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 ($u_{\infty }<20\mathrm{m}/\mathrm{s}$). As $u_{\infty }$ 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 $u_{\infty }\le 20\mathrm{m}/\mathrm{s}$, a higher noise level is measured for the distributed roughness elements with respect to the zigzag strip. In contrast, for $u_{\infty }>20\mathrm{m}/\mathrm{s}$, 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.

在空气动力学和空气声学风洞实验中，通常使用表面粗糙度元素来迫使层流向湍流过渡。在后缘处形成的湍流边界层中压力波动的统计特征和频谱含量会影响远场噪声。为了阐明这种依赖性，实验研究了由随机分布的粗糙度元素和NACA 0012机翼上相同高度的之字形带引起的边界层过渡。首次在通用实验设置中首次解决了粗糙度几何形状对近场流动拓扑，过渡位置和远场噪声的影响。对于固定的粗糙度高度，在低自由流速度下，分布的粗糙度元素比之字形带在强制过渡方面效果不佳（${ü}_{\infty }<20\mathrm{米}/\mathrm{s}$）。作为${ü}_{\infty }$当存在锯齿形条带时，与过渡开始相比，分布粗糙度元件的过渡前沿增加，与粗糙位置相比，移到更接近粗糙度位置，到达相同或什至更上游的位置。远场噪声取决于过渡位置。为了${ü}_{\infty }\le 20\mathrm{米}/\mathrm{s}$，相对于之字形带，分布的粗糙度元素测得较高的噪声水平。相反，对于${ü}_{\infty }>20\mathrm{米}/\mathrm{s}$，与分布的锯齿形带相比，采用分布的表面粗糙度的较早转变会导致较低的噪声水平。数据证实，在风洞实验期间测量远场噪声时，必须对边界层过渡进行适当的表征。