This paper investigates the noise emissions from thin airfoils experimentally. An attempt has been made to study the effect of line distribution of 3 mm diameter holes adjacent to the trailing edge on noise emission characteristics. Thin airfoils having chord length and span of 150 and 240 mm, respectively, are considered for the investigation. The airfoil is subjected to the flow Reynolds number (defined based on the chord length of airfoil) in the range of 2.0 × 105 to 5.0 × 105, and angles of attack of 0°, 2°, and 4°. The spectral results indicate that the modified airfoil with holes is effective in minimizing the lower frequency noise (<3.5 kHz) compared to that of the reference airfoil. The reduction in the low-frequency sound pressure levels for the modified airfoil is found up to 5 dB. The modified airfoil with holes is ineffective in reducing the higher frequency noise components, and in fact, generated a broadband frequency noise at higher Reynolds number. From the spectral studies, a critical Strouhal number is estimated to be around 0.15 that defines the limiting value for the effectiveness of the modified airfoil. The overall sound pressure level studies depicted that the modified airfoil is effective at higher angles of attack with the reduction of around 1–2 dB. The noise generated from the thin airfoils is found to have the directivity towards the upstream.

中文翻译：

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后缘附近孔线分布的薄翼型气动噪声特性

本文通过实验研究了薄翼型的噪声排放。已尝试研究与后缘相邻的 3 毫米直径孔的线分布对噪声发射特性的影响。弦长和跨度分别为 150 和 240 毫米的薄翼型被考虑用于研究。翼型受流雷诺数（根据翼型弦长定义）在 2.0 × 105 到 5.0 × 105 范围内，攻角为 0°、2° 和 4°。光谱结果表明，与参考翼型相比，带孔的改进翼型可有效降低低频噪声 (<3.5 kHz)。发现改进翼型的低频声压级降低了 5 dB。带孔的改进翼型在减少高频噪声分量方面无效，实际上，在较高雷诺数下产生了宽带频率噪声。根据光谱研究，临界 Strouhal 数估计约为 0.15，它定义了改进翼型有效性的极限值。总体声压级研究表明，改进后的翼型在更高的迎角下有效，降低了约 1-2 dB。发现由薄翼型产生的噪声具有朝向上游的方向性。15 定义了改进翼型有效性的极限值。总体声压级研究表明，改进后的翼型在更高的迎角下有效，降低了约 1-2 dB。发现由薄翼型产生的噪声具有朝向上游的方向性。15 定义了改进翼型有效性的极限值。总体声压级研究表明，改进后的翼型在更高的迎角下有效，降低了约 1-2 dB。发现由薄翼型产生的噪声具有朝向上游的方向性。