Abstract
Generating economical, high-resolution and high-quality computational grids for Large Eddy Simulation (LES) of wind flow and convective heat transfer (CHT) around surface-mounted obstacles is not straightforward. When the grid size is used as filter, LES grids should ideally consist of cubic cells, while CHT requires a very high near-wall resolution to resolve the thin viscous sublayer and buffer layer that represent the largest resistance to CHT. To avoid very high cell numbers and the need for excessive computational resources, non-conformal grids can be considered. This paper provides a detailed evaluation of the performance of non-conformal grids with cubic cells, for wind flow and CHT around a wall-mounted cubic obstacle. LES results on non-conformal versus conformal grids are compared with each other and with wind-tunnel measurements of wind speed and surface temperature. Moreover, sensitivity analysis is performed concerning the impact of overall grid resolution, subdomain size and grid refinement ratio. Average absolute deviations between LES on non-conformal versus conformal grids are about 0.9% (0.5 °C) for surface temperature on all cube surfaces. Comparison with experiments shows for the non-conformal grid an average and maximum absolute deviation for surface temperature of 2.0% (1.1 °C) and 7.6% (3.6 °C), respectively. The sensitivity analysis shows minor impact of subdomain size on convective heat transfer coefficients (CHTC) where, on average, absolute deviations of less than 2.2% are observed. This study shows that non-conformal grids can strongly reduce the total cell count (here by a factor up to 30.2) without significantly compromising the accuracy of results.
Highlights
With the advancement of computational power and numerical methods, the use of Computational Fluid Dynamics (CFD) in wind engineering, referred to as Computational Wind Engineering, has seen a rapid growth in the past 50 years, covering a wide range of topics encountered within the lower part of the atmospheric boundary layer (ABL) [1e3]
The results show a good agreement for the flow field and an average absolute difference for surface temperatures of about 0.9% (0.5 C)
Results on the non-conformal grid are compared with wind-tunnel measurements and the agreement is quantified by validation metrics (FAC1.05, FAC1.3, normalised mean square error (NMSE) and hit rate)
Summary
With the advancement of computational power and numerical methods, the use of Computational Fluid Dynamics (CFD) in wind engineering, referred to as Computational Wind Engineering, has seen a rapid growth in the past 50 years, covering a wide range of topics encountered within the lower part of the atmospheric boundary layer (ABL) [1e3]. In the framework of implicit filtering in LES, where the grid size is used as filter, ideally, grids should consist of cubic cells as they present equal filter length in all three directions The combination of these two requirements provides an exceptional challenge in grid generation, certainly for high Reynolds (Re) number flow as encountered in wind engineering, where the overall dimensions of the problem under study can be five orders of magnitude larger than the thickness of the viscous sublayer and the size of the near-wall cells. The main goal is to provide recommendations on how economical LES grids can be generated that still provide sufficient accuracy for engineering analysis Without such recommendations, LES simulations of wind flow and CHT around buildings in an actual urban area will remain practically infeasible due to excessively large total numbers of cells and the associated excessive computational requirements.
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