Single Plume Research Articles

Slab-parallel advection versus Rayleigh-Taylor instabilities in melt-rich layers in subduction zones: A criticality analysis

We show slab-parallel advection and Rayleigh-Taylor instability (RTI) as two competing gravity-driven flow mechanisms in the melt-rich layer atop a subducting slab. Scaled laboratory model results, supported by CFD simulations, indicate a transition of the RTI to advection mechanism at a threshold slab-dip angle (α) between 20° and 30°. The advection mechanism results in updip flow, forming a single plume at the upper edge of the melt-rich layer. Based on lubrication approximation, we present an analytical solution of the linear stability problem to perform a criticality analysis of α for the transition between RTI and advection, as observed in the experiments. The solution indicates this transition to occur at α⁎ = 28°, in favorable agreement with the experimental values. The article finally highlights the implications of this criticality analysis in interpreting the spatial distributions of subduction volcanism.

First Measurements of Gas Flux with a Low-Cost Smartphone Sensor-Based UV Camera on the Volcanoes of Northern Chile

UV cameras have been used for over a decade in order to remotely sense SO2 emission rates from active volcanoes, and to thereby enhance our understanding of processes related to active and passive degassing. Whilst SO2 column density retrievals can be more accurate/sophisticated using alternative techniques (e.g., Differential Optical Absorption Spectrometer (DOAS), Correlation Spectrometer (COSPEC)), due to their higher spectral resolutions, UV cameras provide the advantage of high time-resolution emission rates, a much greater spatial resolution, and the ability to simultaneously retrieve plume speeds. Nevertheless, the relatively high costs have limited their uptake to a limited number of research groups and volcanic observatories across the planet. One recent intervention in this regard has been the introduction of the PiCam UV camera, which has considerably lowered instrumental cost. Here we present the first data obtained with the PiCam system from seven persistently degassing volcanoes in northern Chile, demonstrating robust field operation in challenging conditions and over an extended period of time, hence adding credence to the potential of these units for more widespread dissemination to the international volcanic gas measurement community. Small and weak plumes, as well as strongly degassing plumes were measured at distances ranging 0.6–10.8 km from the sources, resulting in a wide range of SO2 emission rates, varying from 3.8 ± 1.8 to 361 ± 31.6 td−1. Our acquired data are discussed with reference to previously reported emission rates from other ground-based remotely sensed techniques at the same volcanoes, in particular considering: resolution of single plume emissions in multi-plume volcanoes, light dilution, plume geometry, seasonal effects, and the applied plume speed measurement methodology. The main internal/external factors that influence positive/negative PiCam measurements include camera shake, light dilution, and the performance of the OpenCV and control points post processing methods. A simple reprocessing method is presented in order to correct the camera shake. Finally, volcanoes were separated into two distinct groups: low and moderate SO2 emission rates systems. These groups correlate positively with their volcanological characteristics, especially with the fluid compositions from fumaroles.

Plume accumulation effect and interaction of plumes induced by irradiation of a copper target with a burst of nanosecond laser pulses near the ionization threshold

The interaction between plumes induced by irradiation of a copper target in argon background gas with a burst of nanosecond laser pulses is studied numerically. The two-dimensional (2D) simulations are performed based on a model that includes a thermal model of the irradiated target and a kinetic model of plume expansion. The latter is implemented in the form of the direct simulation Monte Carlo method, where the equilibrium ionization and absorption of laser radiation are considered. For the laser spot diameter varying from 20 μm to 200 μm, the 2D plume structure is found to be different from the structure predicted by a one-dimensional model. The simulations reveal a strong plume accumulation effect when the plumes generated by preceding pulses in a burst change the conditions of propagation for plumes induced by subsequent pulses. The inter-plume interaction can result in merging shock waves induced by individual pulses and formation of a single plume. The degree of plume accumulation depends on the inter-pulse separation, laser spot diameter, background gas pressure, and number of pulses. When the irradiation conditions for a single pulse are below the ionization threshold, the conditions of plasma ignition can be reached by increasing the number of laser pulses due to the plume accumulation effect. This results in plasma shielding and reduction of the effectiveness of material removal by the subsequent pulses in the burst. The plume accumulation effect, thus, determines the optimum number of pulses in the burst in applications of laser ablation for material removal.

