Numerical and experimental studies of turbulence in vegetated open-channel flows

A numerical model based on the double-averaged (spatial and ensemble averaged) method has been developed to simulate vegetated free surface flows. The classical k- ε model is modified by including additional turbulence generation and dissipation terms. These new terms consider the large-scale turbulence generated by shear flows near the flow-vegetation interface and the small-scale wake turbulence generated by the flow separation behind vegetation stems. A new damping function is introduced in the turbulence closure model to suppress the wake turbulence inside the vegetation domain if the shear-generated turbulence is dominant. A series of dense emergent and submerged vegetated flows experiments have been conducted and the velocities have been measured by ADV (Acoustic Doppler Velocimetry) and PIV (Particle Image Velocimetry) at different locations. The experimental results show that the turbulence characteristics near each stem in a group of cylinders are similar to that of a single cylinder, even when the vegetation is dense. The measured data are then used to calibrate the new turbulence closure model. The numerical model is further validated against other published laboratory data of both dense and sparse emergent, submerged and floating vegetated flows. The comparisons show that the new model provides consistently better predictions for mean velocity and turbulence kinetic energy than those by using the existing models.