Internal Energy Excitation and Chemical Reaction Models for Rarefied Gases

Models for internal energy exchange and for chemical reactions developed for DSMC simulations are reviewed. 1.0 INTRODUCTION DSMC [1, 2] is a standard tool for the simulation of rarefied gas flows. It has, therefore found widespread application in the aerospace community for the simulation of high altitude vehicle aerodynamics. New developments and new goals in the field pose new challenges for the simulation. Planetary exploration missions entail high speed entries into atmospheres different from air; the same missions ask for simulation of very high speed re-entries into Earth atmosphere where ionization phenomena become relevant and accurate estimation of the emitted radiation is crucial to thermal protection design. Assessment of advanced concepts like the use of Magneto Hydro Dynamic interaction to reduce heat transfer to the vehicle require the modelling of plasma effects. As a result, new physical models need be included in DSMC simulation for the correct treatment of the energy relaxation, chemical kinetics and plasma dynamics of high temperature, partially ionized, chemically reacting gas mixtures. This includes extending available models to different gases (in particular to polyatomic gases and to ionized species). In addition, technological development calls for more stringent requirements on the predictive capabilities of the simulations so that new solutions must be sought, beyond the standard models, to accommodate more details of physical realism in a practically tractable model. This work discusses the available models for internal energy relaxation and chemical reactions to use in DSMC simulation. The focus is on physical content rather than on computational efficiency. Also, the flexibility of the model with respect to application to different gases or to different flow regimes is analysed. Issues of currently available models and challenges for the development of new ones are outlined. Section 2 discusses the modelling of the collision frequency and its connection to the transport properties of the simulated gas; section 3 explains the rationale behind the commonly adopted algorithms for choosing among different available exit channels in a collision event; sections 4 and 5 summarise the models for internal energy exchange and for chemical reactions, respectively; section 6 discusses the issues connected with modelling the interaction of the gas with solid surfaces. 2.0 KINETIC CROSS SECTION The first step in modelling collisions concerns the evaluation of the collision frequency. To this end the total collision cross section is needed as a function of the colliders’energies. Since every collision, either RTO-EN-AVT-194 4 1 Internal Energy Excitation and Chemical Reaction Models for Rarefied Gases Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE JAN 2011 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Internal Energy Excitation and Chemical Reaction Models for Rarefied Gases 5a. CONTRACT NUMBER