Decay of velocity and temperature fluctuations in grid turbulence

The characteristics of turbulence generated in the tip region of a blunt-tipped airfoil were studied using surface pressure measurements. Pressure transducers were located at five chordwise positions on the upper tip-edge, the lower tip-edge, and on the flat tip. The model was an NACA 0012, 0.76-m chord, aspect ratio 2.7, semispan wing section. Tests were performed at flow speeds of 75, 55, and 35 m/s and angles of attack of 6,12, and 16 deg. Reynolds numbers based on the wing chord were 1.9, 3.0, and 4.1 × 10⁶. Pressure fluctuations measured near the primary tip-vortex on the upper, low pressure side of the wing tip were uncorrelated with those on the blunt tip. Fluctuations on the high pressure side of the wing were strongly correlated with those on the flat tip, but 10-20 dB less intense. Spectra measured on the flat tip displayed pronounced peaks at dimensionless frequencies ft/U₀ = 0.8 to 1.3. Cross correlations between some of the flat-tip pressures displayed two echolike groupings. A model is proposed that explains these correlations.