Publikationen

Turbulent mixing and entrainment at the boundary of a cloud is studied by means of direct numerical simulations that couple the Eulerian description of the turbulent velocity and water vapor fields with a Lagrangian ensemble of cloud water droplets that can grow and shrink by condensation and evaporation, respectively. The focus is on detailed analysis of the relaxation process of the droplet ensemble during the entrainment of subsaturated air, in particular the dependence on turbulence timescales, droplet number density, initial droplet radius and particle inertia. We find that the droplet evolution during the entrainment process is captured best by a phase relaxation time that is based on the droplet number density with respect to the entire simulation domain and the initial droplet radius. Even under conditions favoring homogeneous mixing, the probability density function of supersaturation at droplet locations exhibits initially strong negative skewness, consistent with droplets near the cloud boundary being suddenly mixed into clear air, but rapidly approaches a narrower, symmetric shape. The droplet size distribution, which is initialized as perfectly monodisperse, broadens and also becomes somewhat negatively skewed. Particle inertia and gravitational settling lead to a more rapid initial evaporation, but ultimately only to slight depletion of both tails of the droplet size distribution. The Reynolds number dependence of the mixing process remained weak over the parameter range studied, most probably due to the fact that the inhomogeneous mixing regime could not be fully accessed when phase relaxation times based on global number density are considered.

We study distortion of laminar liquid metal flow by a magnetic point dipole in a straight square duct. This basic configuration is of fundamental interest for Lorentz force velocimetry, where the Lorentz force opposing the relative motion of conducting medium and magnetic field is measured to determine the flow velocity. The total force is highly dependent on the velocity profile, which changes its shape due to the acting Lorentz force itself. We are interested in the deflection of the flow and its dependence on magnitude and distribution of the magnetic field. To this end, we perform direct numerical simulations with an accurate finite-difference scheme in the limit of small magnetic Reynolds numbers. The hydrodynamic Reynolds number is choosen to be high enough to allow the generation of vortices and turbulent structures.

We construct a low-dimensional model (LDM) of turbulent mixed convection in a Cartesian cell with in- and outlets and local sources of heat which is narrow in one of the two horizontal space directions. The basis is a high-resolution three-dimensional direct numerical simulation (DNS) record. The model is derived with basis functions, which have been obtained by a proper orthogonal decomposition (POD) using the snapshot method. The POD analysis is applied for a sequence of three-dimensional snapshots aswell as for datawhich are bulk-averaged in the direction of narrowextension. This step is taken since the flow is found to have no significant dependence along this direction in the cell. We compare the three-dimensional and two-dimensional POD modes. This simplification reduces the complexity of the problem significantly and allows us to construct and run a two-dimensional LDM with a small number of degrees of freedom. We study the long-time dynamical behavior of this system using a closure of the LDM based on a mode-dependent viscosity and diffusivity. The LDM has been optimized in terms of the standard deviation of the energy spectrum and the transient energy for different numbers of degrees of freedom by comparison with the original DNS data. We find that the evolution of the coherent structures of flow and temperature agrees well with the two-dimensional original data and determine their contribution to the global transfer of heat. Root-mean-square profiles of the fluctuations of the turbulent fields agree qualitatively well with the original simulation data, but deviate slightly in amplitude. We conclude that the reduction in the dimensionality and the number of degrees of freedom can reproduce the gross features of the mixed convection flow in this particular setup well.