Study On Hydrodynamic And Oxygen Transfer Mathematical Models And Their Characteristics In Coastal Aquaculture Waters

The environmental capacity continues to decline in coastal waters with the continuous increase of population and production density.It is imperative to solve the contradiction between development and protection.The coastal aquaculture area is a place where many factors interwork,involves physics,chemistry and ecology.It is a hot and difficult issue to study hydrodynamic characteristics,transfer mechanism and characteristics of dissolved oxygen in coastal aquaculture waters.This study was carried out by developing mathematical models,numerical simulation and experiments based on the theories of fluid mechanics and turbulent diffusion.The research work and its results are as follows.1.Philosophical thinking on complexity of mathematical models,as well as the ideas and ways of innovation of mathematical models of water environment was analyzed from the perspectives of epistemology and methodology.The mathematical model theories of hydrodynamics and dissolved oxygen transfer in the coastal aquaculture waters was described firstly,including control equation of fluid mechanics and material transfer,turbulence model theories,and field synergy principle of enhanced mass transfer.2.Based on the assumption of "full flow field transmission of turbulent momentum rather than inter-layer transfer",one-dimensional mathematical model of vertical profile distribution of mean velocity was derived by taking probability density distribution as the law of velocity redistribution in turbulence.The calculation results of this model and logarithmic law model were compared with the data reported in the literature.The comparison shows that this model is superior to the classical logarithmic law model.A vertical profile distribution model of SSC was established with the turbulent diffusion coefficient as the object based on the same assumption.And the comparison with Rouse equation showed that the accuracy of this model is better than Rouse equation.Especially,the calculating result of the model avoids the paradox that the concentration is always 0 at the gas-liquid surface.3.Acoustic Doppler velocity meter was used to measure the point velocity of the three-dimensional circulation in a water tank,positioning three-dimensional towing system on water tank in the laboratory experimental study.A basic method for calculating mean velocity,pulsating velocity and turbulent intensity was established by using the fast Fourier transform method.Distribution characteristics of mean velocity,secondary flow,and velocity pulsation intensity were discussed.4.Dynamic grid method was adopted to solve the impeller rotation in the numerical simulation research on the process characteristics of forced circulation flow field in the aquaculture pond was carried out,based on the unsteady Reynolds equation with finite volume method.The reliability of the model is verified by comparing with the measured velocity data.The basic rules of increase and decrease of the flow velocity in the impeller-induced circulation were calculated and analyzed.The numerical simulation method can be used to calculate and analyze the growth and decay process of impeller-mechanically induced circulation,and provides a reference research method for the three-dimensional numerical simulation of aquaculture ponds.5.The convection-diffusion model of material transfer was further adopted as the dissolved oxygen transfer model,and the hydrodynamic and transient process of dissolved oxygen transfer in rectangular and narrow tanks with aeration forced circulation was numerically studied in aquaculture pond circulation.The transient and process characteristics of dissolved oxygen transfer were studied by using the field synergy principle.The relationship among the flow structure,the uniformity of concentration distribution and the transfer rate of dissolved oxygen was analyzed in detail.The transfer mechanism and characteristics of dissolved oxygen in aquaculture ponds was explained.The general principle of aerator arrangement in aquaculture pond was put forward.A preliminary mathematical model was established to describe the vertical profile distribution of one-dimensional turbulent mean velocity and suspended sediment concentration by this research work.The mathematical model provides a new idea and application model for the hydrology environment of Marine aquaculture,especially for the turbulent aquaculture waters.The laboratory experiment and numerical simulation of three-dimensional circulation forced by paddle-wheel aerators in aquaculture ponds were carried out to verify the phenomenon of secondary flow in the flow field.It provides a reference method for the research on pollution particles collection by circulation,on breeding habitat and on energy conservation of aeration machines.The research on the mechanism of enhanced transfer of dissolved oxygen in circulation provides a general principle for the reasonable arrangement of aerators.