1 4 January 2005 DLR, Institute of Propulsion Technology Broadband Shock Noise in Flight Ulf Michel German Aerospace Center Institute of Propulsion Technology Berlin International Symposium on Recent Advances in Aeroacoustics and Active Flow-Noise Control Goa, India, 4-6 January 2005 2 4 January 2005 DLR, Institute of Propulsion Technology^^ Broadband shock noise was one of Shôn’s work fields. Shôn has made important contributions to the acoustic analogy. Some recent papers were very critical with Lighthill’s acoustic analogy. Broadband shock noise has gained renewed interest (fighter aircraft noise emission and cabin noise in cruise). Content of this presentation: It will be shown that broadband shock noise can be described with the acoustic analogy. A theory for broadband shock noise based on very simple turbulence models is presented. Results will be compared with new experimental data on broadband shock noise. Broadband shock noise described with the acoustic analogy 3 4 January 2005 DLR, Institute of Propulsion Technology^^ Recent experimental data for broadband shock noise in flight Norum et al. 2004 Results for M f = 0.44 (---)/0.51(-) M j = 1.36 M e = 1.71 OASPL in forward quadrant higher than in rear quadrant. Avoidable for M j = M e 4 4 January 2005 DLR, Institute of Propulsion Technology^^ Lighthill equation for flight situation U f U i x 3 x 2 x 1 Turbulent flow field of a jet must be described in nozzle-fixed coordinate system (Michalke 1970 - 1983). Turbulent flow field stationary random in this system. Nozzle-fixed coordinate system first introduced by Ribner (1959), who concluded that Doppler amplification is identical with earlier results that used a coordinate system attached to the moving eddy. Ribner did not realize that the choice of the coordinate system has an impact on the frequencies observed in the far field. 5 4 January 2005 DLR, Institute of Propulsion Technology^^ Frequencies in far field of a jet Result assuming coordinate system fixed on the “moving eddy”: Coordinate system fixed to the moving eddy yields higher frequencies in the rear arc of a jet whether static or in forward motion. There is not a single experimental verification of this result. Experimental evidence is: Frequency spectrum of a subsonic static jet remains almost constant for all angles except in the rear arc, where the frequencies go down due to refraction effects. Frequency spectrum in flight is Doppler-shifted according to the flight Mach number of the aircraft. Frequencies increase in forward arc. 6 4 January 2005 DLR, Institute of Propulsion Technology^^ U f U i x 3 x 2 x 1 2 2 2 2 0 1 i i i p U p q a t x x ⎛ ⎞ ∂ ∂ ∂ + − = . ⎜ ⎟ ∂ ∂ ∂ ⎝ ⎠ 2 2 2 0 i j ij i i j i p q uu U x x t x a ρ τ ρ ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ ⎛ ⎞⎛ ⎞ ∂ ∂ ∂ = − − + − , ⎜ ⎟⎜ ⎟ ∂∂ ∂ ∂ ⎝ ⎠⎝ ⎠ Lighthill equation for flight situation Convective Lighthill equation with uniform flow velocity in the ambience (Michalke & Michel 1979). u i = rel. velocity