The cyclonic turning and propagation of buoyant coastal discharge along the shelf

Buoyant coastal discharge typically forms a current flowing along the coast in the direction of Kelvin wave propagation (hereinafter referred to as the downstream direction). In this paper the opposite, upstream penetration of buoyancy-driven current is studied using numerical modeling. Previous models of coastal buoyancy-driven currents repeatedly predicted the upstream spreading while in the field this feature was not commonly observed. The mechanism responsible for the propagation of buoyant flow along the coast upstream from its source is identified as follows. In many cases, the boundary conditions applied for buoyant discharge oversimplify the actual dynamics at the mouth blocking landward flow in the lower layer. This generates a strong cyclonic vorticity disturbance with corresponding upstream turning of buoyant flow at the source. This process initiates the upstream spreading of buoyant flow.Alternative boundary condition maintaining constant net transport but allowing baroclinic adjustment of buoyant inflow is formulated and shown to reduce the generation of cyclonic vorticity at the mouth. The upstream propagation is further enhanced by the vertical mixing. The buoyant water forms an anticyclonic bulge at the river or estuary mouth. While spreading around the center of this anticyclone, the fresher water gradually becomes saltier due to vertical mixing/diffusion. As a result, the pool of lightest water does not coincide with the center of the anticyclone (in the sense of integral streamfunction) but tends to occupy the upstream and inshore segment of the bulge where the buoyant water comes first. This sets a new center for the anticyclonic turning at the surface and promotes the upstream shift of the anticyclonic bulge. This process sustains continuous growth of the buoyant plume upstream. It is shown that the upstream ambient current does not produce a similar effect. Instead, buoyant flow periodically sheds anticyclones advected upstream with the mean current. Under certain conditions, upstream spreading is also possible in nature. For example, Beardsley et al. (1985) reported substantial upstream penetration of the Changjiang River discharge in the East China Sea during the period of high runoff

Interaction of transient shelf currents with a buoyancy-driven coastal current

Observations on the inner New Jersey shelf (1996) showed that transient wind-driven currents of 3–4-day periods strongly interacted with a buoyancy-driven coastal current (or buoyant plume) originating from the Hudson estuary. In particular, the depth-averaged current fluctuations were amplified in the buoyant water. This phenomenon is studied here using a primitive equation numerical model (SPEM5). Transient (periodic) shelf currents are introduced in the form of incident barotropic shelf waves (BSW). The model domain is an idealized channel with open upstream/downstream boundaries and the depth exponentially increasing offshore, which allows the BSW propagation through the model domain. In most cases, the wave period is five days. The buoyancy-driven coastal current is forced by constant buoyant discharge through a coastal gap. Propagating BSWs reduce the growth of a buoyant anticyclonic bulge at the source region while a coastal current downstream from the source contains more buoyant water with a sharper density gradient in the frontal zone compared to the case without BSWs. The amplitude of vertically averaged transient currents increases in the buoyant layer (for instance, by 20–30% for the inflow density anomaly of -3 to -4 kg m-3). This amplification depends on the density anomaly of buoyant inflow. On the other hand, variations in the inflow velocity and/or the net transport do not affect the BSW amplification. The amplification of velocity amplitude is associated with the incident BSW scattering into higher wave modes. The total energy flux should remain approximately the same as BSWs propagate through the domain. The higher modes have lower group speeds and thus their amplitudes should be higher in order to maintain the same energy flux. Such interactions of transient currents with buoyant plumes are important for the mixing processes and the across-shelf exchange on the inner shel

The impact of ambient stratification on freshwater transport in a river plume

The influence of ambient stratification on the river plume dynamics is studied using the Regional Ocean Modeling System (ROMS). The shelf flow is thermally stratified and retains a 15-m deep upper mixed layer. A buoyant inflow into the model domain is systematically varied so that both surface-advected and bottom-advected buoyant plumes are formed. The resulting buoyant plumes are compared with the corresponding numerical solutions when the ambient shelf flow is not stratified. Surface-advected plumes spread on top of the thermocline and are not affected by the ambient stratification. Bottom-advected plumes on the other hand interact with the ambient stratification by deepening the thermocline which in some cases renders the buoyancy current unstable. Bottom-advected plumes under stratified conditions exhibit the formation of a frontal disturbance at an earlier stage downstream of the anticyclonic bulge, with more eddies developing later along the density front. These eddies grow rapidly in both offshore and downstream directions and propagate with the buoyant coastal current. In an extreme case, they spread thrice the offshore extension of the plume in the corresponding nonstratified case. More eddies develop either when the inflow salinity anomaly is smaller or when the stratification is stronger. Eddies form later, grow at a slower rate and are less developed offshore with a gentler bottom slope. Frontal eddies triggered by the ambient stratification reduce (up to 35%) the downstream and enhance the offshore freshwater fluxes compared to the corresponding nonstratified case. Energy conversion diagnostics indicate that both baroclinic and barotropic instabilities contribute to the formation of frontal eddies, with baroclinic instability playing the leading role

