Mode Choice Model for the Elderly: Case of Mashhad City, Iran

Although much research has been conducted on mode choice, very little has focused on the topic for the elderly. Considering the very particular behavior of this population group, different factors are expected to influence their decisions. Travel behavior of the elderly are mostly studied in developed countries. However, developing countries will have a great share of the elderly in a few decades. This paper aims to analyze Mashhad’s elderly travel behavior statistically through the application of multinomial logit model. A questionnaire is then designed based on the literature and the particular sample in Iran and administered to a sample of 499 elderly people through a field survey. The questionnaire contains socio-economic and trip characteristics questions. That includes travel origin and destinations, mode of travel, purpose and frequency of travel, number of the people along with respondents, household size, educational attainment, monthly household expenses, number of cars in the household, elderly car-ownership and having a driving license. To determine the statistically significant variables and Multi Nomial Logit model is used and the results indicate that the elderly in Masshad tend to use public transport more. Also, car users are mostly women and they prefer to be car passengers. Elderly with higher educational attainment and income tend to use car more. Elderly prefer to walk more as they get older and elderly men walk more than women. The walking mode has a negative correlation with the travel distance

Prediction of installed jet noise

A semianalytical model for installed jet noise is proposed in this paper. We argue and conclude that there exist two distinct sound source mechanisms for installed jet noise, and the model is therefore composed of two parts to account for these different sound source mechanisms. Lighthill’s acoustic analogy and a fourth-order space–time correlation model for the Lighthill stress tensor are used to model the sound induced by the equivalent turbulent quadrupole sources, while the trailing-edge scattering of near-field evanescent instability waves is modelled using Amiet’s approach. A non-zero ambient mean flow is taken into account. It is found that, when the rigid surface is not so close to the jet as to affect the turbulent flow field, the trailing-edge scattering of near-field evanescent waves dominates the low-frequency amplification of installed jet noise in the far-field. The high-frequency noise enhancement on the reflected side is due to the surface reflection effect. The model agrees well with experimental results at different observer angles, apart from deviations caused by the mean-flow refraction effect at high frequencies at low observer angles.The first author (B.L.) wishes to gratefully acknowledge the financial support co-funded by the Cambridge Commonwealth European and International Trust and the China Scholarship Council. The third author (I.N.) wishes to acknowledge the UK Turbulence Consortium (UKTC) for the high-performance computing time to carry out the LES simulation on ARCHER under EPSRC grant no. EP/L000261/1 and under a PRACE award on HERMIT

Light trapping in solar cells at the extreme coupling limit

We calculate the maximal absorption enhancement obtainable by guided mode excitation in a weakly absorbing dielectric slab over wide wavelength ranges. The slab mimics thin film silicon solar cells in the low absorption regime. We consider simultaneously wavelength-scale periodicity of the texture, small thickness of the film, modal properties of the guided waves and their confinement to the film. Also we investigate the effect of the incident angle on the absorption enhancement. Our calculations provide tighter bounds for the absorption enhancement but still significant improvement is possible. Our explanation of the absorption enhancement can help better exploitation of the guided modes in thin film devices.Comment: accepted for publication in JOSA

Prediction of installed jet noise

A semianalytical model for installed jet noise is proposed in this paper. We argue and conclude that there exist two distinct sound source mechanisms for installed jet noise, and the model is therefore composed of two parts to account for these different sound source mechanisms. Lighthill’s acoustic analogy and a fourth-order space–time correlation model for the Lighthill stress tensor are used to model the sound induced by the equivalent turbulent quadrupole sources, while the trailing-edge scattering of near-field evanescent instability waves is modelled using Amiet’s approach. A non-zero ambient mean flow is taken into account. It is found that, when the rigid surface is not so close to the jet as to affect the turbulent flow field, the trailing-edge scattering of near-field evanescent waves dominates the low-frequency amplification of installed jet noise in the far-field. The high-frequency noise enhancement on the reflected side is due to the surface reflection effect. The model agrees well with experimental results at different observer angles, apart from deviations caused by the mean-flow refraction effect at high frequencies at low observer angles.The first author (B.L.) wishes to gratefully acknowledge the financial support co-funded by the Cambridge Commonwealth European and International Trust and the China Scholarship Council. The third author (I.N.) wishes to acknowledge the UK Turbulence Consortium (UKTC) for the high-performance computing time to carry out the LES simulation on ARCHER under EPSRC grant no. EP/L000261/1 and under a PRACE award on HERMIT

