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Internal Boundary Layers (internal + boundary_layer)

Distribution by Scientific Domains

Engineering	40%

Selected Abstracts

Turbulent flow over a dune: Green River, Colorado

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 3 2005
Jeremy G. Venditti
Abstract Detailed echo-sounder and acoustic Doppler velocimeter measurements are used to assess the temporal and spatial structure of turbulent flow over a mobile dune in a wide, low-gradient, alluvial reach of the Green River. Based on the geometric position of the sensor over the bedforms, measurements were taken in the wake, in transitional flow at the bedform crest, and in the internal boundary layer. Spatial distributions of Reynolds shear stress, turbulent kinetic energy, turbulence intensity, and correlation coefficient are qualitatively consistent with those over fixed, two-dimensional bedforms in laboratory flows. Spectral and cospectral analysis demonstrates that energy levels in the lee of the crest (i.e. wake) are two to four times greater than over the crest itself, with minima over the stoss slope (within the developing internal boundary layer). The frequency structure in the wake is sharply defined with single, dominant peaks. Peak and total spectral and cross-spectral energies vary over the bedform in a manner consistent with wave-like perturbations that ,break' or ,roll up' into vortices that amalgamate, grow in size, and eventually diffuse as they are advected downstream. Fluid oscillations in the lee of the dune demonstrate Strouhal similarity between laboratory and field environments, and correspondence between the peak frequencies of these oscillations and the periodicity of surface boils was observed in the field. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Aircraft observations of the atmospheric boundary layer over a heterogeneous surface in eastern Siberia

HYDROLOGICAL PROCESSES, Issue 14 2003
T. Hiyama
Abstract This paper outlines specifications and gives preliminary results of aircraft observations made during the Intensive Observation Period in 2000 (IOP2000) over the Yakutsk area of eastern Siberia. The observations were part of the GAME-Siberia project. Spatial and seasonal variation in the fluxes of sensible heat, latent heat, and carbon dioxide were determined using a Russian aircraft equipped with turbulence sensors. Two flight paths covered 12 × 32 km2 grids over heterogeneous forest and grass surfaces on the left- and right-hand banks of the Lena River. The spatial flux distributions were consistent with the underlying vegetation cover. A video camera recorded aerial images of the land, while a spectrometer observed the spectral reflectance of the land surface. These data helped describe the relationship between the atmosphere and the land surface. The vertical structure of the atmospheric boundary layer (ABL) was also observed on long flight paths between the left and right banks of the Lena River. Thermally induced internal boundary layers (TIBLs) developed in the ABL under different thermal and dynamic conditions near the Lena River. The horizontal and vertical distributions of sensible and latent heats in the ABL were consistent with the generation of cumulus, which appeared over the forested area, but not over the Lena River lowland. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Modeling water flux in forward osmosis: Implications for improved membrane design

AICHE JOURNAL, Issue 7 2007
Jeffrey R. Mccutcheon
Abstract Osmotically-driven membrane processes, such as forward osmosis and pressure retarded osmosis, operate on the principle of osmotic transport of water across a semipermeable membrane from a dilute feed solution into a concentrated draw solution. The major hindrance to permeate water flux performance is the prevalence of concentration polarization on both sides of the membrane. This article evaluates the external and internal boundary layers, which decrease the effective osmotic driving force. By modeling permeate flux performance, the role that feed and draw concentrations, membrane orientation, and membrane structural properties play in overall permeate flux performance are elucidated and linked to prevalence of external and internal concentration polarization. External concentration polarization is found to play a significant role in the reduction of driving force, though internal concentration polarization has a far more pronounced effect for the chosen system conditions. Reduction of internal concentration polarization by way of membrane modification was found to improve the predicted flux performance significantly, suggesting that alteration of membrane design will lead to improved performance of osmotically driven membrane processes. © 2007 American Institute of Chemical Engineers AIChE J, 2007 [source]

Modeling and Simulation of Notional Future Radar in Non-Standard Propagation Environments Facilitated by Mesoscale Numerical Weather Prediction Modeling

NAVAL ENGINEERS JOURNAL, Issue 4 2008
ROBERT E. MARSHALL
Normal near surface radio-frequency (RF) propagation in the littorals across the land,sea boundary is rare due to the land,sea temperature difference, coastline shape, ground cover, urban density, coastal topography, and soil moisture content. The resulting frequent existence of coastal non-standard vertical profiles of refractivity and the resulting RF propagation has a profound impact on the performance of Navy ship-borne radars operating within 100 nm of the shore. In addition, these non-standard RF propagation conditions are spatio-temporally inhomogeneous. These spatial and time dependent propagation conditions and the resulting radar engineering implications cannot be revealed by a single vertical profile of refractivity taken near the ship borne radar. The results from single profile analysis techniques provide no spatiotemporal information and may lead to overly conservative radar design. Mesoscale numerical weather prediction (NWP) is a rapidly maturing technology with a strong operational Navy history that can provide a vertical profile of refractivity every 1 km in the battle space and every hour, up to 48 h, in the future. The Sensor Division at NSWCDD has applied mesoscale NWP for the last 2 years to better understand the performance of prototype radar in realistic four-dimensional (4D) coastal environments. Modern RF parabolic equation models are designed to model specific radar designs and to employ 3D refractivity fields from mesoscale NWP models. This allows for a radar design to be tested in realistic littoral non-standard atmospheres produced by stable internal boundary layers, sea breeze events, and the more rare sub-refractive events. Mesoscale NWP is currently qualitative for this purpose, but a research and development program focused on sea testing of prototype radars is described with the purpose of developing a more quantitative mesoscale NWP technology to support radar acquisition, testing, and operations. [source]