Experimental Investigation On Interaction Between Stratification Of Salinity And Suspended Sediment And Flow Turbulence

In estuary and coastal areas,the stratification is a common natural phenomenon.The so-called stratification is a delamination phenomenon caused by nonuniform vertical distribution of substance concentration in water.The stratification of salinity and suspended sediment are two typical stratification phenomena.The stratification will produce buoyancy instability,which will have a certain influence on turbulence.In the meantime,the turbulence in turn will affect water stratification.To investigate the interaction between the stratification and turbulence,a horizontal oscillating-grids flume with double layer grids,which can generate turbulence of similar characteristics in open channel flow,was designed and manufactured.The vertical stratification of salinity and suspended sediment were simulated using the oscillating-grid flume and the interaction between the stratification of salinity and suspended sediment and turbulence was investigated experimentally.The specific research contents and main conclusions are as follows.(1)A multichannel salinity meter and a multichannel sediment concentration meter using optical fiber were developed based on the conductivity and the optical backscatter respectively.The experiment results proves that the multichannel salinity and sediment concentration meter have the characteristics of small probe size,small disturbance to water volume,large measuring range,high precision and synchronous measurement with multi-channel.The mutual influences of salinity and sediment concentration are calibrated,and the modified relations for salinity and sediment concentration measurement are presented respectively.(2)Uniform silicon carbide particles with different particle sizes and natural sand particles with different grading were calibrated.The unified formulas of concentration measurement based on silicon carbide particle size and natural sediment characteristic particle size were obtained by data fitting,and the concentration measurement ranges corresponding to particle sizes were given.The measurements of uniform and natural sediment concentration without calibrating are realized through the unified relation,and the convenience of the sediment concentration meter is greatly improved.(3)The three dimensional turbulence in the central area of oscillating-grid flume was measured.The relationship of root-mean-square velocity and turbulence intensity was obtained by fitting,and the range of uniform turbulence was determined.Experimental results show that turbulent flow field with an zero average velocity can be generated in the central area of the double layer grids,which is similar to the turbulent flow characteristics in open channel,and is more representative for investigating salinity diffusion and sediment movement.(4)A salinity delamination device was used to realize the initial stratification between fresh water and salt water in the oscillating-grid flume.A series of experiments on the interaction between salinity stratification and turbulence under different initial salinity conditions show that uniformly mixed salt water has no effect on turbulence variation,and only salinity stratification can suppress turbulence.The dimensionless number representing the degree of stratification is defined.The relation between the stratification parameter and turbulence suppression rate and eddy viscosity is obtained by fitting.(5)The suspended sediment stratification experiment was carried out in the oscillating-grid flume,and it is indicated that the turbulence intensity is the most direct factor that affects the sediment suspension,and the sediment concentration profile is the result of the equilibrium between sediment turbulence diffusion and gravity.Under different oscillating frequencies,the flume will form obvious high-concentration suspension near-bottom.Sediment suspension stratification can suppress turbulence.The turbulence suppression rate depends on both vertical sediment concentration gradient and turbulence intensity itself.The relations between the stratification parameter and turbulence suppression rate and eddy viscosity are obtained by fitting.(6)Experiments of combined stratification of salinity and suspended sediment were carried out in the oscillating-grid flume.The analysis on experimental data shows that the turbulence suppression caused by salinity stratification will delay the upward diffusion of sediment suspension near the bottom,and increase the suspended sediment concentration near bottom,which more likely leads to high sediment concentration near bottom.The three-dimensional root mean square velocity and turbulence intensity are greatly suppressed under combined stratification.The unified relations between the stratification parameter and suppression rate and eddy viscosity under different stratification conditions are established.