Computational Fluid Dynamics (CFD)
The activities of the CFD department focus on the qualification of CFD-codes for multiphase flows. This includes model development as well as validation basing on suitable experimental data. A more detailed presentation of the background is given below the figure.
The most important topics of present research are:
- CFD-modelling of poly-dispersed bubbly flow including phase transfer (Inhomogeneous MUSIG model, wall boiling models)
- CFD-modelling of free interfaces (AIAD model)
- Concept of a generalized CFD-model for multiphase flows (GENTOP)
- CFD-model development and validation related to the transport, deposition and re-suspension of aerosol particles
CFD-codes are used for multi-dimensional flow simulation. The basic equations to describe the flow are well known for long time. Since they can be solved analytically only for some very simple cases, numerical methods have be used for their solution. The challenge for such simulations arises from the fact that the relevant physical phenomena occur on a wired range of scales. In turbulent flows, there are eddies whose sizes reach from the dimension of the investigated flow volume down to the molecular scale. For this reason a direct simulation of the flow would require the consideration of this entire range of scales, and thus resulting in very small computational cells. For many practical problems that would lead to a number of required computational cells that cannot be processed with present day available computational resources nor with such expected to be available in the next decades. Therefore it is necessary to perform the simulations on grids with larger cells. Phenomena that take place on smaller scales than the size of the computational cell have to be accounted for by additional closure models.
For single-phase flows this concerns in particular the turbulence modelling. Many models with different degree of detail have been developed in the past and are presently under development. In general the models for single-phase have now reached a stage which permits a broad application of CFD codes for practical application. This is the case for example in the automotive or aerospace industry. There CFD simulations with respect to the air resistance and other flow-related phenomena already have replaced to a large extent expensive wind tunnel tests. CFD methods are increasingly used for design, optimization and safety assessment of industrial processes.
However, for multiphase flows (e.g. if liquid and gas is present in the considered volume) CFD-codes have only limited applicability. Today's computational models still show significant deficits in spatial and temporal description of transient multiphase flows. This is caused by the complex interactions between the phases. On the other hand, multiphase flows play an important role in many different sectors of industry. Examples for industrial applications range from multiphase chemical reactors to mineral oil production, hydraulic machines and nuclear light water reactors. Accordingly there is a demand for simulations of such practically relevant multiphase flows in order to support design, optimization and safety analyses of plants and components.
For this reason the long-term objective of the department of Computational Fluid Dynamics lies in the qualification of CFD-codes for industrial applications. This includes the development of theoretical models for basic phenomena of transient, three-dimensional multiphase systems. Local geometry independent closure models for mass, momentum and heat transfer between the gas/steam and the liquid phase are developed and validated. Such models are an essential pre-condition for the application of complex fluid dynamic codes to the simulation of flow related phenomena in chemical or nuclear facilities. Our primary partner for CFD code qualification is ANSYS , the developer of the CFD-code CFX, which is one of the leading CFD codes worldwide. Based on this partnership the models developed within the frame of this R&D program are implemented into the code and thus contribute to the code qualification.
The model development as well as the validation of codes is only possible with high quality experimental data. Such data are required with high resolution in space and time. For this reason the activities of the CFD department are closely related to the ones of the department Experimental Fluid Dynamics. A main topic are experimental two-phase flow investigations using innovative measuring techniques. These experiments supply information on the time-dependent shape and size of the interfacial area which is the basis for mass, momentum and heat transfer between the phases.
TOPFLOW is one of the major scientific facilities at HZDR. It is the central facility of the National Research Initiative of the Federal Ministry of Economy and Technology (BMWi) to qualify CFD codes for nuclear safety research. With the widely used wire-mesh sensor technology as well as with the new developed ultrafast X-ray electron-beam tomography HZDR has a leading position in the area of fast two-phase flow measuring technique with high spatial resolution. In addition the R&D program is closely connected to the activities in the Magneto-hydrodynamics department.