Direct numerical simulation of a wall jet: flow physics

A direct numerical simulation (DNS) of a plane wall jet is performed at a Reynolds number of $Re_{j}=7500$. The streamwise length of the domain is long enough to achieve self-similarity for the mean flow and the Reynolds shear stress. This is the highest Reynolds number wall jet DNS for a large domain achieved to date. The high resolution simulation reveals the unsteady flow field in great detail and shows the transition process in the outer shear layer and inner boundary layer. Mean flow parameters of maximum velocity decay, wall shear stress, friction coefficient and jet spreading rate are consistent with several other studies reported in the literature. Mean flow, Reynolds normal and shear stress profiles are presented with various scalings, revealing the self-similar behaviour of the wall jet. The Reynolds normal stresses do not show complete similarity for the given Reynolds number and domain length. Previously published inner layer budgets based on LES are inaccurate and those that have been measured are only available in the outer layer. The current DNS provides fully balanced, explicitly calculated budgets for the turbulence kinetic energy, Reynolds normal stresses and Reynolds shear stress in both the inner and outer layers. The budgets are scaled with inner and outer variables. The inner-scaled budgets in the near wall region show great similarity with turbulent boundary layers. The only remarkable difference is for the turbulent diffusion in the wall-normal Reynolds stress and Reynolds shear stress budgets. The outer layer interacts with the inner layer through turbulent diffusion and the excess energy from the wall-normal direction is transferred to the spanwise direction.</jats:p

High-contrast self-imaging with ordered optical elements

Creating arbitrary light patterns has applications in various domains, including lithography, beam shaping, metrology, sensing, and imaging. We study the formation of high-contrast light patterns obtained by transmission through an ordered optical element based on self-imaging. By applying the phase-space method, we explain phenomena such as the Talbot and the angular Talbot effects. We show that the image contrast is maximum when the source is either a plane wave or a point source, and it has a minimum for a source with finite spatial extent. We compare these regimes and address some of their fundamental differences. Specifically, we prove that increasing the source divergence reduces the contrast for the plane wave illumination but increases it for the point source. Also, we show that to achieve high contrast with a point source, tuning the source size and its distance to the element is crucial. We furthermore indicate and explore the possibility of realizing highly complex light patterns using a periodic transmission element. These patterns can have more spots in the far field than the number of diffraction orders of the periodic element. We predict that the ultimate image contrast is smaller for a point source compared to a plane wave. Our simulations confirm that the smallest achievable spot size in the image is imposed by diffraction regardless of the imaging regime. Our research can be applied to similar domains, e.g., quantum systems

