Shear velocity is a critical variable used in many hydrodynamic and aeolian applications. The Law of the Wall is commonly used to derive shear velocity as the product of the slope of a measured velocity profile and the von Kármán constant, κ = 0.4. However, a number of hydrodynamic experiments show that there is a substantial apparent decrease of κ in sediment-laden flow, which was explained by: 1) The energy loss to support the sediment particle suspension in the fluid and 2) The buoyancy effect due to stratification. The energy loss is associated with sediment concentration and grain size, and the stratification can be characterized by sedimentological flux Richardson number or gradient Richardson number. Since there is an apparent change of κ, the term “apparent von Kármán parameter”, or κa, was adopted from Wright and Parker to replace κ in sediment-laden flow. There has been no study to attempt to detect and to evaluate the variability of κa during aeolian saltation, which is the purpose of this dissertation research. Two “clear air” runs and fifteen “sediment-laden” runs were conducted at the northeast coast of Brazil. Wind profile data were collected by a stack of cup anemometers; “true” shear velocity was estimated by an ultrasonic anemometer; and sediment mass flux profile and grain size were estimated from the sand samples collected in a stack of vertical hose-style traps. With these estimates, κa, sediment concentration and sedimentlogical Richardson numbers were derived. Regression analysis indicates that there is a statistically insignificant relationship between κa and grain size, which may be caused by small range of grain size in the study site. However, there is strong statistical relationship between κa and bulk, volumetric concentration below 25 mm, S25, and between κa and sediment transport rate Q (kg/m/s) as: ka = -2088.4S25 0.3964 and ka = -3.134Q 0.4011 A strong relationship was also found between κa and sedimentological Richardson numbers in the lower saltation layer, which can be well explained by the stratification theory.

In this thesis I investigate characteristics of turbulent flow in a channel where one of the walls has riblets, superhydrophobic microribs, or a hybrid surface with traditional riblets built on a superhydrophobic microrib surface. PIV measurements are used to find the velocity profile, the turbulent statistics, and shear stress profile in the rectangular channel with one wall having a structured test surface. Both riblets and superhydrophobic surfaces can each provide a reduction in the wall shear stress in a turbulent channel flow. Characterizing the features of the flow using particle image velocimetry (PIV) is the focus of this research. Superhydrophobicity results from the combination of a hydrophobic coating applied to a surface with microrib structures, resulting in a very low surface energy, such that the fluid does not penetrate in between the structures. The micro-rib structures are aligned in the streamwise flow direction. The riblets are larger than the micro-rib structure by an order of magnitude and protrude into the flow. All the test surfaces were produced on silicon wafers using photolithographic techniques. Pressure in the channel is maintained below the Laplace pressure for all testing, creating sustainable air pockets between the microribs. Velocity profiles, turbulent statistics, shear stress profiles, and friction factors are presented. Measurements were acquired for Reynolds numbers ranging from 4.5x10^3 to 2.0x10^4. Modest drag reductions were observed for the riblet surfaces. Substantial drag increase occurred over the superhydrophobic surfaces. The hybrid surfaces showed the greatest drag reduction. Turbulence production was strongly reduced during riblet and hybrid tests.