Jacob Hjelmager Jensen

A morphological stability analysis for a long straight barred coast

A morphological stability analysis is carried out for a long straight coast with a longshore bar. The situation with oblique wave incidence and a wave-driven longshore current is considered. The flow and sediment transport are described by a numerical modelling system. The models comprise: (i) a wave model with depth refraction, shoaling and wave breaking, (ii) a depth integrated model for wave driven currents and (iii) a sediment transport model for the bed load transport and the suspended load transport in combined waves and current. The direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity, neglecting the effects of a bed slope and secondary currents. An instability is found to develop around the bar crest. The instability is periodic in the alongshore direction, and tends to form rip channels and to steepen the offshore face of the bar between the rip channels. The alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile.

Solution processed large area fabrication of Ag patterns as electrodes for flexible heaters, electrochromics and organic solar cells

A simple method for producing patterned Ag electrodes on transparent and flexible substrates is reported. The process makes use of a laser printed toner as a sacrificial template for an organic precursor, which upon thermolysis and toner lift off produced highly conducting Ag electrodes. Thus, the process takes only a few minutes without any expensive instrumentation. The electrodes exhibited excellent adhesion and mechanical properties, important for flexible device applications. Using Ag patterned electrodes, heaters operating at low voltages, pixelated electrochromic displays as well as organic solar cells have been demonstrated. The method is extendable to produce defect-free patterns over large areas as demonstrated by roll coating.

Photochemical stability of electrochromic polymers and devices

The stability of fully printed flexible organic electrochromics based on 11 different conjugated polymers is explored from the fundamental chemical degradation level to the operational device level. The photochemical stability of the electrochromic polymers (ECPs) is studied enabling an analysis of the influence that the chemical constitution of the conjugated polymer backbone has on the photochemical stability. Based on changes in the UV-visible absorption and IR spectra, the polymers were categorized into two distinct groups, each with a separate degradation mechanism. During irradiation (1000 W m−2, AM 1.5G) under ambient conditions the majority of the polymers degraded within 4–5 hours. Three polymers showed increased stability with degradation rates from 0.44 to 1.58% per hour measured as loss of absorption. Application of oxygen and UV barrier foils was found to drastically slow the photochemical decomposition of the polymer films, such that after 2200 hours of continuous irradiation the less stable polymer films were degraded 27% on average, while the degradation of the most stable polymer films was immeasurable thus indicating that such materials can be sufficiently stable for device operations for many years under indoor conditions and for a few years under outside conditions. Finally, functioning electrochromic devices (ECDs) were made and the effect of illumination on the response time and optical contrast was established. This report shows that encapsulated electrochromic devices based on flexible barrier substrates exhibit increased stability and are indeed viable in devices such as shading elements, light management systems, displays with low switching speed requirements and signage.

Digital grayscale printing for patterned transparent conducting Ag electrodes and their applications in flexible electronics

Grayscale (halftone) laser printing is developed as a low-cost and solution processable fabrication method for ITO-free, semi-transparent and conducting Ag electrodes extendable over large area on a flexible substrate. The transmittance and sheet resistance is easily tunable by varying the grayscale value of the color fill between 10% to 20%. The operation of electrodes is demonstrated by fabricating a transparent push button, an electrochromic window and a solar cell.

Overflow Concentration and Sedimentation in Hoppers

AbstractSediment spillage from hopper overflow constitutes a source for sediment plumes and can also impact the turbidity of aquatic environments. The overflowing mixture is often different from the mixture pumped into the hopper (the inflow), because the mixture undergoes compositional transformation as a result of different timescales in the segregation of the various sediment fractions. The heavier constituents in a mixture will have had time to settle, and overflowing sediments are therefore primarily composed of the finer and lighter constituents, whose concentrations potentially exceed those at the inflow. The hopper constitutes a complex system despite its geometrical regularity; the complexities are largely from the settling processes in concentrated polydisperse mixtures. These settling processes can, however, be captured by employing available settling formulas applicable for multifractional sediment mixtures (i.e., polydispersions). Strictly speaking, these formulas have been validated for homo...

Field experimental observations of highly graded sediment plumes.

A field experiment in the waters off the south-eastern coast of Cyprus was carried out to study near-field formation of sediment plumes from dumping. Different loads of sediment were poured into calm and limpid waters one at the time from just above the sea surface. The associated plumes, gravitating towards the seafloor, were filmed simultaneously by four divers situated at different depths in the water column, and facing the plume at different angles. The processes were captured using GoPro-Hero-series cameras. The high-quality underwater footage from near-surface, mid-depth and near-bed positions gives unique insight into the dynamics of the descending plume and near-field dispersion processes, and enables good understanding of flow and sediment transport processes involved from-release-to-deposition of the load in a non-scaled environment. The high resolution images and footages are available through the link provided herein. Observations support the development of a detailed multi-fractional sediment plume model.

Fixation of Carbon Dioxide Using Molecular Reactions on Flexible Substrates

This chapter explores the possibilities of using flexible substrates as basis for carbon dioxide fixation or photocatalytic reduction. Flexible substrates facilitate the processing of the large active areas necessary for targeting the increased atmospheric content of carbon dioxide, and various coating methods and reactor designs from laboratory to industrial scale are presented; also the future challenges are discussed. In designing reactors, we describe how in situ infrared spectroscopy can be utilized as a simple analysis tool. A key challenge in these conversion systems is the necessary precautions that must be taken when evaluating the product outcome. Verifying actual conversion is most convincingly done by using isotopically labeled carbon dioxide, as several groups have found that using isotopically labeled CO2 leads to a product composition of species containing both 13C and 12C. This makes it likely that carbonates or other carbonaceous residues that reside on the chamber or catalyst surface somehow contribute to the products. The photochemistry of commonly used flexible substrate is reviewed, as the photochemical stability of these needs to be enhanced in order for this approach to be viable.

Effects of Mixing on Hopper Sedimentation in Clearing Mixtures

AbstractHopper sedimentation is the result of precipitation of typically fine sediment from a homogenous, high-concentration mixture, which is not completely deficient of turbulence. If hopper sedimentation or loading is accomplished through a single-inflow system, or if the irregularity of the inflow concentrations is pronounced or simply terminated, then the hopper mixture will clear. Whereas turbulent mixing is redundant, when the mixture is homogeneous, it may take an active role when the mixture is clearing. The role of turbulence on hopper sedimentation has been the focus of several studies, and a common perception of turbulence (or at least of mixing) is that it delays sedimentation. Existing measurements of sedimentation rates in a closed-flume experiment, engineered to provide input to a hopper sedimentation model, revealed that turbulence in a clearing mixture is not necessarily associated with a delay in sedimentation. The experiment showed that sedimentation was boosted by adding a current to ...