3D LES computations of a shallow lateral expansion using an immersed boundary method

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732In environmental shallow flows, the phenomenon of flow separation often gives rise to large-scale turbulent structures (vortex shedding). In this study, 3D LES computations of three Shallow Lateral Expansion geometries are performed. The resolved large-scale turbulent structures are studied in detail in order to allow a comparison with laboratory experiments, carried out using the Particle Image Velocimetry (PIV) technique. When LES is applied for practical cases involving flow separation, immersed boundaries are often an essential part of the geometry. These boundaries can cause problems with respect to the Navier Stokes solver used, especially regarding the pressure correction module. A solution to this problem, known as Immersed Boundary Method (IBM), is found by using body forces to ensure the impermeability of internal boundaries. The classical IBM formulation, however, makes a systematic error regarding momentum transfer in the vicinity of solid walls. In this study an adjusted IBM is proposed, based on momentum fluxes instead of body forces. The adjusted model is applied to Shallow Lateral Expansion geometries of various aspect ratios. In order to analyze the real-time large-scale turbulent structures, the vector potential function of the velocity field is computed. This is a very suitable tool to detect large-scale flow structures. The turbulence features observed in the 3D LES computation are compared with the PIV data, especially regarding the vortex shedding behaviour. An analysis of Reynolds stresses and the downstream development of eddy length scales reveals the existence of two different regimes in the vortex shedding behaviour. The difference can be explained by the interaction of shed vortices with the primary and secondary recirculation cells that are present

Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments

Large‐scale morphology, in particular meander bend depth, bar dimensions, and bifurcation dynamics, are greatly affected by the deflection of sediment transport on transverse bed slopes due to gravity and by secondary flows. Overestimating the transverse bed slope effect in morphodynamic models leads to flattening of the morphology, while underestimating leads to unrealistically steep bars and banks and a higher braiding index downstream. However, existing transverse bed slope predictors are based on a small set of experiments with a minor range of flow conditions and sediment sizes, and in practice models are calibrated on measured morphology. The objective of this research is to experimentally quantify the transverse bed slope effect for a large range of near‐bed flow conditions with varying secondary flow intensity, sediment sizes (0.17–4 mm), sediment transport mode, and bed state to test existing predictors. We conducted over 200 experiments in a rotating annular flume with counterrotating floor, which allows control of the secondary flow intensity separate from the streamwise flow velocity. Flow velocity vectors were determined with a calibrated analytical model accounting for rough bed conditions. We isolated separate effects of all important parameters on the transverse slope. Resulting equilibrium transverse slopes show a clear trend with varying sediment mobilities and secondary flow intensities that deviate from known predictors depending on Shields number, and strongly depend on bed state and sediment transport mode. Fitted functions are provided for application in morphodynamic modelin

Mean and variance of the velocity of solid particles in turbulence

Even the simplest velocity statistics, i. e., the mean and the variance for particles moving in turbulence still offer challenges. This paper offers simple conceptual models/explanations for a couple of the most intriguing observations, namely, the enhanced settling rate in strong turbulence and the reduced Lagrangian velocity variance for even the smallest of sinking particles. While simultaneous experimental observation of the two effects still do not exist, we draw parallels between two classical sets of experiments, each exhibiting one, to argue that they are two sides of the same phenomenon: Selective sampling due to particle concentration on fast tracks like those illustrated by Maxey & Corrsin (1986)

Breaching Flow Slides and the Associated Turbidity Current

This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations

Transverse bed slope effects in an annular flume

Large scale morphology, in particular bar dimensions and bifurcation dynamics, are greatly affected by the deflection of sediment transport on transverse bed slopes due to gravity and by helical flows. However, existing transverse bed slope predictors are based on a small set of experiments with a minor range of flow conditions and sediment sizes, and do not account for the presence of bedforms. In morphological modelling the deflection angle is therefore often calibrated on measured morphology. Our objective is to experimentally quantify the transverse slope effect for a large range of near-bed flow conditions and sediment sizes (0.17 - 4 mm) to test existing predictors, in order to improve morphological modelling of rivers and estuaries. We have conducted about 400 experiments in an annular flume, which functions as an infinitely long bended flume and therefore avoids boundary effects. Flow is generated by rotating the lid of the flume, while the intensity of the helical flow can be decreased by counterrotating the bottom of the flume. The equilibrium transverse slope that develops during the experiments is a balance between the transverse bed slope effect and the bed shear stress caused by the helical flow. We obtained sediment mobilities from no motion to sheet flow, ranging across bedload and suspended load. Resulting equilibrium transverse slopes show a clear trend with varying sediment mobilities and helical flow intensities that deviate from typical power relations with Shields number. As an end member we found transversely horizontal beds by counterrotation that partially cancelled the helical flow near the bed, which allows us to quantify helical flow. The large range in sediment mobilities caused different bed states from ripples and dunes to sheet flow that affect near-bed flow, which cause novel nonlinear relations between transverse slope and Shields number. In conclusion, our results show for a wide range of conditions and sediments that transverse bed slope effects are not simple functions of sediment mobility but depend strongly on bed state. We are now focusing on isolating effects of helical flow intensity and near-bed flow patterns, working towards a new transverse bed slope predictor for use in morphodynamic models

Influence of bedforms on the transverse bed slope effect

The deflection of sediment transport on a transverse slope due to gravity determines the large scale morphology by influencing bar dimensions and bifurcation dynamics. However, existing transverse bed slope predictors in morphodynamic models are based on a small range of flow conditions and sediment sizes, and do not account for the presence of bedforms. The objective of the current research is to quantify the transverse slope effect for a large range of flow conditions and sediment sizes, using an annular flume. Preliminary results show that the transverse slope is related to helical flow intensity and sediment size. Also, different bed form types developed during the experiments and appear to have a large influence on flow patterns. In order to quantify the effects of bed forms on the transverse slope, a large eddy simulation model of the annular flume is needed