【巨視的】 特集 5巻3号(1991)  土石流; 災害; 火砕流;
Multi-Phase flow is often related to natural disasters. Especially large flows cause drastic geomorphic changes and can lead to catastrophy in downstream areas. Debris flow is a typical two-phase flow in such phenomena. In this paper, the authors explain the behaviour of debris flow obtained by observations and experiments. A consititutive equation for debris flow is developed by considering the mechanism of energy dissipation in the flow of the mixture of water and sand particles. The consititutive equation is applied to discuss the distributions of velocity and particle concentration in uniform debris flow. The authors describe the characteristics of two-phase flow in catastrophic events by taking such examples as the Mt. St. Helens eruption, Mt. Asama's eruption and the Mt. Ontake land slide.
【巨視的】 特集 5巻3号(1991)  雲; 火砕流; 雪崩;
Phase change of water vapour in the atmosphere produces small water particles in the atmosphere. They are usually suspended quite densely in limited space and are called clouds. When the air temperature falls below -20°C, they are replaced by ice particles. Clouds play an important role in the meteorological processes from local to global scales.Pyroclastic flow is a complicated multiphase flow associated with volcanic eruptions. It consists of a mixture of air and hot volcanic ash and behaves like a gravity current along mountain slopes. Since the air is heated by hot ash, buoyancy in the pyroclastic flow increases like cumulus in which the air is heated by the latent heat of water.Powder snow avalanche is a kind of turbidity currents, which occurs when a large amount of powder snow accumulates in piles along the moutain slope. Once the avalanche begins, turbulence in the avalanche takes the piled snow into the air along the path and the effective density increases. The powder snow avalanche is very destructive, because the velocity reaches about 100m/s in the developed stage.
【巨視的】 特集 5巻3号(1991)  海洋大循環モデル; 海面水温;
Hydrographic observations in the western tropical Pacific show not only the heat content variability related to ENSO but also another decadal trend related to the recent global warming issue. Since the background of SST is high in the western tropical Pacific, even weak SST anomalies may strongly affect the location of the Aleutian Low as well as the Asian monsoon in the extratropics in winter. In the off-equatorial western Pacific the ocean behaves like a dynamic slave to the winter Asian monsoon as demonstrated by the oceanic model simulation. In particular, the winter monsoon variations are responsible for the maturity or immaturity of the cold Mindanao Dome off the Philippine coast. The active (inactive) summer monsoon followed by the increasingly anomalous easterlies (westerlies) over the western tropical Pacific from summer through winter appears to be responsible for the positive (negative) SST anomalies in the western tropical Pacific. This suggests that an interesting positive feedback mechanism may exist in the Asian monsoon system, i. e., a coupled ocean-atmosphere-land system. Since vast amounts of short term natural climate variabilities ranging from several years to decades seem to originate in those SST variations in the western tropical Pacific, it will be necessary to look more closely at the Asian monsoon system.
【巨視的】 特集 5巻3号(1991)  密度成層; 成層乱流; 内部重力波;
Buoyancy force associated with density stratification plays a key role in environmental flows. It damps or enhances vertical motions directly in the stratified flows. Under stable stratification, it induces internal gravity waves and blocking, which cause drastic changes in the flow pattern. In addition to these direct effects, buoyancy force deforms the turbulence structure in the stratified flows, which, in turn, causes drastic changes in the turbulent transport mechanisms of momentum, heat and mass, and therefore in the flow pattern. In this paper, these direct and indirect effects of buoyancy forces are reviewed, with special attention focused on the stratified atmospheric motions.
【巨視的】 総説 6巻2号(1992)  密度成層; 乱流構造; 鉛直混合;
Research work concerning vertical mixing processes in stratified water areas is briefly reviewed. Attention is focused on vertical mixing phenomena which are found in the hydraulic engineering field, such as estuaries, lakes, reservoirs, land locked bays, etc. From the view point of the mixing mechanism, entrainment processes are classified into three categories, i.e., (1) shear induced entrainment (Stype), (2) entrainment due to eddy convection and diffusion in the mechanical stirring fied (DM-type) and (3) entrainment due to thermal convection (DT-type). The turbulence structure and entrainment mechanism for each case are examined and discussed. Experimental and analytical work on entrainment relationships is shown for several types of stratified flow fields among which shear-free turbulence generated by an oscillating grid, stratified shear flows, wind-induced mixing and turbulent penetrative convection are included.