Convection-dominated dissolution for single and multiple immersed sessile droplets

We numerically investigate both single and multiple droplet dissolution with droplets consisting of less dense liquid dissolving in a denser host liquid. In this situation, buoyancy can lead to convection and thus plays an important role in the dissolution process. The significance of buoyancy is quantified by the Rayleigh number , which is the buoyancy force over the viscous damping force. In this study, spans almost four decades from 0.1 to 400. We focus on how the mass flux, characterized by the Sherwood number , and the flow morphologies depend on . For single droplet dissolution, we first show the transition of the scaling from a constant value to , which confirms the experimental results by Dietrich et al. (J. Fluid Mech., vol. 794, 2016, pp. 45–67). The two distinct regimes, namely the diffusively and the convectively dominated regimes, exhibit different flow morphologies: when , a buoyant plume is clearly visible, which contrasts sharply with the pure diffusion case at low . For multiple droplet dissolution, the well-known shielding effect comes into play at low , so that the dissolution rate is slower as compared to the single droplet case. However, at high , convection becomes more and more dominant so that a collective plume enhances the mass flux, and remarkably the multiple droplets dissolve faster than a single droplet. This has also been found in the experiments by Laghezza et al. (Soft Matt., vol. 12 (26), 2016, pp. 5787–5796). We explain this enhancement by the formation of a single, larger plume rather than several individual plumes. Moreover, there is an optimal at which the enhancement is maximized, because the single plume is narrower at larger , which thus hinders the enhancement. Our findings demonstrate a new mechanism in collective droplet dissolution, which is the merging of the plumes, which leads to non-trivial phenomena, contrasting the shielding effect.

The 2018 fire season in North America as seen by TROPOMI: aerosol layer height intercomparisons and evaluation of model-derived plume heights

Abstract. Before the launch of the TROPOspheric Monitoring Instrument (TROPOMI), only two other satellite instruments were able to observe aerosol plume heights globally, the Multi-angle Imaging SpectroRadiometer (MISR) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The TROPOMI aerosol layer height is a potential game changer, since it has daily global coverage, and the aerosol layer height retrieval is available in near real time. The aerosol layer height can be useful for aviation and air quality alerts, as well as for improving air quality forecasting related to wildfires. Here, TROPOMI's aerosol layer height product is evaluated with MISR and CALIOP observations for wildfire plumes in North America for the 2018 fire season (June to August). Further, observing system simulation experiments were performed to interpret the fundamental differences between the different products. The results show that MISR and TROPOMI are, in theory, very close for aerosol profiles with single plumes. For more complex profiles with multiple plumes, however, different plume heights are retrieved; the MISR plume height represents the top layer, and the plume height retrieved with TROPOMI tends to have an average altitude of several plume layers. The comparison between TROPOMI and MISR plume heights shows that, on average, the TROPOMI aerosol layer heights are lower, by approximately 600 m, compared to MISR, which is likely due to the different measurement techniques. From the comparison to CALIOP, our results show that the TROPOMI aerosol layer height is more accurate over dark surfaces, for thicker plumes, and plumes between approximately 1 and 4.5 km. MISR and TROPOMI are further used to evaluate the plume height of Environment and Climate Change Canada's operational forecasting system FireWork with fire plume injection height estimates from the Canadian Forest Fire Emissions Prediction System (CFFEPS). The modelled plume heights are similar compared to the satellite observations but tend to be slightly higher with average differences of 270–580 and 60–320 m compared to TROPOMI and MISR, respectively.

