Modelling the unsteady flow and shock-associated noise from dual-stream turbulent jets

Sustained demand for more fuel efficient, environmentally friendly, and quieter aircraft power plants is promoting the development of higher bypass engines for large transport aircraft. These turbofan engines feature a fan of larger diameter, providing a larger fraction of the thrust by expansion through the secondary (annular) nozzle. This fixed-geometry nozzle is typically run choked at cruise conditions, generating an under-expanded co-axial jet with a significant broadband shock-associated noise signature. The three-dimensional shock-shear layer flow dynamics of a dual stream jet was modelled by Detached Eddy Simulations, using a simplified staggered coaxial round dual-nozzle geometry without a central plug. Predictions were obtained using an in-house up to third order space accurate finite-volume scheme based on the Roe approximate Riemann solver, using a tuneable Sweby flux limiter for capturing the unsteady shocks. Flow conditions were taken as representative of secondary nozzle turbofan engine operations in cruise. The primary nozzle was modelled subsonic and cold, to allow comparison with static jet measurements, for validation purposes. Large Eddy Simulations obtained independently using a high-order shock-tolerant code from CERFACS gave further support to the validation and to the flow analysis. The dual-nozzle jet was found to share broad similarities with the more widely studied under-expanded single jet flow, with the interaction between the unsteady outer shear layer and the shock train generating broadband shock-associated noise. The axial stagger between the primary and secondary nozzles was found to have a strong influence on the first shock cell and on the flow downstream of it. Predictions of the radiating pressure field from the dual-stream jet show that the broad-band shock-associated noise component is satisfactorily predicted by Detached Eddy Simulations, by virtue of this contribution occupying a narrower Strouhal number range compared to broad-band turbulence associated nose, for a range of observers located along the jet sideline.