【巨視的】 解説 7巻3号(1993)  土石流; 災害; 火砕流;
Unzen Volcano began to erupt in November 1990 after 198 years of dormancy and has remained active. Continuous growth of a lava dome and falls of lava rocks have resulted in frequent pyroclastic flows. Since a great volume of volcanic material has been deposited and scattered by the pyroclastic flows, debris flows have frequently occurred along the rivers around Mount Unzen, particularly along the Mizunashi River. On June 30, 1991, August 8, 1992, and April 28, 1993, large debris flows occurred in the Mizunashi River and caused severe damage in the down reach. Over a hundred houses were destroyed by each debris flow, however, no one was killed or injured as the debris flows flooded within the evacuated area. On June 3, 1991, however, 43 lives were lost as a result of a large pyroclastic flow. On June 8 and September 15, 1991, larger pyroclastic flows occurred and burned many houses along the Mizunashi River. As Unzen Volcano is still active, the possibilities of debris flows and corresponding risk to residents have risen up at the wider areas in addition to the risk of pyroclastic flows themselves.
【巨視的】 解説 8巻3号(1994)  海洋大循環; 炭素フラックス; 植物プランクトン;
Ocean plays an important role in global environment in terms of heat transport and carbon cycle. Carbon is distributed in the ratio of 1:50 between atmosphere and ocean, respectively, and therefore even small fluctuation of carbon budget in the ocean will strongly affect the carbon budget in the atmosphere. Biogeochemical cycle in ocean is driven by marine ecosystem, in particular by marine phytoplankton, and complex multi-phase flow exists in the ocean. Controlled experimental ecosystem is useful in order to understand biochemical flux in multi-phase flow of marine ecosystem. Experimental results using marine phytoplankton. Emiliania huxleyi showed the importance of phytoplankton on carbon flux in the ocean.
【巨視的】 小特集 9巻2号(1995)  地震; 予知;
On 5:46Am, January 17, 1995, devastating earthquake with magnitude 7.2 hit western Japan. This earthquake was finally named “Hanshin-Awaji earthquake disaster”, and lost over 5, 500 people. Were we able to predict this earthquake? We introduce the method of earthquake prediction by monitoring electro magnetic phenomena, especially telluric current measurements.
【巨視的】 小特集 9巻2号(1995)  火災旋風; 渦;
In order to elucidate the physical processes of the formation of fire whirl, a series of laboratory experiments on three dimentional temperature profile of crib fire plume in cross wind were performed. From the experimental result, it became evident that a fire in cross wind produces a pair of vortex-like circulation in the downstream side of fire plume. In rare occasions, fire whirls were observed in the series of experiment of multiple fires at the central or leeward part of multiple fires under weak wind.
【巨視的】 小特集 9巻2号(1995)  地震; 考古学; 液状化;
遺跡で検出された地震の痕跡を研究する「地震考古学」によって多くの成果が得られつつある.特に, 液状化現象については, 液状化した地層・地割れ内を上昇しつつある噴砂・地面に広がった噴砂を一連のシリーズとして観察できる.又, 過玄の地震の時期が把握できる利点を生かして, 南海地震や東海地震の周期性を把握することができた.
【巨視的】 小特集 9巻3号(1995)  地震; 液状化; 杭基礎;
The phenomenon of liquefaction is one of the most important subjects in earthquake engineering. After introducing the general description on the liquefaction phenomenon, computer algorithms for liquefaction analyses of fluid-saturated grounds are summarized which use finite element and finite difference coupled numerical methods; the accuracy of several numerical methods are then addressed through comparisons of the analytical solutions and the experimental data as well as observed ground motions during the 1995 Hyogo-ken Nanbu earthquake. To estimate large ground displacement during and after the liquefaction occurrence a typical stress-strain relationship of sand after onset of liquefaction is discussed using the data obtained by torsional shear tests under several conditions.
