Efficiency, energy and stoichiometry in pelagic food webs; reciprocal roles of food quality and food quantity

In most lakes, zooplankton production is constrained by food quantity, but frequently high C:P poses an additional constraint on zooplankton production by reducing the carbon transfer efficiency from phytoplankton to zooplankton. This review addresses how the flux of matter and energy in pelagic food webs is regulated by food quantity in terms of C and its stoichiometric quality in terms of C:P. Increased levels of light, CO2 and phosphorus could each increase seston mass and, hence, food quantity for zooplankton, but while light and CO2 each cause increased C:P (i.e. reduced food quality for herbivores), increased P may increase seston mass and its stoichiometric quality by reducing C:P. Development of food quality and food quantity in response to C- or P-enrichments will differ between 'batch-type' lakes (dominated by one major, seasonal input of water and nutrients) and 'continuous-culture' types of lakes with a more steady flow-rate of water and nutrients. The reciprocal role of food quantity and stoichiometric quality will depend strongly on facilitation via grazing and recycling by the grazers, and this effect will be most important in systems with low renewal rates. At high food abundance but low quality, there will be a 'quality starvation' in zooplankton. From a management point of view, stoichiometric theory offers a general tool-kit for understanding the integrated role of C and P in food webs and how food quantity and stoichiometric quality (i.e. C:P) regulate energy flow and trophic efficiency from base to top in food webs.From a management point of view, stoichiometric theory offers a general tool-kit for understanding the integrated role of C and P in food webs and how food quantity and stoichiometric quality (i.e. C:P) regulate energy flow and trophic efficiency from base to top in food webs.

Investigation of boundary layer development in a multistage LP turbine

This paper describes an investigation of the behavior of suction surface boundary layers in a modern multistage Low Pressure turbine. An array of eighteen surface-mounted hot-film anemometers was mounted on a stator blade of the third stage of a 4-stage machine. Data were obtained at Reynolds numbers between 0.9 × 105 and 1.8 × 105 and 1.8 × 105. At the majority of the test conditions, wakes from upstream rotors periodically initiated transition at about 40% surface length. In between these events, laminar separation occurred at about 75% surface length. It is inferred that the effect of the wakes on the performance of the bladerow is limited and that steady flow design methods should provide an adequate assessment of LP turbine performance during design.

Investigation of boundary layer development in a multistage LP turbine

This paper describes an investigation of the behavior of suction surface boundary layers in a modern multistage Low-Pressure turbine. An array of 18 surface-mounted hot-film anemometers was mounted on a stator blade of the third stage of a four-stage machine. Data were obtained at Reynolds numbers between 0.9 × 105 and 1.8 × 105. At the majority of the test conditions, wakes from upstream rotors periodically initiated transition at about 40 percent surface length. In between these events, laminar separation occurred at about 75 percent surface length. Because the wake-affected part of the flow appeared to be only intermittently turbulent, laminar separation also occurred at about 75 percent surface length while this flow was instantaneously laminar. At all but the lowest Reynolds numbers, the time-mean boundary layer appeared to have re-attached by the trailing edge even though it was not fully turbulent. It is inferred that the effect of the wakes on the performance of the blade row is limited and that steady flow design methods should provide an adequate assessment of LP turbine performance during design.

The development of aerodynamic performance measurements in a transient test facility

Transient test facilities offer the potential for the simultaneous study of turbine aerodynamic performance, unsteady flow phenomena and the heat transfer characteristics of a turbine stage. This paper describes the development of aerodynamic performance measurement techniques in the Oxford Rotor Facility (ORF). The solutions to the technological issues involved with transient testing presented in this paper are expected to achieve levels of precision uncertainty comparable with traditional steady flow test rigs. The theoretical background to the measurement of aerodynamic performance is presented together with a comprehensive pre-test uncertainty analysis. The instrumentation scheme for the measurement of stage mass flow rate is discussed in detail, the measurements of shaft power, total inlet enthalpy, and stage pressure ratio are also outlined. The current working section features a 62% scale, 1-1/2 stage, high-pressure shroudless transonic turbine. The required inlet flow conditions are provided by an Isentropic Light Piston Tunnel (ILPT) with a quasi-steady state run time of approximately 70ms. The testing is conducted at engine representative specific speed, pressure ratio, gas-to-wall temperature ratio, Mach number and Reynolds number.

