Spanwise structure of wall pressure on a cylinder in axial flow

Abstract

The spanwise structure of wall pressure fluctuations was measured in an axtisymmetric turbulent boundary layer on a cylinder parallel to the mean flow at a momentum thickness Reynolds number of 2530 and a boundary layer thickness to cylinder radius ratio of 4.81. The measurements were made using miniature hearing aid type condenser microphones with spanwise separations of 0?, 10?, 20?, 30?, 60?, and 90?. An improved wall pressure power spectrum was obtained at low frequencies by utilizing a two-point subtraction method to remove low frequency acoustic background noise of the wind tunnel. The spanwise correlations indicate that the spanwise coherent length of the wall pressure is 30? (78v/u? or 0.11?). The spanwise coherence is weak and concentrated in a frequency band that is substantially lower than the most energetic frequency band of the wall pressure spectrum. A mode number-frequency decomposition of the wall pressure spectrum indicates that the greatest quantity of energy is in the circumferential modes nearest zero. Modes -4 to 4 contain most of the wall pressure energy. Conditional sampling by pressure peak and VITA detection schemes (where VITA was applied to wall pressure to detect strong pressure gradient events) indicate that the spanwise extent of the high pressure peaks and high wall pressure gradients is 60? (156v/u? or 0.22?.