High resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport
Final Report Abstract
Sediment beds are encountered in many natural systems, such as rivers and coastal environments, as well as technical systems (man-made canals, chemical engineering pipelines). On the one hand, the presence of the sediment influences the flow of fluid through the system, while on the other hand, the (typically turbulent) fluid flow can set the sediment in motion, leading to a modification of its own boundary. In the present project this complex interplay has been analyzed with the aid of data generated through direct numerical simulation, involving the resolution of the phase-interfaces between fluid and particles. The project consortium also involves a group at the University of Aberdeen (funded by EPSRC), where complementary laboratory flume experiments have been performed. On the technical side, we have proposed a number of important improvements over previously available simulation techniques, both concerning the fluid-particle coupling and the inter-particle contact modelling. On the physical side, we have considered configurations with fixed particles and others where sediment erosion and transport takes place in various modes, in all cases concentrating on plane open channel flow. Concerning the cases with fixed particles, we have performed simulations from the hydraulically smooth regime all the way up to the fully rough regime, considering in most cases regular particle arrangements. The scaling of force and torque acting on these particles has been explored in depth, and the gap to the experimental data at much larger Reynolds number could be bridged. Recently we have also begun to explore the impact of the regularity of the particle arrangement upon the results in the fully rough flow regime. We have explored different approaches for the description of the onset of particle erosion. The simulation of a multitude of unrelated single-particle erosion events from a mostly immobile bed has demonstrated the importance of the inter-particle collisions prior to the dislodging which typically takes place due to an intense sweep event. The dynamics of sediment transport well above the threshold of particle motion has been explored by a number of simulations. Finally, we have, for the first time, applied the Double-Averaging Method to mobile sediment with particle clustering assessing the role of form-induced stresses resulting from the uneven distribution of particles upon bed-load transport. These results are physically new and interesting. Furthermore, they provide information on the individual terms of the averaged equations which is relevant for developing models applicable on larger length scales. One of the surprising facts which we have found during the course of the project was the strong influence of wall-roughness upon the particle concentration profile in turbulent open channel flow, even at distances of many multiples of the roughness height away from the wall. Another surprise was the finding that close to the erosion threshold individual particles travelling over the sediment bed play a certain role in the mobilization of particles, constituting a sort of trigger in conjunction with particular flow structures responsible for erosion.
Publications
- Force and torque acting on particles in a transitionally rough open channel flow. J. Fluid Mech., 684:441–474, 2011
C. Chan-Braun, M. García-Villalba, and M. Uhlmann
(See online at https://dx.doi.org/10.1017/jfm.2011.311) - Collision modelling for the interface-resolved simulation of spherical particles in viscous fluids. J. Fluid Mech., 709:445–489, 2012
T. Kempe and J. Fröhlich
(See online at https://doi.org/10.1017/jfm.2012.343) - Turbulent open channel flow, sediment erosion and sediment transport. PhD Thesis, Karlsruhe Institute of Technology, 2012
C. Chan-Braun
(See online at https://dx.doi.org/10.5445/KSP/1000029253) - DNS of horizontal open channel flow with finite-size, heavy particles at low solid volume fraction. New J. Phys., 15(2):025031, 2013
A.G. Kidanemariam, C. Chan-Braun, T. Doychev, and M. Uhlmann
(See online at https://doi.org/10.1088/1367-2630/15/2/025031) - Spatial and temporal scales of force and torque acting on wall-mounted spherical particles in open channel flow. Phys. Fluids, 25(7):075103, 2013
C. Chan-Braun, M. García-Villalba, and M. Uhlmann
(See online at https://doi.org/10.1063/1.4813806) - Fluid–particle interaction in turbulent open channel flow with fully-resolved mobile beds. Adv. Water Res., 72:32–44, 2014
B. Vowinckel, T. Kempe, and J. Fröhlich
(See online at https://doi.org/10.1016/j.advwatres.2014.04.019) - On the relevance of collision modeling for interface-resolving simulations of sediment transport in open channel flow. Int. J. Multiphase Flow, 58:214–235, 2014
T. Kempe, B. Vowinckel, and J. Fröhlich
(See online at https://doi.org/10.1016/j.ijmultiphaseflow.2013.09.008) - Highly-resolved numerical simulations of bed load transport in a turbulent open channel flow. PhD Thesis, TU Dresden, 2015. ISBN 978-3-95908-002-6
B. Vowinckel
- Entrainment of single particles in a turbulent open-channel flow: a numerical study. J. Hydr. Res., 54(2):158–171, 2016
B. Vowinckel, R. Jain, T. Kempe, and J. Fröhlich
(See online at https://doi.org/10.1080/00221686.2016.1140683) - . Spatially-averaged momentum fluxes and stresses in flows over mobile granular beds: a DNS-based study. J. Hydr. Res., 2017
B. Vowinckel, V.I. Nikora, T. Kempe, and J. Fröhlich
(See online at https://doi.org/10.1080/00221686.2016.1260658) - Momentum balance in flows over mobile granular beds: application of double-averaging methodology to DNS data. J. Hydr. Res., 2017
B. Vowinckel, V.I. Nikora, T. Kempe, and J. Fröhlich
(See online at https://doi.org/10.1080/00221686.2016.1260656)
