Experimental and analytical investigation of radiation-induced natural convection in the near shore regions of lakes and reservoirs

Abstract

Radiation-induced natural convection can play an important role in developing mixing and flow exchange within lakes and reservoirs that are well-protected from wind and have a long residence time. In the daytime, although incoming solar radiation transfers the radiative energy uniformly across the water surface, the bathymetry variation between the near shore and deep water regions creates a horizontal temperature gradient, leading to a horizontal convective motion. In addition, as the incoming light penetrates, its intensity attenuates with increasing depth according to Beer’s law. In near shore regions where the water depth is less than the light penetration depth, there exists residual radiation that is absorbed by the bed and re-emitted back to the water body as a heat flux. Therefore, a relatively warmer water layer is formed near the bottom and may become unstable leading to a vertical convective flow. Thus, net transport and mixing in such water bodies in daytime is promoted by these flows, the characterisation of which is the aim of this thesis. In Chapter 2, the vertical convective flow alone is studied on a laboratory scale using concurrent shadowgraph and Particle Image Velocimetry (PIV) measurements and experimental results are compared with previously developed scales. The 3D linear stability of this flow is investigated using a frozen time model in Chapter 3. The effect of different parameters on the critical conditions and the resulting flow pattern are evaluated. Further, the numerical and laboratory experimental determinations of critical wave numbers are compared. In Chapter 4, the coupled convective flow is examined on a laboratory scale using the same concurrent visualisation technique. The flow development, flow structure and its dependency on off-shore distance are quantified and compared with previously developed scales. Chapter 5 concludes the present study through reviewing the main results and providing suggestions for future work