Efficient Generation of Inflow Conditions for Large Eddy Simulation of Street-Scale Flows |
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Authors: | Zheng-Tong Xie Ian P Castro |
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Institution: | (1) School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, UK |
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Abstract: | Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation
of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations
to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent
shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow
conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing
flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor.
The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered
to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data
from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating
plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities,
the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic
boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations. |
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Keywords: | Weather scale Urban CFD Coupling Bluff bodies Channel flow |
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