Three-dimensional Flow Structure of a Simulated Atmospheric Boundary Layer Using a Modified Irwin Spire-Roughness Technique

Irwin spires have been used by many experimental programs to simulate atmospheric flows. The design developed by Irwin relies on a single row of spires. This paper reports an experimental investigation into the structure of a boundary layer simulated in a wind tunnel using the traditional spire-roughness technique in comparison with multiple rows of spires. Mean flow and turbulence characteristics were measured using three-dimensional laser Doppler velocimetry (LDV) measurements in the push through ultra-low speed wind tunnel at the Chemical Hazards Research Center. In addition to comparing a single row of spires with and without surface roughness, the effect of spire configurations on the flow was tested using 1, 2, and 3 rows of staggered array of spires, and exploring various inter-row spacing between 0.25h and h where h is the spire height. Mean velocities, velocity variances, Reynolds shear stress, kinetic energy, and the fluxes of velocity variances and turbulent kinetic energy were measured. The measurements show the essential role of surface roughness in a spire-roughness arrangement to induce a transfer of energy from the inner layer to the outer layer of the flow. Two rows of spires with an appropriate spacing increased the constant shear stress layer from 15% to 83% of the boundary layer depth, which was the intended result of the program. Moreover, turbulence intensities and turbulence variances can be generated in the CHRC wind tunnel within ranges that apply to flat terrain in the planetary boundary layer.