Abstract
For the design of slender structures the precise wind-load determination plays a major role. While CFD Simulations are already used in other industries, wind tunnel tests are still the state of the art in wind engineering. Currently a global research effort is undertaken to test the accuracy and suitability of CFD simulations for reliable wind-load determination. To include all methods and recent developments from other disciplines the Bluff Body Benchmark (BBB) was developed as a test case for a real engineering problem. It ensures tight constraints regarding the atmospheric boundary layer which is essential in wind engineering. However, the benchmark is open to all methods for which the participants check and document their results to the best of their knowledge. The evaluation of the Bluff Body Benchmark will not only provide an overview of the status quo but will also form a database for the validation of further research. We invite any research group to participate with their preferred methods (see table below).
Initiators
Sophie Breitkopf, M.Sc. and Aljoscha Sander, M.Sc.
sophie.breitkopf@tu-dortmund.de, Chair of Structural and Conceptual Design, TU Dortmund
Cases
Background
The development of Computational Fluid Dynamics (CFD) and its applications were mainly driven by aerospace engineering in the 1970s. With the increase of computer power, CFD simulations became available for a broader field and entered the construction sector. Wind tunnels provided the foundation for experimental findings regarding wind in general. Until today scaled wind tunnel tests are the most accepted tool to determine the wind effects on buildings, while CFD simulations are emerging as a valuable tool for the design process.
Therefore, current research aims to evaluate the reliability of CFD simulations with the ongoing increase of computational power. The latter is one of the limiting factors when attempting to run simulations for very high Reynolds numbers and thus in the range of full-scale buildings. Other critical aspects for the accuracy of the simulations in wind engineering are the adequate generation of the atmospheric boundary layer on the one hand and the highly complex fluid structure interaction on the other hand. With these ongoing limitations in mind, the question as to the currently achievable quality arises, when adhering to the current CFD best practice guidelines and considering the most recent findings and developments.
While qualitative benchmarks for engineering problems exist for wind tunnels (Holmes & Tse (2014), Fritz et al (2008)), CFD benchmarks often focus on detailed 2D flows under predefined conditions and often lower Reynolds numbers (Rodi (2004), BARC Bruno et al. (2014)). Therefore it is not known, how well best practice CFD simulations can deal with occurring engineering problems. When codes are a conservative first guess for wind loads on structures, that can be enhanced through wind tunnel tests, it is time to see which quality is delivered by state of the art CFD simulations. At a meeting held during the EAWE PhD Seminar 2021 in Porto, it was decided to develop a method agnostic benchmark for an engineering problem at a high Reynolds number (see table of cases above).
Benchmark set-up
The bluff body benchmark was developed to test the sensitivity of all available methods for a given engineering problem: Representing a standard supporting structure (based on CAARC building) a free standing building of 180 m height and a 6:1 ration (height:width) is chosen. For this building the horizontal wind loads become relevant for the design, while it is still covered by most codes. Three different cross-sections are compared: A square (1:1), a rectangle (2:1) and a circle. A sub-urban terrain with a high 10-min-mean wind-speed (between zone III and IV for Japan, China, Europe, Canada and Australia) is chosen. Simulations can be run for different levels of complexity, namely: the 2D cross-section at the building top or the 3D rigid model. As an outlook a 3D aeroelastic model is included for the highly advanced participants. The required information on the dynamic properties of the structure are provided in the template (see template link below).
Rules of the game
Any research group or individual researcher may participate. Multiple teams from the same organization are welcome. We think it is essential that all groups may apply their own proven methods which applies to the method itself (CFD, wind tunnels, engineering models, codes), the scale, the turbulence generation, solvers and discretizations. It is expected that the results that are handed in, have carefully been obtained and validated through guidelines and experience. To get an overview, the simulations should be documented, at least by filling in the template simulation-form (Version D). Any set-up combination from the case table is a valuable contribution. It is not mandatory to run all possible cases in order to participate. While the upload is not anonymous, the organisers ensure that all results are handled with discretion and anonymously. If you are interested, please send an e-mail to sophie.breitkopf@tu-dortmund.de in order to register for the mailing list.
Timeline
The finalized version of this benchmark was be presented at 8th EACWE (20-23.09.2022) in Bucharest and at the 18th EAWE PhD Seminar (02-04.11.2022) in Bruges. The kick-off meeting for the benchmark took place in November 2022 with a midterm meeting in February 2023. The results will be handed in until 11/2023 through email. All results obtained until then will be included anonymously in a paper for the Journal of Wind Engineering and Industrial Aerodynamics and possibly presented at the BBAA IX Conference in Birmingham. Without a link to the results, we will acknowledge all participants for their valuable help and contributions.
FAQ
Are there reference results?
To the best of our knowledge there is not “exact” solution for this problem and additionally we see a risk in a “exact” dataset to calibrate against. From existing codes reference values for the two main wind directions can be determined for a magnitude of expected results.
Do I have to run CFD or wind tunnel simulations at high Reynolds numbers?
This bluff body benchmark states a high Reynolds number problem for which different approaches shall be compared qualitatively. To use scaled models from which the full-scale results are derived is also a legitimate approach.
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