ENGINEERING AND CFD

Researchers of the Institute of Aerodynamics at RWTH Aachen University used large-eddy simulation and computational aeroacoustics methods to analyze noise sources in turbulent flames and the interaction of the resulting acoustic waves with the flame and the turbulent flow field. To achieve accurate results of the flow and the acoustic field highly resolved large-scale simulations with several hundred million mesh points are necessary. The simulation results have given new insights into fundamental sound-generation mechanisms and their phase-relationship that are important for the prediction and control of thermoacoustic instabilities, and ultimately, the development of more efficient and gas turbines with lower pollutant emissions.

Many wall-bounded flows in nature and technology are affected by the surface roughness of the wall. In some cases, this has adverse effects, e.g. drag increase leading to higher fuel costs; in others, it is beneficial for mixing enhancement or transfer properties. Computationally, it is notoriously difficult to simulate these flows because of the vast separation of scales in highly turbulent flows and the challenges involved in handling complex geometries. The studies are carried out in two paradigmatic and complementary systems in turbulence research, Taylor-Couette and Rayleigh-Bénard flow.

Wind turbine and aircraft design relies on numerical simulation. Current aerodynamic models represent turbulence not directly but model its averaged impact. Such models are only reliable near the design point and require vast experience of the design engineer. Industry wants therefore to enable more accurate methods, such as wall-modeled Large-Eddy Simulation (wmLES) which represents turbulent flow structures directly. R2Wall provides a high-resolution simulation of the NACA4412 airfoil as reference data for the development of wall-models for LES and turbulence models in general. This project has enabled the definition of guidelines for future computations, and the calibration of wall-models.

In this study, researchers in fluid mechanics at ISAE-SUPAERO investigate possible control of shock boundary layer interaction, a well known flow phenomenon occuring on high speed supersonic devices. In particular, low frequency modes have a significant impact on the device load. High fidelity simulations of turbulent flows are performed to understand the modifications of the flow field induced by the microramp vortex generators located upstream the interaction.

Sorting is one of the most fundamental and widely used algorithms. It can be used to build index data structures, e.g., for full text search or for various applications in bioinformatics. Sorting can also rearrange data for further processing. In particular, it is a crucial tool for load balancing in advanced massively simulations. The wide variety of applications means that we need fast sorting algorithms for a wide spectrum of situations. Researchers have developed massively parallel robust sorting algorithms, apply new load balancing techniques, and systematically explore the design space of parallel sorting algorithms.

The dynamic behaviour of fluid motion is driven by processes on a wide range of spatial and temporal scales. In a project run by Heidelberg University scientists, parts of model systems that describe fluid dynamics and temperature evolution were investigated. The models are formulated in terms of velocity, temperature, pressure, and density. The researchers employ a hierarchy of different physical models with an increasing degree of complexity. Additionally, uncertain input parameters are taken into account.

In many turbulent convection flows in nature and technology the thermal diffusivity is much higher than the kinematic viscosity which means that the Prandtl number is very low. Applications of this regime reach from deep solar convection, via convection in the liquid metal core of the Earth to liquid metal batteries for grid energy storage and nuclear engineering technology. Laboratory experiments in low-Prandtl-number convection for Pr < 0.1 have to be conducted in liquid metals which are inaccessible for laser imaging techniques and require analysis by ultrasound or X-rays. Direct numerical simulations of this regime of turbulent convection at high Rayleigh numbers are the only way to reveal the full three-dimensional structure of…

Viscosity represents the most important property of turbulent flows, yet its impact of its variation on turbulence dynamics is not yet fully understood. This project addresses how the dissipation mechanism and self-similarity of turbulent flows are affected by fluctuations in viscosity—questions that can help lead to better turbulent mixing models, and, in turn, improved predictions of pollutants in combustion engines. Highly resolved direct numerical simulations of turbulent shear flows on JUQUEEN with up to 231 billion grid points were performed in pursuit of an answer to this question.

With the rapidly changing massively parallel computer architectures arising in recent years, it became a huge challenge to make efficient use of contemporary supercomputers. Project ExaStencils addresses this problem by providing an easy-to-use, multi-layered domain-specific language to the application programmer such that problems can be formulated in an intuitive way fitting to different levels of abstraction. The necessary code transformations are performed by a special-purpose compiler framework that is able to produce scalable and efficient code that runs on large supercomputers like JUQUEEN of JSC.

