Turbulent structures in smooth and rough open channel flows: Effect of depth.

摘要

In this thesis, detailed experiments are performed to study the effect of the flow depth on turbulent structures in smooth and rough bed open channel flow. Shallow open channel flow is dominated entirely by the wall turbulence with a wall boundary layer that occupies a significant fraction of the flow depth. When the rough bed is introduced in the shallow flow, the local turbulence near the roughness element intensifies and becomes highly heterogeneous. The model roughness under study consists of a train of two dimensional square ribs spanning the whole length of the channel. The height of the ribs ( k) occupy 10-15% of the depth of flow (d) and falls in the category of large roughness. The experimental program was designed to study k-and d-type roughnesses at intermediate flow submergence (6 d/k 10). Velocity measurements were conducted using laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) systems.;Two-dimensional PIV measurements were made in the streamwise-wall normal plane of the smooth open channel flow at d = 0.10 m and R ed = 21,000 (Red = dU0/nu) to further study the influence of the free surface on the turbulent structures. Proper orthogonal decomposition (POD) and swirling strength analysis were employed to investigate the structures present in the flow. Analysis of the POD reconstructed velocity fields reveals the presence of large-scale energetic structures near the free surface. These structures are almost parallel or slightly inclined to the free surface creating long zones with uniform momentum.;When large distributed bed roughness is introduced in the open channel, the anisotropy of the Reynolds stresses is reduced in the outer layer and found to depend on the rib spacing and roughness density. At shallow depth, the presence of roughness increases the turbulence intensities, Reynolds shear stress and higher-order moments in the outer layer of various locations along the rib wavelength. While for the shallow depth, the ratio of the shear contribution of sweep to ejection events is very different from that obtained on the smooth bed, for the deep flow cases, this difference diminishes in the outer layer.;While on the smooth bed, mean velocity scaling in the classical logarithmic format was confirmed from the present experiments, for the deep-flow cases, turbulence quantities were found to be influenced by the free surface. A modified length scale based on a region of constant turbulence intensity is proposed to account for the effect of the free surface. The new length scale provides a better description not only for the mean velocity profiles but also for the Reynolds shear stress profiles and correlation coefficients. With the use of this new length scale, the estimation of the wake parameter is positive and provides for a more accurate estimate of the friction velocity.