OpenFOAM Workshop 26 January 2024

Tom Fahner

Wind Assessment Tool - Pedestrian wind comfort studies for non OpenFOAM experts

Urban planning in recent years is focusing on more densely populated areas, where high-rise buildings are replacing medium to low-rise buildings to accommodate more residents on a smaller surface area. These changes result in challenges in terms of city planning and quality of life for people living in these areas.

One of the aspects that becomes important in the case of tall buildings is pedestrian wind comfort. Predicting the effect of the change in the built environment typically requires a CFD or wind tunnel study. Most architects however lack the technical knowledge to carry out these studies. On the other hand they are interested in a quick assessment of their design in terms of the wind climate effects. Our Wind Assessment Tool provides this service in early stage development. All that is necessary from the architect is a model of their building, the surrounding buildings and some details about the location. Within hours the wind climate is assessed and can be compared to other design variants in a web browser. For more detailed analysis of the results, 3D data can be downloaded. The WAT is a dedicated tool combining the design knowledge of the architect with our knowledge on CFD and OpenFOAM.

Sebastiaan Kuipers

MultiRegionFoam: solving multiphysics problems of the multi-region coupling type within OpenFOAM

Eduard Montella

Modeling granular flow dynamics and structure interaction Insights from sedFoam solver

Gaining insight into granular flow dynamics is crucial in several sediment transport applications. A critical aspect involves understanding how these dynamics drastically differ based on the
initial volume fraction. Under shear deformations, loose granular beds trigger rapid, accelerated
flows, while denser beds show delayed mobility and a creeping flow. The interplay of geometrical granular dilatancy and pore pressure feedback underlies these behaviors. To address these
effects, a dilatancy model has been integrated into sedFoam [1], an OpenFoam-based software used
in sediment transport applications. We’ve validated this model by reproducing numerically the
experiments conducted by [2]. In this setup, a horizontal granular layer immersed in a viscous
fluid was tilted beyond the critical angle to initiate an avalanche. Several initial volume fractions
resulted in different observed behaviors. The successful alignment with experimental data suggests
that sedFoam is able to capture the interplay between porosity changes and fluid pressure during
the initial stages of the granular avalanche.

We extended our findings to 2D configurations were dilatancy plays a key role. Specifically,
attention was directed towards understanding the breaching mechanism and shear failures of granular columns. Initially, we replicated numerically the collapses of small-scale granular columns with
varying initial fractions of [3]. Consistent with experimental results, we noticed that loosely packed
columns initially contract, triggering rapid collapses via positive pore pressure. Conversely, densely
packed columns dilate, generating negative pore pressure that stabilizes the column and leads to
a slower collapse. Extending these discoveries to a larger scale, the model accurately predicts the
dilative behavior and the turbidity currents observed in the breaching experiments of [4].
Previous tests have showcased sedFoam’s capability to predict various natural coastal processes
or human-induced activities such as dredging operations. However, there’s a particular interest
in understanding how fluid and granular flows interact with solid structures. To address this, a
six-degree-of-freedom solver has recently been integrated into sedFoam. This enhancement sheds
light on the interaction between particle and fluid forces on rigid objects within sediment transport
applications. This implementation represents a significant leap towards comprehending intricate
interactions among fluids, particles, and structures, offering deeper insights into sediment transport
and structural responses.

References
[1] J. Chauchat, Z. Cheng, T. Nagel, C. Bonamy, and T.-J. Hsu, “Sedfoam-2.0: a 3-d two-phase flow
numerical model for sediment transport,” Geoscientific Model Development, vol. 10, no. 12, 2017.
[2] M. Pailha, M. Nicolas, and O. Pouliquen, “Initiation of underwater granular avalanches: influence
of the initial volume fraction,” Physics of fluids, vol. 20, no. 11, p. 111701, 2008.
[3] L. Rondon, O. Pouliquen, and P. Aussillous, “Granular collapse in a fluid: role of the initial volume
fraction,” Physics of Fluids, vol. 23, no. 7, p. 073301, 2011.
[4] D. Weij, “On the modelling of the unstable breaching process,” 2020.
[5] J. Steiner, C. Morize, I. Delbende, A. Sauret, and P. Gondret, “Erosion by the unsteady motion of
a disk close to a granular bed,” Bulletin of the American Physical Society, 2023.