Event Title

Modeling of Ablation Using the DSMC Method

Start Date

1-3-2011 8:00 AM

End Date

3-3-2011 12:30 PM

Description

Many of the future materials of interest to NASA for atmospheric re-entry are ablators. When a charring ablator is used, thermal pyrolysis of the heat shield material produces an inner gas which then flows through the surface and into the flow field. Additionally, the exposed surface of the material reacts and eventually looses mass and recesses. At high altitude, the latter phenomenon is not dominant, and the majority of the ablative products come from the inner structure of the material. The pyrolysis process starts early in the re-entry trajectory, often when the vehicle has not yet reached the continuum region. In order to model these types of noncontinuum ablating flow fields, the Direct Simulation Monte Carlo (DSMC) method is used. The DSMC method is a kinetic numerical technique that has been used to simulate gas flows in translational, chemical and thermal nonequilibrium for many applications. The DSMC method involves following a representative group of simulated particles throughout the computational domain, as they move and collide. Individual models are employed to account for physical processes, such as chemical reactions, excitation of internal energy modes and gas-surface interactions. In this work, an existing DSMC code is modified to include the injection of a gas with a specified composition and mass flow rate from the vehicle surface into the flow field. The surface-normal velocity components of the injected particles are sampled from a biased Maxwellian velocity distribution at the specified constant surface temperature. As a first step toward the implementation of a fully reacting surface boundary with blowing, the well documented IRV-2 vehicle test case is used to test the new boundary condition. The free stream conditions at an altitude of 67 km are used, with a vehicle velocity of 6780.6 m/s. The Knudsen number, indicating continuum breakdown, is approximately 0.03 based on the nose radius of the vehicle. This places the 67 km flight condition in the non-continuum flow regime. An isothermal wall temperature of 1600 K is imposed, as is a fixed blowing rate of 0.033 kg/m2/s of CO. The influence of the ablation products on the surface heat flux and shock stand-off distance is investigated. Additionally, the DSMC results are compared to results obtained using the CFD code LeMANS. Qualitative agreement between the two sets of results is observed, however there are significant differences in the predicted CO concentration and flow field temperatures. The latter discrepancy is observed in comparisons of DSMC and CFD flow field results.

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Mar 1st, 8:00 AM Mar 3rd, 12:30 PM

Modeling of Ablation Using the DSMC Method

Many of the future materials of interest to NASA for atmospheric re-entry are ablators. When a charring ablator is used, thermal pyrolysis of the heat shield material produces an inner gas which then flows through the surface and into the flow field. Additionally, the exposed surface of the material reacts and eventually looses mass and recesses. At high altitude, the latter phenomenon is not dominant, and the majority of the ablative products come from the inner structure of the material. The pyrolysis process starts early in the re-entry trajectory, often when the vehicle has not yet reached the continuum region. In order to model these types of noncontinuum ablating flow fields, the Direct Simulation Monte Carlo (DSMC) method is used. The DSMC method is a kinetic numerical technique that has been used to simulate gas flows in translational, chemical and thermal nonequilibrium for many applications. The DSMC method involves following a representative group of simulated particles throughout the computational domain, as they move and collide. Individual models are employed to account for physical processes, such as chemical reactions, excitation of internal energy modes and gas-surface interactions. In this work, an existing DSMC code is modified to include the injection of a gas with a specified composition and mass flow rate from the vehicle surface into the flow field. The surface-normal velocity components of the injected particles are sampled from a biased Maxwellian velocity distribution at the specified constant surface temperature. As a first step toward the implementation of a fully reacting surface boundary with blowing, the well documented IRV-2 vehicle test case is used to test the new boundary condition. The free stream conditions at an altitude of 67 km are used, with a vehicle velocity of 6780.6 m/s. The Knudsen number, indicating continuum breakdown, is approximately 0.03 based on the nose radius of the vehicle. This places the 67 km flight condition in the non-continuum flow regime. An isothermal wall temperature of 1600 K is imposed, as is a fixed blowing rate of 0.033 kg/m2/s of CO. The influence of the ablation products on the surface heat flux and shock stand-off distance is investigated. Additionally, the DSMC results are compared to results obtained using the CFD code LeMANS. Qualitative agreement between the two sets of results is observed, however there are significant differences in the predicted CO concentration and flow field temperatures. The latter discrepancy is observed in comparisons of DSMC and CFD flow field results.