Experimental Study Of Topographic Effects On Particle-driven Gravity Currents

Particle-driven gravity currents are buoyancy-driven flows due to suspended particles which intruding into ambient fluid.One of the main research methods of such flows is lock-exchange experiment.The experimental results could be applied to natural pyroclastic density currents?PDCs?with appropriate scaling method.This thesis reviewed the physical dynamic characters,sedimentation patterns and classification of PDCs,and deduced the fundamental physics and models?box-model for gravity currents?of dilute PDCs.The topographic effects on kinematics character and deposition patterns of flows could be investigated by setting up various barriers in water tank.Two kinds of topography effects on particle-driven gravity currents are investigated in this thesis.One of them is spanning barrier with finite height always shorter than the height of current front,which provided a block to the whole bottom of flows.The other one is vertical buffer connected with one side of the tank which providing a lateral obstruction to the flows and partial block to the bottom of flows.Some primary results of topography-compositional/particle driven current are as following,1.The compositional gravity currents back to "inertial-buoyancy" regime and deceleration model after spanning barrier is related to barrier height.The current back to "inertial-buoyancy" regime required 35.1-38.6,12.0-18.land 2.1-4.3times distance with respect to the barrier height with?1/3,?2/3 and?1/1 height of current front,respectively,and they did not show linear relationship.The current front decelerated as t^5/6,t^3/4and t^2/3.2.The non-dimensional length/time displayed slopes about 2/9,1/9 and 1/16 after particle-driven gravity current passed over spanning barrier with 1/3,2/3 and 1/1 height of current front.The location of barrier did not show significant effects on deceleration behaviours.3.The deposition curve showed gentle slope after shorter spanning barrier than those of higher barriers.The non-dimensional deposition pattern showed exponential relationship about 6.9,7.2 and 15 with barrier height when the barrier with 1/3,2/3 and 1/1 height of current front.4.Lateral partial block showed lag influence on compositional gravity currents.For buffer/tank-wide ratio of 1/4,1/2 and 3/4,the current length showed relationship with time as t^0.771,t^0.700 and t^0.623.5.The buffer effects on deposition are complex.The front speeds of particle driven currents did not influenced by buffers in the very beginning.The declining deposition curves of particles are gentler when particle currents passed through narrower buffer.Curves also declined slower with bigger initial reduced gravity.After the current passed lateral partial block with 1/4 of the tank width,the front speed did not show change in beginning,but they speeded up and beyond reference currents and could take more particles through buffer.After the current passed lateral partial block with 1/2 of the tank width,the front speed did decelerate and reflect particles to upstream.However,when the non-dimensional time T>100,the front speed accelerated and moved faster than reference currents.After the current passed lateral partial block with 3/4 of the tank width,the front speed decelerated and particle deposition decreased,significantly.The experimental results were non-dimensionalized and could be applied to volcanological situations.One of them focused on the interaction between PDCs and caldera edge?barrier with limited height totally blocked bottom boundary of flows?.The other focused on the effects of valley curvature?partial block to bottom boundary with vertical buffer?on PDCs.Deposits of Baiguamiao PDCs in Tianchi volcano?China?and current behaviors of Unzen volcano?1991 eruption,Japan?were tentatively discussed by applying our experimental findings.The research significances of this thesis are providing detailed flow changes when current meet various obstacles and could be applied in hazard assessment.Some technical skills explored in this thesis could provide experiences for development of experimental volcanology.