A Stochastic Unified Convection Scheme (UNICON). Part I: Formulation and Single-Column Simulation for Shallow Convection

AbstractBy extending the previously developed unified convection scheme (UNICON), we develop a stochastic UNICON with convective updraft plumes at the surface randomly sampled from the correlated multivariate Gaussian distribution for updraft vertical velocity w^ and thermodynamic scalars ϕ^, of which standard deviations and intervariable correlations are derived from the surface-layer similarity theory. The updraft plume radius R^ at the surface follows a power-law distribution with a specified scale break radius. To enhance computational efficiency, we also develop a hybrid stochastic UNICON consisting of n bin plumes and a single stochastic plume, each of which mainly controls the ensemble mean and variance of grid-mean convective tendency, respectively. We evaluated the stochastic UNICON using the large-eddy simulation (LES) of the Barbados Oceanographic and Meteorological Experiment (BOMEX) shallow convection case in a single-column mode. Consistent with the assumptions in the stochastic UNICON, the LES w^ and ϕ^ at the surface follow approximately the half- and full-Gaussian distributions, respectively. LES showed that a substantial portion of the variability in ϕ^ at the cloud base stems from the surface, which also supports the concept of stochastic UNICON that simulates various types of moist convection based on the dry stochastic convection launched from the surface. Overall, stochastic UNICON adequately reproduces the LES grid-mean thermodynamic states as well as the mean and variance of ϕ^, including their dependency on the domain size and R^. A sensitivity test showed that the perturbations of ϕ^ as well as R^ at the surface are important for the correct simulation of the grid-mean thermodynamic states.

Distribution of iron in the Western Indian Ocean and the Eastern tropical South pacific: An inter-basin comparison

The Western Indian Ocean (WIO) and Eastern Tropical South Pacific (ETSP) are distinctly different regimes, yet they share several important features. These include a strong upwelling system, a large oxygen minimum zone (OMZ) with active denitrification, a spreading center with extensive hydrothermal activity, and a vast oligotrophic upper water column. Here, we show that the distribution and geochemistry of iron shows remarkable similarities as well. In particular, both basins exhibit large sub-surface plumes of iron derived from sediments underlying the oxygen minimum zones and from hydrothermal sources. Moreover, the behavior of Fe, especially its redox cycling, is remarkably similar in the Arabian Sea and Peruvian oxygen minimum zone, reflecting similar processes associated with the nitrogen cycle in each region. The large Fe plumes under each OMZ are well below the oxygen-free depth, yet they are probably dependent on internal redox cycling and scavenging of Fe within the overlying OMZs. Globally, both the Arabian Sea and Peru margin sediments are hot spots of carbon oxidation, which probably contributes to their importance as Fe sources.The fate of hydrothermally derived Fe in the WIO and ETSP is particularly interesting to compare because their source terms are very different even though their oxidation rates, once they have entered the water column, should be directly comparable. In the ETSP, hydrothermal inputs from along a section of the ultra-fast spreading Southern East Pacific Rise (SEPR) are integrated into a single coherent plume that is exported west across the ocean interior for >4000km. In contrast, hydrothermal inputs to the WIO derive from multiple sources including outflow from the Red Sea via the Gulf of Aden, the Indonesian Throughflow, and from multiple vent-sources along the Carlsbad Ridge, Central Indian Ridge and SW Indian Ridge which are slow, medium and ultra-slow spreading, respectively. As a result, hydrothermally sourced Fe distributions are more complex in the WIO, with several overlapping plumes and distinctly different distributions of two other hydrothermally-derived metals, aluminum and manganese. Here we argue that comparative studies of these basins, with a future emphasis on benthic-water column interactions, will advance knowledge about Fe biogeochemistry and how distributions in the WIO, in particular, are likely to change in the future.

