紊流作用下泥沙释放磷的试验及数值模拟

Experimental and numerical investigation of the effects of turbulence on phosphorus release from sediment

为研究水体紊动对泥沙释放污染物的影响,引入近似均匀紊流模拟装置,开展不同强度紊动作用下泥沙运动释放磷动态过程及基本规律的试验,研究表明:紊动扩散作用对泥沙解吸释放磷有着十分重要的影响,紊动较弱时,泥沙尚未起动,床面泥沙颗粒吸附的磷元素解吸至孔隙水中,在垂向紊动扩散作用下输送至上覆水中,将使得上覆水中溶解态磷浓度增大;紊动作用较强时,泥沙逐渐悬扬至水体中,悬浮泥沙颗粒吸附的磷元素直接解吸至上覆水中,泥沙浓度越高,溶解态磷含量增长速率越大,达到平衡的时间越短.通过耦合紊流模型、水流泥沙数学模型、污染物对流扩散模型和Langmuir吸附动力学方程建立了紊动作用下泥沙释放磷的垂向一维数学模型,针对试验条件进行数值模拟,模拟结果与试验数据吻合较好,平均相对误差低于10%,可为模拟天然水体中泥沙悬浮释放磷的时空演化提供参考.

Nearly isotropic homogeneous turbulence in a water column was generated by an oscillating grid turbulence device. Effects of flow turbulence on phosphorus release from sediments were investigated. Results show that the turbulent diffusion was an important factor in phosphorus release from sediments. Under the low turbulent shear stress, sediments remained at the bottom of water body, and the phosphorus absorption onto bottom sediments released into the pore water. The phosphorus in pore water would be mixed into upper water due to the vertical turbulent diffusion to increase the concentration of phosphorus in the upper water column. While the higher turbulence intensity led to sediment suspension, the phosphorus adsorbed onto suspended sediments would be released into upper water directly through the desorption process. The dissolved phosphorus concentration increased and the equilibrium time decreased with increasing suspension-sediment concentration. Coupling the turbulence model, hydrodynamic and sediment transport model, convection-diffusion model and the Langmuir adsorption kinetics model, a one dimensional vertical numerical model was established to simulate dynamic processes of sediment transport and phosphorus release in the turbulent flow. The numerical results and the experimental data show an agreement and an average relative error of less than 10%. The model can be used to simulate phosphorus release processes in real environments.