A volumetrically heated jet: large-eddy structure and entrainment characteristics

1996 ◽  
Vol 325 ◽  
pp. 303-330 ◽  
Author(s):  
G. S. Bhat ◽  
R. Narasimha
Keyword(s):  
Large Scale ◽  
Laser Doppler Anemometry ◽  
Local Heating ◽  
Integral Model ◽  
Volumetric Heating ◽  
Electrically Conducting ◽  
Cloud Development ◽  
Entrainment Process ◽  
Centreline Velocity ◽  
Heat Injection

We report here an experimental study of a round vertical liquid jet that, after achieving a self-preserving state, is subjected to volumetric heating between two diametral stations. The heat injection is achieved by applying a voltage across the stations, the jet fluid having been rendered electrically conducting by the addition of acid. Using laser-induced fluorescence, digital image processing and laser-Doppler anemometry, the flow properties of the jet have been studied in detail. It is found that, with sufficient heating, the jet no longer grows linearly with height, and the decay of both centreline velocity and turbulence intensity is arrested, and may even be reversed just beyond the zone of heat addition; nevertheless the entrainment decreases, which is at variance with the hypotheses often made for modelling it. This behaviour is here attributed to the disruptive influence that, as the present experiments show, the volumetric heating has on the large-scale vortical structures in the jet, which are known to be largely responsible for the engulfment of ambient fluid that is the first step in the entrainment process. It is shown that a new non-dimensional heat release number correlates the observed data on changes in jet width. An integral model that would describe the effect of local heating is proposed, and implications for cloud development in the atmosphere are discussed.

The entrainment interface

1972 ◽  
Vol 51 (1) ◽  
pp. 97-118 ◽  
Author(s):  
O. M. Phillips
Keyword(s):  
Velocity Field ◽  
Turbulent Flow ◽  
Surface Area ◽  
Large Scale ◽  
Positive Curvature ◽  
Lagrangian Description ◽  
Eulerian Description ◽  
Entrainment Process ◽  
Necessary And Sufficient ◽  
Small Disturbances

A theory is developed to describe the evolution of the entrainment interface in turbulent flow, in which the surface is convoluted by the large-scale eddies of the motion and at the same time advances relative to the fluid as a result of the micro-scale entrainment process. A pseudo-Lagrangian description of the process indicates that the interface is characterized by the appearance of ‘billows’ of negative curvature, over which surface area is, on average, being generated, separated by re-entrant wedges (lines of very large positive curvature) where surface area is consumed. An alternative Eulerian description allows calculation of the development of the interfacial configuration when the velocity field is prescribed. Several examples are considered in which the prescribed velocity field in the z direction is of the general form w = Wf(x – Ut), where the maximum value of the function f is unity. These indicate the importance of leading points on the surface which are such that small disturbances in the vicinity will move away from the point in all directions. The necessary and sufficient condition for the existence of one or more leading points on the surface is that U [les ] V, the speed of advance of an element of the surface relative to the fluid element at the same point. The existence of leading points is accompanied by the appearance of line discontinuities in the surface slope re-entrant wedges, In these circumstances, the overall speed of advance of the convoluted surface is found to be W + (V2 – U2)½, where W is the maximum outwards velocity in the region; this result is independent of the distribution f.When the speed U with which an ‘eddy’ moves relative to the outside fluid is greater than the speed of advance V of an element of the front, the interface develops neither leading points nor discontinuities in slope; the amplitude of the surface convolutions and the overall entrainment speed are both reduced greatly. In a turbulent flow, therefore, the large-scale motions influencing entrainment are primarily those that move slowly relative to the outside fluid (with relative speed less than V). The experimental results of Kovasznay, Kibens & Blackwelder (1970) are reviewed in the light of these conclusions. It appears that in their experiments the entrainment speed V is of the order fifteen times the Kolmogorov velocity, the large constant of proportionality being apparently the result of augmentation by micro-convolutions of the interface associated with small and meso-scale eddies of the turbulence.

