Niels Finderup Nielsen

Numerical calculation of electrostatic field surrounding a human head in visual display environments

Abstract The aim of the present study is to investigate the electrostatic field between a surface such as a video display terminal and the face of an operator. Special attention is given to the field on the surface of the face. A numerical model is employed to calculate the three-dimensional electrostatic field governed by the Laplace equation for the electric potential. The model is based on computational fluid mechanical tools and advanced computational grids in order to analyse the field around the complex shapes of a human face. The advantage of the present numerical model is a high resolution of the calculated field. The electrostatic field is investigated for two different face shapes. For person 1, a female East-Asian, a parameter study is performed for the distance between screen and face and for the screen size. Equations fitting the calculated field strength are given for nose tip, forehead and eye. For person 2, a male Caucasian, emphasis is given to the field at the surface of the face and the results are used for comparison with person 1. The results indicate that for a given face shape, as that of person 1, the field at the nose decreases as l s −0.82 , l s being the distance between screen and nose tip. For both faces very steep gradients were found close to the face. Moreover, local protrusions on the surface of the face such as the nose tip result in a very high electrostatic field strength and different face shapes yield very different overall field distributions.

Particle deposition onto a human head: Influence of electrostatic and wind fields

This study investigates electrostatic fields surrounding the human head and particle deposition onto facial skin and eyes caused by the combined effect of electrostatic and wind fields. The electrostatic fields are calculated by a three-dimensional numerical model calculating the field strength between a field source and a human head. The deposition velocity can be viewed as determined by the sum of two contributions: that of an electrostatic field and that of a wind field. Deposition velocities are calculated by a semiempirical particle deposition model that considers particle transport from the free stream to the human face. The particle deposition model uses the electrostatic field model results as input parameters and is applied to the forehead and eyes of two facial shapes for two different turbulence conditions and aerosol charge distributions. The results of different practical working conditions, under which the potential difference between head (person) and source ranges from 5.6 to 15.0 kV, indicates that the presence of electrostatic fields always increases particle deposition for industrial aerosols. For aged aerosols an effect is only present for submicron particles.

A note on ciliated plane channel flow with a pressure gradient

An envelope model is applied to the case of a two-dimensional channel with ciliated parallel walls. The formulation assumes identical values of the longitudinal and transverse amplitudes, frequency and wavelength of the two walls; it allows for arbitrary phase relations and arbitrary (not too small) spacing, and it includes an externally imposed pressure gradient. General results of a second-order perturbation analysis of creeping flow are presented. The time-averaged steady mean velocity may be viewed as the sum of two contributions: that of the pressure gradient (Poiseuille flow), and that of ciliary-driven motion which, owing to nonlinearities, also depends on the pressure gradient and reduces to pure streaming in the absence of a pressure gradient. For zero pressure gradient, the ratio of the streaming velocity of the channel and that of a single sheet shows the degree to which streaming is augmented or impeded by flow interaction. This ratio increases for the symplectic and peristaltic cases, but decreases for the antiplectic case, as the width of the channel decreases for fixed values of phase relation and amplitudes. The net flow arising from streaming and pressure gradient is shown as pump characteristics, and associated efficiencies are given. The results indicate that propulsion (pumping) is greatest and most effective for symplectic metachronism in ciliated channels with predominantly transverse waves, that it is nearly as good for peristaltic motion, but that it is considerably inferior for antiplectic metachronism in channels with predominantly longitudinal waves.

Applying CFD for Design of Gas Conditioning Towers With Swirling Flow

The aim of the present study is to investigate the gas flow distribution within a Gas Conditioning Tower (GCT) used for cooling hot flue gases by means of atomized water injection. Special focus is placed on developing a design that prevents wall flow separation in the evaporation zone as well as in the tower in general. In due cause a special swirler design is presented introducing the swirling type of flow in the tower (swirl no. in the range of 0.2 to 1.0). The GCT geometry studied includes inlet ducting, swirl generator unit, conical diffuser section and tower section. The swirler is a totally new design while the inlet ducts and conical diffusers studied are modifications of the traditional design. The three-dimensional flow distribution within the GCT is calculated by standard Computational Fluid Dynamical (CFD) tools giving high resolution of the calculation domain. The presented results include intensive validation studies of the swirling flow introduced by the swirler and practical applications. The validation case shows that medium mesh resolution, second order upwind difference scheme and the high Reynolds number form of the standard k-e turbulence model with the law of the wall representation offer a good compromise between accuracy, required number of computational cells, computer time and agreement with experimental data. Studies of practical applications include design flexibility in general, optimization of gas distribution and sensitivity to water droplet size. Depending on the actual parameter in play, the results clearly demonstrate, that the newly developed design reflects the needs of an acceptable gas distribution in GCTs and that the design can be used in other flue gas process units including large opening angle diffusers. Also, the study indicates that the swirl number is a good measure of optimum operation.Copyright