The project EXASTEEL is concerned with parallel implicit solvers for multiscale problems in structural mechanics discretized using finite elements. It is focussed on modern high strength steel materials. The higher strength and better ductility of these materials largely stems from the carefully engineered grain structure at the microscale. The computational simulations used therefore take into account the microstructure, but without resorting to a brute force discretization (which will be out of reach for the foreseeable future). The researchers' approach combines a computational multiscale approach well known in engineering (FE) with state-of-the-art parallel scalable iterative implicit solvers developed in mathematics. 

In many turbulent convection flows in nature and technology the thermal diffusivity is much higher than the kinematic viscosity which means that the Prandtl number is very low. Laboratory experiments in very low-Prandtl-number convection have to be conducted in liquid metals which are inaccessible for laser imaging techniques and require analysis by ultrasound or X-rays. Researchers of the TU Ilmenau and the Occidental College Los Angeles ran direct numerical simulations of this regime of turbulent convection at high Rayleigh numbers to reveal the full 3D structure of temperature and velocity fields.

Leveraging the petascale computing power of HPC system JUQUEEN of the Jülich Supercomputing Centre, researchers at the Chair of Computational Science at ETH Zurich performed unprecedented large-scale simulations of cloud cavitation collapse with up to 75’000 vapor cavities on resolutions of up to 0.5 trillion mesh cells and 25’000 time steps.

Direct numerical simulation (DNS) of turbulent mixing layers has been possible only in relatively recent times. In an ambitious project using HPC system JUQUEEN of JSC, scientists analysed the process of mixing layer formation in a flow configuration with sizeable compressibility effects by numerically reproducing the flow conditions of a well documented flow case with two turbulent streams. Large-scale direct numerical simulation were performed in a wide computational domain which included the two upstream turbulent boundary layers developing on the two sides of a zero-thickness splitter plate and their early merging region. The complexity of the flow, the extent of the computational box and the mesh size made the study extremely…

An international team of scientists conducted high-precision spectral element simulations which resolved the fine-scale structure of turbulent Rayleigh-Bénard convection, in particular the statistical fluctuations of the temperature and velocity gradients.

Liquid chromatography is an industrially relevant application of mass transport through a porous medium, where a fluid mix of chemical substances is separated into its components by passing through a cylindrical tube densely packed with solid, spherical adsorbent particles (the column). The number of chemical substances that can be separated by a column (its separation efficiency) is determined by how the packing particles are distributed over the column volume. 

The process in which a magnetic field is amplified by the flow of an electrically conducting fluid such as liquid metal or plasma, known as dynamo action, is believed to be the origin of magnetic fields in the universe including the magnetic field of the earth. Laboratory experiments using liquid sodium try to investigate the underlying mechanisms. Especially one setup has been successful in producing a dynamo with a free turbulent flow, the Von Karman Sodium experiment in Cadarache, consisting of a cylindrical vessel filled with liquid sodium, stirred by two counter-rotating soft-iron impellers.

The accurate simulation and modeling of turbulent fluid motion is a very important research subject in natural and engineering sciences. Ranging from the application in aircraft design, the aeroacoustic optimization of an automobile and the cooling of modern microprocessor devises, the efficient simulation of turbulence is a key technology in modern engineering and design of technical products and devices.

Turbulence is pervasive in turbomachinery, and predicting its effect on the flow and performance is a major challenge for CFD (Computational Fluid Dynamics) tools. RANS (Reynolds-averaged Navier–Stokes) methods, modelling the averaged impact of turbulence, are currently the industrial standard. Scale-resolving approaches such as DNS (Direct Numerical Simulation) and LES (Large Eddy Simulation) compute either all turbulent structures directly, or only represent the larger, and model the impact of the smallest structures. 

Understanding the mechanisms involved in the turbulent transition in boundary layers is crucial for many engineering domains. The instabilities that develop in to those flows are highly non-linear and unsteady. They are mainly studied by analytical theories supplemented by direct numerical simulations (DNS) of the entire flow dynamics which must be sufficiently accurate to properly take into account all spatial and temporal characteristic scales and their non-linear interactions.