Three dimensional free-surface modelling with a novel validation approach.

Andrew Sleigh, Duncan Borman, Andrew Coughtrie

Friday 3 july 2015

13:30 - 13:45h at Oceania (level 0)

Themes: (T) Water engineering, (ST) Computational methods

Parallel session: 16D. Engineering - Computational

We show that that a three-dimensional two-phase CFD-VOF model can reliably predict the position of waves and hydraulic jumps within the complex hydraulic flow environments of a recreational white-water course and dam release in a natural river. The model is validated to predict the full transient behaviour of white-water ‘features’ that form such courses. A novel application of LIDAR provides high-quality data for validation of the free-surface location. Multiphase free-surface CFD approaches together with greater access to high-performance computing facilities has meant that simulations that would have been unfeasible even five years ago are now a realistic prospect for many industrial hydraulic applications. A RANS CFD model is implemented in this work which incorporates an explicit VOF model to predict the transient free-surface behaviour of white-water phenomena. A SPH free-surface model is also implemented for a comparison of modelling approaches. Experimental studies were undertaken in a recreational white-water course channel that provided a means to vary the flow rates of water and restrict the flow easily as required as well as in river with a controlled flow from a dam release. The addition of obstructions within the channels allowed analysis of the impact on the size, position, velocities associated with hydraulic jumps, waves and other key features necessary for white-water kayaking. The study includes an original application LIDAR which has successfully been used as a novel measurement tool allowing high-quality free-surface data in a situation where other available options would be very difficult - either prohibitively expensive or would have had an impact on the flow being measured. LIDAR is not traditionally used to measure the surface of water as typically there is no return from the water; however the broken water surface of white-water provides enough signal to capture the detail of a stationary wave and other features. The results of the study establish that the free-surface CFD approaches can accurately predict the complex hydraulic behaviour in large-scale open channel flows. In order to reliably capture the full three-dimensional characteristics of the water free-surface using the VOF approach a structured, high resolution mesh with time-steps in order of milliseconds is necessary.