Lateral bed-roughness variation in shallow open-channel flow with very low submergence

Quantifying turbulent fluxes and secondary structures in shallow channel flows is important for predicting momentum and mass transfer in rivers as well as channel capacity and associated water levels. Here, we focus on the flow over a lateral bed-roughness variation with very low relative submergence of the roughness elements, h∕k ={3, 2, 1.5}, where h is the flow depth and k is the roughness height. Measurements were performed in a 1.1 m wide and 26 m long glass flume whose bed was fitted with cubes arranged in two regular side-by-side patterns with frontal densities λf = 0.2 and 0.4 to create a rough-to-rougher variation. Measurements were performed using stereoscopic PIV in two orthogonal planes, in a vertical transverse plane spanning the two roughness types, and in a longitudinal one at the interface between the roughness types. The results show that the bulk velocity difference between the two sides of the channel increases with decreasing h/k. Also, contrary to what is observed at high relative submergence with smooth-to-rough transitions, higher bulk velocities occur on the side with higher roughness. This difference is increasing as the flow becomes shallower and is shown to be due to increasing effective depths ratios, leading to increasingly lower friction factor ratios with lower friction factors on the high-velocity but rougher side. Although increasing streamwise momentum transfer at the interface is needed as h/k decreases, the turbulent and secondary circulation transfer of momentum is increasingly inhibited. A globally-driven secondary-circulation at h∕k = 3 ceases for lower h/k and roughness-scale circulation becomes dominant. Also, even the increased global shear does not lead to large-scale Kelvin Helmholtz instabilities structures. However, the relative importance of the roughness difference on the flow is augmented as the flow becomes shallower and momentum transfer due to lateral dispersive stresses increases

Flow structures in a shallow channel with lateral bed-roughness variation

Highly heterogeneous floodplains can give rise to secondary flow structures responsible for the bulk of lateral momentum exchange. Quantifying the redistribution of momentum is required to predict lateral profiles of flow velocity and the associated water level in a river. In the work herein, we focus on studying secondary flow structures and the momentum redistribution associated with a lateral bed-roughness variation in a channel with low relative submergence of the roughness elements, h=k = 3, 2 and 1.5, where h is the flow depth and k is the roughness height. A series of laboratory experiments were performed in a flume containing rows of cubes. They were arranged in two types of regular patterns, with higher and lower frontal density, and placed side by side such that the bed roughness varies in the lateral direction. The measurements were performed using stereoscopic PIV in a vertical cross plane spanning between the two roughness types. The time-averaged and turbulence statistics of the three components of the velocity field were analyzed. First, we focus on the intensity of the secondary currents. As the flow becomes shallower (lower relative submergence), the cross-stream velocity normalized by the streamwise velocity increases. A large-scale secondary current at the border between the two roughnesses as observed in [1] (though in their case between smooth and rough regions) appears for h=k = 3. As h=k decreases, this structure reaches to the same size as the secondary flow generated by the roughness elements. Also, the discharge distribution between the two sides of the channel becomes less uniform with decreasing h=k. In this sense, the relative importance of the roughness difference increases with decreasing water depth. Moreover, higher discharge is observed on the side with higher equivalent sand roughness, contrary to what is observed for smooth-to-rough transition [1, 2]. Time series of the streamwise velocity fluctuations are calculated using Taylor’s “frozen turbulence” hypothesis. In this representation, streamwise velocity streaks are apparent for h=k = 3, but they appear to lose coherence for the most shallow case of h=k=1.5

Flow structures in a shallow channel with lateral bed-roughness variation

Highly heterogeneous floodplains can give rise to secondary flow structures responsible for the bulk of lateral momentum exchange. Quantifying the redistribution of momentum is required to predict lateral profiles of flow velocity and the associated water level in a river. In the work herein, we focus on studying secondary flow structures and the momentum redistribution associated with a lateral bed-roughness variation in a channel with low relative submergence of the roughness elements, h=k = 3, 2 and 1.5, where h is the flow depth and k is the roughness height. A series of laboratory experiments were performed in a flume containing rows of cubes. They were arranged in two types of regular patterns, with higher and lower frontal density, and placed side by side such that the bed roughness varies in the lateral direction. The measurements were performed using stereoscopic PIV in a vertical cross plane spanning between the two roughness types. The time-averaged and turbulence statistics of the three components of the velocity field were analyzed. First, we focus on the intensity of the secondary currents. As the flow becomes shallower (lower relative submergence), the cross-stream velocity normalized by the streamwise velocity increases. A large-scale secondary current at the border between the two roughnesses as observed in [1] (though in their case between smooth and rough regions) appears for h=k = 3. As h=k decreases, this structure reaches to the same size as the secondary flow generated by the roughness elements. Also, the discharge distribution between the two sides of the channel becomes less uniform with decreasing h=k. In this sense, the relative importance of the roughness difference increases with decreasing water depth. Moreover, higher discharge is observed on the side with higher equivalent sand roughness, contrary to what is observed for smooth-to-rough transition [1, 2]. Time series of the streamwise velocity fluctuations are calculated using Taylor’s “frozen turbulence” hypothesis. In this representation, streamwise velocity streaks are apparent for h=k = 3, but they appear to lose coherence for the most shallow case of h=k=1.5

Experimental evidence of settling retardation in a turbulence column

International audienceSettling experiments were conducted in a turbulence column to investigate the effect of turbulence on the effective fall velocity of solid particles slightly denser than the fluid (ρp≳ρf). Five types of particles of different materials and shapes were tested, their size ranging between o(1)η and o(10)η, where η is the Kolmogorov viscous length scale. Thus, the particles were of finite size with an unknown analytical form for the fluid-particle forces. The density ratio ranged as (ρp−ρf)/ρf={0.13:1.6}, and the still-fluid particle Reynolds number as Re0p={75:981}. The turbulence levels characterized with the integral-scale Reynolds number ranged as ReL={34:510}. Two-dimensional (2D) particle image velocimetry was used to obtain flow statistics, the residual mean circulation, and the turbulence statistics, while 2D particle tracking was performed to measure particle settling velocities. For all types of particles tested, settling retardation is observed as the turbulence intensity is increased. It is found that if both the effective fall velocity Ws and the turbulent fluid velocity Wf,rms are nondimensionalized by the still-fluid particle terminal velocity W0, the settling retardation can be described by a unique relation independent of the particle type, Ws/W0=f(Wf,rms/W0), for the given range of flow regimes. Using analytical descriptions of the loitering and nonlinear drag effects, this scaling is shown to have a solid physical basis