Organized turbulent structures –- The link between experimental data and LES modeling

Abstract

Coherent boundary layer flow structures are assumed to contribute largely to the spatial heterogeneity and temporal variability of atmospheric flow and dispersion phenomena. Even in the case of highly irregular urban flows, coherent motions can have strong influence on the turbulent transport of momentum and scalar quantities, as well as on the turbulent kinetic energy. In this context, coherent structures are treated as large-scale flow patterns with significant temporal persistence. They are embedded in both the mean flow field and the random, small-scale turbulence. In order to characterize unsteady flow and dispersion processes in urban areas, eddy-resolving CFD approaches like Large-Eddy Simulation (LES), as well as qualified laboratory experiments are of special interest and great value. Transient phenomena like the dispersion of exhaust air contaminants, localized vortex shedding or the evolution, progression and disorganization of coherent eddies might be handled adequately with LES approaches. However, these types of models are not only computationally challenging, but also in need of extensive high-resolution validation data. Furthermore, the inflow boundary conditions for such models have to provide a sufficiently realistic spatio-temporal description of turbulent eddies. To address these issues, transient flow phenomena have to be characterized in experimental validation data sets. The filtering of dominant eddies offers the potential to describe and mathematically quantify their influence on the overall dynamics of the flow and furthermore creates a basis for comparisons with numerical results. However, even if sufficiently resolved and accurate information on temporally and/or spatially correlated flow quantities is available, a crucial point remains the problem-oriented choice of unbiased mathematical tools for the detection of turbulent coherent structures. EWTL initialized a comparative wind tunnel study, concerned with the implementation and application of well-established mathematical approaches of eddy structure identification in near-surface turbulent boundary layer flows. Amongst others, proper orthogonal decomposition (POD), linear stochastic estimation (LSE) and the wavelet transform (WT) were applied to turbulent velocity data measured under carefully controlled boundary conditions in a boundary layer wind tunnel facility. The presentation will summarize main results with respect to the potential of either method to serve as a tool for validation purposes and/or for the generation of inflow boundary conditions. Their particular strengths and weaknesses concerning urban-type flow fields will be illustrated by examples, and problems related to turbulent coherent structure characterization will be discussed.