An Experimental Investigation of Wing Tip Vortex Formation Noise

In this study, the far-field acoustics and near-field flow characterizing vortex formation at a wing tip has been investigated experimentally. Measurements have been taken on a rectangular planform NACA 0012 wing with a flat tip and an aspect ratio (span-to-chord ratio) of dd = t/d = 2 at a Reynolds number of dd = 1.5 × 104, and a geometric angle of attack of d = 5◦ in the UNSW anechoic wind tunnel. Acoustic beamforming results obtained with a planar 64-microphone array show that tip noise is a prominent airfoil noise source at mid to high frequencies (3.7 to 11.5 kHz), corresponding to chord-based Strouhal numbers of dtt = t d/d∞ = 8 − 25. Stereoscopic Particle Image Velocimetry measurements reveal information about the size and location of the tip vortex in the near wake. In the vortex core, the axial velocity has a wake-like profile with a velocity deficit of 24%. Snapshot Proper Orthogonal Decomposition is used to extract the dominant coherent structures, which reveals that the two most energetic modes are found to consist of a coherent structure pair within the vortex core region. The unsteady surface pressure field over the wing tip is measured using the remote microphone technique. High magnitude pressure fluctuations are observed near the trailing edge on the tip surface and on the airfoil suction side that may be associated with the tip noise source