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Short Duration, High Altitude, Mixed Continuum/Non-Continuum Flowfield


OBJECTIVE: To develop innovative models for the basic fluid dynamic processes which describe short duration, high altitude events with mixed continuum and free molecular flow regimes. DESCRIPTION: Computational fluid dynamic analysis of short duration, high altitude (45 to 100 km) events, propulsive and/or detonative in nature, continue to prove problematic because of the inherently unsteady and three-dimensional nature of the flows, potentially involving chemical kinetics and two-phase flow features, all within a temporally evolving heterogeneous rarefied/continuum environment. The most robust computational technology for rarefied flows is the Direct Simulation Monte Carlo (DSMC) scheme. Although the method naturally asymptotes to continuum flow, it rapidly becomes computationally intractable as the flow becomes denser. Under the continuum regime, it is more efficient to utilize the traditional Reynolds Averaged Navier Stokes (RANS) formulation. Consequently, the ideal simulation of such events involves the use of hybrid technologies which couple the rarefied and continuum flow regimes, preferably, within a unified framework. To this end, innovative techniques are sought to produce a mixed continuum/non-continuum flowfield capability within an existing computational fluid dynamic (CFD) model suitable for treating chemically reacting two-phase, gas-particle, flows. Special consideration must be given to modeling laminar-turbulent transition, wall boundary conditions, temporal evolution, and the possible interaction with hard bodies for these mixed continuum/non-continuum flows. PHASE I: Innovative technical approaches will be formulated in Phase I to develop a mixed continuum/non-continuum flowfield capability leading to an advanced computational fluid dynamic (CFD) model as a marketable product. These technical approaches must address the key problem areas of coupled continuum-rarefied flow modeling that is requisite for high-fidelity characterization of short duration propulsive/detonative events; namely, mixed continuum/rarefied flows; unsteady effects; chemical kinetics for describing combustion in fuel rich environments, two-phase effects, and generalized 6 degree-of-freedom hard body interactions. Sample test cases will be run and detailed in the Phase I final report to demonstrate the key capabilities of a selected method(s). A detailed roadmap for product development under Phase-II execution will also be delivered. PHASE II: The proposed Phase I plan to develop an advanced, detailed physics-based high-fidelity simulation model or CFD code to accurately model high altitude, transient flowfields as required to support the computational fluid dynamics analyses of propulsive/detonative events will be implemented. Numerical studies will be conducted to address specific components of the new computational model as they apply to problems of interest to the Army. A full discussion of the technology along with the results of the numerical studies will be delivered with the Phase II final report. As a final product, the advanced CFD code should be fully executable on a linux cluster computer requiring no more than 200 CPUs, 16 GB memory per node, and run times within 24 hours. PHASE III: If successful, the end result of this Phase-I/Phase-II research effort will be a validated computational fluid dynamics model to predict short duration, high altitude events. The transition of this product will require additional testing to gauge the validity, accuracy, and applicability of the model. For military applications, this technology is directly applicable to all missile systems which operate at higher altitudes, such as interceptors and space delivery vehicles, where propulsive/detonative events, i.e. jet-interaction thrusters, are of interest. For commercial applications, this technology is directly applicable to advanced propulsion techniques for commercial applications such as high speed supersonic transports and to orbital launch systems. The most likely customer and source of Government funding for Phase-III will be those service project offices responsible for the development of missile interceptors such as the THAAD, SM-3 and GMD programs. REFERENCES: 1. Bird, G.A., Molecular Gas Dynamics and the Direct Simulation of Gas Flows, Clarendon Press, Oxford, 1994. 2. Park, C., Nonequilibrium Hypersonic Aerothermodynamics, Wiley Press, New York, 1990. 3. Boyd, I., Chen, G. and Candler, G.V.,"Predicting Failure of the Continuum Fluid Equations in Transitional Hypersonic Flows,"Physics of Fluids, Vol. 7, No. 1, Jan 1995, pp.210-219. 4. Wilmoth, R.G.,"DSMC Grid Methodologies for Computing Low-Density, Hypersonic Flows about Reusable Launch Vehicles,"AIAA Paper 96-1812, 1996.
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