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Stuttgart Mühlhausen, Germany

Lohner R.,George Mason University | Britto D.,SL Rasch GmbH | Michailski A.,SL Rasch GmbH | Haug E.,ESI France
Engineering Computations (Swansea, Wales) | Year: 2014

Purpose - During a routine benchmarking and scalability study of CFD codes for typical largescale wind engineering runs, it was observed that the resulting loads for buildings varied considerably with the number of parallel processors employed. The differences remained very small at the beginning of a typical run, and then grew progressively to a state of total dissimilitude. A "butterfly-effect" for such flows was suspected and later confirmed. The paper aims to discuss these issues. Design/methodology/approach - A series of numerical experiments was conducted for massively separated flows. The same geometry - a cube in front of an umbrella - was used to obtain the flowfields using different grids, different numbers of domains/processors, slightly different inflow conditions and different codes. Findings - In all of these cases the differences remained very small at the beginning of a typical run, they then grew progressively to a state of total dissimilitude. While the mean and maximum loads remained similar, the actual (deterministic) instantiations were completely different. The authors therefore suspect that for flows of this kind a "butterfly effect" is present, whereby even very small (roundoff) errors can have a pronounced effect on the actual deterministic instantiation of a flowfield. Research limitations/implications - This implies that for flows of this kind the CFD runs have to be carried out to much larger times than formerly expected (and done) in order to obtain statistically relevant ensembles. Practical implications - For practical calculations this implies running to much larger times in order to reach statistically relevant ensembles, with the associated much higher CPU time requirements. Originality/value - This is the first time such a finding has been reported in the numerical wind engineering context. © Emerald Group Publishing Limited. Source


Michalski A.,SL Rasch GmbH | Gawenat B.,SL Rasch GmbH | Gelenne P.,ESI France | Haug E.,ESI France
Journal of Wind Engineering and Industrial Aerodynamics | Year: 2015

The sensitivity of membrane structures to transient wind loads becomes severe at wide spans and low pre-stress levels of the membrane. At stationary wind loads, the elastic behaviour of the flexible membrane leads to deformations with an associated change of the flow conditions and wind pressure distributions. This effect can be enhanced by time dependant fluid fluctuations such as atmospheric or building induced turbulences. Common methods in wind engineering practise like small scale wind tunnel experiments do not fully cover non-linear structural behaviour, contact interaction between membrane and structural elements and the interaction of the flow field with the structural response. Therefore numerical tools are used for the structural design of lightweight membranes. This paper presents results of the first industrial application of the fully coupled fluid structure interaction simulation for aerodynamically sensitive membrane structures situated in a built environment. The dynamic behaviour for gust induced wind loads has been investigated and reaction forces were determined. The application of the fully computational wind engineering method, in time domain, allowing as well for non-linear structural effects as for fluid structure interaction effects, makes the discovery of large dynamic amplification factors possible. The described procedure and results were reviewed and approved by Buro Happold, one of the world leading structural engineering offices. © 2015 Elsevier Ltd. Source


Michalski A.,SL Rasch GmbH | Britto D.,SL Rasch GmbH | Gelenne P.,ESI France | Haug E.,ESI France
6th European and African Conference on Wind Engineering, EACWE 2013 | Year: 2013

This paper presents the work on the structural design of large membrane structures applying numerical simulation tools. This article outlines a virtual design methodology for large lightweight structures under the impact of fluctuating wind loads. It emphasises the work on wind load simulations in urban environment. Source


Lohner R.,George Mason University | Haug E.,SL Rasch GmbH | Michalski A.,SL Rasch GmbH | Muhammad B.,SL Rasch GmbH | And 3 more authors.
Journal of Wind Engineering and Industrial Aerodynamics | Year: 2015

Recent developments that are pertinent to the particular field of computing lightweight structures exposed to windloads are described. The topics covered include computational fluid dynamics (CFD), computational structural dynamics (CSD) and fluid-structure interaction (FSI) for wind vs. aerospace engineering, recent hardware and software trends, butterfly effects, rogue loads, and adjoint estimation of boundary conditions. © 2015 Elsevier Ltd. Source


Lohner R.,George Mason University | Lohner R.,SL Rasch GmbH | Lohner R.,ESI Group | Britto D.,George Mason University | And 8 more authors.
51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013 | Year: 2013

A series of numerical experiments was conducted for massively separated flows. The same geometrya cube in front of an umbrella - was used to obtain the flowfields using different grids, different numbers of domains/processors, slightly different inflow conditions and different codes. In all of these cases the differences remained very small at the beginning of a typical run, they then grew progressively to a state of total dissimilitude. While the mean and maximum loads remained similar, the actual (deterministic) instantiations were completely different. We therefore suspect that for flows of this kind a 'butterfly effect' is present, whereby even very small (roundoff) errors can have a pronounced effect on the actual deterministic instantiation of a flowfield. This has important consequences for the type of runs that have to be conducted to determine windloads for civil engineering structures. © 2013 by The authors. Source

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