Spacecraft often include instruments that are sensitive to contamination or may be composed of lightweight components with a limited ability to withstand high flow rates past component surfaces. For a typical expendable launch vehicle, prelaunch air-conditioning and purge requirements generally entail injection of air or gaseous nitrogen (GN₂) at high flow rates into the payload fairing (PLF) once the spacecraft has been encapsulated. A computational fluid dynamics (CFD) analysis was carried out to characterize the resulting flowfield (velocity, pressure distributions, and flow streamlines) and to investigate flow-induced effects and possible contamination sources and dispersions over spacecraft surfaces.

Overset (or embedded) grids are becoming increasingly popular in CFD applications for the prediction of flowfields about complex three-dimensional geometries. In the last decade, overlapping grids were mostly applied to problems of high-speed aerodynamics, and their applications to low-speed internal flow are relatively few. This report summarizes the application of overset grids for the analysis of PLF/spacecraft internal flow.

Overlapping grids were generated for two PLF/spacecraft configurations. Various grid topologies were utilized for the component grids. Collar grids were considered for defining the intersection regions. The collar grids provide the communication between the intersecting grids, as well as the necessary resolution for viscous flow computation.

The steady-state flowfield is obtained with the aid of a three-dimensional Navier-Stokes code, OVERFLOW, developed by NASA. The code has the capability to handle chimera overlapped grids. Turbulent flow is modeled with the aid of the Spalart-Allmaras one-equation turbulence model governing turbulent kinetic energy. Initially, fluid is set at rest in the entire system; that is, all the velocity components are set to zero. Appropriate boundary conditions have been imposed, including the solid wall and inflow and outflow boundaries. Inflow velocity profiles (and mass flow rate) are specified at the pipe inlet. Because of the subsonic nature of the flow, the static pressure at the outflow boundary needs adjustment for providing the necessary mass flow rate. Convergence is achieved using time-stepping scheme, multigrid cycling, and low Mach number preconditioning. Steady-state solution for this grid system was obtained for pipe Reynolds number of 2.4E5 (based on pipe diameter) and a Mach number of 0.04.

An examination of the steady-state flowfield indicated the complex three-dimensional flow is characterized by areas of vortex flow, flow separation, high degrees of swirl, and reverse flow. Streamline traces from various sources (typical of access and vent openings on the PLF and launch vehicle) suggest the possibility of particle dispersion both upstream and downstream of the sources.
Figure 1 displays the streamline traces emanating from the air-conditioning (AC) pipe/fairing junction. The velocity magnitudes within the fairing are exhibited in figure 2. A typical source streamline from an access opening is displayed in figure 3.