Surveillance of SO<sub>2</sub> and NO<sub>2</sub> from ship emissions by MAX-DOAS measurements and the implications regarding fuel sulfur content compliance

Abstract. Due to increased concerns regarding air pollutants emitted from shipping, feasible technology for the surveillance of these pollutants is in high demand. Here, we present shore-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of SO2 and NO2 emitted from ships under different traffic conditions in China's ship emission control areas (ECAs) in Shanghai and Shenzhen, China. Three typical measurement sites were selected in these two regions to represent the following emission scenarios: ships docked at berth, ships navigating in an inland waterway and inbound/outbound ships in a deep-water port. Using 2-D scanning, the observations show that SO2 and NO2 hot spots can be quickly and easily located from multiple berths. Although MAX-DOAS measurements can not distinguish plumes from specific ships in the busy shipping lanes of the inland waterway area, they certify that variations in the SO2 and NO2 levels are mainly impacted by the ship traffic density and the atmospheric dispersion conditions. In the open water area, which has a lower vessel density, MAX-DOAS measurements can capture the pulse signal of ship-emitted SO2 and NO2 very well; they can also characterize the peak's altitude and the insistent duration of the individual ship plumes. Combined with the ship activity data, information on the rated power of the engine and the fuel sulfur content, it was found that the SO2∕NO2 ratio in a single plume is usually low (< 1.5) for inbound vessels due to the usage of the auxiliary engine, which has less power and uses “clean” fuel with a low sulfur content. Thus, an unexpectedly high SO2∕NO2 ratio implies the use of fuel with a sulfur content exceeding the regulation limits. Therefore, the observed SO2∕NO2 ratio in the plume of a single ship can be used as an index to indicate compliance (or noncompliance) with respect to the fuel sulfur content, and the suspicious ship can then be flagged for further enforcement. Combining the ship emissions estimated by actual operation parameters and the logical sulfur content, shore-based MAX-DOAS measurements will provide a fast and more accurate way to surveil ship emissions.

Steady laminar plume generated from a heated line in polymer solutions

From the theoretical viewpoint, governing equations are derived for a steady laminar plume generated from a heated line in polymer solutions, in which similarity variables and the single polymer chain model are introduced. The resolved solutions imply that polymers promote the velocity in the centerline-near region but suppress that in the region far from the centerline. The equivalent effect of polymers is understood as producing two space-dependent source terms, which can explain the interaction between polymers and fluid flow from the viewpoint of energy transport. There exists a critical Weissenberg number (Wi) beyond which the promotion effect in the centerline-near region disappears, which results from the competition of stretching and relaxation of the polymer chain. Meanwhile, the corresponding modification of similarity velocity and heat transport are illustrated and validated numerically by single plume flow in polymer solutions. This work thus may contribute to the understanding of the polymer effect on single plume flow and further the heat transport enhancement mechanism in the bulk flow of turbulent Rayleigh-Bénard convection with polymers.

Investigations on near-field atomization of flash boiling sprays for gasoline direct injection related applications

Flash boiling sprays have been widely studied recently because of its potential in generating fuel sprays with improved atomization efficiencies. Due to the strong multiphase influence, the exact spray atomization mechanisms of flash boiling sprays are not well understood yet, especially in the near-field where the optical depth is usually high for optical measurements. This investigation utilized a two-hole gasoline direction injection fuel injector to analyze the near-field atomization performance of a single spray plume under flash boiling conditions with a lower injection pressure to avoid the interference from dense sprays under higher injection pressures. High-speed backlit and Schlieren techniques were used to capture the liquid phase and gas phase distributions in the near-field and a microscope was incorporated to obtain detailed structure information of the fuel spray near the nozzle exit. Boundary conditions including fuel temperatures and ambient pressures were varied to study fuel atomization under different superheat indices and Cavitation numbers. Plume characteristics at different stages of the injection were investigated and significant multiphase influence can be found within the transitional flash boiling regime, including a two-zone atomization behavior at the beginning of the injection, a liquid skirt structure during the fuel injection, and a fishnet ligament pattern at the end of injection, respectively. This work also examined the impact of Cavitation number, superheat index, and ambient pressure on the properties of the external spray and such influence was represented by the near-field cone angle defined from near-field measurements.

