Boundary Layer Contributions Research Articles

Seasonal variations of the boundary layer turbulence at Dome C

The atmospheric boundary layer contribution to the total seeing at Dome C is presented in this paper. With a mean value of 0.07'' , this is one of the most stable boundary layer encountered. We discuss the temporal evolution of this part of the atmosphere and its relationship with meteorological parameters. In winter, the combined effects of the low wind speeds and low inversion layer translate to the creation of little thermal turbulence above 30 m.

Seeing, outer scale of optical turbulence, and coherence outer scale at different astronomical sites using instruments on meteorological balloons

Here we analyze 168 optical turbulence profiles made at nine different locations worldwide by means of free flight balloons equipped with instrumentation. Seeing eFWHM, optical turbulence outer scales Lo, and wavefront outer scales £o are derived for the different locations and taking into account the contribution of the surface layer (0, 50 m), the boundary layer (0, 1 km) and the free atmosphere (1 km, 30 km). Noticeable changes are found between the different locations, mainly due to the boundary layer contribution as well as the surface layer contribution. The free atmosphere contribution seems almost the same for each site. There is a very good coherence between the outer scale derived by us and that measured using the Generalized Seeing Monitor (GSM) technique.

Stagnant Mobile Phase Mass Transfer in Chromatographic Media: Intraparticle Diffusion and Exchange Kinetics

Pulsed field gradient nuclear magnetic resonance has been successfully applied to a direct and detailed experimental study of topological and dynamic aspects involved in the exchange of small, nonsorbed fluid molecules between the intraparticle pore network and the interparticle void space in chromatographic columns packed with spherical-shaped, porous particles. The approach provides quantitative data about the effective, intraparticle diffusion coefficients (and tortuosity factors) and about the associated, diffusion-limited mass transfer kinetics, including stagnant boundary layer contributions. In view of the recorded exchange kinetics, an analytical description for solute diffusion into/out of spherical particles is offered and addresses the influence of the particle size distribution and particle shape on the observed mass transfer rates and calculated diffusivities. The combined analyses of the steady-state intraparticle pore diffusion data and the associated exchange kinetics with Peclet numbers up to 500 reveals the existence of external stagnant fluid where all the interparticle fluid-side resistance to diffusion is localized. It is represented by a thin stagnant boundary layer around the particles and can be accounted for by the introduction of a hydrodynamically effective particle diameter which is found to depend on the Peclet number. The approach appears to be promising for a selective, detailed study of the boundary layer dynamics. Concerning the investigation of different chromatographic media and intraparticle morphologies, we demonstrate that the actual correlation (or randomness) of interconnection between intraparticle pores of different size has a profound effect on the observed tortuosity factors and the diffusion-limited stagnant mobile phase mass transfer kinetics. Compared to intraparticle pore networks with a random assignment of different pore sizes, hierarchically structured bidisperse porous particles offer a superior network topology, which can form the basis for an increased chromatographic performance.

BOUNDARY LAYER INDUCED NOISE IN AIRCRAFT, PART I: THE FLAT PLATE MODEL

The importance of boundary layer contributions to aircraft cabin noise levels implies a need for a simple model capable of providing sufficient physical insight to address the problem at the design stage. This paper described the initial form of such a model, which is based on the sound radiated by a single, flat, elastic plate under boundary layer excitation. A numerical study of the model is presented, the main conclusions of which are that, for bare cabin walls, increasing the structural damping and decreasing the skin stiffness and number of reinforcements will reduce the radiated sound. High excitation levels, due to matching between the boundary layer and elastic wave velocities, are difficult to avoid, but the effect is not overly sensitive to small variations in cruising speed of structural parameters.

Coastal boundary layers in ocean modelling: an application to the Adriatic Sea

Boundary layers play an important role in modelling geophysical fluid-dynamical flows, in as much as they constitute regions of ageostrophic dynamics in which the physical balances characterizing the main interior of the water mass break down. A short synopsis is given of important boundary layers in ocean circulation modelling with specific emphasis drawn upon side wall boundary layers, namely those adjacent to the coastlines of the considered basin. Application of boundary layer analysis is thereafter made for one specific phenomenological situation, namely the Northern Adriatic Sea and the problem posed by its wintertime seasonal circulation. The analysis furnishes a mathematical model for the coastal strip adjacent to the Italian shoreline, treated as a boundary layer in the density field, starting from general model equations valid throughout the interior of the northern Adriatic. The boundary layer model is consequently used to modify the side wall boundary condition for the interior density field. Related numerical experiments are shown and compared with previous standard experiments in which the boundary layer contribution to the density field has not been considered.

Viscomagnetic Heat Flux Experiments as a Test of Gas Kinetic Theory in the Burnett Regime

In a rarefied polyatomic gas streaming through a rectangular channel, an external magnetic field produces a heat flux perpendicular to the flow direction. Experiments on this “viscom agnetic heat flux” have been performed for CO, N2, CH4 and HD at room temperature, with different orientations of the magnetic field. Such measurements enable one to separate the boundary layer contribution from the purely bulk contribution by means of the theory recently developed by Vestner. Very good agreement is found between the experimentally determined bulk contribution and the theoretical Burnett value for CO, N2 and CH4 , yet the behavior of HD is found to be anomalous.

A simplified technique for determining the boundary layer voltage loss in MHD generators

MHD generator performance predictions require an accurate determination of the voltage losses in the channel; however, most techniques for determining these losses need substantial calculations and/or computer storage space. This paper proposes a simplified method for calculating the ohmic boundary layer contribution to the overall voltage losses. Voltage drop regions are discussed and a description of the turbulent boundary layer contribution is derived. Appropriate simplifying assumptions on the basic transport and MHD concepts are used to express the conductivity as a function of temperature only. Weighting functions for averaging the resistivity in turbulent boundary layers are determined and the nature of these functions is presented. Our results are compared with more precise descriptions and with experimental results.

Mass transfer and optimization of faradaic yields in a chlorate cell process

A theoretical approach to the hydrodynamic optimization of the current yields in a chlorate cell process is given by a mathematical analysis of charge and mass transfer of electroactive species within the anode diffusion layer. Two sets of mathematical relationships were obtained: (a) the concentration profile of both active and elemental chlorine within the anode boundary layer, (b) combined diffusional and hydrolysis contributions to the formation of hypochlorous species near the electrode surface. Finally the equations for the current efficiency in cells with black-mix-flow of solution and electrolysers with plug-flow of electrolyte along the anode surface were derived. It is shown that the general mathematical relationships are in agreement with the experimental data obtained in chlorate cells.

Thermal insulation provided by dry, single-layer clothing materials

At 20 ± 2°C for a wide range of clothing fabrics, which usually have high air-volume voidages, the thermal resistance to ‘dry’ heat transfers (apart from boundary layer contributions on their faces) is proportional to the fabric thickness. This resistance exhibits a maximum at a fabric density of approximately 90 kg m −3.