A Numerical Simulation Of Tephra Transport And Deposition For The Millennium Eruption Of The Tianchi Volcano In The Changbai Mountains

Volcanic eruptions, especially explosive volcanic eruptions, can cause tremendous disasters to humanity, among which the disaster produced by t(?)phra fallout is a main type in both intensity and incidence. So, it has great significance to simulate its process quantitatively.According to the structure of the eruptive column, the tephra sediments are divided into three kinds: diffusion, projectile and collapse. This thesis focuses on the study of the diffusion and projectile.1 Diffusion Mode)This work adopts the mathematic model for tephra dispersion proposed by Suzuki (1983). Because of the differences of air density, temperature and viscosity, and the difference of the wind speed and direction at different altitudes, the altitude is divided into many layers, on which particle masses are added in the end. Due to the escape of the gas in the partials, various particles have different densities, which are also considered. The parameter P(z) that denotes the diffusion is amended in this thesis.Based on above theories, this work simulates the tephra dispersion of the Tianchi volcano that erupted in 1199. There are different particle densities because of the different content of air bubbles. According to the wind speed, I adopt there models: Model 1, wind speed fixes at 30m/s ; Model 2, which is also called MW1, assumes a linear increase of wind velocity up to the tropopause and a constant wind velocity of 0.75 of the tropopause velocity in the stratosphere, respectively; Model 3, which is also called MW2, consists a linear increase of wind velocity up to the tropopause and a linear decrease of wind velocity up to 20 km above tropopause, above which the wind velocity is fixed at 10% of the wind velocity at the tropopause.1.1 The conclusions from simulation of diffusion of the Tianchi volcano in 1199 are as follows:(1). The differences of settling rates for different diameter particles are very obvious. For the particles of diameter in the range 2-1 cm, the settling rate has reached 8.95m/s, but for the particles of diameter in 0.005-0.001cm, the settling rate is only 0.03m/s.(2). There is a turning point in the relation chart of particle diameters and the settling rates. It is speculated that this turning point is possibly related with transformation of the eruption column from the convective thrust region to the umbrella region.(3). The value of the dispersal parameterβof the Tianchi volcano in 1199 is 0.45, which indicates most of tephra congregated on the top of the volcano eruption column.(4). The probability densities of diffusion are different for different diameter particles. Big particles start to diffuse at the bottom of the eruption column, but small particles only start to diffuse near the peak of the eruption column. And the probability densities of diffusion of small particles are much bigger than that of big particles.(5). In the three models the diffused distance simulated by the first model is farther than that by MW1 model, but the width is smaller. MW2 model is not suitable for this region.(6). The relation between the ash deposit thickness and the distance from the volcanic crater is index relation in the downwind direction 20km outside the crater.It is nearly 800 years after the eruption, and it can only be estimated that the downwind is SE120°, so in this model does not consider the change of the wind direction with altitude. In addition, there are also some problems in the numerical simulation which need to be solved, for examples, the shape of the volcanic ash particles, the error near the crater, the compaction after sediment action. Although this simple model has provided invaluable insight to the diffusion process, it does not have explicitly forecasted kinetic process, and has not considered the microphysics process.1.2 The influences of the change of input parameters on the results are analyzed. The simulated results to different wind speeds, the medium diameter of the particle, the diffuse parameter, the height of the eruptive column and the eruptive velocity are compared and analyzed. The conclusions are as follows:(1) Wind speed has the largest influence on the diffusion of the volcanic ash, the bigger wind speed is, the bigger diffuse scope becomes, the smaller greatest deposit thickness is, the farther location of greatest thickness is; the smaller diffuse scope;(2) The parameters that affect the diffusion of the volcanic ash include the medium diameter of the particle, the altitude of eruptive column, the variance of particle size and the diffuse coefficient, which don't affect the deposit scope, but the greatest deposit thickness and the location of greatest deposit thickness. With the changes of the medium diameter of the particle b(?)coming bigger, the biggest deposition becomes thicker, but the location does not change. The lower eruption column is, the bigger greatest thickness value becomes, but the position of the greatest thickness does not change obviously. The variance of particle size does not influence the biggest deposit thickness remarkably, but it can make its position far away to the crater. The diffuse coefficient changes slightly which can cause the greatest thickness value to change in a large measure, but the position is invariable;(3) In the case that wind field changes not obviously, the influence of volcanic eruption speed to the final deposit thickness is not serious;(4) There is the second big thickness in every chart of volcanic ash deposit thickness, and the position is all 50km away from about crater, that is created by Aggregation effect of the tiny volcanic ash particles.1.3 The phenomenon of the second largest thickness is analyzed:The second largest deposit thickness has appeared in many volcanic eruptions, which some scholars think as the function of aggregation, and is gathered by many tiny particles. In the moist environment, surface tension associated with any liquid film on particle surfaces may be sufficient to bind particles together. In dry conditions, fragile dry aggregates may be bound by a combination of mechanical interlocking of irregular grains, and electrostatic forces.However, in virtue of the aggregation occurring in the inner of column, it is difficult to observe and study, and there is no perfect way to simulate the aggregation. This is an issue that remains to be studied.2 Projectile ModelThe ejection process, which is seriously harmful for the people lived near the crater, is an important way of the tephra deposition, and mainly composed by big particles in the gas thrust region of the column. In this work I firstly introduce the theory of ejection process, compile the program, then compute the ejection distances, and finally discuss the affection of parameters on the ejection distance. Through the simulation of the Millennium Eruption of the Tianchi Volcano, I get the following conclusions:(1) For the sphere particle, the distance is becoming further with the particle diameter increasing. The distance is 2470 m for 0.1m diameter particle, and 4566 m for 1.6m diameter particle.(2) The particle shape takes an important role on the ejection distance. The ejection distance of cubic particle is further than that of sphere particle when other conditions are same, and the affection of the particle diameter on the ejection distance is smaller for cubic particles than sphere particles;(3) The smaller drag coefficient is, the further the ejection distance becomes when other input parameters are same;(4) The bigger particle density is, the further the ejection distance becomes when other input parameters are same.For the ejection process of the volcanic particles, the process can be simplified, and the ejection distance and the height of every particle can be calculated. However, there are several difficulties for getting the final sediment thickness: (1) The particle size and shape are different for certain distance. (2) The width of the crater and the particle range in which particles can shoot off the crater are difficult to ascertain for the simulation. (3) The particles could be deformed when falling to the ground, the gas may expand in the particle, the particles may freeze and concrete from the surface to the inside, and so on, which make the sediment thickness difficult to determine.