Stereoscopic Particle Image Velocimetry Measurements in an Urban-Type Boundary Layer: Insight into Flow Regimes and Incidence Angle Effect

Abstract

An experimental investigation of the three-dimensional flow through an urban-type array (four rows of three cuboid Plexiglas blocks) in a laboratory modelled neutrally stratified atmospheric boundary layer is presented. We concentrate on the effect of the streamwise spacing between adjacent rows defining two different flow regimes (wake interference and skimming flow) as well as the effect of the incident angle of the approaching boundary layer. Stereoscopic particle image velocimetry measurements provide all three components of the velocity field in closely spaced two-dimensional planes in a region located in the middle row downstream of the centre block. It is found that the maximal exchange rate between the fluid within the street and the flow above is for the wake interference regime. Two regions are apparent: one influenced by streamwise velocity fluctuations, the other by spanwise fluctuations. In addition, the incidence angle of the incoming flow has a much more dramatic effect for the wake interference regime that would greatly favour dispersion. Coherent-structure identification tools are applied to obtain information on the shape, extent and localisation of vortical structures.

Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions

“…It is also crucial to be able to accurately determine 556 R. Vinuesa et al vortex shedding frequencies in complex geometries for adequate structural design in order to avoid large vibrations induced by resonance. For instance, the flow around one building under the influence of various surface layer conditions was studied experimentally by Corke et al, [3] and pollutant dispersion in a number of urban environments was characterised by Nagib and Corke [4] and Monnier et al [5] by means of wind tunnel testing. The relation between surface mounted cylinders and jets emanating from the surface has been studied and characterised by Fric and Roshko.…”

The Structure of Urban Flows

“…In conclusion, the study presented by Monnier et al [71] shows how the PIV strategy can produce relevant results that deepen the understanding of the fundamental drivers of urban flows.…”

“…In Figure 7 we show the schematic representation proposed by Hunt et al [121], where a total of 4 vortical structures are identified: the horseshoe vortex (A) which forms around the obstacle; the roof vortex (B) and the vortices on the obstacle sides (C), which have a close interaction with the wake; and finally the well-known arch vortex (D) on the leeward side of the obstacle, which will be discussed in detail below. Note that there are a number of methods used to identify coherent structures, and although they do not exhibit significant differences when applied to turbulent flows [122], they may emphasize slightly different phenomena when applied to urban environments [71]. Furthermore, we argue that the use of adequate thresholds may help to discern intense event inherent to turbulent fluctuations and large-scale effects induced by the geometry.…”

“…However, the size of the urban canopies is usually a limiting factor for such experiments. Nevertheless, the PIV technique, when properly implemented, is a very powerful tool to study complex flows.Monnier et al[71] presented a research-oriented study that aimed to test the applicability of PIV methodologies in the investigation of flows over an urban-like obstacle array. The authors relied on work of Nagib et al[72] to design the experimental strategy.…”

“…The experiment mainly consists of PIV measurements varying the incidence angle of the incoming freestream flow in a scaled urban environment. The measurement system introduced in the wind tunnel consists of an array of three hot wires mounted over a vertical traverse system enabling measurement of the velocity profiles starting from a position close to the lower wall and extending approximately 400 mm above it[71].…”