Turbulent stratified open channel flow with solar heating up to Pr=7 using Direct Numerical Simulation

This paper investigates the turbulent structure of stratified open-channel flow subjected to a radiative volumetric heat source modelled by the Beer–Lambert law, for Prandtl numbers ( P r ) varying from 0.07 to P r = 7 . Direct Numerical Simulation (DNS) was employed to model the open-channel flow. To overcome the increased computational resources required to resolve the thermal fields when P r > 1 , a multi-resolution method using quadratic interpolation was employed to resolve the temperature and momentum fields on different spatial and temporal resolutions. This scheme was implemented in an in-house computational fluid dynamics (CFD) code. To further reduce the computation cost, the DNS of P r = 2 . 2 and 7 fluids were initialised using the outputs of minimal channel simulations. The simulations were conducted for P r = 0 . 07 , 0.22, 0.71, 2.2, and 7 under neutral ( λ = 0 ), near-neutral ( λ = 0 . 1 ), and stable ( λ = 0 . 5 ) thermal stratification. The results demonstrate that P r significantly affects the flow structure and turbulence characteristics of stratified flows, particularly near the free surface. This includes higher velocity, temperature gradient, and buoyancy effects for P r = 7 compared to lower P r values. For stratified P r = 7 flow, examination of the Reynolds stresses and turbulent heat flux reveals significant damping of turbulence near the surface, with flow displaying near-laminar behaviour. • DNS data for thermally stratified open-channel flow for Pr ≤ 7. • Prandtl number significantly affects stratification, flow, and turbulence. • For stably stratified Pr = 7 flow Reynolds stresses suppressed at surface.