Finite-size Particle Dynamics and Scalar Transport in Turbulent Open Channel Flow Over a Mobile Sediment Bed
Institute for Hydromechanics, Karlsruhe Institute of Technology (KIT)
Local Project ID:
HPC Platform used:
Hazel Hen and Hawk of HLRS
The quality of surface water typically depends upon a complex interplay between physical, chemical and biological factors which are far from being completely understood today. While in most developed countries the quality of the majority of the surface water bodies does satisfy the applicable standards under normal circumstances, strong rainfalls can cause sewage overflow events in urban areas, which typically lead to a discharge of contaminated water into adjacent streams. Such storm-water discharges typically carry pathogens in the form of bacteria, viruses and/or protozoa.
Most practical water quality predictions for rivers or streams rely on a very much simplified formulation for the concentration of fecal indicator bacteria downstream of a pollutant source. So far, turbulent transport processes have only been "lumped" into average transport coefficients which rely heavily upon empirical input. The yielding models have limited predictive capabilities and overlook the spatio-temporal heterogeneous nature intrinsic to turbulent flow problems.
This GCS large-scale project aims at pushing the modeling boundary further by performing – for the first time ever – massively-parallel computer simulations of a mathematical model which resolve all scales of: hydrodynamic turbulence in river-like flows, the micro-scale flow around rigid, mobile particles, as well as the concentration field of suspended bacteria. The computational model is applied to the transport of suspended and dissolved contaminants resulting from a sewage overflow event into a streaming water body (such as a river or a canal), where the bacteria are initially present both in freely-suspended form as well as attached to the surfaces of the particles.
While the simulation of turbulent flows itself is already a challenge requiring immense computational resources, the coupling of the flow problem with the transport of around 100,000 fully-resolved particles makes high-performance computers a necessity. Furthermore, the simulation of the transport of dissolved contamination poses stringent requirements on the resolution of the simulated fields, due to which approximately 67 billion mesh nodes had to be deployed.
The simulations within this project were conducted for several months on the HPC systems Hazel Hen and Hawk at HLRS and utilized up to 8,192 cores per run. In total, the generated dataset has a size of several hundreds of terabyte which are stored predominantly on tapes at the bwDataArchive and moved to the Large-Scale Data Facility (LSDF) at KIT on demand.
The results already provided important insights into the interaction of particles with turbulent flows, and on their capabilities to release and manipulate the concentration of dissolved contaminants. One key finding is that the velocity at which a solid contamination settles can be substantially faster than what is assumed in current water-quality prediction models which neglect turbulent effects on particle dynamics. This finding is directly related to the inhomogeneity of turbulent flows, as settling particles self-organize and segregate into cluster and void regions while settling, which is the cause of this discrepancy.
Furthermore, the usefulness of the data is not solely limited to problems related to water-quality analysis. Since the full simulated system consists of various sub-problems itself, the dataset also helps to shed light on more fundamental processes such as pattern formation in the sediment bed. In fact, within this project the evolution of a ripple-like sediment pattern has been computed from scratch at the highest value of the Reynolds number ever up to this date. This phenomenon is vividly discussed in recent literature and the data obtained within this project contributes greatly to the understanding of it, while merely serving the purpose of an initial condition to the water-quality problem.
Michael Krayer1, Markus Scherer1, Markus Uhlmann1 (PI)
1Institute for Hydromechanics, Karlsruhe Institute of Technology (KIT), Karlsruhe
M. Uhlmann, "An immersed boundary method with direct forcing for the simulation of particulate flows", Journal of Computational Physics, 2005.
J. Qian, E. Walters, P. Rutschmann, M. Wagner and H. Horn, "Modelling the influence of total suspended solids on E. coli removal in river water", Water Science and Technology, 2016.
A. G. Kidanemariam, M. Uhlmann, "Formation of sediment patterns in channel flow: minimal unstable systems and their temporal evolution", Journal of Fluid Mechanics, 2017.
M. Scherer, M. Uhlmann, A. G. Kidanemariam, M. Krayer, "On the role of turbulent streaks in generating sediment ridges", submitted to Journal of Fluid Mechanics, 2021.
Computational Fluid Dynamics group
Institute for Hydromechanics
Karlsruhe Institute of Technology (KIT)
Kaiserstraße 12, D-76131 Karlsruhe (Germany)
e-mail: markus.uhlmann [@] kit.edu
Local project ID: GCS-PASC