Numerical simulation of thermally driven exchange flow between open water and aquatic canopies

Maria Tsakiri, Panayotis Prinos

Monday 29 june 2015

14:20 - 14:35h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 2G. Environment – Ecohydraulic

Thermally driven exchange flow is a convective exchange flow which is produced due to temperature difference in a fluid. In nature this flow can be observed ether between shallow and deep water or between open water and aquatic vegetation. The latter occurs because the water in the open area is more affected by the solar energy and its temperature is higher than that in the vegetated region which is shaded by the vegetation. This temperature difference induces density difference and produces convective currents. Moreover vegetation can influence the propagation of the currents as it produces an additional drag resistance. In the present study, thermally driven exchange flow between open water and aquatic canopies, due to different radiation absorption, is investigated numerically, in order to examine the effect of differential heating and vegetation on the hydrodynamics. The simulation is referred to lock-exchange flows in a tank with an area with open water and an area with aquatic canopy which represents the vegetation. The surface of the open region is heated with constant heat flux. The vegetation is simulated by circular cylinders and is characterized by the porosity. The numerical simulation is based on the experimental model described by Zhang & Nepf (2009) and is referred to a tank with length equal to 0.60 m and height equal to 0.40 m. The water depth is 0.10 m. The surface heat flux is 157 W/m2. Three cases of vegetation porosity have been investigated, 0.97, 0.92 and 0.85. A commercial code is used for solving the unsteady Navier-Stokes equations in conjunction with the energy equation. The effect of vegetation on the flow is taken into account through additional terms in the momentum equations and the absorption of radiation through additional term in the energy equation. These terms are included in the model through User Defined Functions. The Boussinesq approximation is used for taking into account the density difference due to temperature difference. Numerical results for the fluid velocity and the variation of temperature in the fluid are presented. The results are compared and are found in satisfactory agreement with available experimental data (Zhang & Nepf, 2009). REFERENCES: Zhang, X. and Nepf, H. (2009). Thermally driven exchange flow between open water and an aquatic canopy, J. Fluid Mech., Vol 632, pp. 227-243.