Aerosols and Climate

2016 ◽  
Author(s):  
Bjørn H. Samset
Keyword(s):  
Global Warming ◽  
Greenhouse Gases ◽  
Black Carbon ◽  
Large Scale ◽  
Cloud Condensation Nuclei ◽  
Local Heating ◽  
Climate Impact ◽  
Cloud Properties ◽  
Wide Range ◽  
The Stability

Among the factors that affect the climate, few are as diverse and challenging to understand as aerosols. Minute particles suspended in the atmosphere, aerosols are emitted through a wide range of natural and industrial processes, and are transported around the globe by winds and weather. Once airborne, they affect the climate both directly, through scattering and absorption of solar radiation, and indirectly, through their impact on cloud properties. Combining all their effects, anthropogenic changes to aerosol concentrations are estimated to have had a climate impact over the industrial era that is second only to CO2. Their atmospheric lifetime of only a few days, however, makes their climate effects substantially different from those of well-mixed greenhouse gases. Major aerosol types include sea salt, dust, sulfate compounds, and black carbon—or soot—from incomplete combustion. Of these, most scatter incoming sunlight back to space, and thus mainly cool the climate. Black carbon, however, absorbs sunlight, and therefore acts as a heating agent much like a greenhouse gas. Furthermore, aerosols can act as cloud condensation nuclei, causing clouds to become whiter—and thus more reflecting—further cooling the surface. Black carbon is again a special case, acting to change the stability of the atmosphere through local heating of the upper air, and also changing the albedo of the surface when it is deposited on snow and ice, for example. The wide range of climate interactions that aerosols have, and the fact that their distribution depends on the weather at the time and location of emission, lead to large uncertainties in the scientific assessment of their impact. This in turn leads to uncertainties in our present understanding of the climate sensitivity, because while aerosols have predominantly acted to oppose 20th-century global warming by greenhouse gases, the magnitude of aerosol effects on climate is highly uncertain. Finally, aerosols are important for large-scale climate events such as major volcanoes, or the threat of nuclear winter. The relative ease with which they can be produced and distributed has led to suggestions for using targeted aerosol emissions to counteract global warming—so-called climate engineering.

scholarly journals Light absorption by marine cyanobacteria affects tropical climate mean state and variability

2018 ◽  
Vol 9 (4) ◽  
pp. 1283-1300 ◽  
Author(s):  
Hanna Paulsen ◽  
Tatiana Ilyina ◽  
Johann H. Jungclaus ◽  
Katharina D. Six ◽  
Irene Stemmler
Keyword(s):  
Light Absorption ◽  
Large Scale ◽  
Local Heating ◽  
Light Attenuation ◽  
Tropical Climate ◽  
Climate System ◽  
Radiative Heating ◽  
Subsurface Water ◽  
Marine Cyanobacteria ◽  
Tropical Ocean

Abstract. Observations indicate that positively buoyant marine cyanobacteria, which are abundant throughout the tropical and subtropical ocean, have a strong local heating effect due to light absorption at the ocean surface. How these local changes in radiative heating affect the climate system on the large scale is unclear. We use the Max Planck Institute Earth System Model (MPI-ESM), include light absorption by cyanobacteria, and find a considerable cooling effect on tropical sea surface temperature (SST) in the order of 0.5 K on a climatological timescale. This cooling is caused by local shading of subtropical subsurface water by cyanobacteria that is upwelled at the Equator and in eastern boundary upwelling systems. Implications for the climate system include a westward shift of the Walker circulation and a weakening of the Hadley circulation. The amplitude of the seasonal cycle of SST is increased in large parts of the tropical ocean by up to 25 %, and the tropical Pacific interannual variability is enhanced by approx. 20 %. This study emphasizes the sensitivity of the tropical climate system to light absorption by cyanobacteria due to its regulative effect on tropical SST. Generally, including phytoplankton-dependent light attenuation instead of a globally uniform attenuation depth improves some of the major model temperature biases, indicating the relevance of taking this biophysical feedback into account in climate models.

