This computer program can be used to predict the gas dynamic and chemical properties of underexpanded rocket plumes from sea level to the altitude above which the viscous continuum flow assumption, with distinct shocks, is no longer valid. The program computes the plume shock structure while simultaneously accounting for turbulent mixing, nonequilibrium chemistry, and gas/particle nonequilibrium effects. The program also has the ability to calculate plume properties in the subsonic region downstream from the Mach disc, downstream of the shock reflected from the triple point, and in the far field. The program can readily be used to determine plume optical and electrical properties, which are necessary data for calculating the infrared radiation pattern and radar cross section. This program was used to calculate deposition rate of various nitrogen oxides into the stratosphere caused by the Space Shuttle exhaust plumes.
The AIPP code is based on the Multitube code developed by Boyton (which incorporates the finite difference streamtube calculation technique) and has been expanded to treat particle/gas nonequilibrium, chemical kinetic, and turbulent mixing effects within exhaust plumes. For the gas flow upstream of the Mach disc, the governing elliptic Navier-Stokes equations are reduced to a hyperbolic system (including lateral pressure gradients) by neglecting diffusion of mass, momentum, and energy along streamlines as compared with that across streamlines. The conservation equations are then written in a streamline oriented coordinated system. Shocks are treated as thin bounding surfaces of the flow across which the Rankine-Hugoniot relations are applicable. In the subsonic region behind the Mach disc the inviscid flow governing equations are taken to be elliptic. The equations of flow must be solved within the supersonic and subsonic regions, while simultaneously maintaining the equality of pressure and flow direction along the dividing streamline. In meeting these boundary conditions several approximations are employed to reduce the amount of computer time and storage required. For the condensed phases present within the flow a continuum particle cloud assumption is made and field conservation equations for continuity, momentum, and energy can be written for the particles.
A finite difference formulation of the gas phase and particle cloud governing equations is utilized on a grid which lies along and perpendicular to the streamlines. The gas flow equations are solved via an explicit finite difference marching technique. The chemical production terms utilize an implicit finite difference formulation because an explicit formulation leads to an impractically small integration step for near-equilibrium chemistry. The particle equations have no wave or diffusive nature and are solved explicitly via a finite difference formulation. Initially, all flow properties, streamline positions, and angles must be known along an orthogonal surface. The streamlines are extended an incremental distance forming a streamtube. The properties at the upstream surface are used, along with the governing equations, to determine all the necessary properties at the downstream surface. Below the Mach disc a similar procedure using assumed streamlines is performed.
The computer program is written so that only minimal judgment by the user is required to operate it. The input data required are nozzle exit conditions along a surface orthogonal to the exit streamlines, uniform supersonic external flow conditions, and a suitable reaction mechanism and rate coefficients. The output contains the results of all calculations in a highly readable format. Care must be taken that the program is not used to predict plume characteristics at altitudes above which the assumption of continuum flow ahead of the Mach disc starts to break down. ( Aerochem Research Labs., Inc. for NASA Langley)
This program was released through COSMIC as program LAR-12203. The italicized text above is from the official COSMIC release.