Computational analysis of free jets from different nozzle geometries

A computational study on the effects of nozzle exit geometry and boundary layer swirl on subsonic and underexpanded supersonic jet evolution and comparison with experimental data is presented in this report. The purpose of this study was to answer the following questions: (1) Can reasonable subsonic and underexpanded supersonic jet evolution information be calculated by the three-dimensional Proteus computer code? (2) What are the effects of nozzle exit geometry and inducing boundary layer swirl on mixing characteristics of subsonic and supersonic jets? (3) Does the shock adaptive grid system improve the results of supersonic solution? In this study, subsonic and supersonic jets emerging from the rectangular nozzle and notched nozzle with and without boundary layer swirl vanes are investigated for their mixing characteristics. Subsonic jet has nozzle exit Mach number of 0.06; supersonic jet has nozzle exit Mach number of 1.526. The nozzle aspect ratios for the subsonic and supersonic nozzles were 2.8 and 5.0 respectively. The computational analysis is conducted by running the three-dimensional Proteus code developed at NASA Lewis Research Center. A structured grid system was used for subsonic cases and an improved grid system produced by shock adaptive grid method was used for the supersonic cases. The code solves the three-dimensional, Reynolds-averaged, unsteady, compressible Navier-Stokes equations in strong conservation law form. For the supersonic study, a structured grid is applied first and based on the results of calculations using the structured grid, a shock adaptive grid is produced and the code is run again with the shock adaptive grid. In this study, shock adaptive grid system based on the nozzle centerline shock locations is used to compute the supersonic jet characteristics. The nozzle centerline distributions of axial velocity, static pressure, and mass flow rate are plotted and compared with experimental data along with axial velocity contour maps at the center planes. For the analysis of subsonic jet, the contour plots of axial velocity at several locations downstream of the nozzle exit plane are presented. Static pressure contour maps at the center planes are also included for supersonic jet analysis. It is concluded that the three-dimensional Proteus code can be used to analyze subsonic and supersonic free jet mixing characteristics. Flapping oscillation, spanwise oscillation, and pumping motion are observed in the supersonic study with frequency of about 7500 Hz matched with experimental data in the case of rectangular nozzle without boundary layer swirl. It is also shown that the change of nozzle exit geometries and induction of boundary layer swirl (S = 0.0, 0.41, and 1.0) strongly affects favorably the jet mixing characteristics. In addition, the computational calculation using shock adaptive grid technique improved the simulation of the underexpanded supersonic jet.