【巨視的】 小特集 9巻3号(1995)  地震; 高架橋梁;
As you know we had the great Hanshin Earthquake at early morning on Jan.17, 1995, so beautiful Kobe city was almost crumbled. This report is made on basis of a field investigation on almost all bridge structures located in the related area. By the inspection of damaged or collapsed bridges it is clear that a large number were designed more than 20 years ago, and also these structures had not retrofitted to increase their resistant capacity against large earthquakes. Finally, this earthquake showed more than ever, the effect of vertical acceleration like a shock on the failure mechanism of collapsed structures.
【巨視的】 特集 23巻3号(2009)  気象; 雲; 数値計算;
【巨視的】 特集 24巻1号(2010)  ラジオグラフィ; 火山; 火動流;
This paper reviews the principle of cosmic-ray muon radiography (muography) and its application to the analysis of gas-liquid two-phase flow in a conduit during volcanic eruption. Muography is a useful technique for imaging internal density distribution of a big object (e.g., a volcano, a blast furnace, etc.). By measuring muon path lengths and absorption along different paths through the object, one can deduce average density along the path inside the object. According to numerical modeling of gas-liquid two-phase flow in the volcanic conduit during a dome-forming eruption, magma porosity (i.e., density) inside the conduit largely changes depending on magma flux even if the porosity at the surface is kept low. The increase in the porosity inside the conduit may induce the transition to explosive eruption. This implies that high-resolution measurement of the porosity distribution inside the conduit is essential for predicting the transition of eruption styles. Recent progress in a muon detector system enables us to perform this measurement, such as in Mt. Iwodake of Satsuma-Iwojima Volcano, Japan. Spatial resolution of muography is adequate for detecting the variation of the porosity inside the conduit. Applications to industrial plant monitoring are also discussed.
【巨視的】 特集 24巻2号(2010)  気象レーダ; 雲; 豪雨;
This paper introduces the basic theory of advanced weather radar which consists of polarimetric technique and dopplar measurement. Moreover, equations representing the cloud microphysical processes for both precipitation particls development and air mass movement are explained briefly.
【巨視的】 特集 26巻1号(2012)  地震; 長期予測; 古地震;
The 2011 M9.0 off the Pacific Coast of Tohoku Earthquake (Tohoku-oki earthquake) brought a great impact on the long-term forecasting of subduction earthquakes around the Japanese islands. Short historic data together with dogmas of modern seismology, such as conventional asperity model, characteristic earthquake model, and earthquake scaling law prevented us to have anticipated the size of M9 earthquake offshore Pacific coast of Tohoku. It may suggest that longer than 1000-year earthquake occurrence history is required to properly evaluate the size and frequency of mega-thrust events, same as the M~7 destructive earthquakes associated with inland active faults. The Tohoku-oki earthquake has significantly changed the state of crustal stress in northeast Honshu island from EW compression to EW extension, in which numerous widespread triggered earthquakes have been occurring. Here I introduce the coseismic stress transfer due to the Tohoku-oki earthquake onto the major active faults, and then demonstrate the importance of the transient changes of state of stress on the faults for long-term earthquake forecasting during the next few decades.
【巨視的】 特集 26巻1号(2012)  地震; 津波; 災害;
In Iwate, Miyagi and Fukushima Prefectures, more than 1,000 people were killed or missing each due to the 2011 off the Pacific coast of Tohoku Earthquake and Tsunami. It is thought that damaged local governments have faced a lot of difficult problems concerning social responses caused by the wide-area catastrophic damage. The purpose of my paper was to examine problems concerning social responses under the Great East Japan Earthquake Disaster that are peculiar to “a wide-area catastrophic disaster". I define “a wide-area catastrophic disaster" to refer to a disaster that causes catastrophic damage to two more prefectures. For the purpose of this study, first, the method of disaster categorization in terms of the degree of damage and the extent of damage was proposed. Secondly, the degree of damage and the extent of damage of the 2011 event were discussed in terms of tsunami height, inundation area, the number of dead and missing people and the amount of disaster debris. Thirdly, particular problems concerning social responses under the 2011 event were discussed by using limited reports and data that are published on the Internet. As a result, 5 types of particular problems under the 2011 event as a wide-area catastrophic disaster were pointed out: the limit of social response of damaged local governments, the limit of social response of supporting local governments, competition of resource support during damaged local governments, instability of policies, uncertainty of problem concerning social response. In conclusion, understanding the particular problems concerning social response under a wide-area catastrophic disaster is considered essential for reducing damages of next catastrophic disasters. It is recommended that particular problems concerning social responses under the 2011 event are discussed by using more data that will be published and hearing the local governors in the damaged area and so on.