Numerical approach to analyze propane flash boiling spray using modified gas-jet model

In engine simulation, coarse mesh size is usually used to analyze a variety of cases utilizing chemistry which consists of many species and reactions. However, it is difficult to predict spray morphology exactly with coarse mesh. In this study, a model to predict structure deforming flash boiling spray even for the coarse mesh has been developed. A flash breakup model combined with a modified version of a gas-jet model was utilized to analyze propane flash boiling spray. The gas-jet direction was calculated by averaging the directions of spray from each nozzle. A high-velocity region near the axis of gas-jet flow confined the plumes to form a single plume. Gasoline spray was analyzed to determine proper Kelvin-Helmholtz-Rayleigh-Taylor (KH-RT) model constants. Gasoline spray characteristics such as spray tip penetration and morphology were captured using the final model constants. Using the KH-RT model constants, the flash breakup model was validated for propane flash boiling spray. Sauter mean diameter (SMD) of simulation and experiment showed similar results. However, spray morphology could not be captured by the simulation using only the flash breakup model. A modified gas-jet model was applied to the simulation along with the flash breakup model to improve spray morphology. Using the modified gas-jet and flash breakup models, spray structure penetrating through the central axis was successfully predicted and this resulted in a better spray tip penetration prediction. SMD trend depending on ambient pressure was captured and maximum error between experimental and simulation results was 11%.

Thermal Flow Characteristics of the Triple Plasma Torch System for Nanoparticle Synthesis

Triple dc plasma torch system was used for the synthesis of nanoparticles. Three thermal plasma jets are ejected from each plasma torch and they are merged into a strong single plasma plume. Since the feedstock is injected into the jet merging area, this system is suitable for refractory nanoparticle synthesis and enable to high production yield caused by the complete evaporation is possible. In this paper, numerical simulation was carried out to analyze thermal plasma characteristic inside a reactor according to the plasma-forming gas and carrier gas compositions: Ar, Ar–H2, and Ar–N2 plasma; Ar, He, and N2 carrier gas. Then, it was comparatively interpreted with the actually synthesized nanoparticle in order to understand the condensation process of nanoparticles. It was revealed that the thermal environment is able to control readily in the triple torch system by the operating condition depending on the target materials.

Reproducibility assessment and uncertainty quantification in subjective dust source mapping

Abstract Accurate dust-source characterizations are critical for effectively modeling dust storms. A previous study developed an approach to manually map dust plume-head point sources in a geographic information system (GIS) framework using Moderate Resolution Imaging Spectroradiometer (MODIS) imagery processed through dust-enhancement algorithms. With this technique, the location of a dust source is digitized and recorded if an analyst observes an unobscured plume head in the imagery. Because airborne dust must be sufficiently elevated for overland dust-enhancement algorithms to work, this technique may include up to 10 km in digitized dust-source location error due to downwind advection. However, the potential for error in this method due to analyst subjectivity has never been formally quantified. In this study, we evaluate a version of the methodology adapted to better enable reproducibility assessments amongst multiple analysts to determine the role of analyst subjectivity on recorded dust source location error. Four analysts individually mapped dust plumes in Southwest Asia and Northwest Africa using five years of MODIS imagery collected from 15 May to 31 August. A plume-source location is considered reproducible if the maximum distance between the analyst point-source markers for a single plume is ≤10 km. Results suggest analyst marker placement is reproducible; however, additional analyst subjectivity-induced error (7 km determined in this study) should be considered to fully characterize locational uncertainty. Additionally, most of the identified plume heads (>90%) were not marked by all participating analysts, which indicates dust source maps generated using this technique may differ substantially between users.

