Two-point correlations of sound pressure in the far-field of a jet: experiment

Correlation of sound pressure between two microphones yield substantially more information than single microphone data; hence, they provide a useful diagnostics tool for jet noise. In the present experiment, the two microphones were located at (R, θ, φ) and (R, θ, φ′), respectively, in spherical polar coordinates. The jet was directed along the polar axis (θ = 0) with the nozzle at the origin. Two sets of space‐time correlation were carried out (1) φ′ = 0; 0 ⩽ θ ⩽ 180°; θ′ = 0, 30°, 45°, 60°, 90°, 120°, 135°; (2) φ′ = 0; 0 ⩽ θ ⩽ 180°; θ′ = 0, 30°, 45°, 60°, 90°, 120°, 135°. Jet diameters were 1, 2, and 3 in. with a range of nozzle velocities from 500 to 1000 ft/sec, and R/D was 166 for the 1‐in. jet. Nineteen carefully phase‐matched microphones were employed. Polar plots of these correlations show a definite cusp at the origin (φ = φ′), unlike polar plots of p2. among other interesting features.