火山噴火における混相流現象 (火山噴煙の数値シミュレーション)
【巨視的】 特集 29巻2号(2015)  火山; 噴煙; 数値計算;
Explosive volcanic eruption is one of the most hazardous natural phenomena. During explosive eruptions, a mixture of volcanic ash and gases is ejected from a volcanic vent into the atmosphere. For hazard risk assessment, it is important to comprehensively explain various observed data during eruptions and to understand the dynamics of explosive eruptions and the mechanism of volcanic ash dispersal. We have developed a pseudo-gas model of eruption cloud dynamics and ash dispersal. Our model has successfully reproduced the heights of eruption cloud and the distribution of fall deposits during large eruptions such as the Pinatubo 1991 eruption and those during small eruptions such as the Shinmoe-dake 2011 eruption. For more accurate estimates of volcanic hazard risks, two-way coupled models of multiphase flow are required.
【巨視的】 特集 29巻2号(2015)  土石流; 雨; 災害;
Large-scale debris flow disasters take place due to heavy rainfall recently. The present study discusses the relationship between the trigger and the inherent factor of debris flow occurrence. Three debris flow disasters in Izuoshima, Nagiso-town and Hiroshima-city are chosen for investigation. Debris flow occurrence due to rainfall is greatly dependent on two factors: previous prolonged rainfall and the last short strong rainfall. Rainfall index R' can express with one value combining the long-term effective rainfall Rw and the short-term effective rainfall rw. In addition, it is necessary to clarify the inherent factors of a natural place, such as topography, geology and vegetation. Especially, slope gradient has strong influences on the sediment runoff characteristics, such as flow depth, velocity, discharge and sediment concentration. We should learn more about formation in land, land-use history and the history of disasters as preparations to a natural disaster.
【巨視的】 特集 29巻2号(2015)  高潮; 気象; モデリング;
Surge-Wave-Tide coupled model (SuWAT) has been developed and improved its applicability. Effect of tidal variation on storm surge is discussed by a simplified bathymetry test and hindcast simulation in Korean west coast. Effect of wave setup is also confirmed by hindcast of storm surge in Kochi coast caused by typhoon Anita. SuWAT became enabling to use time history data of atmosphere as a boundary forcing and applied to hindcast simulation of typhoon Vera, which caused the national worst storm surge disaster. Climate change experiment result by GCM is employed for future coastal disaster projections, and the results indicate future storm surge would increase in the Ise Bay and the west Seto Inland Sea.
【巨視的】 特集 31巻4号(2017)  津波; 数値計算; 可視化;
This paper, first of all, explains a two-dimensional depth-integrated tsunami flow model, which is often used in practice. The validity and utility of the model were examined by applying the model to tsunami induced by The 2011 Tohoku Earthquake. Furthermore, this study expounds a three-dimensional fluid analysis model “DOLPHIN” that can calculate solid-gas-liquid multiphase flow, as well as a visualization system of tsunami simulation using a three-dimensional numerical tool “OpenFOAM”, which provides dynamic information so as to enable to intuitively understand the characteristics and risks of the disaster.
【要素的】 特集 32巻4号(2018)  ダウンスケーリング; 雲微物理スキー ム; シミュレーション;
The current status of convection-permitting regional climate models is reported. First, the outline of regional climate models is described. Next, the representation of clouds in regional climate models is mentioned. Subsequently, the performance of numerical simulations using convection-permitting regional climate models is presented. In particular, the effects of high-resolution topography on the performance of simulations are discussed. Finally, an example of future climate projections using convection-permitting regional climate models is introduced.