Influence of wood component on physical and chemical transformations during high temperature heating of composite fuel based on bituminous coal

Experimental studies of ignition delay time for mixtures of dispersed hard coal of two types and milled wood over a wide temperature range have been performed. Time of total completion of organic part pyrolysis of both components at different concentrations have been established in order to assess the prospects for such fuels application in large- and small-scale power engineering (combustion in boiler furnaces of different capacities). It has been established that simultaneous thermal decomposition of mixture of coal and wood particles leads to a significant change of pyrolysis temperature range and release of anthropogenic gases (sulfur and nitrogen oxides) in case of high-temperature heating of the mixture based on lean coal. The same effect, but in much smaller scale, has been recorded for a mixture based on long-flame coal. A hypothesis has been formulated on the mechanism of sulfur and nitrogen oxides precipitation during thermal decomposition of the mixture of lean coal and wood particles as a result of interaction between transitory gaseous and solid pyrolysis products of coal and wood in the temperature range up to 1000°C. Prospects of applying the milled coal and wood mixtures as fuel for steam and hot water boilers have been substantiated. With a small decrease of energy characteristics of such fuels, compared to homogeneous coals, significant improvement of ecological and economic characteristics of the fuel combustion processes can be achieved. It has been shown that vital synergistic effect of co-combustion of coal and wood particles is achieved only with certain coals

Analysis of composite fuel application possibility based on coal and oats husks in industrial power engineering

Experimental studies were conducted to determine the energy and technical characteristics of composite fuels from T grade coal (Alardinsky deposit) and oats (Shegarsky district, Tomsk region). An effective concentration of composite fuel of 50% / 50% is established, at which the heat of combustion is reduced by less than 2%, the ash content is up to 44%, and the fly ash output is up to 48%

Research of Heat and Mass Transfer Processes in the Nodes of Free-Flow Micro-Hpp With the Use of 3D Technology

In the article the analysis of existing micro-hydro turbines. Created 3D experimental model. Designed and developed an experimental stand for testing micro-turbine. Investigated options for generators on permanent magnets, selected the optimum scheme and developed a prototype three phase generator with permanent magnets

The impact of spatial wind variations on freshwater transport by the Alaska Coastal Current

The Alaska Coastal Current (ACC) is located in a region with prevailing downwelling-favorable winds, flows over a long stretch of coastline (over 2000 km), and is driven by multiple sources of freshwater discharge totaling 24000 m3 s–1 along its length. Using the Regional Ocean Modeling System (ROMS) we attempt to determine how spatially variable winds affect the downstream transport of freshwater along a long coastline with nearly continuous sources of freshwater. The model domain represents a fraction of the ACC region and periodic boundary conditions are applied to allow propagation of the buoyant flow from upstream. The model is forced by multiple freshwater sources in the central part of the domain and by both constant and spatially-varying, predominantly downwelling-favorable, winds. Freshwater flux gain in the coastal current (as opposed to spreading offshore) is calculated by taking a 30-day averaged difference between freshwater fluxes at the downstream and upstream edges of the buoyancy forcing region. Model runs are split into two categories: relatively high gains (50 – 60% of total discharge) were observed under moderate wind stress (∼0.05 Pa) or no wind conditions while lower gains (35– 45%) were observed under light average wind stresses (∼0.025 Pa), especially when wind varied alongshore. The offshore freshwater transport is eddy-driven and is enhanced in the areas of converging wind forcing. Eddy generation is associated with the wind-induced deepening of the buoyant layer near the coast. When the surface boundary layer is thin under light wind conditions, this deepening translates into enhanced vertical shear of the alongshore current through the thermal wind balance. Reversal of alongshore wind to upwelling-favorable wind effectively blocks the downstream freshwater transport and spreads the buoyant layer offshore

Applying composite fuels based on coal and finely dispersed wood in heat power engineering

Results of experimental research of thermal decomposition of composite fuels based on 2B brown coal (Borodinskoe deposit) and wastes of timber industry (finely dispersed wood) are presented. Elemental composition of researched fuels has been defined and technically analyzed. Kinetic constants have been calculated within Arrhenius model of the first order. It has been determined that with an increase of wood concentration up to 50% in composite fuel, its energy characteristics decrease by less than 2%, the maximum temperature of fuel thermal decomposition reduces by 9%, while SOx and NOx yield reduces by 50%, and fly ash – by 75%. An effective composite fuel composition of 50%/50% has been established. Results of performed experimental studies illustrate possible applications of composite fuels based on brown coal and wood at thermal power plants