scholarly journals Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation

2016 ◽  
Vol 2 (4) ◽  
pp. e1501227 ◽  
Author(s):  
Lin Zhou ◽  
Yingling Tan ◽  
Dengxin Ji ◽  
Bin Zhu ◽  
Pei Zhang ◽  
...  
Keyword(s):  
Light Absorption ◽  
Self Assembly ◽  
Metallic Nanoparticles ◽  
Large Scale ◽  
Strong Field ◽  
Local Heating ◽  
Field Enhancement ◽  
Solar Irradiation ◽  
Steam Generation ◽  
Solar Steam Generation

The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as critical for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an average measured absorbance of ~99% across the wavelengths from 400 nm to 10 μm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber–based solar steam generation has over 90% efficiency under solar irradiation of only 4-sun intensity (4 kW m−2). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufacturing of other nanophotonic structures and devices.

The Statistical Forecasting Models of a Hail for the Western Part of the North Caucasus and the Black Sea Coast Constructed on Output Production of Global System of Forecasts (GFS NCEP)

2018 ◽  
Vol 931 ◽  
pp. 1037-1041
Author(s):  
Arthur Kh. Kagermazov
Keyword(s):  
Large Scale ◽  
Convective Cloud ◽  
The Black Sea ◽  
Model Data ◽  
North Caucasus ◽  
Independent Variables ◽  
The North ◽  
Black Sea Coast ◽  
Cloud Development ◽  
Gfs Model

Statistical models of the hail forecast are proposed for the two regions of the North Caucasus, developed from the output of the global atmosphere model GFS NCEP. Statistical schemes are obtained as a result of discriminant analysis conducted using statistical software package SPSS. Independent variables in these schemes are the most informative predictors of strong convective cloud development, calculated on the basis of the global GFS model data related to local atmospheric instability and large-scale synoptic processes. Based on the results of the operational audit, the estimates of the success of the hail forecasts according to existing criteria are given, the high values of which assume a reduction in damage from hailstorms, when using them.

scholarly journals Empirical Validation and Numerical Predictions of an Industrial Borehole Thermal Energy Storage System

Energies ◽  
2019 ◽  
Vol 12 (12) ◽  
pp. 2263
Author(s):  
Emil Nilsson ◽  
Patrik Rohdin
Keyword(s):  
Energy Storage ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Large Scale ◽  
Storage System ◽  
Flow Characteristics ◽  
Heat Extraction ◽  
Short Term ◽  
Numerical Predictions ◽  
Heat Injection

To generate performance predictions of borehole thermal energy storage (BTES) systems for both seasonal and short-term storage of industrial excess heat, e.g., from high to low production hours, models are needed that can handle the short-term effects. In this study, the first and largest industrial BTES in Sweden, applying intermittent heat injection and extraction down to half-day intervals, was modelled in the IDA ICE 4.8 environment and compared to three years of measured storage performance. The model was then used in a parametric study to investigate the change in performance of the storage from e.g., borehole spacing and storage supply flow characteristics at heat injection. For the three-year comparison, predicted and measured values for total injected and extracted energy differed by less than 1% and 3%, respectively and the mean relative difference for the storage temperatures was 4%, showing that the performance of large-scale BTES with intermittent heat injection and extraction can be predicted with high accuracy. At the actual temperature of the supply flow during heat injection, 40 °C, heat extraction would not exceed approximately 100 MWh/year for any investigated borehole spacing, 1–8 m. However, when the temperature of the supply flow was increased to 60–80 °C, 1400–3100 MWh/year, also dependent on the flow rate, could be extracted at the spacing yielding the highest heat extraction, which in all cases was 3–4 m.

