Large boundary layer wind tunnels, such as the wind tunnel at Saint Anthony Falls Laboratory (SAFL), provide an excellent platform for fundamental studies of the atmospheric boundary layer (ABL). Field studies on SFS turbulence are generally limited to homogeneous terrains and may have undesirable external variables that can affect accurate measurements of turbulence (e.g. non-stationarity of wind, weather, unknown boundary layer heights etc.). To extend the experimental study of SFS physics to boundary layers over heterogeneous surfaces it is necessary to have control of the flow conditions and to specificy the heterogeneous surface properties. It is also important to have the ability to obtain spatial information of the velocity in the boundary layer without using Taylor's frozen flow hypothesis (which breaksdown in advective conditions such as flow over a step change in roughness).

A measurement technique well-suited to this task is Particle Image Velocimetry, a Lagrangian technique that measures instantaneous fields of velocity by essentially computing the displacement of particles in the flow using a pulsed dual Nd:YAG laser sheet and high-resolution CCD cameras. PIV has advantages over other more traditional wind tunnel measurement techniques that measure velocity (i.e. pitot tubes and hot-wires) in that it provides turbulent velocity measurements that are not sensitive to the wind tunnel environmental conditions (i.e. temperature) and do not rely on Taylor's hypothesis. Furthermore, the increased capability of recent PIV systems (2048x2048 pixel CCD cameras) such as the system (TSI UltraPIV) currently used at SAFL make it an excellent choice for studying subfilter-scale quantities.

Overview of Wind Tunnel Specifications:

The boundary layer wind tunnel at the Saint Anthony Falls Laboratory has a plan length of 37.5m with a main test section fetch of roughly 15m. There is a contraction with a 6.6:1 area ratio upstream of the main test section along with flow conditioning/turbulence control consisting of a coarse wire mesh and honeycomb flow-straightener. The tunnel is driven by a 200 hp fan and operates as a closed loop, though it can also be operated with two open ends as a blow-down wind tunnel. (more wind tunnel info found here).