Takatoshi Yanagisawa

Experimental investigation of natural convection induced by internal heat generation

Dilatation of a convection cell with respect to its Rayleigh number, one of the problems in internally heated convection, was quantitatively investigated by analyzing temperature field in a cell. The temperature field visualized by a thermo-chromic liquid crystal (TLC) expresses the cell dilatation. A calibration system was developed to convert the visualized photographs of the temperature field to the temperature field. A calibration curve correlating color information extracted from the photograph and temperature was determined from the approximately linear temperature distribution in the horizontal fluid layer using the hue method. Photos taken at various internal Rayleigh numbers were converted to the temperature field by the obtained curve. Extracting individual cells from a temperature field achieves a quantitative expression of the cell dilatation as the variation of the wavenumber of the cell with Rayleigh number increases. The temperature profile in a cell shows that high temperature areas appear at the apexes of the cell, largely different from the profile obtained by linear theory.

Low-altitude remote sensing of volcanoes using an unmanned autonomous helicopter: an example of aeromagnetic observation at Izu-Oshima volcano, Japan

The results of aeromagnetic observations at Izu-Oshima, Japan using an unmanned autonomous helicopter are reported. A practical observation system was assembled, adopting a bird-type magnetometer installation, and dense observations of the northern half of the caldera area including the central cone were made from a very low altitude. In the detailed magnetization intensity mapping deduced from the collected data, low magnetization intensity at the vent and three rows of high magnetization intensity on the caldera floor were found. The former is interpreted as the presence of high-temperature materials, such as magma or hot rock, or vacant space in the conduit. The latter is considered to be due to solidified dykes. Low magnetization intensity suggesting a magma body (or subsidiary magma chamber) was not detected below the caldera floor. The observation results confirmed that aeromagnetic observation using an unmanned autonomous helicopter had sufficient performance for volcanic observations, and could also be utilized as a low-altitude platform for other sensors.

Mantle evolution in Venus due to magmatism and phase transitions: From punctuated layered convection to whole‐mantle convection

A series of numerical models of magmatism and mantle convection with a stagnant lithosphere are developed to understand the mantle evolution in Venus. Magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting, and the solid-state convection of mantle materials with temperature-dependent Newtonian rheology is affected by the garnet-perovskite transition and the postspinel transition. In our preferred models, the mantle evolves in two stages: The earlier stage is characterized by layered mantle convection punctuated by repeated bursts of hot material from the deep mantle to the surface. Mantle bursts induce vigorous magmatism and also cause the basaltic crust, enriched in heat-producing elements (HPEs), to recycle into the mantle. A part of the recycled basaltic crusts accumulates along the postspinel boundary to form a barrier, and this basalt barrier causes mantle convection to become layered. At a later stage, when the HPEs have already decayed, in contrast, the basalt barrier disappears and whole-mantle convection occurs more steadily. Mild magmatism is induced by small-scale partial melting at the base of the crust and hot plumes from the deep mantle. The internal heating by the HPEs that recycled into the mantle in the earlier stage allows the magmatism of the later stage to continue throughout the calculated history of mantle evolution. The two stages arise when the barrier effect of the postspinel transition is weak and the lithosphere is mechanically strong enough. The two-stage evolution model meshes with the observed history of magmatism and the lithosphere on Venus.

Cell pattern of thermal convection induced by internal heating

A numerical simulation of thermal convection induced by internal heating in a three-dimensional thin flat box is performed. The upper boundary is held at a constant temperature and the lower boundary is thermally insulated with a uniform internal heating in a fluid layer. We observe hexagonal cells with downwelling at their center in moderate Rayleigh number (RaH≲30RaHc). As the Rayleigh number increases, the cell size increases. We also observe a pattern transition: sheet-like downwellings gradually develop around the columnar downwelling with increasing Rayleigh number. At ∼30RaHc, hexagonal cells with sheet downwelling at their center (spoke pattern) is observed. These sheets can be seen only near the upper boundary. At 30RaHc∼70RaHc, the cells experience successive transitions to the pattern which are dominated by sinking sheet.

