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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/gmd-2018-7
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Development and technical paper
12 Mar 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).
Implementation of an Immersed Boundary Method in the Meso-NH model: Applications to an idealized urban-like environment
Franck Auguste1, Géraldine Réa1, Roberto Paoli3,4, Christine Lac2, Valery Masson2, and Daniel Cariolle1,2 1CECI, CNRS, CERFACS, Toulouse, France
2CNRM, CNRS, Météo-France, Toulouse, France
3University of Illinois at Chicago, Department of Mechanical and Industrial Engineering, Chicago, USA
4Argonne National Laboratory, Argonne, USA
Abstract. This study describes the numerical implementation, verification and validation of an Immersed Boundary Method (IBM) in the atmospheric solver Meso-NH for applications to urban flow modelling. The IBM represents the fluid-solid interface by means of a LevelSet Function and models the obstacles as part of the resolved scales.

The IBM is implemented with a three-steps procedure: first, an explicit-in-time forcing is developed based on a novel Ghost-Cell Technique that uses several image points instead of the classical one mirror point. The second step consists in an implicit step projection where the right-hand side of the Poisson equation is modified by means of a Cut-Cell Technique to satisfy the incompressibility constraint. The condition of non-permeability is achieved at the embedded fluid-solid interface by an iterative procedure applied on such modified Poisson equation. In the final step, the turbulent fluxes and the wall model used for Large-Eddy-Simulations (LES) are corrected and a wall model is proposed to ensure consistency of the subgrid scales with the IBM treatment.

In the second of part of the paper, the IBM is verified and validated for several analytical and benchmark test cases of flows around single bluff bodies with increasing level of complexity. The analysis showed that MNH-IBM reproduces the expected physical features of the flow, which are also found in the atmosphere at much larger scales. Finally the IBM is validated in the LES mode against the Mock Urban Setting (MUST) field experiment, which is characterized by strong roughness caused by the presence of a set of obstacles placed in the atmospheric boundary layer in nearly-neutral stability conditions. The Meso-NH IBM-LES reproduces with reasonable accuracy both the mean flow and turbulent fluctuations observed in this idealized urban environment.

Citation: Auguste, F., Réa, G., Paoli, R., Lac, C., Masson, V., and Cariolle, D.: Implementation of an Immersed Boundary Method in the Meso-NH model: Applications to an idealized urban-like environment, Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2018-7, in review, 2018.
Franck Auguste et al.
Franck Auguste et al.
Franck Auguste et al.

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Short summary
The numerical implementation of an Immersed Boundary Method in the atmospheric solver Meso-NH is presented. This technique models the fluid-solid interaction and allows to simulate the urban flows considering the buildings as part of the resolved scales. This study constitutes a first robust step towards a better understanding of the interactions between Weather and Cities and better predictions of such interactions.
The numerical implementation of an Immersed Boundary Method in the atmospheric solver Meso-NH is...
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