Surface roughness elements are often used to force laminar to turbulent transition in aerodynamic and aeroacoustic wind-tunnel experiments. The statistical features and spectral content of the pressure fluctuations in the resulting turbulent boundary layer at the trailing edge can affect far-field noise. To elucidate this dependence, boundary-layer transition induced by randomly distributed roughness elements and a zigzag strip of the same height over a NACA 0012 airfoil is investigated experimentally. The effects of roughness geometry on the near-field flow topology, transition location, and far-field noise are addressed in the common experimental setting for the first time. For a fixed roughness height, distributed roughness elements are less effective in forcing transition than the zigzag strip at low freestream velocity (u∞<20  m/s). As u∞ increases, the transition front for the distributed roughness elements moves closer to the roughness location, reaching the same or even further upstream locations compared to the transition onset in the presence of the zigzag strip. The far-field noise depends on the transition location. For u∞≤20  m/s, a higher noise level is measured for the distributed roughness elements with respect to the zigzag strip. In contrast, for u∞>20  m/s, the earlier onset of transition with the distributed surface roughness leads to a lower noise level than that with the zigzag strip. The data confirm that an adequate characterization of the boundary-layer transition is necessary when measuring the far-field noise during wind-tunnel experiments.@en

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.

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Surface roughness elements are often used to force laminar to turbulent transition during aerodynamic and aeroacoustic experiments in wind tunnels. The statistical features and frequency content of the forced turbulent boundary layer can influence the far-field trailing edge noise. To study this dependence, boundary layer transition induced by randomly distributed roughness elements and a zigzag strip of the same height over a NACA 0012 airfoil is investigated experimentally. The effects of roughness type on the near-field flow topology, transition location, and far-field noise are addressed. At a fixed roughness height, the distributed roughness elements are less effective in forcing transition than the zigzag trip at low freestream velocity (u∞ < 20 m/s). As u∞ increases, the transition front for the distributed roughness elements moves close to the roughness location, reaching the same or even further upstream location compared with the zigzag strip. The far-field noise intensity depends on the transition location. For u∞ < 20 m/s, a higher noise level is measured for the distributed roughness with respect to the zigzag strip. Whereas, for u∞ <20m/s, the earlier onset of transition leads to a lower noise level for the distributed roughness elements with respect to the zigzag strip. The data confirms that an adequate characterization of the state of the boundary layer is necessary when measuring far-field noise in wind tunnel experiments.@en