Modelling of the Unsteady Dynamics of Turbulent Disperse Bubbly Flows

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Bubbly flows are found in a large number of industrial processes including oil wells, power generation as well as in the food, pharmaceutical and chemical industry. In this way chemical goods worth billions of dollars are produced worldwide every year. Therefore the advantages of the ability of a reliable numerical simulation tool in regards of process safety and process control are enormous.Typically, two phase flow simulations in industry are based on the Reynolds averaged Navier-Stokes equations (RANS). However even for single phase flow, after decades of research, no satisfactory turbulence closure model is available. Bubbly flows are characterized by spatially and temporally deforming interfaces. The manifold interaction of bubble dynamics, turbulence and the bubbles itself are so complex, that the availability of a universal and accurate RANS model for the prediction of bubbly flows appears unlikely in near future. Large eddy simulation (LES) offers a good compromise between RANS based closures and the direct numerical simulation (DNS) of the underlying equations. Combined with an interface advection technique LES offers the advantage, that many physical processes can be resolved to a large extent which is in contrast to modelling the whole range of length and time scales in RANS. In the context of two phase flows with moving boundaries the LES equations contain additional unknown terms. The development of closures for these terms has just started and no model has been demonstrated in the literature to yield satisfactory results in a real LES, yet. The goal of this project is to establish an LES model which is able to predict disperse bubbly flows of practical interest with good accuracy. The project is subdivided into two phases. First a DNS data base will be generated. To this end vertically rising single bubbles as well as small bubble swarms will be simulated in a quiescent fluid as well as in a turbulent counterflow configuration. Subsequently the data will be filtered to obtain pseudo LES data (a-priori analysis). In this way model ideas can be compared with the exact unclosed terms determined from DNS. The true evaluation of a model requires however to perform a real LES. The models from phase one which appear most promising will be implemented in the LES code and compared with DNS and very detailed experimental data available from a research partner. This so called a-posteriori analysis will guide the qualification of an LES model for bubbly two phase flows. In a continuation of this research project it is planned to use a multiscale approach in order to enable the simulation of large bubble swarms. This will allow to reach the global goal, which is the simulation of configurations of practical interest.