Quadtree mesh for combined hydrodynamic and water quality modelling

Water quality modelling is usually coupled with hydrodynamic simulation, where calculation of concentration is performed after the flow field has been established as an output of hydrodynamic simulation. The compulation of concentration is largely confined to the hydrodynamic grid. In practice this is generally acceptable, as usually the most prominent mixing process with high gradient in concentration is also where the flow is rapid and the resolution of hydrodynamic grid is fine. However, there are cases in which locations of the largest concentration gradient do not coincide with large hydrodynamic flow gradient (such as contaminant spilling from a point source into tranquil ambient water), or where the location of intense mixing is not of special interest (concentration should be estimated somewhere else in more steady flow condition). Under these circumstances, we may wish to design a specific mesh for estimating the concentration, rather than using the same mesh as that with hydrodynamic computation. The writers propose using coupled quadtree grids for hydrodynamic and water quality simulation. The grids are adaptive and allow liner resolution in regions with high gradients of flow and concentration, which constantly vary as the system evolves. As a demonstration of the proposed technique, a test case was presented in which a plume is issued from a multiple ports diffuser system. The results obtained are compared against analytical calculation. The achievable accuracy and the computational efficiency are discussed.