Spatial Statistical Assessment of Groundwater PCE (Tetrachloroethylene) Diffuse Contamination in Urban Areas

Contamination by chlorinated solvents is typically associated with point sources, which are able to release high concentrations and to generate well defined plumes. Nevertheless, in urban settings (especially in functional urban areas—FUAs), multiple-point sources are frequently present, consisting of a series of unidentifiable small sources clustered within large areas, generating a diffuse, anthropogenic contamination. This situation results in the coexistence of single plumes with higher contaminant concentrations, and larger areas where the concentration is lower but still higher than the maximum admissible concentration limits. This paper proposes a methodology devised to cope with the diffuse contamination by chlorinated solvents within shallow aquifers due to multiple-point sources in FUAs. The approach is based on a Bayesian model that helps to spatially evaluate the likelihood of having active multiple-point sources, and to relate their impact on the shallow aquifer to the hydrogeological features of the area. Moreover, the approach allows testing of the efficiency of the monitoring network to properly characterize the contamination in the aquifer. The consistency of the results of the analysis was also checked for the Milan FUA (Italy) by a comparison to a previous study, performed through an inverse numerical modelling approach within a Monte Carlo statistical framework to identify the areas with the highest likelihood to host potential multiple-point sources.

Investigation of two-hole flash-boiling plume-to-plume interaction and its impact on spray collapse

Flash boiling can enhance fuel breakup and atomization via the quick growth and eruption of bubbles when the high temperature fuel exiting the nozzle. For multi-hole fuel injectors, a high level of flash boiling might cause the plumes to merge into a single plume. Such phenomenon is known as spray collapse, which will change the preset spray targeting in real engines, and is not desirable for practical application due to a longer penetration it might cause. Although extensive works have been done on the collapse of flash-boiling, no consensus has been reached for its core mechanisms yet. This work used a two-hole injector to study the flash-boiling plume-to-plume interaction, which is considered to be a key inducement of spray collapse. Optical measurement approaches were used to examine the influence of fuel temperature, ambient pressure, and injection pressure on the morphology of the flash boiling spray. It was found that a secondary plume was generated by two primary plumes side-collision in the spray central region under some flash-boiling conditions, and its behavior had a clear relation with the level of flash-boiling and other boundary conditions. Additionally, elevating injection pressure and increasing ambient pressure are two doable options for controlling the flash-boiling targeting in real engines, through which non-collapse flash-boiling sprays can be obtained. Finally, the relation between the secondary plume and the spray collapse was clearly demonstrated, and possible collapse mechanism was discussed in details.

Petrogenesis of the southern Qiangtang mafic dykes, Tibet: Link to a late Paleozoic mantle plume on the northern margin of Gondwana?

AbstractThis study presents 13 new U-Pb zircon ages obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) together with whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data for a mafic dike swarm in the southern Qiangtang area of Tibet. These data provide the basis for a new model of the late Paleozoic evolution of the Tethys. Combined with the results of previous zircon U-Pb dating, the magmatic zircon grains extracted from mafic dikes yield latest Carboniferous to Early Permian ages (317–279 Ma). The geochemistry of the southern Qiangtang mafic rocks indicates the presence of low-Ti (QLT) and high-Ti (QHT) suites. The magmas that formed the QLT suite underwent higher degrees of partial melting (>5%) and display evidence of crustal contamination, whereas the QHT suite was derived from magmas generated by low-degree (1%–5%) partial melting of a garnet-bearing mantle source, with a greater extent of fractional crystallization than the QLT suite, and no evidence of crustal contamination. We propose that the QHT and QLT suites may have been derived from magmas from different parts of a single mantle plume. The formation of the southern Qiangtang mafic dikes (latest Carboniferous to Early Permian; 317–279 Ma) may have been related to the northward drift of the Cimmerian continent from the northern Gondwana margin, which resulted in the opening of the Meso-Tethys Ocean.

