From http://www.hq.nasa.gov/hpcc/insights/vol5/overflow.htm:
The overset grid flow solver OVERFLOW was developed as part of a collaborative effort between NASA Johnson Space Center in Houston, Texas and NASA Ames Research Center (ARC) in Moffett Field, Calif. The driving force behind this work was the need for evaluating the flow about the Space Shuttle launch vehicle. Developed in the early 1990s by NASA's Pieter Buning, now at Langley Research Center, Dennis Jespersen at ARC and others, the code is an outgrowth of earlier codes F3D and ARC3D, and a result of ARC's long history of flow-solver development.
Scientists use OVERFLOW to better understand the aerodynamic forces on a vehicle by evaluating the flowfield surrounding the vehicle. While wind tunnel testing provides limited data at many flow conditions, computational fluid dynamics (CFD) simulations provide detailed information about selected conditions. CFD also provides a much-needed distribution of forces on the vehicle, aiding in structural design.
Most advanced CFD systems are based on the Navier-Stokes equations of motion for fluids. These partial differential equations express conservation of mass, momentum and energy. CFD codes usually incorporate some simplifications in these equations to reduce the computational burden to an acceptable level. Once the equations to solve a problem have been selected, they are converted to finite difference approximations in which the solution is determined on a grid of points spaced in some regular pattern representing the flow domain and boundary points.
OVERFLOW is a compressible 3-D flow solver that solves the time-dependent, Reynolds-averaged, Navier-Stokes equations using multiple overset structured grids. It provides an alternative, cost-effective means of simulating real flows. “A body-conforming mesh represents each object in the flow field or each aircraft component,” explains Buning. “No limitations are placed on mesh interaction except that the entire volume of the flow field must be filled by grid points.”
See also www.psc.edu/~nystrom/jnnie/appb/overflow/overflow.ps [PostScript]
Source: http://ad-www.larc.nasa.gov/~buning/codes.html#overflow
OVERFLOW Flow Solver
OVERFLOW is a Navier-Stokes flow solver for structured grids. It can use single block grids or Chimera overset (structured) grid systems. Right-hand side options include central differencing with Jameson 4/2 dissipation and Roe upwinding. Left-hand side options include the Pulliam-Chaussee diagonalized (scalar pentadiagonal) scheme and the LU-SGS scheme. Low-Mach number preconditioning is available for accuracy in computing low-speed steady-state flows.
First-order implicit time advance is used. A time-accurate mode is available, or local timestep scaling can be selected for acceleration to steady state. Grid sequencing and multigrid are also implemented for convergence acceleration.
Performance on the NAS Cray C-90 is 440 MFLOPS per processor; the code runs in a multitask mode with an average concurrency of 6 CPUs for three-dimensional problems. CPU time required is about 8 microseconds/point/iteration.
Memory required is 35 words per grid point of the largest grid block.
(see http://halfdome.arc.nasa.gov/cfd/CFD4/New_Page/Overflow-D2.htm)
OVERFLOW-D is a general purpose Navier-Stokes solver for problems that may involve relative motion between configuration components. The code uses overset structured grids to accommodate arbitrarily complex geometries while, on a component-wise basis, retain the computational advantages inherent to structured data. OVERFLOW-D has recently been released for BETA-testing and comes with complete documentation, a set of examples, and an easy-to-use overset grid generation package called OVERGRID.
OVERFLOW-D is based on the well known NASA OVERFLOW code, but has been significantly enhanced to accommodate moving body applications, facilitate accuracy control via solution adaption, and run efficiently on scalable computers. Makefiles to compile the software on IBM-SP, Origin 2000, Sun 10000, and Cray T3E platforms are provided with the code. OVERFLOW-D uses MPI to enable inter-processor communication.
OVERFLOW-D employs a powerful discretization paradigm that partitions the problem domain into near-body and off-body regions. The near-body region includes the surface geometry of all configuration parts being considered and the volume of space that extends a short distance above the respective surfaces. The near-body portion of the domain is discretized in a classical "Chimera" fashion. Near-body grids are generated in a pre-process using standard grid generation packages (OVERGRID is especially well-suited to this task).
The off-body portion of the domain encompasses the near-body domain and extends to the far-field boundaries of the problem. OVERFLOW-D automatically discretizes the off-body domain with uniform Cartesian grid components (structured) of varying levels of resolution capacity. By default, off-body resolution capacity is set based on proximity to near-body components. Users can run simulations on the near-body and default off-body grid systems, or can enable solution adaption. With adaption enabled, OVERFLOW-D allocates off-body grid resolution based on proximity to near-body components and results of estimates of solution error. Of course, error estimation is carried out automatically by OVERFLOW-D. In all cases, OVERFLOW-D organizes grid components into groups of equal size. Then, on parallel computers, groups are assigned to processors. Scalability is realized in a group-wise fashion.
OVERFLOW-D can be used to simulate moving body applications that involve arbitrary rigid-body motion, prescribed motion, or maneuvers. OVERFLOW-D has a general 6-degrees-of-freedom model (6-DOF) that allows body motion to respond to aerodynamic loads as well as applied forces and moments associated with separation mechanisms. All OVERFLOW-D functionality is tightly coupled, including 6-DOF, domain connectivity, solution adaption, etc., in order to maximize computational efficiency for such applications.
OVERFLOW-D has been tested on a range of applications that have practical importance, several of which are described in AIAA Paper 99-3302-CP (see proceedings of the 14th AIAA CFD Conference, pp. 469-483). Details about the overset grid generation package that comes with OVERFLOW-D (viz., OVERGRID).
Algorithm basics of OVERFLOW-D