Aeromagnetic survey using an unmanned autonomous helicopter over Tarumae Volcano, northern Japan

Unmanned aerial vehicles (UAVs) have recently received attention in various research fields for their ability to perform measurements, surveillance, and operations in hazardous areas. Our application is volcano surveillance, in which we used an unmanned autonomous helicopter to conduct a dense low-altitude aeromagnetic survey over Tarumae Volcano, northern Japan. In autonomous flight, we demonstrated positioning control with an accuracy of ~10 m, which would be difficult for an ordinary crewed vehicle. In contrast to ground-based magnetic measurement, which is highly susceptible to local anomalies, the field gradient in the air with a terrain clearance of 100 to 300 m was fairly small at 1 nT/m. This result suggests that detection of temporal changes of an order of 10 nT may be feasible through a direct comparison of magnetic data between separate surveys by means of such a system, rather than that obtained by upward continuation to a common reduction surface. We assessed the temporal magnetic changes in the air, assuming the same remagnetising source within the volcano that was recently determined through ground surveys. We conclude that these expected temporal changes would reach a detection level in several years through a future survey in the air with the same autonomous vehicle.

An aeromagnetic survey of Shinmoe-dake volcano, Kirishima, Japan, after the 2011 eruption using an unmanned autonomous helicopter

In January 2011, magmatic eruptions at Shinmoe-dake, Kirishima, Japan, started for the first time in 300 years. After three sub-plinian explosive events, lava accumulation started at the summit crater. The accumulation lasted for about three months, and the final volume of the accumulated lava reached 1.5 × 107 m3. We conducted aeromagnetic surveys using an unmanned autonomous helicopter over the Shinmoe-dake summit crater in late May 2011, and late October to early November, 2011, in order to investigate the magnetization of this area and changes in the magnetic field associated with the 2011 eruption. The averaged magnetization intensity around Shinmoe-dake is 1.5 A/m. A demagnetized area elongated in the north to northwest direction from Shinmoe-dake has been detected. We also detected a clear change in the total magnetic field between the two aeromagnetic observations characterized by positive and negative changes in the south and north, respectively, of the Shinmoe-dake edifice. These changes are well reproduced by a model in which 20–70% of lava accumulated in the summit crater was cooled down below the Curie temperature and has been magnetized.

Dilatation and Pattern Formation of Cells in Internally Heated Convection

The dilatation and the pattern formation of convection cells in natural convection driven by internal heat generation, first investigated during the 1970s, are reexamined in this paper using an improved experimental apparatus in order to reduce uncertainties as much as possible. The convective motion in the fluid layer was visualized using reflecting particles (Kalliroscope). Cell dilatation was confirmed in the improved experimental apparatus and then investigated quantitatively by extracting the pattern wavenumber from Fourier analysis of the images recorded by a digital camera. The pattern wavenumber was found to decrease with increasing Rayleigh number. We compare our results with earlier investigations and discuss the influence of the thermal boundary condition at the bottom of the fluid layer on the variation of the wavenumber. Two different structures were observed in the same fluid layer at a relatively high Rayleigh number; an additional cell appears in the original cell (Double cell structure) and the descending flow region around the center of the cell expands laterally like a spoke of wheels (Spoke-like structure). Factors for the formation of such complex structures are discussed.

Vortex tracking on visualized temperature fields in a rotating Rayleigh–Bénard convection

AbstractWe established a vortex detection method using instantaneous temperature fields that were visualized using thermochromic liquid crystals (TLCs) to investigate behaviors of vortical structures in a rotating Rayleigh–Bénard convection. Experimental testing was performed at a fixed Rayleigh number

Numerical models of Martian mantle evolution induced by magmatism and solid-state convection beneath stagnant lithosphere

Ra = 1.0 \times 10^7