Numerical simulation of the eff[|#11#|]ect of a low bypass cooling stream on supersonic jet noise

Jet flow and noise simulations are performed for realistic tactical aircraft engine nozzles with a cooling fan-stream. A hybrid approach combining time-accurate CFD simulations and the acoustic analogy is used. A multiblock structured mesh topology is used to represent the complex nozzle geometry, including the faceted inner contours representing flaps and seals, fan-core splitter and the finite nozzle thickness. A reasonable agreement of the predicted noise spectra with the acoustic measurement is found to reach to St ≈ 2.0. The results show how the bypass cooling flow introduces a low-temperature, low-speed buffer between the heated high-speed core jet and the stationary ambient. This initial buffer reduces the velocity gradient in the jet turbulent mixing layer near the nozzle exit. In turn, this results in a reduction in medium shear layer, which results in a reduction of the turbulent mixing noise when the same core temperature is retained. The difference diminishes after full mixing appears further downstream. As a further analysis tool, the Proper Orthogonal Decomposition is applied to the beamformed far-field acoustic pressure and the near-field pressure fluctuations obtained from the unsteady CFD computations. The dominant modes show a wave packet form. When inserted into the pressure wave equation, the wave packets reproduce well the Mach wave radiation in terms of its phase, wavelength and radiation direction.