Experiments have been performed on a series of high-Reynolds-number flat-plate turbulent boundary layers formed over rough and smooth walls. The boundary layers were fully rough, yet the elements remained a very small fraction $({<}1.4\,\%)$ of the boundary-layer thickness, ensuring conditions free of transitional effects. The wall-pressure spectrum and its scaling were studied in detail. One of the major findings is that the rough-wall turbulent pressure spectrum at vehicle relevant conditions is comprised of three scaling regions. These include a newly discovered high-frequency region where the pressure spectrum has a viscous scaling controlled by the friction velocity, adjusted to exclude the pressure drag on the roughness elements.

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This paper presents the results of an experimental investigation of the flow-induced sound produced by a smooth, wall-mounted finite length airfoil with flat ended tip and a tripped turbulent boundary layer. Acoustic measurements have been taken in the Stability Wind Tunnel at Virginia Tech with a microphone array at a range of Reynolds numbers (ReC = 7.9 × 10 5 − 1.6 × 10 6 , based on chord), angles of attack (α = 0 − 12 • ) and for a variety of airfoil aspect ratios (airfoil length to chord ratio of AR = L/C = 1 − 3). Spectral data show the dominant noise sources are airfoil trailing edge noise and tip vortex formation noise. Acoustic data are also used to evaluate semi-empirical prediction of wall-mounted finite airfoil trailing edge and tip noise with the so-called Brooks, Pope and Marcolini (BPM) model. The prediction method employs the BPM trailing edge noise model modified to incorporate span-wise variations in flow properties in combination with the BPM flat tip noise model. Three-dimensional trailing edge noise predictions agree well with measured spectra at a Strouhal number of StC < 18, based on airfoil chord. The BPM tip noise model under-predicts the peak level and frequency of tip noise contributions at StC > 18. A new empirical model of flat tip airfoil noise is presented that provides accurate estimation of the wall-mounted finite airfoil dominant tip noise contribution to within 1.7 dB.

Accurate wind tunnel measurements of the lift and drag of airfoil sections are critical for the design and performance evaluation of wind turbine blades. As blades continue to increase in size, the demand for highly accurate wind tunnel results at progressively larger Reynolds numbers has also increased. Performing these wind tunnel measurements requires precise experimental control, and three challenges for these measurements are model surface quality, pressure tap effects, and model deflections under aerodynamic loading. These challenges were systematically studied in the Virginia Tech Stability Wind Tunnel using a DU96-W-180 airfoil geometry at a chord Reynolds number (Rec) of 3.0 × 106. Naphthalene sublimation showed turbulent wedges caused by surface imperfections; removing these imperfections increased the lift curve slope by 3%. Pressure tap diameter effects were investigated by placing taps of varying size at the same chord location on the airfoil. These measurements showed a steady pressure bias correlated to tap diameter when making measurements in turbulent boundary layers, and naphthalene visualizations showed a turbulent wedge created by pressure taps at the leading edge. Finally, laser distance sensors were used to measure model deflections/rotations under aerodynamic loading, improving upon the traditional angle of attack measurement. Addressing these challenges has improved the accuracy of lift measurements in the Stability Wind Tunnel and emphasized the need for precise experimental controls when performing these types of wind tunnel measurements.

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