Numerical simulation of steady and pulsed flows through thrust augmenting ejectors

A thrust augmenting ejector is a shroud that surrounds a jet nozzle which entrains secondary air from the surroundings with the aid of the higher velocity primary jet flow from the nozzle, resulting in increased thrust, compared to that due to the nozzle jet alone. It has application to fixed-wing airplanes that have Vertical/Short Take off and Landing requirements. The thrust augmentation due to the ejector is defined as the ratio of the total thrust at the ejector outlet to the thrust of the jet nozzle with a corresponding steady flow without the ejector. In recent years it has been shown experimentally that the thrust augmentation due to an ejector can be significantly increased by pulsating the primary jet. The goal of this dissertation is to numerically simulate the steady and pulsed flow fields of axisymmetric and 3D ejector configurations and compare the results with the experimental data. The numerical solver employed in the computations is FLUENT. For this study three ejector area ratios 7.6, 11.0 and 12.0 were considered. The compressible Unsteady Reynolds-Averaged Navier-Stokes solver in conjunction with the one-equation Spalart-Allmaras turbulence model was employed in this study. Three primary jet Mach numbers of 0.3, 0.65 and 0.80 were considered. The computations were performed for both steady and pulsed (frequencies 70 Hz, 125 Hz and 220 Hz) flows of the primary jet. The free jet (primary jet without the ejector) flow field was obtained for both steady and pulsed cases and was employed to show similarities with the experimental free jet model. Using the available experimental data, the boundary conditions for the computational model were defined and the flow fields for various Mach numbers and area ratios were computed. For both the steady and the pulsed primary jet flow cases for various Mach numbers and pulsation frequencies for both an axisymmetric and a 3D model of the ejector shroud with different area ratios, excellent agreement with the experimental data was obtained for thrust augmentation. The flow fields for each of the cases were generated and were compared to the available experimental Particle Image Velocimetry images, showing very similar contours.