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Computational fluid dynamics CFD
Dipl.-Ing. Donald Stubbe
Scientific analysis of flows
Computational fluid dynamics (CFD) enables complex information about the flow, the replacement of simple but lengthy experiments as well as constructive and/or physical parameter studies. CFD methods also form the basis for numerical aeroacoustics, which deals with the propagation of flow noise.
For the calculation of turbulent flows, mesh generation as well as the careful selection of suitable boundary conditions and the correct modeling of turbulent properties are of fundamental importance. CFD has a wide range of models to capture turbulent flow phenomena. RANS (Reynolds-Averaged-Navier-Stokes) models are the most widely used approaches to turbulence modeling. Practical examples are the k-Omega model (especially SST) and k-Epsilon-2 equation models as well as the RSM (Reynolds Stress Model) for highly turbulent flows. As a supplement to the RANS models, scale-resolving turbulence models offer the possibility of actually resolving certain turbulent effects in space and time (e.g. LES (Large-Eddy Simulation)). The requirements for the computational network and the time step in the context of transient flow simulations are considered and applied in detail.
Numerical simulations are often carried out in parallel to measurements. The interaction of measurement and numerics is essential in numerous projects and should on the one hand increase the variety of possible configurations and on the other hand, by validating the CFD simulation through measurement results, significantly reduce the number of experiments with preceding construction of test samples. This approach is applied to all relevant areas of investigation - fluid mechanics, acoustics and energy.
At the Institute of Air Handling and Refrigeration, flow simulation is used, among other things, to investigate phenomena in indoor air flow. Simulation tools from ANSYS or OpenFOAM are used for this purpose.
Furthermore, the institute's own development and the use of highly specialized, efficient flow solvers [CARIBOU] for real-time CFD is to be mentioned. These applications enable, among other things, the energy-efficient and comfortable operation of HVAC systems, the networking of all components such as heating, cooling, ventilation, sun protection, etc. and thus form a system for intelligent and self-optimizing room air control.
Flow simulation in room structures and in context of building services
For buildings with people occupying the building, the temperature and velocity conditions are typically determined under specification of loads and building physical conditions using numerical flow simulation. Statements on other room air parameters, such as room air humidity and quality (CO2 content) are further elements of such investigations.
The evaluation of the thermal comfort in the living area of the persons is very often part of the result analysis. In addition, the aim of the simulations is to show the functionality of the cooling, heating and ventilation system for the defined use cases and the associated evaluation criteria, taking into account environmental influences (e.g. summer and winter conditions).
In nature, multiphase flows usually occur. These can be considered in a simulation from mass transport to the explicit resolution of phase boundaries and the calculation of individual droplets.
Fire and smoke simulation
With the simulation of fire scenarios it can be proved whether the protection goals, which follow from the „Leitfaden – Ingenieurmethoden des Brandschutzes“, are achieved. The protection goals are valid 2.50 m above the floor and are considered equally for fire scenarios. The protection goal visibility is treated separately, because it is not a direct result of the simulation. Visibility is linked to the spread of carbon monoxide and calculated from its local concentration.
In the verbal representation the terms plume and ceiling jet are used. Plume means the temperature and pollutant plume rising directly from the source of the fire. Within the plume, temperature and pollutant concentrations are extremely high. It is therefore almost impossible to comply with the protection goals inside the plume and in its immediate vicinity.
If the plume meets a ceiling without openings, it will be distributed according to the ceiling geometry. The wall jets that form on the ceiling are called ceiling jets. Ceiling jets can be used to transport large quantities of pollutants to distant parts of a building. Plume and Ceiling Jets are central engines that jeopardize compliance with the protection goals.
The pictures show exemplary simulation results for the distribution of flow velocity and CO concentration in a building complex. From the findings, practice-relevant optimization hints can be derived.
Numerical optimization of components | Product development
The flow simulation of assemblies with the help of numerics enables the further development as well as the improvement of, for example, efficiencies without the time-consuming production of all optimization stages.
Contra-rotating axial fan
The contra rotating fan (CRF) impellers increase the efficiency by reducing the usual swirl at the impeller outlet. The CRF is characterized by the use of only one motor and the omission of a gearbox or other mechanical components to achieve contra-rotating operation. Characteristic for this fan is the contactless power transmission into the rotating system, which was additively developed for this application.
Condenser | Heat exchanger
The CFD simulation of a section of a condenser (heat exchanger) has the objective to numerically determine the flow and temperature behavior as well as the cooling and condensation effectiveness.
Transient CFD of a cooling process with sliding meshes
The goal of the transient CFD simulation with sliding mesh is to optimize a cooling process both thermofluiddynamically and with respect to the time sequence. The basic task here is a flow-mechanical revision with the specifications of minimizing the energy input and the process duration for the cooling process.
Simulation of vehicles
The flow simulation of rail and motor vehicles is an alternative to cost-intensive investigations, for example in the wind tunnel.
It enables the optimization of external and internal flows already in an early phase of development or when improving existing vehicles.
Real-Time Simulation CFD | Solver Development
Within the scope of research work, a new, highly efficient flow solver for rapid simulation of indoor air flows was developed, parallelized and extensively tested. With the flow solver laminar and turbulent flows with and without heat transfer and buoyancy effects can be calculated significantly faster than with general purpose simulation programs. The efficiency and accuracy has been proven with numerous fluid mechanical test cases. In contrast to simulations with RANS approaches, the method can also be used to calculate flows in complex geometries very accurately using LES.