Date of Graduation

5-2026

Document Type

Thesis

Degree Name

Bachelor of Science in Chemical Engineering

Degree Level

Undergraduate

Department

Chemical Engineering

Advisor/Mentor

Tom Spicer

Committee Member

Heather Walker

Second Committee Member

Chad Smith

Abstract

Atmospheric Dispersion Models (ADMs) are computer-based simulations used to model gas releases for enforcing regulatory compliance, designing facilities, and responding to emergencies. Because ADMs rely on complex mathematical models, they require experimental validation to ensure accuracy. Wind tunnels offer a controlled, repeatable method for gathering validation data. The Chemical Hazards Research Center (CHRC) at the University of Arkansas operates an ultra-low speed wind tunnel for this purpose. A key feature of this tunnel is its simulated Atmospheric Boundary Layer (ABL). The simulated ABL is a turbulent layer in which wind speed increases logarithmically with height from near zero at the surface to the freestream velocity at the top of the layer, replicating the behavior of Earth's lower atmosphere. At low wind speeds, however, the simulated ABL does not reach sufficient depth to conduct large-scale experiments without introducing molecular effects due to the required reduction in geometric scale. The depth of the simulated ABL is influenced by tunnel geometry, in particular, trapezoidal turbulence generators called modified Irwin spires placed at the beginning of the tunnel. This demonstration investigated the effect of modified Irwin spire quantity on ABL depth by testing four configurations, 14, 21, 27, and 41 spires at a low wind speed. For each configuration, centerline velocity profiles were collected using Laser Doppler Velocimetry (LDV). From these velocity profiles, a Log Law fit (Stull, 1988) was applied, freestream velocity (U∞) was determined by matching Castro’s (2013) line, and boundary layer depth (δ₉₉) was defined as the height at which the velocity reached 99% of 𝑈∞. Horizontal tunnel symmetry was also characterized and confirmed prior to spire configuration testing. For the 14, 21, and 27 spire configurations, boundary layer depth increased linearly with spire quantity from 38.7 to 62.6 cm. The 41-spire configuration, which required a two-row arrangement, produced anomalous results and was excluded from the correlation. These results demonstrate that modified Irwin spire quantity is an effective and controllable parameter for tuning the simulated boundary layer depth in the CHRC wind tunnel, providing a practical tool for future atmospheric dispersion modeling studies.

Keywords

Atmospheric Boundary Layer; Irwin Spires; Wind Tunnel; Laser Doppler Velocimetry; Boundary Layer Depth

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