The flow across a street canyon of variable width—Part 1: Kinematic description

Measurements have been made of the scalar dispersion of smoke released from a two-dimensional slot in the wall perpendicular to a boundary layer flow and located parallel to and midway between two square obstacles placed on the wall. The Reynolds number of the boundary layer at the slot location without the obstacles in place is Rθ≈980Rθ≈980. Two optical systems with CCD cameras facing each other have been used to measure simultaneously the velocity and scalar concentration fields, respectively, with PIV and Mie scattering diffusion. In Part B of this paper the data will ultimately provide detailed information about the scalar fluxes for this environmentally relevant geometry. Here in Part A the results of the velocity field measurements in the streamwise plane will be reported for spacings between the obstacles of 1–10 obstacle heights.The mean flow measurements reveal the increasing complexity of the canyon flow with increasing obstacle spacing. A primary vortex, with negative spanwise vorticity, occurs within the canyon for all spacings and is driven by the flow above. The circulation region of this vortex extends above the level of the tops of the obstacles. For spacings of 2h2h and greater, a secondary vortex with positive vorticity appears in the upstream corner of the canyon, and a tertiary vortex with negative spanwise vorticity first appears in the downstream corner of the canyon for an opening of 6h6h. The spatial distribution of the level of turbulence within and around the canyon is indicated by the contours of two-dimensional turbulent kinetic energy, 12(u2¯+v2¯). The region of elevated turbulence in the shear layer created by the upstream obstacle penetrates deeper into the canyon with increasing canyon opening. For all openings, the Reynolds shear stress is negative above the canyon. The vertical extent of the high negative stress region increases as the canyon opening increases, and it also penetrates well within the canyon. This region of negative stress is due to the transport, by the primary canyon vortex, of low momentum fluid from deep within the canyon upward and high momentum fluid from above the canyon downward.