Vortex filament simulation of trailing vortex merger

Aircraft in high-lift configuration shed multiple vortices. These generally merge to form a downstream wake consisting of two counter-rotating vortices of equal strength. The understanding of the merger of two co-rotating trailing vortices is important in evaluating the separation criteria for different aircraft to prevent wake vortex hazards during landing and take-off. There is no existing theoretical method on the basis of which such norms can be set. The present study is aimed at gaining a better understanding of the behaviour of wake vortices behind the aircraft. Two dimensional studies are carried out using the vortex blob method and compared with Bertenyi's experiment. It is shown that inviscid two dimensional effects are insufficient to explain the observations. Three dimensional studies, using the vortex filament method, are applied to the same test case. Two Lamb-Oseen profile vortices of the same dimensions and initial separation as the experiment are allowed to evolve from a straight starting condition until a converged steady flow is achieved. The results obtained show good agreement with the experimental distance to merger. Core radius and separation behaviour is qualitatively similar to experiment, with the exception of rapid increases at first. This may be partially attributable to the choice of filament-element length, and recommended further work includes a convergence study for this parameter. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Hydrobiological status of a lesser Himalayan river

It is generally observed that a variety of physical and chemical variables have considerable impact on the biological life in a mountain river which include plankton, benthic algae, benthic invertebrate and finally the fishery resources. They are often subjected to extreme hydrological disturbances particularly during rainy seasons when increased volume of water create 'Wash Off' situation for existing fauna and flora. A case study of a lesser Himalayan river - the Gaula, which drains the south-central part of Kumaon region in Uttaranchal is presented here. Since this river does not get snow-melt water, it mainly depends on steady flow of the groundwater round the year. The climate of the area is characterized with long winters, short summer and good amount of rain during monsoon months. The infiltrated groundwater which seeps into the surface soil layers during monsoon seasons is the chief source of discharge of water during winter and summer season

Improving the performance of a valveless pulse combustor using unsteady fuel injection

This paper describes an experimental investigation into the effect of unsteady fuel injection on the performance of a valveless pulse combustor. Two fuel systems were used. The first delivered a steady flow of ethylene through choked nozzles, and the second delivered ethylene in discrete pulses using high-frequency fuel injectors. Both fuel systems injected directly into the combustion chamber. The high-frequency fuel injectors were phase locked to the unsteady pressure measured on the inlet pipe. The phase and opening pulse width of the injectors and the time-averaged fuel mass flow rate through the injectors were independently varied. For a given fuel mass flow rate, it is shown that the maximum pressure amplitude occurs when fuel is injected during flow reversal in the inlet pipe, i.e. flow direction is out of the combustor. The optimal fuel injection pulse width is shown to be approximately 2/9th of the cycle. It should, however, be noted that this is the shortest time in which the injectors can reliably be fully opened and closed. It is shown that by using unsteady fuel injection the mass flow rate of fuel needed to achieve a given amplitude of unsteady pressure can be reduced by up to 65% when compared with the steady fuel injection case. At low fuel mass flow rates unsteady fuel injection is shown to raise the efficiency of the combustor by a factor of 7 decreasing to a factor of 2 at high fuel mass flow rates. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

Performance of choked unsteady ejector-nozzles for use in pressure-gain combustors

