3D-3C Mean Velocity Measurements Below the Building Height in an Urban Canopy Flow*

The flow structure below the building tops in an urban canopy is of interest because it influences momentum transport into the canopy, contaminant dispersion, and pedestrian safety.  Measurements within the canopy also illuminate mechanisms by which wall roughness modifies boundary layers.  We have acquired 3-component magnetic resonance velocimetry measurements throughout an array of 48 cubical buildings with a single building 3X taller.   The spatial resolution of 0.06 building lengths allows identification of separation bubbles and dominant vortex structures around each building.   The mean-flow structure is highly sensitive to the freestream flow orientation relative to the street canyons.  When they are aligned, separation bubbles fill the gap between subsequent buildings and vertical axis vortices form behind the rear corners.  Separation bubbles are much larger when the flow is skewed and the streamwise mass flux within the canopy is significantly reduced.  The skewed building array has larger variations in the local vertical velocity due to the presence of streamwise vortices shed from the upper building corners.   In the skewed case, streamlines released at half building height meander around separation bubbles resulting in rapid spanwise transport.