【巨視的】 特集 33巻1号(2019)  底質移動動態; 高波浪; アウターバー形状;
The field observations of cross-shore sediment transport from the swash zone to the offshore side of the outer bar using fluorescent sand tracers were conducted in 2014 and 2015 at the Hasaki coast, Japan facing the Pacific Ocean during storm wave conditions. The topography difference between the two years data set is that outer bar shape existed in 2014 and not existed in 2015. During the observations, wave height and wave period were observed, and sand core samplings were conducted. By using the observed wave data and the analyzed results of collected fluorescent sand tracers from the cores, the sediment movements from the offshore side of the outer bar area to the swash zone during storms were investigated. The results indicate that the sediment movements during the storms were affected by the outer bar shape. If the outer bar existed, the sediment of the onshore side of the outer bar tend to move landward, and it also moves to seaward due to undertow until the outer bar location. The sediment at the offshore side of the outer bar tends to move to onshore ward due to bedload sediment transport. However, the transport rate decrease in the trough region. The beach topography without the bar shape, the bed load sediment transport rate at the trough region was higher than that with the bar shape. Thus, the sediments at the offshore side of the bar also could move until the swash zone.
【巨視的】 特集 33巻1号(2019)  砂浜消失; Bruun則; 土砂生産量;
Future beach loss in Japan are reported to be more than 90% due to sea level rise in the worst case. Beaches are affected not only by the sea level rise but also by rainfall and wave characteristic changes and others. This paper reviews past and future beach loss in Japan in a national scale and introduces projection method of future beach loss due to sea level rise and rainfall characteristic change. Even past beach loss and sediment budget are not easy to be quantitatively elucidated in coastal catchments in a national scale, then realistic future beach loss projection is much more challenging. Both further past data analysis and beach profile model improvement/development are necessary for better beach loss projections.
【巨視的】 特集 33巻2号(2019)  海岸侵食; 土砂フラックス; 土砂輸送量;
Coastal erosion is a severe problem elsewhere along the Japanese island, and efforts have been paid to cope the situation. For sophisticated management of coastal erosion, we must understand the spatial and temporal distribution of sediment fluxes, however, this item is usually very difficult to observe directly. The alternative way to estimate this item is measurement of bottom topographies, and then assess the sediment flux indirectly using the continuity equation between temporal change of bottom elevation and divergence of sediment flux. In this article, estimation of coastal sediment transport in the field is discussed, and then an attempt to estimate the sediment motion at the river mouth is introduced.
【巨視的】 特集 33巻2号(2019)  沿岸砂州; 組織的変動; 土砂輸送;
The variabilities of multiple bar system have been investigated in inter-annual time scale based on long-term field surveys. The field sites include Ishikawa Coast and Chirihama Coast facing the Sea of Japan. On both sites, large scale sandbar system has developed and the morphological variabilities are characterized by cyclic and systematic migration of multiple bars. On Ishikawa Coast, representative spatial patterns during individual cycle are analyzed to describe the three-dimensional morphological evolutions. Considerable changes in plan shape are observed such as quasi-regular crescentic patterns, parallel alignment of oblique bars, disconnection and realignment of bars. In contrast, bar migration on Chirihama Coast is quasi two-dimensional. The transition of the multiple sandbar systems is well reproduced by an empirical parametric model.
【巨視的】 特集 33巻2号(2019)  海岸侵食; 汀線変動; 海浜変形;
This manuscript demonstrated the characteristics of morphological change in Shichiri-mihama Ida coast, located in the southern part of Mie prefecture, by means of discriminant analysis based on shoreline change characteristics analyzed by network camera and wave data, and SfM/MVS analysis using UAV. Additionally, the correlation between the run-up height and the morphological change was discussed. As a result, discriminant analysis showed the usefulness of estimating advance, retrogression and stay events of shoreline behind artificial reefs at Ida beach. In the results of SfM/MVS analysis, the effectiveness of artificial reefs installed in Ida beach to reduce erosion behind artificial reefs, while it was confirmed that the function of artificial reefs to decline erosion was decreased by the long period waves with a significant wave period more than 13s. The total volume of Ida coast has increased due to the recently implemented beach nourishment. However, because the large beach nourishment material makes the foreshore slope of the Ida coast steep which is possible to accelerate beach erosion, the necessity of ongoing monitoring was suggested. Finally, the possibility to predict the maximum altitude of morphological change in Ida coast foreshores from the calculation of the run-up height by considering the wave direction of the wave after passing through the artificial reef was confirmed.
