An investigation of a plug-nozzle for supersonic aircraft concepts

A plug-nozzle with a cylindrical cowl is studied for thrust and noise characteristics. The geometry is adopted following commercial supersonic aircraft concepts considered in an ongoing NASA program. For the given cowl, plugs of different shapes are studied including straight conic shapes, shapes developed by a numerical optimization study for best thrust performance, as well as porous plugs. Experimental data on noise and flow field are discussed with an eye for minimum noise at low nozzle pressure ratios (NPR≈2) representing landing and takeoff (LTO) conditions. Up to NPR≈5 could be covered in the experiments. Results show that a plug designed for optimum thrust at cruise (NPR = 5.9) is noisier than a plug designed for optimum thrust at LTO, over the NPR range covered. Limited thrust data with accompanying numerical simulation results are presented. Overall trend in the simulation results agree with that of the experimental results. The experiments also show that the plugs can generate transonic tones and excess broadband noise (EBBN) at low NPR, apart from the well-known screech tones and broadband shock associated noise (BBSN) at high NPR. A long porous plug is found to effectively suppress all these aberrant noise components while suffering less than 1% loss in thrust coefficient.