Viscous sublayer flow visualizations at Rθ≂1 500 000

Abstract

Plan view flow visualization experiments were conducted in the atmospheric surface layer that flows over the Great Salt Lake Desert at the U.S. Army Dugway Proving Ground, Dugway, Utah. Measurements were acquired on a nonconductive, polyethylene platform made flush with the desert floor. Surface conditions upstream of the measurement site were flat, devoid of vegetation, and because of the dried mud/clay/salt composition, essentially dust free. Local surface variations ranged between 1 and 3 mm, which corresponded to three to ten viscous units during the experiments. Flow visualizations were accomplished by continuously injecting theatrical fog through a tangential slit covering a smoke reservoir buried under the platform. During the visualizations, the atmospheric surface layer flow was near neutral thermal stability. Flow velocities at 2.0 m above the surface maintained directional constancy, with a magnitude of about 1.5 m/s. A single element hot-wire probe positioned near y+=3.4 was used to measure the wall shear. Visualization results indicate the existence of the pocket and streak motions seen at much lower Reynolds number. The average inner normalized streak spacing was found to be λ+=93.1, with a positively skewed, nearly lognormal distribution. The average maximum inner normalized pocket width was found to be w+=127.2, with a positively skewed distribution. The average time between pockets was determined to be T+=36.6. Comparisons are made with existing low Reynolds number results, and a brief discussion is provided regarding the physics underlying the present observations.