School of Engineering and Materials Science
Research Student Awards
PhD Thesis: Open channel turbulence modelling using layer-averaged large eddy simulation.
Author: OSAWE, MO
Supervisor(s): John Williams
Incompressible turbulent flow with periodic and wall boundary conditions is investigated using the layer-averaged Large Eddy Simulation model. The open channel flow, maintained by a balance of gravity and bed shear stress, is simulated using filtered conservation equations with the assumption of hydrostatic pressure distribution. The resulting quasi-3D equations was found to be incapable of maintaining a turbulent flow field; thus necessitating the design and integration of a stochastic body force to maintain a turbulent flow field. The body force due to the inclination of the channel is made random by using a first order Markov model in order to achieve the effects of the full set of filtered equations. Closure is effected by the use of the traditional Smagorinsky model.
The discretised conservation equations are coded up in a computer program ISOKEN III using second order Adam-Bashforth time-marching scheme and Central Differences for the spatial discretisation. Simulations are advanced in time to obtain statistical equilibrium, after which the flow statistics are calculated. Turbulent flow is simulated at a Reynolds number of 3450 based on the bulk flow velocity for a wide channel problem, and at a Reynolds number of 97,000 for a narrow rectangular channel problem with a channel aspect ratio (breadth/depth) of 2. Simulations are also carried for varying grid resolution and Smagorinsky model constant. The results of the simulations are found to be in qualitative agreement with experimental data, and in particular, the hydrodynamic formulation is found to be quite capable of predicting the occurrence of the known secondary currents structure in narrow rectangular channels, as well as the associated dip of the location of the maximum streamwise velocity component.