Plume-to-plume interactions in the Pearl River Delta in winter

River plumes are important carriers of terrigenous materials being transported into coastal seas. Previous studies of river plumes focused on large and single river plumes and paid less attention to the plume-to-plume interactions among the small river plumes. In this study, we used the ROMS (Regional Ocean Modeling System) model to study the plume-to-plume interactions in the Pearl River Delta area during wintertime, when the river discharge is relatively low and downwelling-favorable winds prevail. During the wintertime, the river plumes inside the Pearl River Estuary are well developed, and their interactions are extensive. Both the downstream and upstream plumes affect each other, and the interactions include both barotropic and baroclinic effects. The barotropic effects of a sub-plume could repel or suppress the spread of other plumes into its territory, but with limited spatial coverage. The baroclinic effect (buoyancy input) could alter salinity stratification, generate current fields, and greatly influence the dispersal of other sub-plumes over a broader spatial extent. The river plumes in the Pearl River estuary further affect the dispersal of waterborne materials in the downstream Modaomen estuary. Our results have distinct implications for regional environmental management.

Two-photon fluorescence laser sheet imaging for high contrast visualization of atomizing sprays

Two-photon excitation laser induced fluorescence (2p-LIF) is used here for imaging an optically dense atomizing spray. The main advantage of the approach is that very little fluorescence interference originating from multiple light scattering is generated. This leads to high image contrast and a faithful description of the imaged fluid structures. While point measurement 2p-LIF imaging is a well-known approach used in life science microscopy, it has, to the best of the authors’ knowledge, never been tested for analyzing liquid structures in spray systems. We take advantage of this process, here, at a macroscopic scale ($\sim 5\times 5$∼5×5 mm field of view) by imaging the central part of a light sheet of 10 mm height. To generate enough 2p-LIF signal at such a scale and with single-shot detection, ultra-short laser pulses of 25 fs, centered at 800 nm wavelength and having 2.5 mJ pulse energy, have been used. The technique is demonstrated by imaging a single spray plume from a 6 hole commercial Gasoline Direct Injection (GDI) system running at 200 bar injection pressure. The proposed approach is very promising for detailed analysis of liquid breakups in optically dense sprays and can be used for other fluid mechanics related applications.

Numerical study on the dynamic process of single plume flow in thermal convection with polymers

A direct numerical simulation of single plume flow in thermal convection with polymers was carried out in a domain with 1:3 as the width to height ratio. The heat transport ability is weakened by adding polymers within the here-investigated governing parameter range. However, it is promoted when the maximum polymer extension L is increased. The distribution of vertical velocity and temperature indicates that the plume in the polymer solution case is speeded up and widens bigger as compared to that in the Newtonian fluid case. Inside the plume, polymer chains tend to release energy at the position where the velocity is decelerated. The ratio of Nusselt numbers (Nu/NuNew) shows the power-law scaling relation with the governing parameter L2/Wi in polymer solution cases, which is only applicable for moderate Wi and small L. The present study can give direct insight into the observation about plumes in turbulent thermal convection experiments. It is therefore useful for the analysis of heat transport in thermal convection with polymers.

Spectroscopic investigation of molecular formation in laterally colliding laser-produced carbon plasmas.

We report detailed spectroscopic investigation of carbon dimer production in the stagnation layer formed due to laterally colliding plasma plumes under vacuum and N2 ambience. Two spatially separated Nd:YAG laser pulses are used to generate plasma plumes in close proximity to a solid graphite target. A comparative optical emission spectroscopic study of a stagnation layer and a single carbon plasma plume is carried out in order to find the optimum experimental conditions for carbon dimer (C2) production. Significant enhancement in the C2 production is observed in the stagnation layer as compared to that observed in the single plume. Further, a substantial increase in carbon dimer formation is monitored by introducing N2 gas. Along with carbon dimer, carbon nitride (CN) band emission is also observed in the emission spectra. Spatiotemporal evolution of C2 and CN emission reveals that the yield of carbon dimer production is at a maximum close to the target surface, and decreases with an increase in time as well as spatial position. On the contrary, CN emission increases with the target distance. The dominant pathways for CN formation are suggested, and it is demonstrated that C2 acts as a precursor to CN formation. The above observation is correlated with cooling of ejected carbon species in the collisional regime and the estimated vibrational temperature of C2.