To describe the coupled flow, heat/mass transfer and particle/droplet dynamics in various small scale systems such as experimental devices, the computational fluid dynamics (CFD) model FLUENT (Ansys Inc., Canonsburg, PA, USA) combined with the Fine Particle Model (FPM, Particle Dynamics GmbH, Leipzig) is applied. The FLUENT model, which solves for the fluid dynamics equations, is commercially available and one of the leaders amongst the available CFD-models. It allows the simulation of nearly any small scale flow problem. The FPM model is an Eulerian particle dynamics model and solves for the particle dynamics equations. Equipped with its own graphical user interface (GUI), it is fully integrated into FLUENT. This means, mass and heat transfer between particle and gas phase are considered in the simulations. In the past years, the FPM has been continously further devoloped. It is now capable to describe hygroscopic growth, activation and dynamic growth of various non-ideal behaving particles (Voigtländer, 2010), as well as ice particle nucleation processes (Hartmann et al., 2011).

The numerical simulations with the FLUENT/FPM model are used to design and optimize experiments investigating particle nucleation and growth carried out within the framework of the TROPOS activities (e.g. LACIS: Stratmann et al., 2004, Hartmann et al., 2011; or CLOUD: Voigtländer et al., 2012), to determine suitable initial and boundary conditions for these experiments, to interpret the experimental data (Hartmann et al., 2011), and to evaluate and derive micro physical expressions and parameterizations (e.g., mass accommodation coefficient of water vapor on water, Voigtländer et al., 2007).

To describe droplet freezing processes as investigated at LACIS, the FPM model was further developed comprising a) the introduction of an additional ice-phase including the treatment of phase transfer processes, and b) the implementation of models and parameterizations for the calculation of homogeneous and heterogeneous ice particle nucleation rate coefficients (Hartmann, 2011). Based on the Modal Aerosol Dynamics method (MAD, Whitby and McMurry, 1997), the newly-developed phase transition model transfers particles from the seed particle/droplet mode to either the homogeneous or heterogeneous ice mode via the respective sink/source terms.

A comparison of FLUENT/FPM simulations with LACIS data from experiments investigating homogeneous (diluted ammonium sulfate) and heterogeneous (mineral dust) ice nucleation is exemplary shown in Fig. 2 – Fig. 5 (Fig. 2: calculated temperature and saturation profiles; Fig. 3: mass fraction of water indicating the transfer between the gas and particle phases; Fig. 4: calculated freezing rates; Fig. 5: ice fraction compared to LACIS data). The FLUENT/FPM simulations with different model approaches (blue, green and red lines) agree well with the experimental data, indicating that the FLUENT/FPM is a suitable tool for describing the complex fluid/particle dynamical and phase transition processes taking place in LACIS. Therefore, the performed model developments and realized improvements significantly enhanced the LACIS facilities' capabilities concerning the investigation of heterogeneous ice nucleation processes. Having more reliable and accurate methods for the characterization and theoretical description of droplets and ice particles available has already helped and will help in the future to further strengthen the position of LACIS as one of the worldwide key facilities in ice nucleation research.

  • S. Hartmann, D. Niedermeier, J. Voigtländer, T. Clauss, R.A. Shaw, H. Wex, A. Kiselev, P. Stratmann, Homogeneous and heterogeneous ice nucleation at LACIS: operating principle and theoretical studies, Atmos. Chem. Phys., 11, 1753-1767, 2011.
  • E. Herrmann, H. Lihavainen, A.P. Hyvarinen, I. Riipinen, M. Wilck, F. Stratmann, M. Kulmala: Nucleation simulations using the fluid dynamics software FLUENT with the fine particle model FPM, J. Phys. Chem A, 110(45), 12448-12455, doi: 10.1021/jp064604m, 2006.
  • D. Niedermeier, S. Hartmann, R. A. Shaw, D. Covert, T. F. Mentel, J. Schneider, L. Poulain, P. Reitz, C. Spindler, T. Clauss, A. Kiselev, E. Hallbauer, H. Wex, K. Mildenberger, F. Stratmann, Heterogeneous freezing of droplets with immersed mineral dust particles - measurements and parameterization, Atmos. Chem. Phys., 10 (8), 3601-3614, 2010.
  • M. Schütze and F. Stratmann: Numerical simulation of cloud droplet formation in a tank, Comput. Geosci., 34(9), 1034-1043, doi: 10.1016/j.cageo.2007.06.013, 2008.
  • F. Stratmann, A. Kiselev, S. Wurzler, M. Wendisch, J. Heintzenberg, R.J. Charlson, K. Diehl, H. Wex, S. Schmidt: Laboratory studies and numerical simulations of cloud droplet formation under realistic supersaturation conditions, J. Atmos. Oceanic Technol., 21 (6), 876-887, 2004.
  • F. Stratmann, E. Herrmann, T. Petaja, M. Kulmala: Modelling Ag-particle activation and growth in a TSI WCPC model 3785, Atmos. Meas. Tech., 3(1), 293-281, 2010.
  • J. Voigtländer, F. Stratmann, D. Niedermeier, H. Wex, A. Kiselev: Mass accommodation coefficient of water: A combined computational fluid dynamics and experimental data analysis, J. Geophys. Res., 112, D20208, doi:10.1029/2007JD008604, 2007.
  • J. Voigtländer: Hygroscopic growth and CCN activation of slightly soluble organic and inorganic compounds : evaluation of experimental LACIS data with FLUENT/FPM, Ph.D. Thesis, University Leipzig, d-nb.info/1000773639, 2010.
  • J. Voigtländer, J. Duplissy, L. Rondo, A. Kürten, F. Stratmann: Numerical simulations of mixing conditions and aerosol dynamics in the CERN CLOUD chamber, Atmos. Chem. Phys., 12, 2205-2214, doi: 10.5194/acp-12-2205-2012, 2012.