Laboratory Experimental Study On The Influence Of Flow Velocity On Dispersion Coefficient And Its Scale Effect

In this paper,an indoor one-dimensional solute transport experiment was carried out.Based on the experimental data,the influence of the velocity on the scale effect of the dispersion coefficient and the relationship between the dispersion coefficient and the velocity were studied.In this paper,a PVC pipe with a length of 3.25 m and a diameter of 10cm was designed.River sand with a diameter of 0.125mm-0.3mm and0.5mm-0.7mm was selected as the filling medium,sodium chloride was used as the tracer,and one-dimensional indoor solute transport test was carried out by placing the tracer in an instantaneous manner.According to the experimental data,the measured C-t curve is drawn,analytical and numerical methods are used to fit the measured C-t curve,and the dispersion coefficient is solved.In addition,the existence of the scale effect of the dispersion coefficient,the influence of the flow velocity on the scale effect of the dispersion coefficient,and the relationship between the dispersion coefficient and the flow velocity are analyzed,and the specific conclusions are as follows:(1)A hydraulic experiment and homogeneity analysis were carried out on the two sand columns used in the experiment.The results of the hydraulic experiment showed that the flow velocity and hydraulic slope conformed to Darcy's law;the homogeneity analysis showed that the sand column could be regarded as a relatively homogeneous medium.The permeability coefficient of the sand column was measured.The result of the measurement is:the average permeability coefficient of the sand column with a particle size of 0.125mm-0.3mm is 9.60 m/d,and the average permeability coefficient of the sand column with a particle size of 0.5mm-0.7mm is52.23 m/d.(2)Each of the two sand columns carried out 6 sets of solute transport experiments with different flow rates.Among them,the flow rate range of the sand column with a particle size of 0.125mm-0.3mm is 2.18 m/d-5.85 m/d,and the flow rate range of the sand column with a particle size of 0.5mm-0.7mm is 2.45 m/d-5.56m/d.A total of 78 measured C-t curves at different distances and flow rates are obtained.(3)A numerical calculation program considering the effects of sampling was compiled,and the applicability and accuracy of the numerical method were tested.The test results show that,under the experimental conditions in this paper,the numerical method can be used to solve the problem,and the numerical method can better simulate the sampling behavior of the experiment.(4)Analytical and numerical methods are used to fit the measured C-t curves of the two sand columns,and the calculation results of the dispersion coefficient are obtained:for the sand column with a particle size of 0.125mm-0.3mm,the analytical solution of the dispersion coefficient is between 0.0021 m~2/d and 0.0043 m~2/d,and the numerical solution of the dispersion coefficient is between 0.0017 m~2/d and 0.0300m~2/d.For a sand column with a particle size of 0.5mm-0.7mm,the analytical solution of the dispersion coefficient is between 0.0080 m~2/d and 0.0049 m~2/d,and the numerical solution of the dispersion coefficient is between 0.0074 m~2/d and 0.0420m~2/d.(5)Under the experimental conditions in this paper,as the sampling distance x increases,the dispersion coefficient D increases significantly,and there is an obvious scale effect.There is an obvious linear relationship between the diffusion coefficient and the sampling distance,and it can be expressed by D=ax+b.With the increase of the flow velocity,the slope a has a more obvious increasing trend,indicating that the larger the flow velocity,the more obvious the scale effect of the dispersion coefficient.(6)Within the flow rate range tested in this paper,as the flow rate increases,the dispersion coefficient increases significantly,and there is a relatively obvious linear relationship between the two.This paper has 56 pictures,22 tables,and 67 references.