POD characterisation of extreme wake patterns of turbulent wind fields past rectangular buildings

Abstract

The turbulent wind fields around three different rectangular building models in two types of simulated atmospheric boundary layer are measured using particle image velocimetry. Proper orthogonal decomposition (POD) is then applied to recognize and extract the coherent structures of large energy of fluctuating velocities. The extreme flow patterns of the first two POD modes are identified from the instants of occurrence of peak POD coefficients. The way how these extreme patterns influence the instantaneous flow fields is discussed. The results demonstrate that these extreme patterns can contribute to the instantaneous building wake flow in completely different manners but similar influence patterns can be summarized as described from the wake flow data of different rectangular building models. In the broad sense, one extreme wake pattern strengths the time-averaged mean flow field of the wake such as a recirculating bubble behind the building on the vertical planes and two symmetric counter-rotating vortices on the horizontal planes. However, the other extreme wake pattern causes air to flow largely in the opposite directions as the main flow. On the longitudinal vertical planes behind the building, a momentary fluid source formed at some point in the wake drives air to flow upwards with some flow backwards toward the building. On the horizontal planes, one extreme pattern reveals a pair of asymmetric vortices attached to the building leeward wall. The instantaneous peak flow characteristics influenced by the extreme POD modal patterns, apparently reported for the first time, could have important implications in building wake effect assessment especially for hazardous situations.