3D-modelling of the thermal circumstances of a lake under artificial aeration

Abstract

A 3D-model was developed to study the effects of hypolimnetic aeration on the temperature profile of a thermally stratified Lake Vesijärvi (southern Finland). Aeration was conducted by pumping epilimnetic water through the thermocline to the hypolimnion without breaking the thermal stratification. The model used time transient equation based on Navier-Stokes equation. The model was fitted to the vertical temperature distribution and environmental parameters (wind, air temperature, and solar radiation) before the onset of aeration, and the model was used to predict the vertical temperature distribution 3 and 15 days after the onset of aeration (1 August and 22 August). The difference between the modelled and observed temperature was on average 0.6°C. The average percentage model error was 4.0% on 1 August and 3.7% on 22 August. In the epilimnion, model accuracy depended on the difference between the observed temperature and boundary conditions. In the hypolimnion, the model residual decreased with increasing depth. On 1 August, the model predicted a homogenous temperature profile in the hypolimnion, while the observed temperature decreased moderately from the thermocline to the bottom. This was because the effect of sediment was not included in the model. On 22 August, the modelled and observed temperatures near the bottom were identical demonstrating that the heat transfer by the aerator masked the effect of sediment and that exclusion of sediment heat from the model does not cause considerable error unless very short-term effects of aeration are studied. In all, the model successfully described the effects of the aerator on the lake's temperature profile. The results confirmed the validity of the applied computational fluid dynamic in artificial aeration; based on the simulated results, the effect of aeration can be predicted.

Assessing vertical diffusion in a stratified lake using a three‐dimensional hydrodynamic model

“…From a lake management perspective, simulating the vertical diffusion coefficient may also help in the design of infrastructure for lake water-quality management and restoration efforts. Hypolimnetic aeration by water transfer between epi-and hypolimnion (Tian, Pan, Köngäs, & Horppila, 2017) or by bubble plume diffusers (Gantzer, Bryant, & Little, 2009) requires a quantitative estimate and solid planning for the necessary aeration intensities (Tian et al, 2017). Understanding the dynamics of vertical diffusion can provide guidance regarding the amount of aeration required and the layer of the lake to which the aeration should be applied.…”