Hydrodynamic Mechanism Of Accumulation For Typical Algae In Red Tide In A Complex Flow

The hydro-environment pollution and hydro-ecology damage caused by the red tide have become an increasingly serious problem.Hydrodynamics play a significant role in the migration,distribution and accumulation of algae.The mechanism of algal accumulation in complex flows is essential to understand and predict the evolution of red tides.In this dissertation,the swimming characteristics of typical algae in red tide were investigated for the still water and three-dimensional vorticity field.A mathematical model was established to quantitatively describe the migration and aggregation of algal cells.The trajectories and distribution of algal cells in the complex flow were simulated to reveal the mechanism of accumulation mechanism of typical algae in red tide.The occurrence of red tide in different areas of Daya Bay was explained from the view of hydrodynamics.Results may provide scientific basis and technology support for the prediction and prevention associated with ecological disasters such as red tide.The main results are as follows:(1)The swimming characteristics of typical algae in red tide were investigated.The probability distribution of the swimming velocity,the reorientation time,B,and the rotary diffusion coefficient,Dr,are obtained,based on the trajectories of algal cells in the still water which was measured by use of the laser-induced fluorescence technique.Results show that the swimming of H.akashiwo has no preference in the horizontal direction and upward preference in the vertical direction.It is also shown that the mean swimming velocity of H.akashiwo is 144 ?m/s,the reorientation time B is 5.6 s,and the rotary diffusion coefficient Dr is 0.046 rad2/s.(2)The swimming velocity and translational diffusion of H.akashiwo were studied for the three-dimensional vorticity field.The relationship between the mean swimming velocity,translational diffusion tensor and vorticity was established based on the Fokker-Planck equation,which reveals the swimming characteristics of algal cells in the three-dimensional vorticity field.Results show that the algal cells are able to swim in a given swimming direction and exhibits obvious anisotropy for the weak horizontal vorticity.As the vorticity increases,the cells cannot swim in a given direction and the translational diffusion tends to become isotropic.The vertical vorticity may retard the process that the vertical swimming velocity of algal cells tends to zero,which makes the algal cells still maintain a certain swimming speed under the strong vorticity,and hinders the occurrence of gyrotactic trapping.(3)The hydrodynamic mechanism of accumulation of algal cells in a complex flow were investigated.The velocity field and concentration field of H.akashiwo were measured by use of the particle image velocimetry(PIV)and plane laser induced fluorescence(PLIF)technology.The high concentration cloud due to accumulation was observed in the complex flow,which differs the thin layer of phytoplankton in the two-dimensional flow.Comparison between the distribution of active and dead cells shows that there is great difference,no accumulation appears for the dead cells while cloud-like accumulation exists symmetrically in the both sides of jet nozzles due to the gyrotactic trapping.The results showed that the swimming characteristics of algal cells play an important role in the distribution and accumulation.(4)The active particle model for migration of algal cells was established for the complex flow.The active particle model of cells'migration was developed to take the randomness and gyrotaxis into account.The trajectories and concentration distribution of algal cells were simulated in the complex flow.The numerical results are in good agreement with the experimental results.The model can simulate the cloud-like accumulation of the algal ceslls in the vortex area.(5)The hydrodynamic mechanisms of the red tide in Daya Bay were studied.The wind speed and direction are found to be the key factors that trigger the red tide,based on the analysis of previous field observation and relevant literatures.The influence of wind on the flow in Daya Bay and the influence of wind-drived flow on the distrnbution of algae were investigated,which explains the occurrences of red tide in different areas of Daya Bay for various wind speed and direction.