Unsteady Structure Measurement of Cloud Cavitation on a Foil Section Using Conditional Sampling Technique

1989 ◽  
Vol 111 (2) ◽  
pp. 204-210 ◽  
Author(s):  
A. Kubota ◽  
H. Kato ◽  
H. Yamaguchi ◽  
M. Maeda
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Laser Doppler Anemometry ◽  
Low Frequency ◽  
Sampling Technique ◽  
Experimental Result ◽  
Small Scale ◽  
Conditional Sampling ◽  
Cloud Cavitation ◽  
Structure Of Flow

The structure of flow around unsteady cloud cavitation on a stationary two-dimensional hydrofoil was investigated experimentally using a conditional sampling technique. The unsteady flow velocity around the cloud cavitation was measured by a Laser Doppler Anemometry (LDA) and matched with the unsteady cavitation appearance photographed by a high-speed camera. This matching procedure was performed using data from pressure fluctuation measurements on the foil surface. The velocities were divided into two components using a digital filter, i.e., large-scale (low-frequency) and small-scale (high frequency) ones. The large-scale component corresponds with the large-scale unsteady cloud cavitation motion. In this manner, the unsteady structure of the cloud cavitation was successfully measured. The experimental result showed that the cloud cavitation observed at the present experiment had a vorticity extremum at its center and a cluster containing many small cavitation bubbles. The convection velocity of the cavitation cloud was much lower than the uniform velocity. The small-scale velocity fluctuation was not distributed uniformly in the cavitation cloud, but was concentrated near its boundary.

Mechanisms for magnetic field reversals

2010 ◽  
Vol 368 (1916) ◽  
pp. 1595-1605 ◽  
Author(s):  
F. Pétrélis ◽  
S. Fauve
Keyword(s):  
Magnetic Field ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Large Scale ◽  
Conducting Fluid ◽  
Similar Model ◽  
Electrically Conducting ◽  
Electrically Conducting Fluid ◽  
Simple Mechanism ◽  
The Magnetic Field

We present a review of the different models that have been proposed to explain reversals of the magnetic field generated by a turbulent flow of an electrically conducting fluid (fluid dynamos). We then describe a simple mechanism that explains several features observed in palaeomagnetic records of the Earth’s magnetic field, in numerical simulations and in a recent dynamo experiment. A similar model can also be used to understand reversals of large-scale flows that often develop on a turbulent background.

Microwave Curing and Drying of Cellular Concrete

1992 ◽  
Vol 269 ◽  
Author(s):  
Walter M. van Loock
Keyword(s):  
Production Process ◽  
Feasibility Study ◽  
Building Material ◽  
Large Scale ◽  
Scale Production ◽  
Volumetric Heating ◽  
Microwave Curing ◽  
Thermal Insulator ◽  
Large Scale Production ◽  
Cellular Concrete

ABSTRACTTraditionally the heating and curing of cellular concrete takes place in an autoclave. This building material is a thermal insulator so the curing and drying processes are energy and time consuming. Volumetric heating is an attractive method for improving the throughputs.A feasibility study leads to the conclusion that microwave processing is not likely to be applied in a large scale production process.

Boundary Layer Transition on a Low Pressure Turbine Blade due to Downstream Potential Interaction

2012 ◽  
Author(s):  
Véronique Penin ◽  
Pascale Kulisa ◽  
François Bario
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Turbulence Intensity ◽  
Large Scale ◽  
Laser Doppler Anemometry ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Pressure Potential ◽  
Layer Transition ◽  
Transition Mode

Engine manufacturers wish to reduce the size and weight of their engines, and one way of achieving this is by reducing the rotor-stator gap. It follows that rotor-stator interactions become stronger, especially the influence of the pressure potential, which, despite its rapid spatial decay, becomes significant as the inter-row gap is reduced. Here we examine the upstream potential effect generated by downstream moving cylindrical rods on an upstream turbine blade. A large scale rectilinear blade cascade was constructed to improve access to the boundary layer. The Reynolds number was 1.6 × 105. Pressure measurements and two-dimensional Laser Doppler Anemometry around the blade were performed to study the boundary layer behavior. At low turbulence intensity (Tu−in = 1.8%), the laminar boundary layer experiences separation once per rod period. There are two transition modes which alternate during a rod period: separation transition mode and bypass mode. At high turbulence intensity (Tu−in = 4.0%), no boundary layer separation occurs. The boundary layer follows a bypass transition mode during an entire rod period.

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