If the conventional steady flow combustor of a gas turbine is replaced with a device which achieves a pressure gain during the combustion process then the thermal efficiency of the cycle is raised. All such 'Pressure Gain Combustors' (e.g. PDEs, pulse combustors or wave rotors) are inherently unsteady flow devices. For such a device to be practically installed in a gas turbine it is necessary to design a downstream row of turbine vanes which will both accept the combustors unsteady exit flow and deliver a flow which the turbine rotor can accept. The design requirements of such a vane are that its exit flow both retains the maximum time-mean stagnation pressure gain (the pressure gain produced by the combustor is not lost) and minimises the amplitude of unsteadiness (reduces unsteadiness entering the downstream rotor). In this paper the exit of the pressure gain combustor is simulated with a cold unsteady jet. The first stage vane is simulated by a one-dimensional choked ejector nozzle with no turning. The time-mean and rms stagnation pressure at nozzle exit is measured. A number of geometric configurations are investigated and it is shown that the optimal geometry both maximizes time mean stagnation pressure gain (75% of that in the exit of the unsteady jet) and minimizes the amplitude of unsteadiness (1/3 of that in the primary jet). The structure of the unsteady flow within the ejector nozzle is determined computationally. Copyright © 2009 by J Heffer and R Miller.

Displacement and mixing ventilation driven by opposing wind and buoyancy

The effect of an opposing wind on the stratification and flow produced by a buoyant plume rising from a heat source on the floor of a ventilated enclosure is investigated. Ventilation openings located at high level on the windward side of the enclosure and at low level on the leeward side allow a wind-driven flow from high to low level, opposite to the buoyancy-driven flow. One of two stable steady flow regimes is established depending on a dimensionless parameter F that characterizes the relative magnitudes of the wind-driven and buoyancy-driven velocities within the enclosure, and on the time history of the flow. A third, unstable steady flow solution is identified. For small opposing winds (small F) a steady, two-layer stratification and displacement ventilation is established. Exterior fluid enters through the lower leeward openings and buoyant interior fluid leaves through the upper windward openings. As the wind speed increases, the opposing wind may cause a reversal in the flow direction. In this case, cool exterior fluid enters through the high windward openings and mixes the interior fluid, which exits through the leeward openings. There are now two possibilities. If the rate of heat input by the source exceeds the rate of heat loss through the leeward openings, the temperature of the interior increases and this flow reversal is only maintained temporarily. The buoyancy force increases with time, the flow reverts to its original direction, and steady two-layer displacement ventilation is re-established and maintained. In this regime, the increase in wind speed increases the depth and temperature of the warm upper layer, and reduces the ventilation flow rate. If, on the other hand, the heat loss exceeds the heat input, the interior cools and the buoyancy-driven flow decreases. The reversed flow is maintained, the stratification is destroyed and mixing ventilation occurs. Further increases in wind speed increase the ventilation rate and decrease the interior temperature. The transitions between the two ventilation flow patterns exhibit hysteresis. The change from displacement ventilation to mixing ventilation occurs at a higher F than the transition from mixing to displacement. Further, we find that the transition from mixing to displacement ventilation occurs at a fixed value of F, whereas the transition from displacement to mixing flow is dependent on the details of the time history of the flow and the geometry of the openings, and is not determined solely by the value of F. Theoretical models that predic t the steady stratification profiles and flow rates for the displacement and mixing ventilation, and the transitions between them, are presented and compared with measurements from laboratory experiments. The transition between these ventilation patterns completely changes the internal environment, and we discuss some of the implications for the natural ventilation of buildings. © 2004 Cambridge University Press.

Tomographic reconstruction of OH* chemiluminescence in two interacting turbulent flames

The tomographic reconstruction of OH* chemiluminescence was performed on two interacting turbulent premixed bluff-body stabilized flames under steady flow conditions and acoustic excitation. These measurements elucidate the complex three-dimensional (3D) vortex-flame interactions which have previously not been accessible. The experiment was performed using a single camera and intensifier, with multiple views acquired by repositioning the camera, permitting calculation of the mean and phase-averaged volumetric OH* distributions. The reconstructed flame structure and phase-averaged dynamics are compared with OH planar laser-induced fluorescence and flame surface density measurements for the first time. The volumetric data revealed that the large-scale vortex-flame structures formed along the shear layers of each flame collide when the two flames meet, resulting in complex 3D flame structures in between the two flames. With a fairly simple experimental setup, it is shown that the tomographic reconstruction of OH* chemiluminescence in forced flames is a powerful tool that can yield important physical insights into large-scale 3D flame dynamics that are important in combustion instability. © 2013 IOP Publishing Ltd.