【巨視的】 特集 33巻2号(2019)  津波流; 限界流速; 粒子骨格構造;
At the Great East Japan Earthquake of 2011, coastal structures were weakened by ground scouring and the consequent loss of the wave resistance function led to large scale damage. As this could seriously affect the economy of a country, urgent measures need to be taken to minimize the damage. In addition, floods arising from frequent heavy rains cause water levels to rise and prolonged high water levels lead to riverbed fluctuations, erosion, and destruction of river levees, and erosion due to overflow. In studies on the scouring phenomenon, tractive force has been widely used for evaluation. Evaluation by tractive force is done from a microscopic viewpoint, focusing on the balance between the shear force generated by the fluid on the ground surface and the effective weight of a soil particle in the outermost ground layer. However, when we consider the scouring phenomena that occurs around structures and on account of overtopping, the scale of scouring is such that it cannot be explained merely from the viewpoint of tractive force. Moreover, it has been pointed out in recent studies that the effect of fluid force does not only to affect the ground surface, but also causes stress changes inside the ground, thereby promoting scouring. Therefore, besides the microscopic view, it is necessary to elucidate the phenomena regarding the meso-scale of soil elements and to consider a wide range of perspectives of the macro scale of structures. Therefore, this paper demonstrates some of the findings from our research on the scour phenomenon focusing on soil-fluid interaction in multiple scales.
地球惑星科学の混相流 -キッチン地球科学の視点から-
【巨視的】 特集 34巻3号(2020)  地球科学; 教育; キッチン実験;
“Kitchen Earth Science,” which is scientific, educational, and public outreach activities, aims at understanding a natural phenomenon in Earth and planetary sciences by analogue experiments using goods and tools in our daily life. Analogue experiments have a function to reveal the fundamental physics governing the phenomenon. On the other hand, they essentially include uncertainties so that unexpected results are frequently obtained, which have a potential for surprising discoveries. These findings also provide a good opportunity for deeply thinking and raise new questions to explore. Such experiences are precious not only for young researchers, but also non-expert people who need a scientific thinking to live wisely. Here, we introduce our recent activities of “Kitchen Earth Science” and the experimental studies rerated to the multiphase flow in Earth and planetary sciences.
【要素的】 特集 34巻3号(2020)  マグマ; 爆発的噴火; 気泡;
Explosive volcanic eruptions produce pumice including bubbles (mainly H2O gas) which form by the vesiculation process under decompression during the magma ascent in the volcanic conduit. Shape of bubbles varies from spherical to tubular or irregular depending on the eruption style, intensity and magnitude. Interestingly, huge eruptions such as caldera-forming eruptions characteristically include elongated bubbles. We conduct the bread baking experiments to examine what controls the shape of bubbles by using the vesiculation process due to the fermentation of yeast. We use two experimental setups, baking within and without a glass conduit, to evaluate the effect of 1 dimensional expansion flow on the bubble shape. Results show that breads baked within the glass conduit typically include the elongated bubbles. We make the textural analysis for the cross section of bread after fermentation and baking with a time interval of 5 min to understand the evolution of bubble texture and with variable concentrations of yeast to evaluate the effect of amount of yeast on fermentation, consequent expansion of dough and bubble texture. By experiments with the variable concentration of yeast, it is found that the maximum expansion occurs at the certain concentration of yeast approximately 0.5 g per 50 g dough, and that the number of bubbles monotonically decreases with increasing the concentration of yeast. Time series experiments show that the number of bubbles decreases and average size of bubble increases with time for both within and without conduit. Aspect ratios of bubbles within a glass conduit are larger in smaller size of bubbles and smaller (i.e., elongated) in larger size. Numerical model simulating the evolution of bubble shape under 1 D expansion demonstrates that smaller bubbles relax to the spherical shape by surface tension effect and larger bubbles maintain the elongated shape formed by 1 D expansion.
