5 Questions: Simulation

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Pengfei Liu

Head of Road Construction Technology (Simulation)


+49 241 80 22780



How can a machine interact with material?

The machine material interaction here mainly refers to the interactions between the paver / roller compactor and the asphalt mixtures during the pavement construction.The pavement materials are well paved from paver, and further compacted from roller compactor. For the paving stage, the auger of the paver keeps rotating to distribute the asphalt mixture along the width of the road. During the compacting stage, the screed plate of the paver or the steel drum of the roller compacts the asphalt with a certain frequency of vertical vibration. On the one hand, the vertical vibrating of the machine offers the normal pressure to the material. On the other hand, during the compaction, there is not only vertical compressive force, the compaction machine also moves forward while it compacts the material. So, there is also horizontal shear force that comes from the frictional behavior between the compacting machine and the material. If there is shear force generated at the contact point between the aggregates, there will be a rearrangement of the aggregates, and there will be a reduction in volume and particle rotation occurs even at very low stress. These processes are complex and of significance for developing durable pavement in practice, therefore, machine material interactions are investigated in our research group to better understand the processes and optimize the manufacturing operation.

What is meant by cyber-physical system?

A cyber-physical system (CPS) or intelligent system is a computer system in which a mechanism is controlled or monitored by computer-based algorithms. In cyber-physical systems, physical and software components are deeply intertwined, and they are able to operate on different spatial and temporal scales.The physical and software components interact with each other in ways that change with context. CPS involves transdisciplinary approaches, merging theory of cybernetics, mechatronics, design and process science.

If we take the asphalt compaction for example, the cyber-physical system basically has the physical object, paver or roller compactor, which creates data, as can be called a digital shade. The data goes to the software or digital model and then the digital model can translate this data into information. This information is the base of the knowledge, which we have gained to see what is actually the goal in laboratory experiments or the engineering practice, what is the best microstructure of the asphalt mixtures that we should have, and what we should produce in the asphalt. This information can then be used maybe in real time to operate the machinery in a way that it can produce this microstructure, guarantee the right quality, and even replace the workers at the construction site with these kinds of algorithms because these algorithms can optimize the construction process in real-time. This is a long time goal and we are very far away from this at the moment. But what we're doing at the moment is to develop the methods towards this.

Which objects does multiscale simulation consider?

The multiscale simulation aims to deeply investigate the detailed mechanical/thermal/hydraulic responses of asphaltic materials at different length scales through the numerical simulations, like finite element method, discrete element method, molecular dynamic method. The different length scales can be defined from molecular scale (mineral components of aggregates and chemical components of bitumen), nanoscale (bitumen), microscale (asphalt mastic and asphalt mortar), mesoscale (asphalt mixture), and macroscale (asphalt pavements). Some significant critical stress/strain behavior within the asphaltic materials at smaller scales can be accurately exhibited, which is the specialty of the multiscale simulations and cannot be achieved by laboratory experiments. According to such critical strain/strain responses, the initial flaws or micro-cracks within asphalt pavements can be made up by small laboratory consumptions. The cross-scale relations can be effectively developed according to the mechanical behavior at different scales.With a deep insight into the multiscale mechanical behavior and cross-scale relations of asphaltic materials, the asphalt pavement design can be effectively improved.

In which areas are there overlaps with the other specialist areas of tunnel technology and traffic engineering?

As I answered for the second question, the CPS consists of the different types of large-scale infrastructures, such as pavements, tunnelsand also bridges, that have a large amount of information and physical components. To achieve efficient and safe road transportation is one of the motivations to carry out the research on CPS and is one of the most challenging areas, as it possesses the information, physical and social features. The road traffic system is that the people are the key element inthe road traffic system. All development and changes of the traffic system are caused by travel behavior of people. This determines the problem that the cyber-physical system applied in traffic is not only the integration of the physical process of the material and energy information, but also the integration of information processes and behavior processes. Furthermore, in the traffic cyber-physical system, the information should have significant effect on the traveler behavior, when the interaction between physical systems and biological systems (people) is established by information flow, to realize information value. For the road traffic control, our CPS, which mainly focuses on the mechanical and functional performances of the infrastructures, can make road traffic more safely and efficiently with cooperation with tunnel and traffic departments. Based on this, the research from our research group can be well connected to the other specialist areas of tunnel technology and traffic engineering.

What makes working with simulations particularly interesting?

Well, I think the most interesting thing about simulation work is that we can give full play to our knowledge and imagination to realize virtual experiments that are difficult to achieve in the laboratory. Unlike the laboratory work and analytical calculations, simulation allows us to create the physical structures of pavements with various sizes and material types. We can build any structures with any loading and boundary conditions. It feels like there is a whole laboratory and test track in computer, we can try any combinations of materials and structures without exhaustive labor work. In addition, simulation provides us with deep insights into the detailed mechanical behavior of structures. In most of the recent researches of asphaltic materials, scholars tried to investigate the mechanism of asphalt binders and aggregates through developing more advanced laboratory equipment, but they can hardly determine the actual behavior of the fundamental constituents with respect to the realistic loadings. However, in simulations, such issues can be easily addressed by providing the visualizations of stress-strain contour plots. From global view, the mechanical performances are effectively determined; from local view, the critical responses and detailed interactions between internal parts are clearly exhibited.