ExaQUte by Blazon Publishing and Media Ltd

Simulations for complex engineering problems Simulations can play an important role in civil engineering projects, enabling staff to assess how different wind conditions will affect a structure and adapt the design accordingly. We spoke to Professor Fabio Nobile and Professor Riccardo Rossi about the work of the ExaQUte project in constructing a framework to help address complex engineering problems. The local wind

patterns are an important consideration in the design and construction of civil engineering structures, and sophisticated simulation techniques are used to ensure that buildings, bridges and other structures are resistant to different conditions. It is not possible to simulate all possible wind scenarios, so the aim instead is often to simulate a sufficient number to be broadly representative of the conditions in which the structure will function, which raises several questions. “How many of those simulations are required? How accurate should they be? Can we improve the design of the structure by selecting certain types of scenarios?” asks Fabio Nobile, Associate Professor of Scientific Computing and Uncertainty Quantification at EPFL. These questions are central to Professor Nobile’s work in the ExaQUte project, an EU-backed initiative which brings together researchers from across Europe. “We are addressing these questions in the project in probabilistic terms, with a type of Monte Carlo simulation,” he explains.

Exascale systems This research is built on leveraging the power of the next generation of Exascale supercomputers, systems of interconnected central processing units (CPUs) that can

perform up to 1018 floating point operations per second. The aim is to utilise the power of these systems, in particular the potential for parallelism. “A simulation can take hours and use a lot of resources. We don’t want to waste these resources, we want to take advantage of them, so we have to build some maths to back up what we are doing,” says Riccardo Rossi, Research Professor at CIMNE and the project’s Principal Investigator. One of the challenges in exploiting the

available resources efficiently, with the aim of ‘quantifying the uncertainties’ in the outputs of interest. Some simulations may take a longer time than others, which is another important consideration in terms of the project’s overall agenda. “The more resources we have - so the larger the computer - the more difficult it is to orchestrate the simulations. This is where the maths plays a very important role, to devise a strategy to do those simulations in

The more resources we have - so the larger the computer - the more difficult it is to orchestrate the simulations. This is where the maths plays a very important role, to devise strategy to do those simulations in parallel. potential of these Exascale systems is that great care has to be taken with respect to the amount of information that needs to be transferred between the processors. “If there is a constant need to exchange information between CPUs, this has a negative impact on performance,” continues Professor Rossi. “The communication time between those processors effectively reduces the power of the simulation.” The challenge here is in orchestrating the simulations effectively and using the

parallel,” explains Professor Rossi. Multiple simulations can be run in parallel if there is no need to exchange information between processors, while others require interactions that need to be carefully scheduled. A second major topic on the project’s agenda centres around mixing simulations of varying levels of accuracy and with different costs. Running simulations for civil engineering structures can be extremely costly, so Professor Nobile is investigating the balance between the accuracy of the

simulations and the number of scenarios that are represented, using a Multi-Level Monte Carlo (MLMC) approach. “Maybe we could run many simulations, but at a lower level of accuracy. Or maybe we could run just a few simulations with a higher level of accuracy, but representing a smaller number of scenarios,” he outlines. Maths plays a central role here in determining the optimal allocation of number of scenarios to be run at each accuracy level, taking also into account the available resources and the need to orchestrate the ensemble of simulations. “We try to find the optimal allocation by building a body of knowledge. We run several scenarios, and based on this we then adapt our approach and reach decisions on the time-span of the simulations and the level of accuracy,” continues Professor Nobile.

Optimising design The wider goal of the ExaQUte project is to help optimise the design of a building, which is intrinsically an iterative process. A simulation is run on a building for different wind conditions and the results are then collected. “We start from certain scenarios and we assess the dynamic loads on the building. Then we change the different parameters a little bit, and we run more scenarios until we reach an optimal design that is robust with respect to the prevailing conditions,” says Professor Nobile. The Multi Level Monte Carlo approach is key here to reducing the overall computational cost. This represents an alternative to traditional experimental approaches to simulating the effects of wind on a building, which are known to have some significant shortcomings inherent to the experimental setup. “We believe that some of those shortcomings could be addressed through the use of computers.

However, computational wind engineering is a relatively novel field, as it’s only relatively recently that we have developed the required computational power, tools and knowledge,” outlines Professor Rossi. A significant degree of progress has been made over the course of the project, with mathematicians and engineers from the different partners benefiting from the opportunity to share knowledge, ideas and expertise. A number of new tools and methods have been developed in the project, with all of the developments open-source, while Professor Rossi says studies into the possible commercial use of some of the technologies are also planned. “This is about assessing the realistic potential applications of what we are doing outside academic settings,” he says. The primary focus of the project is wind engineering, yet some of these tools and methods could eventually be applied in other areas of technology. “There are the same kinds of problems in other fields. So, once we’ve learned how to manage the extreme level of parallelism needed to tackle otherwise unfeasible robust optimization problems, then we can definitely look to move into other fields,” stresses Professor Rossi. There are still many open questions in this area however, and Professor Nobile says there is great scope for further research, which will form an important part of his agenda in future. “We have learned a lot in this project, yet it’s also raised new questions,” he outlines. “We will definitely continue our research in this field. For example we are trying to develop a sound algorithm to optimise robustly the design of a building in unsteady flow conditions, and there is a lot of margin for improvement there.”

ExaQUte EXAscale Quantification of Uncertainties for Technology and Science Simulation Project Objectives

The ExaQUte project aims at exploiting next generation HPC capabilities in tackling Uncertainty Quantification (UQ) problems and Optimization Under Uncertainties (OUU). Such technologies will be demonstrated in application to “wind engineering” with the ultimate objective of improving the design procedure of buildings with respect to wind loads.

Project Funding €3,124,255

Project Partners

http://exaqute.eu/#consortiums

Contact Details

Project Co-investigator Professor Fabio Nobile EPFL - SB - MATH - CSQI MA B2 444 (Batiment MA) Station 8 CH-1015 Lausanne Switzerland T: +41 21 69 34244 E: fabio.nobile@epfl.ch W: http://csqi.epfl.ch Professor Fabio Nobile Professor Riccardo Rossi

Dr Riccardo Rossi, Civil Engineer, is an Associate Professor at the UPC. He is active in the field of CFD and Multiphysics. He is the head of the “Kratos Multiphysics” group at CIMNE where he serves as a Full Research Professor, and coordinator of the EU projects ExaQUte and EdgeTwins.

Views of flow and pressure field around the CAARC (Commonwealth Advisory Aeronautical Council) tall building model.

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