over the blade, which result in reduced thrust/power output, higher specific fuel consumption and higher maintenance costs (Bons, 2010). Due to the wide parameter space of the distributed roughness accumulated on the turbine blades (height, streamwise extent, roughness concentration etc), modelling its effect is difficult and consequently, the mechanism of distributed roughness induced transition is not clearly understood (Durbin, 2017). Previous studies have demonstrated underperformance of gas turbines due to distributed roughness (simulated using sandpaper strips; Yun et al., 2005); however, these studies do not look at the flow features and mechanism of transition. It is expected that a better understanding of the transition mechanism will help in accurately predicting the transition location and length of the transition zone, which is presently a challenging task (Langel et al. 2015).In this work, we investigate the phenomenology of boundary layer transition downstream of a strip of coarse distributed roughness (24-grit industry-grade emery sheet) in a flat plate boundary layer using particle image velocimetry and hot-wire anemometry. The experiments are conducted in an open circuit wind tunnel at the Department of Aerospace Engineering, Indian Institute of Science-Bangalore. The roughness strip is characterized using a laser scanner and it shows the presence of roughness particles of different heights. Pre-transitional region of the boundary layer shows the presence of steady high and low speed streaks similar to those observed downstream of an isolated roughness. There is substantial spanwise variation in the streak strength likely due to the spanwise inhomogeneity of roughness concentration and this suggests that single point measurements may be unsuitable for characterizing the effect of coarse distributed roughness. In the transitional region, streak instability is found to be spanwise localized and develops near the regions of strong streaks. The instability of the steady streaks manifests as unsteady streaks in the wall normal plane, resembling unsteady streaks observed in freestream-turbulence induced transition (Durbin, 2017). Interestingly this is observed for both distributed roughness as well as isolated roughness. These results could have interesting implications for modelling bypass transition under the influence of roughness and/or free-stream turbulence.ReferencesJ.P.&nbsp;Bons (2010),&nbsp;A review of surface roughness effects in gas turbines, J. Turbomach. 132, pp.&nbsp;021004-1–021004-16.Yun, Y. I., Park, I. Y., and Song, S. J. (2005). “Performance Degradation due to Blade Surface Roughness in a Single Stage Axial Turbine”. Journal of Turbomachinery, 127(1), 02, pp. 137–143Langel CM, Chow R, Van Dam CP, Maniaci D, Ehrmann RS, White EB (2014) A computational approach to simulating the effects of realistic surface roughness on boundary layer transition. AIAA Paper 2014-0234Durbin PA (2017), Perspectives on the phenomenology and modelling of boundary layer transition. Flow, Turbulence and Combustion, 99(1), 1-23.

Wind-Tunnel Experiments on Boundary Layer Transition Caused by Coarse Distributed Roughness

Paper Proceedings: https://asmedigitalcollection.asme.org/GTINDIA/GTINDIA2021/volume/85536

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