Pressure Fluctuations in a High-Reynolds-Number Turbulent Boundary Layer over Rough Surfaces of Different Element Spacing

摘要

The pressure fluctuations under a high Reynolds Number rough-wall turbulent boundary layer have been studied. Hemispherical roughness elements of 3-mm radius and varying sparseness ratio, λ = [0.052, 0.13, 0.33], were considered. Pinhole microphone measurements of the wall pressure fluctuations and hotwire measurements of the velocity field were made. It was found that the low frequency pressure levels of the raw spectrum increase with increasing sparseness ratio, λ, up to approximately λ = 0.13. After this point increases in sparseness ratio result in lower pressure levels at all frequencies. Well-known pressure spectrum scalings in the low- and mid- frequency regions exhibit poor collapse for the high λ cases, especially λ ＞ 0.13. The high-frequency scaling of Meyers et al. (2015) appears to be valid for all A tested. The effect of microphone location relative to roughness element was found to be an important consideration in testing. Lastly an analysis of evanescent pressure decay from the roughness tops to the measurement location at the substrate was conducted. This analysis revealed that evanescent decay likely plays a significant role in shaping the measured pressure spectrum, particularly at mid and high frequencies.