Frank Obermeier

Dynamic processes occurring during the spreading of thin liquid films produced by drop impact on hot walls

Abstract When liquid drops impinge on a wall heated up above the boiling temperature of the liquid, bubbling and convection are observed in the emerging film which spreads along the wall. Experimental investigations of these processes and of the connected evaporation of the liquid are aimed at an improved understanding of the underlying mechanisms important for many technical applications. Typical Weber numbers of the impinging drops were varied between 50 and 700, the temperature of the polished aluminium wall between 70°C and 400°C, i.e., above the boiling temperature of ethanol used in the experiments. The growth rate of bubbles, their maximum diameter and their collapse as well as the formation and destruction of convection cells in the liquid film were recorded and analysed.

Modelling of unsteady electromagnetically driven recirculating melt flows

A numerical model for the recirculating flow of an electromagnetically stirred iron melt in a cylindrical induction furnace crucible is presented. Due to the parameters of the problem, the full set of magnetohydrodynamic flow equations decouples into magnetic and hydrodynamic parts. The diffusion approach of the magnetic vector potential describes the magnetic part of the model. The hydrodynamic part of the model is based on the unsteady ensemble-averaged Navier–Stokes equations in conjunction with a Reynolds stress turbulence model. Such an approach is commonly termed the unsteady Reynolds averaged approach. Here the influence of the electromagnetic field is included by means of a Lorentz force density.The diffusion equation is solved by a finite-element method. The Lorentz force density in the melt can be deduced from the results. Then the unsteady Reynolds averaged equations are solved by the finite-volume method.The results of the unsteady Reynolds averaged approach are analysed and compared to the results of the steady Reynolds averaged equations of the flow under consideration and to experimental results obtained from other studies. It is found that the toroidal vortex pair, which is the dominating structure within the flow, interacts by intermittent streamline connections.

Numerical simulation of fluid flow and disperse phase behaviour in continuous casting tundishes

A new Euler-Lagrange model of steel melt flows including dispersed secondary phases in continuous-casting tundishes is presented. The system of fundamental equations for the flow field is closed by different turbulence models. The calculation of tundish flows and a comparison with experimental results show that the flow fields of the steel melt and the behaviour of the dispersed secondary phases in the melt are described adequately by the model. However, the quality of the results for the dispersed phase depends essentially on the choice of turbulence model. Criteria for deciding which turbulence model is best suited for a specific flow situation are suggested. These criteria are deduced from investigations of different tundish flows.

Sound Generation by Low Mach Number Flow Through Pipes with Diaphragm Orifices

Aerodynamic noise generated by low Mach number flow through a pipe with two differently sized diaphragm orifices is investigated both theoretically as well as numerically. Such a flow may be considered as a simple model of an aircraft climate control system. The theory is based on an acoustic analogy introduced by Mohring and Obermeier in the seventies. They identified by means of the so-called Green’s vector function acoustic sources in unsteady flows as the unsteady motion of vorticity “vortices are the voice of flows”. In addition, the unsteady flow through the pipe, the aerodynamic sources, and the spectra of sound are evaluated numerically. Here effects of different numerical algorithms (PISO—PIMPLE) and of different numerical boundary conditions at the outlet of the pipe (advective, convective—wave transmissive) are identified and discussed.