Numerical Investigation Of Gravitaxis Bioconvection

Bioconvection is the name given to pattern-forming motions set up as a result of hydrodynamic instabilities in suspensions of swimming micro-organisms. Researching bioconvection can help better understanding the properties of its forming, evolution, patterns and flow field. It has vast potentials in micro-organism research, environment protection, cell behavior in micro-gravity environment and relative areas.In this thesis numerical simulation methods is adopted to analyze the bioconvection phenomena caused by gravitaxis micro-oranisms in a 2D rectangle chamber and a 3D cylinder with non-slip walls. The equations are discreted using CVM. Simulation of convection pattern is carried out and influence of cell density and swimming speed is studied.The result of 2D rectangle chamber shows that the micro-organisms accumulate rapidly at the top when calculation starts, forming early stage plumes in 50s. After a short time the small plumes combine to a primary one which reaches much deeper in the middle of chamber. The evolution of plume ends at 300s. At steady state most micro-organisms concentrate in the plume. Cells in the top layer are drawn to the plume and transported to the bottom, then swim upwards and back to the top. The plume pattern is affected by both Cell densityρand swimming speed V_c. Decreasingρand increasing V_c will enhance cells' movement, shorten the plume and induce cells gather at the upper surface. The difference isρhas more effect on stability but limited influence upon the center of vortex. Simulation of 3D cylinder indicates a similar plume evolution progress, cell concentration rises with time and finally state is steady at about 400s. What's different from 2D case is there is regularly arranged array of incomplete plumes besides the prime plume in the center. These small plumes distributes at the circumference of radius 0.002m, with a length of 1/5-1/10 of fluid depth. Put into a larger scale space, these incomplete plumes would extend to the bottom and become nearby full-length plumes like the center one. Cell densityρand swimming speed V_c has similar effect as in 2D. Changingρvalue will obviously alter the stability of bioconvection. A lagerρgreatly prolongs the time to reach steady, extend the depth of small plumes, induce adjacent plumes to combine, and enlarge distance between nearby plumes.