Collective motion, sensor networks, and ocean sampling

This paper addresses the design of mobile sensor networks for optimal data collection. The development is strongly motivated by the application to adaptive ocean sampling for an autonomous ocean observing and prediction system. A performance metric, used to derive optimal paths for the network of mobile sensors, defines the optimal data set as one which minimizes error in a model estimate of the sampled field. Feedback control laws are presented that stably coordinate sensors on structured tracks that have been optimized over a minimal set of parameters. Optimal, closed-loop solutions are computed in a number of low-dimensional cases to illustrate the methodology. Robustness of the performance to the influence of a steady flow field on relatively slow-moving mobile sensors is also explored © 2006 IEEE.

一种基于粒子的牛顿流体与粘弹性流体统一模拟方法

在越来越受到人们关注的基于物理流体动画领域,目前分别模拟牛顿流体或粘弹性流体的方法很多,但很少有统一模拟两者的方法.文中基于光滑粒子流体动力学方法,通过对传统纳维-斯托克斯方程添加弹性应力项,提出了一种新的统一模拟牛顿流体和粘弹性流体的方法.通过实验说明该方法不仅有效,易于实现,而且具有良好的可控性,仅仅通过调节参数就可以模拟不同粘弹性、不同类型的流体现象.

Computer simulation of reactive extrusion processes for free radical polymerization

The reactive extrusion for polymerization is an integrated polymer processing technology. A new semi-implicit iterative algorithm was proposed to deal with the complicated relationships among the chemical reaction, the macromolecular structure and the chemorheological property. Then the numerical computation expressions of the average molecular weight, the monomer conversion, and the initiator concentration were deduced, and the computer simulation of the reactive extrusion process for free radical polymerization was carried out, on basis of which reactive processing conditions can be optimized.

Physical modeling of localized scour for the Yangtze Estuary Waterway Improvement Project, Phase I

In order to examine the effectiveness of engineering protection against localized scour in front of the south groin-group of the Yangtze Estuary Waterway Improvement Project, Phase I, an undistorted physical model on a geometric scale of 1:250 is built in this study, covering two groins and their adacent estuarine areas. By use of rinsing fix-bed model as well as localized mobile-bed model the experiment is undertaken under bi-directional steady flow. According to the experimental results, waterway dredging leads to the increase in steram velocity, the increase being larger during the ebb than during the flood. Construction of the upstream groin has some influence on the flow patterns near the downstream groin. Localized scour in front of the groin-heads is controlled mainly by ebb flow. In the case of a riverbed composed entirely of silt, the depths of localized scour in front of the two groin-heads are 27 m and 29 m, respectively. In reality, the underneath sediment of the prototype riverbed is clay whose threshold velocity is much higher than the stream velocity in the Yangtze Estuary; therefore, the depths of localized scour will not be much larger than the thickness of the silt layer, i. e. 7.4 m and 4.7 m, respectively. The designed aprons covering the riverbed in fron of the groin-heads are very effective in scour control. Aprons of slightly smaller size can also fulfill the task of protection, but the area of localized scour increases significantly.

Alternative method to evaluate discharge coefficients. Part 2: case study

The purpose of this article is to apply an alternative method whereby discharge coefficients can be estimated for the flow through a poppet valve at various lifts. Presented is the development of an operational quasi-steady flow rig. An engine cylinder head poppet valve was used as the case study. The requirement to directly measure mass flowrates using a standard conventional steady flow apparatus has been eliminated. Transient mass flowrates, pressures and temperatures of air during an inflow test for a poppet valve at various lifts were measured. Mass flowrates were also calculated from measured cylinder gas pressures and corrected for heat transfer. Using both methods to determine the mass flowrates, isentropic discharge coefficients were calculated and shown to compare within +/- 4.0 per cent of steady flow data. A computational fluid dynamics (CFD) validation of the quasi-steady flow rig is also presented.