In studies of turbulent boundary layers at high Reynolds number, the term “roughness transition” is generally an implicit reference to the case of a streamwise step-change in roughness length (whether the roughness length is associated with surface fluxes of momentum, temperature, humidity, or some other quantity). This roughness configuration and flow response has received broad attention. Here, in contrast, we consider turbulent wall-bounded flows over transverse roughness transitions using large-eddy simulation. This is accomplished simply by aligning the boundary layer freestream direction parallel to momentum roughness length transitions, instead of perpendicular. In the present cases, the bounding surface is composed of two “high roughness” strips placed between three “low roughness” strips. The influences of two parameters are evaluated: (1) λ, the ratio of the high roughness length to the low roughness length; and (2) Ls, the width of the high roughness strips. In the immediate vicinity of the roughness change, the abrupt wallstress variation induces transverse turbulent mixing which is the source of a δ-scale secondary flow, recently described as a low momentum pathway (LMP) by Mejia-Alvarez et al. [“Structural attributes of turbulent flow over a complex topography,” Coherent Flow Structures at the Earth's Surface (Wiley-Blackwell, 2013), Chap. 3, pp. 25–42] and Mejia-Alvarez and Christensen [“Wall-parallel stereo PIV measurements in the roughness sublayer of turbulent flow overlying highly-irregular roughness,” Phys. Fluids, 25, 115109]. LMPs are spatially stationary and flanked by δ-scale counter-rotating vortices which serve to pump fluid vertically from the wall, ultimately leading to a spanwise variation in the boundary layer depth (for flows over surface roughness with a converging-diverging riblet pattern, spanwise variation of δ was also found in recent experiments by Nugroho et al. [“Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered and direction surface roughness,” Int. J. Heat Fluid Flow 41, 90–102 (2013)]. Mean velocity and transverse Reynolds stresses are used to determine the mixing length associated with transverse mixing. In general, we find that variations in Ls and λ have a strong and mild impact on the secondary flow pattern, respectively.

Short title: Barium isotopic composition of mainstream SiCs 10 NuGrid collaboration, http://www.nugridstars.org. ABSTRACTWe present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in δ( 138 Ba/ 136 Ba) values are found, down to −400 ‰, which can only be modeled with a flatter 13 C profile within the 13 C pocket than is normally used. The dependence of δ( 138 Ba/ 136 Ba) predictions on the distribution of 13 C within the pocket in AGB models allows us to probe the 13 C profile within the 13 C pocket and the pocket mass in asymptotic giant branch (AGB) stars. In addition, we provide constraints on the 22 Ne(α,n) 25 Mg rate in the stellar temperature regime relevant to AGB stars, based on δ( 134 Ba/ 136 Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ( 134 Ba/ 136 Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process (i-process), which is activated by the Very Late Thermal Pulse (VLTP) during the post-AGB phase and characterized by a neutron density much higher than the s-process.These two grains may have condensed around post-AGB stars. Finally, we report abundances of two p-process isotopes, 130 Ba and 132 Ba, in single SiC grains. These isotopes are destroyed in the s-process in AGB stars. By comparing their abundances with respect to that of 135 Ba, we conclude that there is no measurable decay of 135 Cs (t ½ = 2.3 Ma) to 135 Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135 Cs decayed.

We characterize EUV TOF for trace analysis using NIST glasses and demonstrate nanoscale imaging on uranium oxide particles.