arXiv subjects

Franziska Tuttas

Publications and source records attributed to Franziska Tuttas.

4 recordsLinked to original sources

arXiv2026

Kinetic modeling of molecular beam formation in a cryogenic buffer-gas cell

Cryogenic buffer-gas cells are widely used to produce cold molecular beams, but the microscopic dynamics governing beam formation remain challenging to model. Here we present fully kinetic simulations of a cryogenic buffer-gas cell using the Direct Simulation Monte Carlo method implemented in the PICLas framework, treating the buffer gas and ablated molecules within a single unified model. We capture characteristic features of cryogenic buffer-gas sources, including plume cooling, directed transport toward the aperture, and the formation of a slow molecular beam, while also resolving energy transfer from the hot ablation plume to the helium buffer gas that is inaccessible to existing approaches relying on the background-gas approximation. Our results demonstrate that fully kinetic simulations can provide detailed insights into buffer-gas cell dynamics and open a route toward a systematic optimization of such sources.

physics.atom-ph↗

arXiv2026

A Multispecies ESBGK Model for Gas Mixtures with Variable Hard Sphere Transport: Theory and Verification

A multi-species Bhatnagar-Gross-Krook (BGK) model for gas mixtures is presented that achieves the correct species-wise relaxation of velocities, temperatures, and pressure tensors according to the Boltzmann collision integral, as well as the correct mixture Prandtl number, while retaining a single relaxation term per species. The model extends the ellipsoidal statistical BGK (ESBGK) model by introducing relative relaxation targets for each species, derived from the Variable Hard Sphere (VHS) production rates of the Grad 13 approximation. Three approaches for the species relaxation frequency are proposed and analyzed: a Grad 13-based per-species frequency, a mixture-averaged frequency, and an empirical harmonic mean of the two. The model is implemented in the particle-based code PICLas and verified against Direct Simulation Monte Carlo (DSMC) results for a range of test cases, including 0D reservoir relaxation, mass diffusion, supersonic Couette flow, and hypersonic flow around a 70° blunted cone for binary and ternary gas mixtures. Across all test cases, the proposed model reproduces the correct Prandtl number, species temperature, velocity relaxation rates and pressure tensor relaxation, with the empirical relaxation frequency consistently yielding the best agreement with DSMC.

physics.flu-dyn↗

arXiv2026

A harmonically-coupled-anharmonic-oscillator approach for polyatomic chemistry modeling in DSMC

Atmospheric entry processes are characterized by high-enthalpy gas flows in strong thermo-chemical non-equilibrium. Accurate simulations of such conditions remain challenging due to the extreme conditions and the complex influence of internal energy modes. In particular, the common assumption of uncoupled harmonic vibrations may break down, and excited internal energy states can directly influence reaction rates. Previously, an anharmonic oscillator model has been developed by Civrais et al. to improve the accuracy of the Direct Simulation Monte Carlo (DSMC) method under such conditions. However, this extension has so far been limited to diatomic molecules. To increase the accuracy of the DSMC method in the open-source code PICLas, the anharmonic oscillator model is extended to include polyatomic species. The proposed model explicitly considers anharmonic effects and intramolecular energy redistribution. Vibrational degrees of freedom are treated in a local mode basis, in which anharmonic stretching modes are harmonically coupled by harmonic bending modes. The coupling allows for the redistribution of localized vibrational excitation. Dissociation can occur by the strong excitation of a stretching mode, and specific modes can be coupled to the reaction coordinate of the transition state in bimolecular exchange reactions. The newly developed model is evaluated by the comparison to high-fidelity calculations for a set of representative processes. Investigated are different dissociation reactions, which exhibit a high degree of energy redistribution, and the hydrogen-exchange reaction between methane and a hydrogen radical, in which only selected modes contribute to the reactive process. In addition, the recombination-dissociation equilibrium system has been investigated for methane.

physics.chem-ph↗

arXiv2026

A Shakhov-based Bhatnagar-Gross-Krook model for polyatomic molecules and for atomic as well as polyatomic mixtures

The implementation of the Shakhov Bhatnagar-Gross-Krook (SBGK) method in the open-source particle code PICLas is extended for modeling of polyatomic molecules, as well as mixtures including atoms and molecules, while accounting for non-equilibrium in the internal degrees of freedom. The conservation properties of the model are shown and the model parameter for the Prandtl number is derived. In order to determine the viscosity and thermal conductivity of gas mixtures, the first approximation of the transport properties using collision integrals is employed. The model is verified with simulation test cases of a supersonic Couette flow and a hypersonic flow around a 70° blunted cone with different flow parameters and gas compositions. The results are compared to the Direct Simulation Monte Carlo (DSMC) method as well as the Ellipsoidal Statistical BGK (ESBGK) method to assess the accuracy of the model, where overall good agreement is achieved. In particular, the proposed SBGK model captures the shock in front of the 70° blunted cone more precisely than the ESBGK model.

physics.flu-dyn↗