【要素的】 特集 34巻4号(2020)  気泡; 非ニュートン流体; 火山;
We observe sounds associated with continuous bubbling in fluid in daily lives. We call them ‘Buku-buku’ sounds. Because similar processes occur at active volcanoes, ‘Buku-buku’ phenomena provide good Kitchen Earth Science subjects. This study investigated bubble sounds using hair-gel solutions with two viscosities. Various waveforms were observed in the air and in the liquid. We focused on two types of airwaves. The one was observed with the relatively low-viscosity fluid and generated by bubble oscillation on the surface. The other was observed with the higher viscosity and generated by bubble opening at the surface. Both types of waves showed frequency gliding from low to high, for which we proposed models. Scaling issues and implications for volcano acoustics are discussed.
Convection in an Internally-Heated Two-Layer System
【要素的】 特集 34巻4号(2020)  マントル; スケール則; レイリーベナール対流;
The Earth's mantle is chemically heterogeneous and includes primordial material inherited from early planetary processes, which probably led to an initial depth-dependent composition of radioactive elements. One consequence is that its internal heat sources are not distributed homogeneously. Mantle convection induces mixing, such that the flow pattern, the distribution of heterogeneities and the thermal structure are continuously evolving. We studied these phenomena in the laboratory using a unique microwave-based experimental set-up for convection in internally-heated systems. We characterize the development of convection and the progression of mixing in an initially stratified fluid made of two layers with different physical properties and heat production rates. In analogy to the Earth's mantle, the upper layer is thicker and depleted in heat sources compared to the lower one. Two different convection regimes are identified, a dome regime and a stratified regime. In the dome regime, large domes of lower fluid protrude into the upper layer and remain stable for long time-intervals due to their enhanced heat production. In the stratified regime, cusp-like upwellings develop in association with deformation of the interface separating the two fluids. Upwellings are similar in size and morphology to those that would be generated by heating through the tank base, implying that mantle plumes are not necessarily due to heating by the Earth's core. These plumes are made of heated upper layer fluid and enriched lower fluid in variable proportions giving rise to a range of plume compositions. Mixing proceeds by two mechanisms: shearing of thin slivers by viscous coupling at the interface between the two fluids, and trapping of upper fluid within the lower fluid through folding. Empirical scaling law for the mixing rate allows extrapolation to planetary mantles.
【要素的】 特集 34巻4号(2020)  隕石衝突; 粉体; 流動化;
Many spacecraft in various countries have succeeded in exploring planets, satellites, and small bodies so far, and the time has come when a large amount of data can be viewed easily by friendly tools. An approach that simulates the morphologies on other celestial bodies by accessible kitchen experiments may facilitate the understanding of them by the public as well as by experts. In this article, we will introduce the fluidized ejecta craters on Mars and the kitchen experiment on the interaction between a vortex ring and a particle layer.
【巨視的】 特集 35巻3号(2021)  太陽; 熱対流; 機械学習;
A method for evaluating the plasma motion on the solar surface is developed by a combination of numerical simulation and machine learning. On the solar surface, we can observe the thermal convection of plasma fluid. Since thermal convection relates to several phenomena such as magnetic field generation and coronal heating, the estimation of convection velocity is essential. While we can evaluate the line-of-sight velocity with the Doppler effect, the horizontal velocity field cannot be directly obtained. On the other hand, magnetohydrodynamic numerical simulations have been developed and can reproduce solar convection. We can get precise quantities such as velocities and magnetic fields on the solar surface in the simulations. The horizontal velocity, which is challenging to observe, is also obtained in the numerical simulations. Here, we construct a neural network to evaluate the horizontal velocity field on the solar surface with the observable quantities such as radiation intensity and the corresponding horizontal velocity field obtained from the simulation and applying them to the observation. The correlation coefficient between the horizontal velocities obtained by the neural network and the simulated data is 0.84 only with radiative intensity and 0.90 with radiative intensity, the line-of-sight velocity, and the line-of-sight-magnetic field. Even when we apply the network to the observation data, the correlation coefficient of 0.6 is kept.