Absorption enhancement in thin-film silicon solar cells

Thin-film silicon technology is a major candidate to comply with the ever-increasing global energy demand. The small thickness of silicon allows high industrial throughput and low material usage and subsequently opens new avenues to mass-production of low-cost solar cells. This small thickness, together with the indirect bandgap of silicon and thus its relatively weak absorption of long-wavelength light, requires methods to improve absorption. This is often called light trapping. Light absorption can be enhanced in a thin film solar cell by introducing light scattering functionalities, for example textured interfaces. Of common interest is to know: 1) how far the photocurrent and efficiency of a cell can be improved by applying these geometrical changes, 2) which geometries can satisfy the criterion of high photocurrent generation, and 3) how much the incident angle affects the two previous points. This thesis addresses the mentioned questions. Specifically, amorphous silicon (a-Si) cells are the main subject of focus in the thesis, however, the results and conclusions are applicable to other types of solar cells, and similar structures such as light-emitting diodes. The first part of the thesis is devoted to the electromagnetic theory for thin film multilayers and the numerical methods which were used for optical simulations during the PhD work. These methods are described and compared, and some common sources of numerical error in them are identified. To address the first and the third questions, the limits of light absorption enhancement, photocurrent generation and efficiency in thin-film solar cells are studied. As a result, we obtain the limits of absorption enhancement in thin films with periodic texture, over a wide angular and wavelength range. More specifically, first we extend the statistical temporal coupled-mode theory to the case of thin films with wavelength-scale grating couplers. Then, we use this theory to study the effect of the incident angle and the grating period on the absorption enhancement in an idealized thin film with a thickness of 200 nm and refractive index n=4. We show that absorption in a thin-film solar cell depends strongly on the grating period and angle of incidence; therefore, consideration of oblique incident of light in these cells is a necessity. We provide guidelines for the design of thin-film solar cells with periodic texture. Afterwards, we obtain the limit of absorption enhancement for different structures including a full thin-film a-Si solar cell stack for different grating geometries. We show that for thin-films, hexagonal gratings enhance absorption more significantly compared to square gratings. We identify parasitic absorption as a major bottleneck for photocurrent generation. To deal with realistic cases, we investigate the guided modes of thin-film a-Si solar cells by rigorous simulations (the second and the third question). First, we extract the guided modes of the cells and study them in an equivalent planar model. We show that a plasmonic mode exists for very thin buffer layers. Then, we focus on the effect of texture geometry over a broad angular range by comparing the short-circuit current density (Jsc) of the cells for different grating patterns. We find that based on the cell configuration, the optimal texture may be symmetric or asymmetric. We show that TM polarized light produces higher photocurrent at large incident angles regardless of the texture geometry. In the final part of the thesis, we study two novel configurations for thin-film solar cells. First a plasmonic a-Si cell is considered which does not include a buffer layer. We demonstrate that the Jsc of the plasmonic cell is sensitive to the thickness of the n-doped silicon layer and we find that for an n-Si thickness of less than 10 nm, the plasmonic cell outperforms a conventional a-Si cell. Then we simulate an a-Si cell with a periodic array of ZnO nanowires inside the active layer. Our simulations indicate that assuming a periodicity of around 500 nm, the Jsc is highest for a nanowire diameter of about 300 nm

Large-eddy simulation of the interaction of wall jets with external stream

AbstractLarge eddy simulations are performed for a wall jet with an external stream. The external stream is in the form of a heated boundary layer. This is separated from a cold wall jet by a thin plate. The Reynolds number based on the displacement thickness, for the incoming boundary layer is 2776. A series of jet velocity ratios in the range M=Uj/U∞=0.30–2.30, is considered. The wall jet and outer stream velocities are Uj and U∞, respectively. The jets with M⩽1.0 develop von-Karman type shed vortices in the wake region. The higher velocity ratio jets with M>1.0 undergo Kelvin–Helmholtz instability and develop closely spaced counter-clockwise rolling structures. These structures determine the mean flow field behaviour and near wall heat transfer. At any given streamwise location adiabatic film-cooling effectiveness for M<1.0 increases rapidly with increasing M. For M>1.0 it decays slowly with further increase in M. For M<1.0 heat transfer from the hot outer stream to the wall depends on two factors; mean wall normal velocity and wall normal turbulent heat flux. For M>1.0 only a wall normal turbulent heat flux is responsible for heat transfer to the wall. The scaling behaviour shows that the near wall flow scales with wall parameters for all values of M. However, scaling in the outer region is highly dependent on M. The flow develops towards a boundary layer in the farfield for M<1.0 and towards a wall jet for the highest velocity ratio M=2.30

Angular behavior of the absorption limit in thin film silicon solar cells

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n^2 limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure which is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances which can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to get more practical values for the absorption enhancement, we consider the effect of parasitic loss which can significantly reduce the enhancement factor