Computational fluid dynamics

Summary

Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems. Ongoing research yields software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial validation of such software is typically performed using experimental apparatus such as wind tunnels. In addition, previously performed analytical or empirical analysis of a particular problem can be used for comparison. A final validation is often performed using full-scale testing, such as flight tests. CFD is applied to a wide range

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CFD simulation of an aerosol mixing chamber

Andrea Giordano

In the last years, the correlation between air pollution and health issues related to respiratory, cardiovascular and digestive systems has become evident. Today, urban aerosols raise the interest of both scientific community and public opinion. METAS, the Swiss Federal Institute of Metrology, takes part in AeroTox, a European Union’s research project involving the development of a reference aerosol calibration infrastructure - a so-called mixing chamber. In this chamber, pure air and particles are injected on top and the resulting aerosol is sampled at the bottom. The quality of this aerosol is assessed according to its concentration homogeneity: the purpose of this master’s project is to improve it. In addition, two research questions were addressed. How much can the mixing chamber dimensions be reduced without affecting the concentration homogeneity? Dimensions are crucial because the mixing chamber must be transportable. Also, how much can the flow rates be reduced without affecting the concentration homogeneity? Computational Fluid Dynamics (CFD) simulations and experiments were employed. Numerical simulations were performed in COMSOL Multiphysics, implementing a particle tracing and a diluted species model. This allowed to investigate the structure of the flow and the involved mixing mechanisms: diffusion, convection and turbulent dispersion. However, only the diluted species model was successful. The simulated concentration at the outlet is perfectly homogeneous. Experiments were carried out using two particle size distributions: NaCl (size peak at 80 nm) and Polystyrene Latex (PSL, size peak at 900 nm). Empirical data validate simulations and show a concentration homogeneity within 5%. Furthermore, uncertainty on the measurements is of 4.24%: the simulated concentration homogeneity thus lies within the uncertainty of the experimental findings. Moreover, experiments show that salt particles reach a higher concentration homogeneity than PSL particles. Finally, in case of salt particles, experiments prove that the flow rates can be halved and even equalized and the length of the mixing chamber can be reduced to 50% without drastically affecting the concentration homogeneity.

2020

Hydrocontest: Computational Fluid Dynamics of hydrofoils

Christian Frederik Kanesan

This project is developed within the scope of HydroContest which is an inter-school competition for the design of a racing boat with a high focus on energetic efficiency; the goal is to maximize the speed of the boat under the constraint of a limited power source. Hydrofoils are especially interesting since they offer an important reduction of drag at high speeds while remaining cost efficient. Within the contest, this project aims at delivering a prediction tool for the hydrofoil performance using numerical simulations of the incompressible Navier-Stokes equations approximated by the means of the Finite Element method with suitable stabilization techniques, such as the Variational Multiscale Method; we consider P1 Finite Elements with a second order BDF time discretization scheme. An automated meshing script was developed to handle arbitrary foil geometries and angles of attack. The numerical simulations were conducted using the LifeV Finite Element Library in a parallel setting. Satisfactory results have been obtained using this approach for Reynolds numbers up to 1 million.

2014

Additive manufacturing (AM) is a group of processing technologies which has the potential to revolutionize manufacturing by allowing easy manufacturing of complex shapes and small series. One AM-method which is of high interest for processing of metals is laser powder bed fusion (LPBF), where a powerful laser is used to melt powder in a layer-by-layer fashion into a consolidated part.Many defects can form during LPBF processing, but copper, copper alloys, and similar metals are particularly prone to the formation of pores.The reason why these metals tend to have these defects is the high reflectivity decreasing the effective energy input, and the high thermal conductivity transporting the absorbed energy away. Several ways of decreasing the pore formation tendency are known: increasing the power, increasing the absorptivity, and alloying to decrease thermal conductivity.The relative effectiveness of these strategies are however not known, making it difficult to choose the correct strategy for the different industrial applications of these metals.In this thesis a simple analytical model for predicting the occurrence of these defects is proposed, allowing a lowest possible processing power given the material properties to be estimated. The model agrees well with experiments, and can be used to predict the transition from so called balling mode processing to conduction mode processing, and for qualitative analysis of different processing strategies. Two strategies of decreasing the porosity are proposed and tested: coating of the powder with an absorbing layer and processing with a green laser, which is mor e strongly absorbed by copper than the conventional IR-lasers.The copper powder was coated with thin layers of tin and nickel using a simple and cheap immersion deposition method, and it is shown that the amount of porosity is decreased more than can be expected of the alloying alone, showing both that the method is working and that laser interaction with the solid is important for the heat balance of the system.The interaction with the solid is shown through computational fluid dynamics simulations to arise from periodic fluctuations of the molten metal which cause laser light to be reflected forward, pre-heating the powder bed.The preheating is larger the higher the solid absorptivity is, explaining the porosity decrease when using coated powders, and shows that processing using a green laser increase the laser coupling more than the absorptivity of the liquid would.Single line tracks made from fine 5 µm pure copper with a green laser source show that the desired conduction mode melting can be achieved at a power below 70 W, an order of magnitude lower than what is needed for processing of the conventionally used 45 µm powder with an IR-laser.

EPFL2021