Acoustic Characteristics of a Circulation Control Airfoil

The sound generated by a 2-D, 20% thick elliptical circulation control airfoil with single slot blowing is experimentally investigated in the University of Florida Aeroacoustic Flow Facility. Experiments are conducted at a variety of freestream and slot jet velocities for three different slot heights h. The airfoil’s lift characteristics compare favorably with data from a previous research study in a conventional closed-wall wind tunnel. A large aperture microphone array reveals low noise contamination in the UFAFF test section. Data from farfield microphones indicate both tonal and broadband noise sources. Vortex shedding from the rounded trailing edge occurs when jet momentum levels are insufficient to prevent flow separation. High frequency tonal noise is also identified due to vortex shedding, where fD/Ujet = 0.21, from the D = 0.28 mm thick slot lip. For large jet velocities, high frequency broadband slot jet noise is identified as the dominant noise source for fh/Ujet > 0.1. Jet velocity amplitude and frequency non-dimensionalization collapse the high-frequency portion of the spectra reasonably well when freestream velocity is low or zero. For curvature and slot noise generated by the external boundary layer flow, local mean velocities at the trailing edge and above the slot are not more effective than freestream and jet velocities when non- dimensionalizing frequency. Finally, freestream amplitude and Strouhal number scaling show a remarkable collapse of spectra for a variety of freestream velocities, jet velocities, and slot heights for a constant momentum coefficient.