Parametric study of the generation of shocks in near-critical turbofan nozzles

This research is part of a larger project to better understand the mechanisms of noise generation in commercial turbofan engines and develop tools to aid in quiet design and quick computational noise prediction. The purpose of this investigation is to identify the factors that contribute to internal shocks in the nozzles of engines operated at high-sub-critical pressure ratios and determine whether these factors can be controlled through design of the nozzle geometry. A number of parametrically generated nozzle geometries were generated and tested using the computational fluid solver Wind-US, and the peak Mach numbers in the jet stream and the size and geometry of the shock were extracted from the computational solutions. Axisymmetric and three-dimensional calculations found that regions of high curvature near the nozzle exit were the strongest factor to contribute to the generation of internal shocks. The strength of these shocks could be controlled by altering the nozzle design to move high-curvature regions upstream without changing the total nozzle length. Shocks could also be weakened and even eliminated by stretching the nozzle axially, reducing the curvature throughout. If a given nozzle profile is extended axially in this manner, there is a monotonic and nearly linear relationship between the length of the nozzle and the thrust it can produce before generating an internal shock. Finally, small corners or rough areas on a nozzle wall in the critical region near the nozzle exit can exacerbate the turning effects that cause internal shocks to form