Boundary layer transition induced by distributed roughness array

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Abstract

The effects of a finite, spanwise-periodic array of cylindrical roughness elements on boundary layer transition over a NACA 0012 airfoil are investigated at a chord-based Reynolds number of 1.44×105 by using hotwire anemometry and infrared thermography. Both the number and the spanwise spacing of roughness elements in the array are varied in order to study their effect on the wake flow topology. Spanwise interaction between the roughness elements has an effect on the connection and the merging of neighbouring low-speed regions, which results in the formation of merged low-speed blobs (MLSs) that modify the spatial distribution and the amplitudes of the velocity streaks. When the spanwise distance between adjacent roughness elements equals 1.5 times the cylinder diameter, the transition location moves rapidly upstream. In this case, the two neighbouring low-speed regions overlap with each other in the near wake of the roughness, leading to the maximum growth in the velocity streak amplitude and the velocity fluctuations. The number of roughness elements affects the total number of MLSs within the boundary layer. For a single MLS behind a pair of cylinders, the Kelvin-Helmholtz instability dominates the growth of velocity fluctuations around the three-dimensional shear layers. When three cylinders are placed in the array, two MLSs appear in the near wake, which coalesce in to one low-speed blob downstream before the onset of transition, revealing the importance of Kelvin-Helmholtz instability.

Original languageEnglish
Title of host publicationProceedings of the 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019
Subtitle of host publication30/07/19 → 2/08/19 Southampton, United Kingdom
Number of pages6
Publication statusPublished - 2019
Event11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019 - Southampton, United Kingdom
Duration: 30 Jul 20192 Aug 2019

Conference

Conference11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019
CountryUnited Kingdom
CitySouthampton
Period30/07/192/08/19

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  • Cite this

    Ye, Q., Avallone, F., Ragni, D., Choudhari, M., & Casalino, D. (2019). Boundary layer transition induced by distributed roughness array. In Proceedings of the 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019: 30/07/19 → 2/08/19 Southampton, United Kingdom